Notes giving general background information on global climate change (the "greenhouse effect") follow. As you'll see, there are many relationships between this topic and issues that we have discussed previously, such as human population growth, agriculture, and tropospheric ozone pollution. More information on the liklihood of global climate change, feedbacks affecting it, human and ecological consequences associated with it, and policy steps that may be taken to address it, are in the sections of notes called "Probabilities, consequences and policies related to global climate change." If you want to use the study guide covering global climate change, or to obtain additional references on this topic, click on study guide .
There are many web resources that can be used to learn more about the prospect of global climate change. I list only a few here, but you will find hundreds when you search on your own, of course! I have tried to list only key sources that approach the subject from rerlatively unbiased perspectives.
The US Global Change Research Information Office (GCRIO) provides access to data and information on global climate change research, adaptation/mitigation strategies and technologies, and global change-related educational resources on behalf of the various US Federal agencies and organizations that participate in the US Global Change Research Program.
The National Center for Ecological Analysis and Synthesis, funded by the National Science Foundation, the State of California, and UC Santa Barbara provides information on a variety of topics related to global climate change (and others as well).
NASA's Goddard Space Flight Center's Distributed Active Archive Center offers NASA Mission to Planet Earth resources to study the greenhouse effect, El Nino's effect on weather and climate, the role of oceans in global climate and more. Provided are data, images, and documentation.
The Consortium for International Earth Science Information Network offers a gateway site, leading to additional sources of information pertinent to the study of global environmental change.
The Environmental Protection Agency offers state climate change impacts sheets, which summarize expected climate change in each state and explain the projected consequences on a variety of resources. Additional resources include a bibliography, glossary, and internal search engine.
The Environmental Protection Agency also supports a site that summarizes data on US greenhouse gas emissions and sinks for the years 1990 through 1999.
The National Oceanic and Atmospheric Administration maintains a National Geophysical Data Center, which leads to information on paleoclimatology, meteorology, ice and snow, and many other types of geophysical information.
Summaries of key climate change data sets, graphs, data tables, and so forth are available through "Trends Online," from the Carbon Dioxide Information Analysis Center (CDIAC) at Oak Ridge National Laboratory. This is referred to as the "single best source" of information on fossil fuels use, CO2 and temperature trends.
Information on potential impacts of climate change on the US is provided by scientists at NOAA and other experts. Prospects for climate change in the Pacific Northwestern US is provided at a site hosted by OSU.
The Intergovernmental Panel on Climate Change (IPCC) is a consortium of over 2000 scientists and policy analysts from over 100 nations. This group, convened by the UN and the World Meteorological Organization (WMO), is charged with conducting state-of-the-science reviews of knowledge about climate change, and has produced a major report on the topic four times (1990, 1995, 2001 and 2007). Their reports are highly anticipated and respected by the international climate change community.
An excellent site for information about renewable and conventional energy -- pros and cons, new developments, etc -- is sponsored by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL).
To help you find what you are looking for in this topic, notes that follow are organized acording to the following outline. Click on the topic to move to that place in the notes. After you have looked at the topic, you can either continue scrolling through the subsequent topics, or return to the following outline to click on the topic that you want. You can use the box labelled "CONTENTS" to move to the master directory for the BI 301 home page. (For more information on navigating within and among these documents, click "Naviga te".)
Disclaimer: I have updated most, but not all, statistics on this topic for 2007, so if I tell you something in lecture that differs from what you read here, take the lecture information as most current.
I. What is the "greenhouse effect?"
ll. Carbon dioxide (CO2)
III. Global temperatures
WHAT IS THE "GREENHOUSE EFFECT"?
Based on the position of Earth relative to the sun and characteristics of Earth's surface, the average temperature of Earth's surface should be -18o C. However, the mean temperature is closer to +15oC. Why this difference?
Earth and its atmosphere receive radiant energy from the sun. This radiation arrives in various wavelenths:
About 1/3 of this incoming radiation is reflected from Earth's surface or from clouds and particles in atmosphere, without being absorbed. The remaining 70% is absorbed by the atmosphere or by Earth's surface.
This absorbed energy is then ultimately re-emitted at longer, lower energy wavelengths, such as infrared wavelengths (longer wavelengths are less energetic), by the atmosphere and Earth's surface and organisms. (Remember the law of thermodynamics that says that energy is degraded with each transfer?)
Some of the energy radiated from atmosphere goes out to space, while some travels to Earth, where it is absorbed and, ultimately, radiated out again. Energy that is radiated out from Earth either:
(1) passes through the atmosphere and out to space (approximately 10% of the energy radiated from Earth), or,
(2) is absorbed by clouds, gases, and particles in the atmosphere, after which much is then returned to Earth as slightly longer wavelength (heat) radiation (approximately 90% of the energy radiated from Earth).
This energy then warms Earth. Without an atmosphere capable of trapping and re-radiating energy, the earth's surface would be below freezing (about - 18oC) rather than the current +15 o C (59 o F). Thus, Earth is warmed largely from trapping and re-radiation of heat -- infrared radiation -- by gases and particles in the atmosphere. This trapping and re-radiation of heat by gases in the atmosphere is called the "greenhouse effect."
The analogy to a greenhouse is loose, but is based on the fact that the glass in a greenhouse is transparent to incoming radiation, but blocks some of the outgoing, longer wavelength radiation, causing the greenhouse to be warmer than the outside air. The atmosphere acts similarly, in that it allows much of the incident shorter wavelength radiation in -- is relatively transparent to it -- but doesn't allow as much of the outgoing, longer wavelength radiation to pass on through. (In fact, much of the warming in a real greenhouse is simply caused by lack of mixing of air.)
The gases that are most active at trapping this radiant heat energy are referred to as "greenhouse gases" or "radiatively active gases."
Thus, Earth's temperature is not simply
a function of our distance from the sun, but also of gases in
our atmosphere.
The greenhouse effect, and humans' understanding of it, is not
new. People have understood that Earth is warmed by greenhouse
gases in the atmosphere for well over a century. For example,
people realized long ago that water vapor is important as a greenhouse
gas, and that one of the most important greenhouse gases in the
atmosphere is carbon dioxide (CO2). The relationship between CO2
and global temperatures was pointed out as long ago as 1896 by
a Swedish chemist named Arrhenius, who suggested that coal combustion
in industrializing Europe might increase atmospheric concentrations
of CO2 and result in global warming! (As we'll see, CO2 only comprises
about 0.03% of Earth's atmosphere (about 368 ppm) but is very
important in our energy balance.)
SO WHAT'S THE HOOPLA?
If the greenhouse effect is a natural phenomenon, why be concerned about it? The concern is about an acceleration of the greenhouse effect, caused by human-influenced emissions of radiatively important gases, including CO2. We often hear about it as though the greenhouse effect itself is a product of humans polluting the atmosphere, but that's not true; there was a greenhouse effect long before there were humans. The real concern is about an increase in atmospheric concentrations of radiatively active gases.
In the following sections, we'll look at several of these gases; their sources, effectiveness at trapping and re-radiating heat, and trends in atmospheric concentrations. We'll begin with CO2, and then we'll look at other trace gases, including water vapor, tropospheric ozone, methane, nitrous oxide, and halocarbons.
Carbon dioxide (CO2 ) is one of the most important greenhouse gases (second only to water vapor), and is one of the gases of most concern. Its concentration in the atmosphere has increased greatly over recent decades, owing to human activities.
Before we discuss ways by which human activities can affect concentrations of CO2 in the atmosphere, we need to understand a bit about the natural cycling of carbon on earth and its atmosphere.
One important component of carbon cycling is the biological carbon cycle. I'm sure that you are familiar with this. It basically involves:
CO2 uptake by plants in the process of photosynthesis, with conversion of a fraction of that CO2 into organic material (sugars, initially) and
Release of CO2 by respiration of plants, animals, and microbes.
The biological carbon cycle involves a huge amount of carbon; more than 20 times the quantity that is released each year by fossil fuel combustion!
Until recently, the biological carbon cycle was usually balanced; as much was taken up by plants in photosynthesis as was given off in respiration and decomposition. That is, uptake of CO2 from the atmosphere was balanced by its release back to the atmosphere. Thus, there was no net gain or loss of atmospheric CO2, and the biomass on earth was roughly constant.
There have been times, historically, when biomass accumulated. One notable example is the Carboniferous era, when much of the biomass that later became our fossil fuels was deposited. This was a warm and damp period, during which uptake and storage of carbon from the atmosphere must have been greater than its release via decomposition and respiration. That is, there was a net increase in biomass (carbon storage) during this time.
Currently, the biological carbon cycle in the broadest sense may not be balanced, because of human influences on it (see below for details). It seems that more CO2 may be being released via respiration, decomposition, and burning than is being taken up by plants. That is, we may be experiencing a net loss of biomass on Earth at present. However, this is not certain -- we don't know for sure to what extent deforestation is being balanced by aggrading forest area. Many scientists do believe that "anthropogenic land conversion" (basically, deforestation, and largely in the tropics) constitutes a large net input of CO2 to the atmosphere, but some suspect that aggrading forests elsewhere take up enough CO2 to balance a good bit of the CO2 given off in deforestation.
Deforestation does two things that affect CO2 concentrations in the atmosphere:
1. It removes a large sink for CO2
(a "sink" is just what it sounds like; an area or entity
that removes more of something than it releases), and it
2. adds a large source of CO2 to the atmosphere (via burning
after logging, or and decomposition)
Until 1992, most scientists thought that this land conversion would constitute a net addition of CO2 to the atmosphere, particularly because most tropical deforestation is not followed by regrowth of forest, but rather by conversion of former forest to vegetation that does not take up and store as much carbon as do forests. However, new evidence suggests that this anthropogenic land conversion may be at least partially offset by accelerated uptake (aggrading forests) in some areas of the world, particularly parts of North America and Europe (see below), as well as China, which has been conducting a vigorous reforestation and afforestation program in recent years.
Returning to our discussion of global carbon cycling in general, it is important to remember that this biological cycle is only a small component in a much larger and slower-behaving cycle in terms both of the total amount of carbon involved and time scales. The geochemical carbon cycle governs the transfer of carbon between rocks, atmoshpere, biosphere, and oceans over long (geological) time scales. (See article from Scientific American on the supplementary reading list for more information of this geochemical carbon cycle.) Briefly, the major pool of carbon on Earth is in sedimentary rocks, formed from past living creatures. Carbon is released from these rocks back into the atmosphere, but over very long time scales, through plate tectonic activity, volcanism, and so on. Thus, the geochemical cycle controls atmospheric CO2 concentrations over extremely long time windows. And, over the long term, the geochemical carbon cycle is generally balanced. As for the biological carbon cycle, there have been times of considerable fluctuation in carbon pools in the past. For example, 100 million years ago, it is estimated that there was about 10 times more CO2 in the atmosphere than there is now, and it was much warmer during that time than it is at present. It may be that there was tremendous volcanic activity as the ancient continent of Pangea broke up, with resultant degassing of CO2 from geological pools into the atmosphere.
The point is; what is happening to concentrations of CO2 in today's atmosphere is likely to be just a small "blip" in the long term story of Earth. While the current blip is likely to be small, and short term, from a geological perspective, it has potential to cause serious consequences for humans and other ecosystem components, as we'll see. I simply think it useful for us to put our influences on the atmosphere into some perspective, and to be humbled a bit by the recognition that we may not be the "big guys" that we think we are!
Now let's look explicitly at the question: How do humans affect the carbon cycle?
What human activities affect atmospheric concentrations of CO2? Two kinds of activities are most important:
(1) Fossil fuel burning
(2) Changes in land use (largely, deforestation)
The CO2 concentration in the atmosphere has increased nearly 30%
since preindustrial times (~ 1860 or so). Back then, the concentration
was about 280 ppm, whereas by now (2004) it is slightly over 370
ppm!
Why this increase in concentration? Well, what are the human-influenced inputs or fluxes of carbon to the atmosphere?
(1) Burning of fossil fuels -- the biggest single human-influenced net source of CO2 to the atmosphere. This combustion oxidizes organic carbon, with carbon and oxygen combining to yield CO2. The major period of fossil fuel burning on Earth has, of course, been since the dawning of the industrial age in the mid- to late 1800's.
(2) Anthropogenic land conversion. Recall that this concerns deforestation, in particular.
I already mentioned that, historically, CO2 taken up in the biological carbon was approximately equaled by CO2 released in the biological cycle. Thus, historically:
GGP-- global
gross primary production (the amount of carbon fixed by plants)
equalled
ER (global ecosystem respiration, comprised of respiration
by plants (AR – for autotrophic respiration) plus respiration
by all other living things on land (HR – for heterotrophic
respiration))
There was no net flux of carbon to or from the atmosphere
on a global basis, and there was no net change in carbon storage
in terrestrial ecosystems (globally).
However, humans have recently been converting forested landscapes to grazed, cultivated, or urban landscapes. This:
(1) Removes a large sink for atmospheric carbon (because forests take up and store larger amts of carbon than do other terrestrial ecosystems), and
(2) Adds a large source for atmospheric carbon (when the trees decay or are burned, releasing carbon). (About 80% of the wood removed during tropical deforestation is destroyed (burned or decayed) or used as fuel wood, so the carbon stored in it is released rapidly as CO2, as opposed to the delayed slow release that accompanies its being used for lumber.)
A mature forest stores a large amount of carbon. When cut, it is often replaced by an ecosystem that stores less carbon, (pasture or crops) with the difference in carbon content of the ecosystems being balanced by a flux of CO2 to the atmosphere.
Thus, there is an increased flux of carbon from terrestrial ecosystems to the atmosphere, resulting from this land conversion. It is estimated that the net input of CO2 to the atmosphere from ALC is about 1/5 - 1/3 as much as from fossil fuel burning -- quite a range of estimates!
Most of this increased flux now comes from tropical Africa and Asia, but until about 1920, North America actually provided the largest ALC flux to the atmosphere.
You can see result of this ALC in the size of pools stored in the terrestrial compartment. These pools were thought to be continuing to shrink when this figure was created a few years ago (and certainly have shrunk since preindistrial times!)
However, there is a much uncertainty concerning the magnitude of fluxes associated with tropical deforestation, and whether it does in fact represent a net flux. (That is, some people now believe that deforestation's inputs are counterbalanced by aggrading forests – see below). The data shown in this systems figure represent just one estimate. The current range of estimates for fluxes from tropical deforestation is from 1.1 - 3.6 billion metric tons of C/year, which would be between 20-65% as much as from fossil fuel emissions. Quite a huge spread in estimates! Most estimates agree that deforestation (and other anthropogenic land changes) contribute between 1/5 -1/3 as much CO2 to the atmosphere as does fossil fuel combustion.
Why is there so much variation in estimates of the size of fluxes resulting from deforestation in the tropics? Several reasons, all relating to uncertainty in estimating the factors that contribute to the fluxes:
(1) What
are actual rates of deforestation (and what counts as deforestation
-- e.g., does selective logging count, or must it be clearcutting?)
(2) What is the fate of deforested land? Is it cleared
permanently or temporarily, and what proportion is treated in
each way?
(3) How big are the stocks of carbon stored in these forests?
Estimates of the standing stock differ by a factor of two!
It is important to realize that contributions to CO2 fluxes from logging per se are considered negligible. For example, much logging in the tropics is selective, and while much is harmed in getting the logs out, re-growth compensates. Most important in terms of CO2 fluxes are the conversions of forest lands to pasture acreage, croplands, acreage devoted to shifting agriculture, and degraded lands. None of these types of vegetation take up and store as much carbon as do forests.
While deforestation does certainly contribute to a CO2 flux to the atmosphere, there is recent evidence suggesting that this increased flux may be counterbalanced to an unknown extent by growth of forests (with consequent carbon uptake) in some places in the world. Places where forests may be taking up and storing enough carbon to counterbalance the increased flux from tropical deforestation include portions of Brazil, Europe, China, and North America. Biomass of forests in many of these regions has, apparently, increased over recent decades, with consequent storage of carbon.
In terms of sources and sinks, then, tropical deforestation does constitute a source of carbon to the atmosphere, but forests in other areas of the world might sinks for carbon that counter balance some of this source. (See the IPCC's report, referenced on the supplementary reading list. )
Why might biomass (and carbon storage) of some areas be increasing? There are several potential reasons, including changing land use (e.g., afforestation of areas that were formerly fields, pastures; drainage and conversion to forest of wetlands; and fire suppression, which allows woody vegetation to encroach on areas that weren't formerly occupied by woody vegetation), and possible effects of CO2 fertilization and/or nitrogen fertilization from atmospheric deposition. In addition, in some areas, humans are fertilizing forests deliberately, fostering their growth (are troubles from this fertilization likely down the road, as we are experiencing with fertilizer inputs in agricultural systems?) Currently (early 2000's), changes in land use are believed to be the dominant force behind increased carbon storage in forests, with contributions from CO2 fertilization or nitrogen fertilization believed to be relatively minor, although there is uncertainty about that too.
Soils also store much of the carbon found in the terrestrial compartment. In fact, more carbon is stored in soils (including peat) than in all of the vegetation of the world! Carbon storage in soils has also shrunk since preindustrial times. Respiration and decomposition increase with ALC, and under most kinds of agricultural systems, such that carbon storage in soils is decreasing.
ARE THERE SINKS FOR CO2 IN ADDITION TO THOSE PROVIDED BY VEGETATION AND THE ATMOSPHERE?
Where does the added CO2 go, besides into the atmosphere? It is estimated that only about 50 % of the CO2 emissions actually stay there -- where do the rest go? The oceans represent a major sink for carbon. Oceans take CO2 up through chemical and biological means.
Chemically, ocean waters absorb CO2 by the formation of carbonic acid:
CO2 + H2O <-----> H2CO3
The double-headed arrow on this equation indicates that this is an equilibrium reaction. Hence, as CO2 in the atmosphere increases, more is taken up by the oceans, "pushing" the reaction towards formation of H2CO3 (carbonic acid). It is estimated that the oceans are now taking up more CO2 from the atmosphere than they have in the last 20 million years! There is also recent evidence that the oceans may actually be becoming slightly more acidic over the past century as a consequence of this accelerated uptake of CO2, with possible consequences for much of the oceanic ecosystem (including creatures whose shells are made of calcium carbonate, which is broken down when conditions are acidic enough. There are a couple of papers that address this concern on your supplementary reading list for this unit (and the IPCC report addresses it too).
Oceans also take up CO2 biologically, largely through photosynthesis of plankton and other algae. This "fixed" carbon is eventually removed from the water by biochemical processes (for example, the algae are eaten by shell fish, which die and sink to the ocean floor, eventually forming carbonates and entering the long term geochemical cycle that we discussed above.
The oceans hold 50-60 time more carbon in various forms than does the atmosphere, which holds it mostly as CO2. Some parts of the ocean are major sinks; such as the North Atlantic during the spring planton bloom (population explosion). On the other hand, some areas of the oceans are net sources, such as the equatorial Pacific. On balance, however, oceans are net sinks for carbon.
Can ocean uptake counter human-influenced net fluxes of CO2 into the atmsophere? The oceans are currently taking up more carbon than they are releasing, but we don't know how rapidly they can take it up in response to increases in atmospheric CO2 concentration. As we'll see, this is a big uncertain factor in attempts to model likely changes in atmospheric CO2 and climate!
Thus, there is much uncertainty in estimates of what fraction of extra CO2 emissions the oceans are really taking up. Fairly recent estimates (IPCC 1995) put the net input to the atmosphere at 7 billion tons (from ALC and fossil fuel burning). Of that, about half (3.4 billion tons) are estimated to stay in the atmosphere, with a net influx into the oceans of 2 bill tons, or about 28 % of the total extra CO2 emitted into the atmosphere. (Net influx means ocean uptake in excess of its giving off of CO2 back to the atmosphere).
Notice anything wrong here? Seven billion tons into the atmosphere and only a total of 5.4 billion of those tons accounted for! The remaining 1.6 billion tons represents the "missing carbon mystery!" What happens to the remaining ~ 20% (or more)? (See articles on your supplementary reading list for more on this mystery.)
No one really knows where that missing carbon is – perhaps component estimates are in error (increasingly it is thought that the oceans are actually taking up more than was previously thought...) perhaps it is being taken up by the aggrading forests that we discussed previously – no one knows yet!
These uncertainties aside, it is clear that humans are putting tremendous quantities of CO2 into the atmosphere, and that fossil fuel combustion is currently the most important contributor to that flux. While this is true at present, over the period 1860-1980, the global emission of CO2 from landscape changes has about equaled that from fossil fuel burning. At present, however, fossil fuel burning is a more important source of CO2 to the atmosphere than is anthropogenic land conversion (ALC).
What are trends in CO2 emissions from fossil fuel combustion?
Not surprisingly, emissions increased rapidly over the last century (see the graph of these emissions on your handout from class). A temporary decrease (about 7%) that occurred between 1979 - 1983 (recession-related) wasn't sustained, as emissions increased again 1883-91. Between 1990 and 1991, global CO2 emissions from fossil fuel burning did decrease again (about 0.2% /yr), largely because of changes in Eastern Europe and the former USSR. Those areas of the world experienced both increasing efficiency of fossil fuel use (owing in part to decontrol of prices and the closing of inefficient, uncompetitive industries) and decreased economic activity during this time.
These decreases in E. Europe and the USSR were big enough to offset increases in emissions from lesser developed countries (LDC's) temporarily. Hoever, global emissions have basically climbed over the last century, and continue to do so. Most projections about future emissions are not cheery; if present trends continued, global emissions would approximately triple before stabilizing, as population and industrialization increase in LDC's! Let's hope, however, that present trends don't continue, as they shouldn't, under provisions of the climate change treaty that nations are trying to ratify.
WHAT IS THE GLOBAL DISTRIBUTION OF EMISSIONS OF CO2 FROM FOSSIL FUEL BURNING?
Not surprisingly, the US contributes almost 25% of total world CO2 emissions (and our emissions increased more than 10% during the 1990's alone!). China was second until the 21st century, but by 2007, as passed ther US in terms of CO2 emissions and her emissions are continuing to grow rapidly. Brazil, India and Indonesia all had big increases during the 1990 - 1995 period as well (20, 28, and 40% increases respectively). In general, emissions are increasing from the LDCs and leveling (or even decreasing) from the more developed nations.
In per capita terms, the US is still the
big leader, as indicated by the following data for 1992 (tons
of carbon per person per year):
.
US 5.4; Canada 4.2; Russia 4.0; Germany 3.1; Japan 2.4; and China
0.6.
TRENDS IN ATMOSPERHIC
CO2 CONCENTRATION
OK, given these emission trends, what is happening to concentrations of CO2 in the atmosphere?
There is no doubt that CO2 concentration in the atmosphere has been increasing over recent decades. We know this because this gas has been measured in the atmosphere for several decades. This is a nice example of the usefulness of having long term monitoring data – if CO2 concentrations hadn't been monitored for long, we could say little about whether increased emissions really were significant enough to increase atmospheric concentrations. Unfortunately, there are many phenomena in environmental science for which such long term data are lacking, crippling our ability to make policy decisions. The case of CO2 is NOT one of these, however!
In 1958, Dr. David Keeling established a site to monitor atmospheric CO2 at Mauna Loa Observatory in Hawaii. The figure that I gave you in class, based on monitoring from this site, is by now quite famous. It is clear that CO2 concentrations have increased progressively since monitoring began there. The average rate of increase since 1958 has been about 0.4%/year, which is an absolute increase of about 1.5 ppmv per year. (Recall, gas concentrations in air are often measured in "ppmv" which is parts per million by volume.)
The increase since the mid-nineteenth century (or preindustrial time) has, of course, been greater than this and is over 30% (from about 280 ppmv to the current ~ 370 ppmv). Current concentrations are the highest they have been in the last 150,000 years! (We'll see in a bit how we know what atmospheric CO2 concentrations were 150,000 years ago.)
Note the annual ups and downs in the CO2 concentrations displayed in the Mauna Loa data? These result from the net uptake of CO2 by vegetation during the growing season and its net release during less favorable seasons. That is, it essentially represents the biota breathing. I think that it is fascinating that this seasonal signal from the biota is big enough to show clearly as annual ups and downs in atmospheric concentrations! (It reflects, largely, seasons in the northern hemisphere, which has more land mass and vegetation than does the southern hemisphere.)
Monitoring data from elsewhere in the world mirror the trend and pattern displayed in the Mauna Loa data set, although none come from as long a record.
Note that, because the actual concentration of CO2 in the atmosphere is quite low (370 ppm = 0.037 %), its concentration can be increased readily by additional inputs. That is, a relatively small input in absolute terms can mean a large change in terms of percentage.
CO2 persists for quite a long time in the atmosphere; its atmospheric residence time is on the order of decades to a century or so.
This increase in atmospheric CO2 is predicted to cause an increase in global temperatures and in global precipitation; how much, how fast, and how distributed globally are the important questions. We'll discuss these questions and present understanding of answers later.
There are greenhouse gases other than CO2 whose concentrations are also increasing and will discuss them in a moment.
LINKAGES BETWEEN ATMOSPHERIC CO2 AND TEMPERATURES?
What evidence do we have that concentrations of CO2 in the atmosphere are really related to global temperatures? Evidence comes from a variety of sources, and reflects relationships between gas concentrations and temperatures over a wide range of time scales.
Evidence from the ancient past links timing of plate tectonic activity (which is associated with abundant volcanic activity and degassing of CO2) with climate changes, and evidence from the more recent past links atmospheric [CO2] with climatic fluctuations during the ice ages in the Pleistocene.
How do we know what CO2 concentrations in the atmosphere were thousands of years ago? One source of information is polar ice, which forms from snowfall accumulating over centuries. Annual layers are formed in this ice, and air bubbles in the ice trap gases from the time when the ice was formed. Concentrations of gases in these ancient air bubbles can be measured.
How about ancient temperatures? The ratio of various isotopes of oxygen (16 and 18, based on the number of neutrons per atom) and of various isotopes of hydrogen in the water comprising the ice is dependent on the temperatures prevailing at the time of its formation, hence analysis of these isotopic ratios gives insights into temperatures.
I gave you a figure in class representing fluctuations in CO2 and temperature over the past 160,000 years. This data set comes from a 2 km long (2000 m) ice cores from Antarctica (the Vostoc ice core). This 160,000 years encompasses our current interglacial period, the last 100,000 yr ice age, a previous interglacial, and an even earlier ice age. It shows clearly that atmospheric CO2 varied in parallel with temperatures; when temperatures were up, so was CO2. (Concentrations of another trace gas, methane, also varied in parallel with CO2 – we'll talk about methane later.) For example, temperatures fell in synch with CO2 at the onset of the last glacial period and then these rose together as the ice retreated about 10,000 yr ago.
Globally, temperatures were about 5 degrees C cooler during glacials than during interglacials, while concentrations of CO2 were about 25% lower during glacials than during preindustrial interglacials. (And, by now, we've seen another ~ 30 % increase since preindustrial times...) (Note that data in the figure I gave in class are from the ice core area, so the difference in temperature between glacials and interglacials shown in the figure is greater than this global average. That is, temperature differences between glacials and interglacials are greater at high latitudes and less at equator -- as we'll see, the same pattern is being found in today's warming.)
We hit a post-glacial climate optimum about 6800 years ago, and a general cooling has been in place since then.
The question raised by these data, of course, is which came first, the chicken or the egg?
That is, did temperature increases cause CO2 concentrations to increase, because of effects on biological processes and changes in ocean circulation or vice versa? Scientists still aren't sure, but it is likely that they are interactive. Orbital forcing may initiate (trigger) the changes in climate, and then gases, changing in response to the change in temperature, amplify the temperature changes.
For example, ocean uptake of gases such as CO2 increases as cooling occurs (remember gases have higher solubility in colder water). This pulls CO2 out of the atmosphere, and would serve to amplify the orbitally-induced cooling. As another example, the volume of oceans decreases with cooling (as more water is frozen), which results in nutrients in the water becoming more concentrated, and fostering algal blooms, which pull CO2 out of solution, again amplifying cooling. (These algal blooms also result in the production of sulfate [from DMS – dimethyl sulfide -- produced by the algae], which, as we'll see enhances cooling still further.)
However, bottom line is that it doesn't really matter which comes first; what matters is that we see that there is global climate sensitivity to radiative gases. The current best guess is that trace gas changes account for about half of the glacial/interglacial climate changes.
A side note: what triggers glacial periods? It is believed that they are driven (or at least initiated) by small changes in the amount of sunlight reaching Earth at various latitudes and seasons, with these changes caused by three orbital effects related to whether Earth's orbit around sun is more circular or elliptical, and to the tilt of its axis relative to sun. These changes are believed to trigger the ice ages, but are not considered to be enough to produce the size of climate changes that occur. Changes in gas concentrations, as indicated above, are considered essential to cause changes of the magnitude actually observed.
WE ALSO HAVE MORE RECENT RECORDS OF TEMPERATURE AND CO2
I gave you a figure in class representing fluctuations in temperature and CO2 concentrations over the past 100 years. Data such as these are derived from a variety of sources, including:
· corals, which preserve the oxygen
isotope ratio of the water (remember its temperature sensitivity)
and can be dated
· glacial ice, whose rate of retreat can be calibrated
to temperature (incidentally, all measured glaciers in various
parts of the world, including those in the Cascades, are retreating)
· CO2 trapped in layered lake sediments
· carbon isotopes in tree rings, which can, of course,
be dated
As illustrated on the figure I gave you in class, during the last 100 years, atmospheric CO2 increased another ~ 30% above its interglacial level. (Concentrations of methane doubled again too.)
SO, WHAT IS HAPPENING TO GLOBAL TEMPERATURES?
Data on temperatures from a variety of recording
stations and instruments give us reasonable surface air temperature
data for the past 100 years or so. After applying corrections
for urban heat, and other factors, we can see that recent decades
have been unusually warm. (See figures from class.)
I used to say that the 1980's were the warmest decade on record
(since instrumental monitoring began) but the 1990's beat the
80's.
El Nino was responsible (partly) for some of the recent extra-warm years, such as 1998, but even after correcting for the influence of these events (El Nino's) the 90's remain the warmest decade since instrumental monitoring of temperatures began.
A BRIEF PRIMER ON EL NINO
El Nino is a situation that occurs when a pool of warm water that
is usually centered in the western Pacific expands to the East,
usually by December. This tropical warmth displaces the jet streams
that then steer unusual weather into various regions of the world,
typically warming the northern US. Here's what happens:
Normally, tropical trade winds blow E to W across the equatorial Pacific, dragging water along and dumping evaporated water as monsoons over Indonesia. The surface water moves W as well, and near the equator, gets diverted poleward by Earth's rotation. The divergent flow (that is, net movement of water from E to W) causes upwelling of deeper cooler water, especially in the E Pacific (because surface water is being blown away from there) where the thermocline is shallower. As deeper cooler water comes up, it brings nutrients from beneath that feed productive food chains off Peru and Chile. During an El Nino, the trade winds weaken, warm water stays in the E Pacific, monsoons fall over the ocean instead of over SE Asia, the thermocline along the coast of Chile and Peru flattens and deepens, and marine life declines as nutrients that support it fail to be recharged by upwelling.
El Nino's used to come every 7-8 years, but during the 1990's, there were several in a row. No one is sure if global warming is related to this increased frequency and duration of El Ninos or not, but global warming effects are likely to intensify the effects of weather extremes associated with El Ninos.
BACK TO TEMPERATURE TRENDS:
You can see that 1992 and 1993 were cooler than adjacent years (after correcting for 1992 being an El Nino year), probably because of Mt. Pinatubo's eruption in June of 1991. What should a volcano have to do with global temperatures?
A volcano injects large quantities of SO2 into the atmosphere, much of which is converted to SO4 (sulfate) in the stratosphere. Much of the sulfate is present as very small, aerosol-sized particles, which cause cooling by reflecting incoming radiation. The sulfate particles also serve as condensation nuclei for high thin clouds, which also increase the albedo (reflectance, essentially) of the stratosphere. The aerosol sizes are so small that they are more effective at reflecting shortwave solar radiation than they are at attenuating longer wavelengths of radiation emitted from Earth.
Climate modelers predicted that the eruption of Mt. Pinatubo would decrease global mean temperatures by about 0.5 degrees C -- enough to temporarily mask warming -- and this did occur between the summers of 1991 and 1992 (briefly impeded by an El Nino early in 1992). Most of the particles resulting from the eruption then precipitated out of the atmosphere, so by 1993, global temperatures were back on an upswing. Climate modelers were pleased that global temperatures responded as predicted to the eruption of this volcano, as the correspondence between model predictions and actual events serves to validate the models; they accurately predicted these climate consequences.
The warming over recent decades is uneven over the globe. It tends to be greatest over mid- latitude continental areas (40 - 70 degrees N). For example, summer temperatures in N Siberia have recently been hotter than any time in the past millenium. The Arctic region has warmed at more than twice the global rate in the last 50 years.
In 2007, the IPCC's summary indicated that the global average temperature has inceased 0.76 degrees C (~ 1.4 degrees F) when comparing current (2001 - 2005) average temperatures with those from 1850 - 1899, an increase that is unprecedented during the last 1000 years. Remember that these data reflect global means; warming has been more in some places and less in others. The warming of the last 25 years has taken place faster than that during any comparable period in the record; that is, it seems that the rate of warming is accelerating.
DOES THIS WARMING MEAN THAT WE ARE EXPERIENCING THE PREDICTED ACCELERATION OF GREENHOUSE WARMING?
This is, of course, the "$64,000 question!" It is critical to know whether this is really unnatural warming. That is, are we outside of the normal range of variability in temperatures? In the summer of 1988, NASA scientist James Hansen testified to the US congress that "greenhouse warming is here." At that time, he was attacked by many others as being premature in his claim. We still don't know for sure whether the warming trend is unnatural. There is much natural variability in the climate system, probably related to changes in ocean circulation, perhaps to variations in the sun's intensity, volcanic activity, etc.
To determine whether warming is unusual and potentially attributable to human activity, what do we need to know?
(1) What natural climate variability is like over relevant time scales.
(2) Whether human influences on radiative gases could be enough to cause the observed warming. To address this, scientists use large computer models that we'll say more about later.
(3) What is the "signal to noise ratio." That is, how large is the trend compared to the background variability in temperatures. Scientists run computations, and test whether the ratio exceeds some statistical detection threshold, using a variety of statistical techniques ("fingerprint methods.")
Let's look at the first and third questions: What is natural climate variability like over relevant time scales and how does the current variability compare to that?
(A) As an average over the Northern hemisphere for summer, recent decades have been the warmest since at least 1400, judging from the data that are available. (Prior to about 1400, data are insufficient to provide good temperature estimates on a hemispheric basis.)
(B) The warming over the past century began during one of the colder periods of the last 600 years.
(C) Ice core data suggest that we are now as warm as any century in the past 600 years, although the recent warming is not exceptional everywhere.
(D) Large and rapid climate changes occurred during the last glacial (20,000-100,00 years before the present) and during the transition towards the present interglacial (the last 10,000 years, known as the Holocene). During this time, we know that there were changes in annual mean temperatures of about 5 degrees C over a few decades in at least a few places (Greenland, Antarctica, and the N Atlantic), probably linked to changes in ocean circulation.
(E) Temperatures have been less variable during the past 10,000 years and it is unlikely that global mean temperatures have varied by more than 1 degree C in a century during this time.
(F) Statistical analyses of current variability compared to that in the past 10,000 years and comparisons of model predictions of what should happen to climate given increases in gases compared to what has actually happened have led to the following conclusions:
(1) Schneider (see references by him on your supplementary reading list) argued that 0.5 degree C anomalies occur 1 or 2 times per millennium and that there is an 80-90% heuristic likelihood that the 0.5 degree C warming trend observed over the past century at the time he was writing these papers is not wholly natural. (By now, in 2007, the warming is coser to 0.76 degrees C, which would be even less likely to occur for natural reasons.)
(2) Less conservative is the same James Hansen referred to above. He points out (pg 36 in "The Challenge of Global Warming") that the standard deviation of annual global mean temperatures about the 30 year mean (1951-1980 climatology) is 0.13 degrees C. Thus, the recent warming we've observed is over 3 standard deviations from that mean. The probability of a chance warming of that much is about 1% -- that is, 99% of the time, these data would be indicating a real warming trend rather than a chance fluctuation.
(3) Most important, the UN's IPCC (Intergovernmental Panel on Climate Change) (a consortium of over 2500 scientists, convened by the UN and the World Meteorological Organization (WHO) has become increasingly certain that human influences are detectable in the current warming.
In their 1990 report, they stated, "...observed warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability....the observed increase could be largely due to this natural variability..."
In their 1995 report, they concluded: "The balance of evidence suggests a discernible human influence on global climate."
Their 2001 report strengthened this language, making it clear that they no longer entertain any doubt that the current warming has been influenced by human-related emissions of greenhouse gases. They wrote, "There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities."
In 2007, they wrote, "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." (Note -- they define "very likely" as having probability > 0.9, and "anthropogenic greenhouse gas" excludes water vapor.)
Most nations that attended the Kyoto conference on global climate in December of 1998 took the position that humans are already influencing the climate – and are sure to do so more in the future – and so we better take steps to minimize this influence. Strikingly, even the governments of oil producing nations such as Saudi Arabia and Kuwait joined the consensus, which was affirmed at a UN conference of 500 delegates from about 120 nations at Rio in 1992 (the "Framework convention on climate change".)
(We'll say more about it later, but the purpose of the Kyoto conference was that parties to the framework convention on climate change, ratified by the US and more than 120 other nations, would meet again to put "teeth" into the Framework Convention. The Framework simply called for stabilization of greenhouse gas concentrations at levels that will protect human interests and nature, but it did not target particular concentrations, nor was it binding. At Kyoto, the parties to the convention met to try to prepare a clear protocol for implementing the convention (that is, move beyond a general agreement to stabilize, to agreement about what levels to stabilize at and a timeframe.) There have been additional meetings since then, and, as of 2007, the US has continued to refuse to ratify the treaty.).
It is important to recognize that in each of its five year reports, the IPCC has increased its certainty that humans are now influencing the global climate. Its 1990 report was quite uncertain, concluding cautiously that, "Observed warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability.....the observed increase could be largely due to this natural variability. Alternatively, this variability and other human factors could have offset a still larger human-induced greenhouse warming." This cautious message came in part because of the following problems:
(1) We would expect a steady warming from the fairly steady increase in greenhouse gases, but have seen instead (please refer to the figure from class that shows recent links between CO2 and temperature):
(a) A period of rapid warming until the end of World War II (and why so much warming then -- before emissions of greenhouse gases had increased hugely -- is unknown, but, as of 2001, is largely believed to have resulted from natural causes, including higher solar output and many years with low amounts of volcanic activity)
(b) slight cooling through the mid 1970's (maybe related to a lot of volcanic activity then, especially in the Northern Hemisphere, with much aerosol causing cooling, but weak evidence). Others say that Earth was in a natural cooling time, and that the cooling during this time would have been much greater without counteracting greenhouse gases.
(c) A second period of rapid warming since the mid-1970's, which, in the opinion of most scientists, can't be accounted for by natural phenomena, as in "a," just above.
(2) Computer models suggest that there should have been warming, given the increases in atmospheric concentrations of CO2 and other radiatively active trace gases. However, early models predicted more warming than we have had, given the increase in radiatively active gases.
However, as understanding of factors regulating climate has improved, so have models, and so has the correspondence between model predictions and reality. In particular, modelers : (a) Realized that early models underestimated the ability of oceans to take up atmospheric warming (that is, they credited the oceans with more thermal inertia than they have). (b) Recognized that early models didn't account well for the involvement of other pollutants (such as SO4 aerosols), which slow warming basically by reflecting incident radiation away from Earth and by modifying the shortwave-reflective properties of clouds. For example, SO4 in clouds increases their albedo (provides a greater abundance of very small droplets high in the atmosphere, which can mean reduced warming). Once these (and other) effects were included in the models, predictions of temperature change attributable to greenhouse gases match much more closely the observed changes
Thus, the general consensus in the scientific community has changed since 1990 to the view that warming is already being detected, that human activities are important drivers of it, and that it is sure to increase in the future as emissions continue to increase.
As another example, the American Geophysical Union (comprised of over 35,000 international earth and planetary scientists) issued a statement in 1999: "There is no known geologic precedent for the trnasfer of carbon from the Earth's crust to the atmoshpere in amounts comparable to fossil fuel burning withour simultaneous change in the climate system." "There is a compelling basis for legitimate public concern." and "The present level of scientific uncertainty does not justify inaction in the mitigation of human-induced climate change."
Even for those who are reluctant to agree that we are already experiencing the predicted warming, there is little doubt that eventually warming would occur. The open question is still how much and how fast, and how drastically will emissions of radiatively active trace gases need to be reduced to prevent further change.
I should mention a recent concern; a concern that has moved in the past year from being considered rather a hare-brained notion to gaining wider recognition as a real possibility. This is that global warming, once in place, could trigger a sudden change (a shut down, basically) in an ocean current that warms Northern latitudes (thermohaline circulation, also called meridional overturning circulation). As I understand it, this current carries warm equatorial waters north in the Atlantic; as the water moves north, it cools, becomes denser, sinks, and returns south. If the circulation stopped, we could see sudden and dramatic cooling of Northern Europe and the NE US and Canada, as this ocean "conveyor belt" that warms these areas stopped. This has apparently happened in the past in response to dramatic climate changes. From the past, there are records of as much as 10 degree C temperature swings in just a few years in regions affected by this ocean current. This shut down is apparently caused by pulses of fresh water coming into the N Atlantic from melting glaciers and ice caps, and from the increased precipitation associated with the previous warming. (These waters are less dense than salt water, and are also warmer than the cooled ocean water, so could prevent the sinking of water that fosters movement of the "conveyor belt.") Whether this kind of abrupt change may be on the horizon is hotly debated at present -- certinaly there are pulses of fresh water entering the N Atlantic from melting ice! . In any case, phenomena such this serve to remind us that the global climate system is probably full of surprises! (In their 2007 report, th IPCC concluded that it is likely that some slowing of this circulation is likely but that it is very unlikely that a large change will occur -- and further, that any cooling induced by its slowing would be more than offset by warming caused by the increased atmospheric burden of greenhouse gases.)
I mentioned above that other trace gases (in addition to CO2) are important in causing the greenhouse effect.
Their importance should not be underestimated: other trace gases contribute almost as much warming as does CO2! (This excludes water vapor, which, by itself, contributes more warming potential than is contributed by the sum of CO2 plus O3, CH4, N2O, and halocarbons.) Furthermore, the concentration of many of them has been increasing as rapidly or more rapidly than that of CO2 recently (which has been increasing at about 0.4%/yr).
WATER VAPOR is an important trace gas, whose contribution to warming is the greatest of any gas -- actually greater than the sum of CO2 plus the other tace gases discussed here. Water vapor absorbs radiation of about the same wave length as CO2. (When I say above that other trace gases contribute almost as much warming potential as does CO2, I am excluding water vapor.)
It is not clear if human activities are having any net global effect on the concentration of water vapor in the atmosphere, hence controls on water vapor aren't being discussed at present in negotiations about controlling emissions of greenhouse gases. However, if we have significant warming, will have more evaporation and hence more water vapor in the atmosphere. Whether this will amplify or dampen warming is unclear, as the effects of water vapor in the atmosphere depend on the droplet sizes and their height in the atmosphere.
TROPOSPHERIC OZONE is another greenhouse gas. Of course, human activities are affecting its concentration in some areas of the world, as we have already discussed. Increases in its concentration may contribute to warming. This is particularly so since one mole of ozone contributes 2000 times as much warming potential as one mole of CO2. This is because it absorbs radiation of different wavelengths than CO2; essentially it "seals in a crack" around the window of wavelengths absorbed by CO2.
In contrast to most other greenhouse gases, ozone has a short (60 hr) atmospheric residence time, which means that it doesn't accumulate in the atmosphere to the same extent that they do.
It is difficult to assess its concentration globally, and it is probably variable hemisphere to hemisphere. Best estimates are that radiative forcing from it is about 15% of that contributed by the longer-lived radiative gases.
Methane (CH4) is another very important greenhouse gas. While it is present in lower concentrations in the atmosphere than CO2 (about 1.7 ppmv vs about 368 ppmv for CO2), it is very effective at causing warming because absorbs radiation of a different wavelength than CO2. Like ozone, it fills in a crack around the CO2 absorption window. Methane is about 25 - 30 times more effective at causing warming than is CO2, mole for mole, largely for this reason. Methane currently contributes about 1/4 the warming effect that CO2 does.
The atmospheric concentration of methane has been increasing for about the past 300 years, and over geological time its concentration has tended to vary in parallel with CO2's (and hence with temperatures). Its recent increase began before the recent rapid increase in CO2, beginning about 300 years ago, as compared to about 100 years ago for CO2. Its concentration has increased more than 100% in the last 100 years (that is, has more than doubled; actually increased 145% since preindustrial times). In contrast, concentrations of CO2 "only" increased a bit more than 25% since preindustrial times. The average rate of increase in methane over the 1984-1994 decade has been about 0.6% per year or 10 ppbv.
Sources of methane:
About 80% of atmospheric methane has biological sources. Note, however, that biological doesn't necessarily mean "natural," as humans have affected many of the biological sources. In fact, anthropogenically-related sources contribute about twice as much as natural sources (340 vs 160 tg/yr). Methane is produced by:
The largest single source is wetlands; followed by mining, processing and use of coal; extraction and use of oil and natural gas; "enteric fermentation" (mainly cattle); and rice paddies (120, 100, 80, 50 tg/yr respectively)
Note that all of these sources are linked to the rising human population and to agriculture! How?
Methane has direct warming effects on its own, and it also contributes to the production of CO2, ozone, and water vapor in the atmosphere, which contribute about as much warming as the methane itself.
Methane has approximately a 12 year atmospheric residence time, which is shorter than that of CO2 (which is about 100 years) or halocarbons, which are also about 100 years (see below).
The increase in methane concentrations in the atmosphere began slowing in the 1980's and in mid-1992, the rate of increase dropped sharply in the Southern hemisphere and actually went to zero in the Northern hemisphere. By now, the rate of increase than held during most of the 1980's has been resumed. Why these changes in the rate at which its atmospheric concentration has been changing? Particularly, why did the increase slow or even stop for a while?
(1) Decreased source: Russia has major natural gas fields and pipe lines, and has plugged many leaks in them. This might be enough, actually, to account for a fair percentage of the decrease.
(2) Decreased source: A cooler climate after the eruption of Mt. Pinatubo in June, 1991 probably led to decreased rates of decomposition and may have decreased biological emissions from wetlands.
No one really knows, but it is back to "normal" now (1999) in terms of concentration increases.
Nitrous oxide has mostly "natural" sources, which contribute about twice the anthropogenic sources. It is produced by bacterial action as part of the nitrogen cycle. It is produced by aerobic nitrification, in which NH4 is oxidized to NO2, releasing N2O along the way, and also by anaerobic denitrification, in which NO3 and NO2 are reduced to molecular nitrogen, and by other bacterial transformations.
However, anthropogenically related emissions are increasing, and, as for methane, many are connected to human population and agriculture:
It is hard to measure its concentration in the atmosphere, but concentrations appear to have increased about 0.25% per year over last decade. (Its increase, like that of methane, slowed temporarily in 1991-1992.) Its concentration in the atmosphere has increased about 12% since preindustrial times (275 up to 312 ppbv).
HALOCARBONS (chlorofluorocarbons and HCFC's)
The only source of these compounds is anthropogenic, as they are not naturally occurring. We discussed these when we talked about stratospheric ozone depletion. Recal: these are synthetic chemicals. They are halogenated carbon compounds, such as CFC11 (CFCl3 or Freon) They all contain carbon and halogens, such as Cl (chlorine), F (fluorine), or Br (bromine), and, in the case of the HCFC's, they also contain H (hydrogen). They were until recently, used in refrigeration, aerosols, for puffing foams, as solvents for cleaning in the electronics industry, and in automobile air conditioners.
In addition to their effects on stratospheric ozone, these are important greenhouse gases. They are tremendously effective at producing warming because, even though they are present in low concentrations in the atmosphere, they absorb heat radiation of different wavelength than CO2. They are 12,000 - 15,000 times more effective at causing warming than is CO2, mole for mole.
Their concentrations in the atmosphere have been monitored since the late 1970's and they increased steadily and rapidly over most of that time; at rates of 3-5% per year.
However, both production and emissions fell precipitously from 1989 on, as result of international treaties intended to halt destruction of stratospheric ozone,and now their concentrations in the atmosphere are actually beginning to decline as well. The decline in atmospheric concentrations lagged greatly behind the decline in emissions, because these compounds are very long-lived (atmospheric residence times on the order of 75 - 120 years).
Replacements for CFC's (largely hydrochlorofluorocarbons (HCFC's) and hydrofluorocarbons – (HFC's)) are also greenhouse gases, but are expected to make a relatively small contribution to the global warming potential contributed by other greenhouse gases. Current models suggest that warming due to all halocarbons (CHC's , halons, and their replacements) will be at most 4-10% of the total expected greenhouse warming by 2100.
In fact, the net radiative forcing from halocarbons is probably smaller than might be thought, because they are depleting stratospheric O3, which has a negative radiative forcing (e.g., tends towards cooling). The cooling results from indirect effects of Cl and Br, and also of reactions involving other constituents that are speeded in the presence of increased ultraviolet radiation (as results from loss of stratospheric ozone); constituents that have negative radiative forcing effects
THE MAJOR POINTS ABOUT OTHER TRACE GASES ARE THAT:
· There are other greenhouse gases
besides CO2,
· Collectively they are adding about as much warming as
CO2
· All are increasing under human influence
· Many are involved in more than one environmental problem
(for example, tropospheric ozone causes
problems in its own right and also contributes to excess warming;
CFC's, deplete stratospheric ozone
and also contribute to warming
Models of effects of gases on climate must take all of these gases into account. This is a great challenge, as they have complex atmospheric chemistry, it is challenging to predict trends in their production, and there are complex feedbacks and interactions among them. Policy decisions must also take this variety of gases into account. Thus, it is a mistake to think that the prospect of global climate change is reducible to CO2 alone.
To move to notes on predictions for climate in the future, click "predictions;" to move to implications for ecological systems, click "ecology;" and to move to implications for humans and policy steps that are being taken, click "policy."
This page is maintained by Patricia
S. Muir at Oregon State University.
Page last updated (PARTIALLY!) March 11, 2007.