This section of notes includes the following topics. You can click
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NOTE: I have updated to 2008 data some, but not all, of the statistics in this section of notes -- if I say something in lecture that disagrees with data in these notes, go with the lecture information as most current.
General background information on the "greenhouse" effect
and the gases involved, as well as a list of links to other sites
that contain information about global climate change, is in another
section of these notes, and you can jump to the contents for that
section by clicking on "greenhouse".
1. What trends are atmospheric concentrations
of greenhouse gases likely to follow?
2. What climatic consequences are likely?
3. What ecological effects might be
anticipated from climate changes?
4. What effects might climate change have on human
systems?
4. What policy steps can (or should)
be taken?
TRENDS
IN ATMOSPHERIC CONCENTRATIONS OF GREENHOUSE GASES?
To make predictions about changes in atmospheric concentrations
of greenhouse gases, we need to know two basic things: What trends
are emissions of these gases likely to follow? How will
concentrations in the atmosphere change as a result of
a given set of emissions?
What are the likely trends in emissions?
Trends in emissions of CO2 will
depend on trends in
(1) fossil fuel combustion and
(2) land use, particularly deforestation and reforestation.
These, in turn, will be influenced by the outcome of international
treaty negotiations, what happens with the rate of population
growth, how aggressive research on and development of alternative
sources of energy are, and other factors,
It is likely that the global distribution of sources will change
as lesser developed nations undergo development. Such changes
have already been seen. For example, in 1950 the US was responsible
for over 50% of global emissions of CO2, while lesser developed
nations (LDC's) contributed about 7%. At present (2006), the US
contributes "only" about 25% of global CO2 emissions,
while the LDC's contribute over 20%. The trend of increased emissions
from LDC's is likely to continue (barring policy steps to avert
that), as evidenced by the following data:
Between 1990 and 1998, the following trends in fossil-fuel related
CO2 emissions occurred:
US increase by 10.3% (1990 - 2003, the increase was about 16%)
China increase by 28% (1990 to 2003, the increase was close to
47%...........)
India increase by 55%
European Union increase by 0.7%
Japan increase by 5.6%
Russia decrease by 24% (largely because of economic difficulties)
World increase by 6.3%
(Incidentally, 15-25% of US CO2 emissions are from transportation,
and collectively the US transportation sector alone accounts for
about 5% of all global CO2 emissions!! )
There is much uncertainty in trying to project
what will happen with fossil fuel use and CO2 emissions in the
future, of course. In part, it depends on whether signatories
to the Kyoto treaty meet their commitments
to decrease emissions -- and, of course, on what the US does!
Emissions from LDC's are likely to increase as well, as they undergo
continued economic development.
As you can imagine, projections concerning trends in emissions
of other trace gases are
very difficult as well.
CH4 sources are not well understood and may be hard to control (capturing "waste" emissions, as is done at the Corvallis landfill promises some relief as do changed diets of ruminants
O3 emissions trends will depend on what happens with emissions of precursor pollutants globally, as well as what happens with tropical deforestation and burning
CFC's are on their way out already, but time lags before
concentrations decrease are great. In addition, some of the main
current substitutes for CFC's (such as HCFC's and HFC's) are
also potent greenhouse gases
N2O emissions are very hard to control (but decreasing
N fertilizer, deforestation and fossil fuel use will help)
CO2 EQUIVALENTS:
Predictions of climatic effects are often based largely on CO2,
because its trends are better understood, recognizing that it
is only about 1/2 the story.
Most of the big computer models that attempt to simulate and
predict global climate ("GCM's" -- global climate models
(or general circulation models) are based on "CO2-equivalents,"
which are the concentration of CO2 that would give the same amount
of radiative forcing as the given mixture of CO2 and other greenhouse
gases. Essentially, they sum the radiative forcings of all trace
gases and treat the total forcing as if it comes from an "equivalent"
CO2 concentration.
HOW FAST WILL ATMOSPHERIC CONCENTRATIONS INCREASE IN RESPONSE
TO INCREASING EMISSIONS?
There is considerable uncertainty here, as we'll see.
However, model projections basically assume that same processes
that remove these gases from the atmosphere as operate now will
continue to operate. For example, they assume that the same fraction
of CO2 emissions will remain in the atmosphere as is currently
the case (roughly, 50%). However, we don't fully understand all
of the processes that influence how much of each gas stays in
the atmosphere at present, much less whether those processes
will continue to operate.
FEEDBACK PROCESSES may become very important here, and
are poorly understood. As CO2 emissions increase, [atm] are likely
to reflect that increase to some extent, at least initially (as
it has been doing). However, at some point feedbacks may become
very important, and some of these would be positive feedbacks
while others would be negative feedbacks.
A few examples will illustrate the point, although there are
many that could be given!
1. The rate of uptake of CO2 by vegetation may increase:
If photosynthesis is stimulated by increased atmospheric
concentrations of CO2, then plants will tend to decrease atmospheric
CO2 concentrations, pulling more out of the atmosphere. (Make
two boxes, one representing vegetation uptake of CO2 and one
representing atmospheric CO2 concentrations. Negative sign on
the arrow that represents effect of vegetation uptake on atmospheric
CO2 (as uptake goes up, concentration goes down. Positive sign
on the arrow from atmospheric concentration to plant uptake as
atmospheric concentration increases, plant uptake increases.
Whole thing then = negative, or stabilizing feedback.)
This feedback could be especially important
if it increased storage of carbon in trees, as that is relatively
long term storage. There is some (slim, in Pat's view...) evidence
that this might be occurring -- some of the "missing"
CO2 in the global budget may be fertilizing additional plant
growth. The seasonal amplitude of atmospheric CO2 cycles, like
we saw for Mauna Loa in Hawaii, has been increasing since the
mid-1960's, perhaps suggesting a larger total biosphere.
However, recent evidence suggests that,
while CO2 uptake by vegetation may increase, at least initially,
under enriched CO2 conditions, the overall cycling rate for that
carbon also increases (respiration and decomposition increase
as well). That is, the carbon may simply be being passed more
rapidly through the system, without then having a net dampening
effect on atmospheric concentrations.
2. The
rate of uptake by vegetation may decrease:
Alternatively, and probably equally likely, plants may be
stressed under conditions of changed climate, particularly in
areas where it becomes drier as well as warmer. In this case,
the rate of uptake of CO2 by vegetation would decrease with increasing
atmospheric CO2, rather than increasing. (The sign on the arrow
from the vegetation uptake compartment to the atmospheric concentration
compartment would remain negative if uptake decrease, concentrations
would increase BUT the sign on the arrow running from the concentration
to vegetation uptake would be negative as well, in this case
(as concentration increased, uptake would decrease.) Two negative
signs mean positive, or destabilizing feedback.
3. The
rate of uptake (or retention) of CO2 by oceans may change:
If the oceans warm, as projected, they
will begin to de-gas CO2 into the atmosphere because warmer waters
can't hold as much gas as can colder waters. In this case, as
CO2 increased in the atmosphere (and temperatures warmed), the
oceans would amplify that increase in CO2 concentrations (rather
than damping it through net uptake of CO2 as is presently the
case). Since the oceans hold about 50 times more carbon than
is held in the atmosphere, a change that alters their retention
of carbon even slightly has potential to make a big difference!
4. Increasing temperatures may increase rates of decomposition:
Dead organic matter in the top meter of global soils stores
over twice as much carbon as is stored in the atmosphere. When
this material decomposes, CO2 is given off. If temperatures increase,
then decomposition may speed up (in places where it isn't too
dry). We know that permafrost is melting in some areas, such
as along the N. Slope in Alaska. The permafrost line has moved
about 100 km to the north in northern Canada and Siberia in the
last 100 years. When formerly frozen soils melt and are very
wet, evolution of both CH4 and CO2 from them increases greatly
(CH4 when soils are saturated
with water, and CO2 when they aren't, basically).
7. Cloud and water vapor
(This one is related to feedbacks influencing global temperature,
not so much gas concentrations, but I think it is important for
you to hear something about it.) Recall that water
vapor is an important greenhouse gas (under certain conditions).
Possible feedbacks involving water vapor -- and to clouds --
are very complex and poorly understood (this is one of the major
sources of uncertainty in projecting what will happen to temperatures,
actually). Increased atmospheric temperatures would mean increased
water holding capacity in the atmosphere. Warmer surface temperatures
would also mean increased rates of evaporation. Thus, the atmosphere
is likely to get "wetter." HOWEVER, whether this will
be a net warmer or a net cooler is uncertain it appears that,
influences of atmospheric water on temperatures will depend on
its height in the atmosphere, on the thickness of clouds, on
the droplet sizes that make up the clouds, and so forth. Thus,
there is still much debate about whether cloud feedbacks will
be positive or negative forcings on global temperatures.
BOTTOM LINE: MOST SCENARIOS PREDICT PARSIMONIOUSLY THAT ROUGHLY
1/2 OF THE NEWLY INJECTED CO2 WILL REMAIN IN THE ATMOSPHERE
If this is true, and if emissions continue at early 1990's
rates, concentrations of CO2-equivalents in the atmosphere will
double over preindustrial levels (about 1860), conservatively,
by about 2100.
WHAT EFFECT
WILL A DOUBLING OF ATMOSPHERIC CO2-EQUIVALENTS HAVE ON CLIMATE?
This is, of course, the $64,000 question. No clear information
is available from historical (and prehistorical) records. We do
know that CO2 concentrations nearly doubled between 160,000
yrs ago and now, but concentrations were lower then and it
isn't clear that there will be a linear relationship. We also
know that, at the peak of the last ice age (18,000 yr ago), temperatures
were about 5 C cooler than at present (globally averaged) and
that CO2 was about half what it is now ([CO2]atm was 180-200 ppm;
pre-industrial was about 280 ppm; now about 379 ppm). Thus, we
may be looking at a temperature change of a similar magnitude,
but in the opposite direction.
HOW DO WE KNOW WHAT TO PREDICT FOR GLOBAL CLIMATES?
Obviously, we can't do lab experiments with climate too
big and complex. (You could argue that we are running a big experiment
right now putting gases into the atmosphere and we'll see what
happens. Not a very good experiment, however no replication, no
controls)
What scientists do instead is rely on mathematical climate models
called global climate models (or general circulation
models) ("GCMs") (or CGCMs for coupled
general circulation models). Several of these exist at various
laboratories and universities in the world. These models have
components representing the ocean, atmosphere, and land, and include
equations that represent what we know about the behavior of the
present climate system, interactions between the atmosphere and
the oceans, and chemical reactions that occur in the atmosphere.
These models run at very large spatial scales; they are coarse-grained
and not good for predictions in small areas. In model runs, the
modeler specifies concentrations of greenhouse gases and runs
it. These are mega-models; simulating global climate for one year
takes several hours on the fastest super computers.
How are the models validated? That is, how do people know that
they are generating reasonable predictions? They are generally
verified by comparing model output of climate for current gas
[]'s, and models then can be refined as needed. Alternatively,
the modeler puts in past concentrations of CO2 and checks model
output against what we know of past climates. The performance
of the models has improved greatly in the last 10 years or so,
although none of them are "perfect" yet.
WHAT DO THE MODELS PREDICT FOR FUTURE CLIMATE?
While the models disagree in details, they are in general agreement:
The IPCC makes projections of
global surface air temperatures for the year 2100 based
on various emissions and atmospheric concentration scenarios,
ranging from optimistic (e.g., major reductions in emissions)
to pessimistic (continued rapid growth in emissions). In their
2007 report, the most likely range of temperature increases, over
this century (comparing ~ 2100 [2090 - 2099] to ~2000 [1980
- 1999]) is predicted to be 1.8 - 4.0 degrees C, with a possible
range between 1.1 and 6.4 degrees C,depending on assumptions.
If temperatures do increase in this range by 2100, that rate of
global warming would be, as far as we know, unprecedented over
the last 10,000 years.
The range of potential increases depends
on model assumptions about the degree of climate sensitivity to
gas concentrations (including compounds that act as cooling agents,
such as sulfate (SO4)) and on emissions scenarios.
Temperature increases are predicted to be greater at high latitudes
(especially at northern high latitudes) and lower towards the
equator. Mid-continental regions will probably warm more than
coastal areas and, at our latitude here in Oregon, more increases
are predicted for winter than for summer. (The average
increase for Oregon is predicted to be about 4 C [or roughly 8
degrees F] -- averaged across all seasons) The average temperatures
in high latitude regions have increased at several times the global
mean increase, which you'll recall has been about 0.76
degrees C in the past 100 years or so.
Such changes would diminish the temperature differential between
the climate of equatorial regions and polar regions, with possible
effects on global patterns of air and water circulation, which
are largely driven by temperature differences between these regions.
Consequences of disrupted global circulation patterns are hard
to anticipate because controls over and effects of these global
circulation patterns are not well understood.
Now, a 1.1 - 6.4 degree C increase doesn't sound like much,
does it? Don't be fooled by apparently little numbers! Such a
rapid increase in global mean temperatures would be unprecedented
in human history. The change would be comparable to the 5 degree
C warming between the peak of the last
ice age 18,000 yr ago and today, but would happen about 10
times faster. I think you have a concept of how different climate
and ecosystems were then (at the peak of the last ice age) from
now; much of North America was covered with ice, for example,
and biogeographic zones were shifted way to the South of where
they are now.
The rate of change is very important. Back then,
global mean temperatures increased at a rate of about 2 C/1000
yrs. (Some places were faster, such as Greenland.) We are now
talking about warming by more than 2 degrees C in a century or
less. Projected temperatures for 2100 would make Earth then the
warmest it has been for the past 2 mill yrs. This rate of change
in climate is very important when considering the capacity of
ecological (and human) systems to adapt or move to new places.
There is increasing concern that surprises may lurk in the climate
system -- things we don't understand that may suddenly flip up
to faster change (just like we didn't understand how the presence
of surfaces in the stratosphere would speed up destruction of
ozone there). For example there is evidence that there can be
sudden shifts in ocean circulation patterns with dramatic climatic
consequences (potentially responsible for the very rapid warming
that took place in the vicinity of Greenland near the end of the
last ice age). We know that huge ocean circulation patterns are
very influential on climate bringing warm waters to coastal Northern
Europe, for example. It appears that these circulation patterns
(which are described as huge conveyor belts snaking their way
through the oceans) can either be shut off, or can reverse direction
abruptly (in less than a decade), at least partly in response
to the amount of fresh water coming into the oceans. If warming
leads to melting of ice and thus to big infusions of fresh water
into the oceans, then big climate changes could happen very fast
(and could result in cooling of northern latitudes rather than
warming!) That is, "surprises" can arise from nonlinear
behavior of the climate system when rapidly forced, nonlinear
systems are prone to unexpected behavior!
ASSOCIATED CHANGES
Other things likely to change
include:
Precipitation:
Models aren't as well validated for precipitation as for temperatures,
nor is output from various models as convergent in what is predicted
for a given region as they are for temperature predictions. However,
models do predict that we will have a wetter world, globally,
largely because of increased rates of evaporation. While mean
precipitation will increase globally during the 21st century,
patterns will vary geographically -- increases are expected over
higher latitudes, but decreases are anticipated for subtropical
regions. Mid-continents (such as the US Midwestern breadbasket
region) are predicted to receive less precipitation in summers
than they do at present. Despite globally increased precipitation,
we should remember that evaporation and transpiration will increase
too, owing to warmer temperatures. (These increases in evaporation
and transpiration, of course, are what fuel the global increase
in precipitation that is anticipated.)
For Oregon, models generally predict that our winters will be
warmer and wetter (sorry!) and summers will be both warmer and
effectively drier. (I say "effectively" because there
might be slightly more rain in summer than there is now, but the
warmer temperatures will cause higher rates of evaoptranspiration,
potentially offsetting that increase.) Snowpack is expected to
decrease by as much as 60% (more winter precipitation will come
as rain rather than as snow, and snow levels should increase by
about 1000'). This is likely to lead to disruptions in water supply
here, as described under consequences for
humans in another section. These changes may also tend to
increase fire frequencies in our area.
Frequency and severity of storms and unusual weather events:
Influenced in part by a changing and destabilized relationship between oceans, lands, and atmosphere (changing temperature differentials), the frequency of such events is likely to increase (signs of instability in the climate system). For example, we may see more years of back-to-back droughts, increases in frequency and severity of hurricanes and storms, and so forth.
There is evidence already of an enhanced hydrological cycle (1999). Precipitation has increased by about 10% across the contiguous US since 1910, mainly in winter, and, globally, the proportion of precipitation coming in very heavy events has increased as well. In 1995 it was announced that, for the US, the Greenhouse Climate Response Index has been high since the 1970's (from a report by National Climatic Data Center scientists) and that this index is high globally as well. (The index combines information on above normal temps, rain intensity, heavy storms, drought and so forth. It was high briefly in the 1930's and in the drought period of the 1950's, but has had a sustained high since the late 70's.) These scientists estimate that there is a 95% probability that we are seeing abnormal weather variability for the lower 48 states.
IPCC's 2007 report notes that the following anomalies have been observed in recent years:
Sea level rise:
A global temperature rise of a few degrees between 1990 and 2100
is likely to cause sea levels to rise by 0.18 - 0.59 meters (0.59
m = ~ 2 feet) (IPCC's 2007 report).
Sea levels have risen an average of 17 cm (range 10-25 cm) globally
over the past 100 yrs (after correcting for changes in vertical
land movements), averaging 3 mm per year between 1993 and 2003,
and averaging 1.8 mm / yr between 1961 and 2003 (i.e. the rate
of rise seems to be accelerating).
When temperatures increased 5 degrees C (globally averaged) after
the peak of the last ice age 18,000 yr ago, sea levels rose 100-130
m (the huge ice cap that had covered much of N. America melted).
Nothing of that scale is predicted this time around the current
distribution of ice is such that much will not melt, unlike then
when there was much ice in marginal areas (like N America).
Why is a rise in sea level predicted to occur?
(1) Thermal expansion of oceans is most important (oceans in some
parts of the world have already warmed). Remember, warmer water
occupies more volume than an equivalent mass of cooler water.
Substantial warming of many ocean waters has been reported for
depths between the surface and 3000 m..
(2) Melting of mountain glaciers and ice caps is another factor
contributing to the anticipated sea level rise. Glaciers across
the world are retreating, including those in the Cascades Range.
Two thirds of Glacier National Park's ice fields have vanished
since 1850, and if present retreat rates continue, there will
be none left there within 30 - 40 years. (Again, it isn't completely
clear if there is a "human fingerprint" behind this
melting or not, but it seems increasingly likely that there is.)
Polar ice caps are also losing mass to various degrees, but they aren't, generally, melting from the top: rather, warmer waters under ice in places like Greenland may be causing ice to break off and then melt (and same in Antarctica, which has lost several large ice sheets recently). Greenland's ice sheets in some areas have thinned by over 6 m since 1992. (Particularly important from the perspective of sea level rise is ice that was over land and that melts; ice that was floating in attached ice masses is less important, as it is already displacing water.) Recent data from Greenland suggest that most loss of ice there is taking place at low elevations rather than at high elevations.
In July 2000, the N Pole was completely
ice-free, which has apparently not been the case in over 50 million
years. (Poor Santa and his elves!!) Arctic ice is, in general,
melting rapidly, its average thickness has decreased by 40% since
the 1950's. Between 1978 and 2001, Arctic sea ice cover (area
covered by ice) has decreased by about 15%. If present melting
trends continue there, the entire Arctic could be ice free during
mid summers. While shipping agencies may like the idea of a short
cut across the top of the world (and the Russians did send two
ships through en route to the Bahamas in 1999!), the implications
for Arctic people and wildlife, who depend on ice for calving
sites, for feeding near ice margins, and so on are not as cheery.
In addition, the major influx of freshwater that this melting
would allow into oceans could greatly affect the patterns in ocean
currents, as described above. Antarctica has lost some large ice
masses in the past few years as well (particularly from the tip
of the Antarctic Peninsula), but the main mass of Antarctic ice
is not expected to disintegrate.
These changes in sea level would be likely to cause trouble for
both natural and human systems. For example, the Everglades ecosystem
and Miami Florida would be inundated by rising sea waters, and,
in fact, 20% of the world's human population lives on lands likely
to be dramatically changed by rising waters. A 1 foot rise in
sea level would lead to the loss of 20-40% of US wetlands (many
of which are coastal), damaging fisheries and wildlife habitat.
To move to the next section of these notes, which deals with ecological and agricultural consequences of climate changes, click ecological effects here. To move to notes on implications for humans and policy steps that are being taken, click "policy." To jump back to the master Table of contents for these pages, click Contents here, and for reminders on how to navigate within and among these pages, click Navigate.
Page maintained by Patricia Muir at Oregon State University. Last updated (partially) March 11, 2007.