Yeah, the August issue of Scientific American does a really good job of explaining this. Here's the link...
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=B1182F51-E7F2-99DF-30CB2EAAC975FE93
Unfortunately, you can't read it for free. But if you want a really fantastic explanation it's well worth either paying to see the article or going to the store/library to read it.
Here is part of it, but you have to pay to see the whole thing:
The Physical Science behind Climate Change
By William Collins, Robert Colman, James Haywood,
Martin R. Manning and Philip Mote
"Why are climatologists so highly confident that
human activities are dangerously warming the earth?
Here some of the participants in the most recent
and comprehensive international review of the
scientific evidence summarize the arguments
and discuss what uncertainties remain
For a scientist studying climate change,
"eureka" moments are unusually rare.
Instead progress is generally made by a
painstaking piecing together of evidence from
every new temperature measurement, satellite
sounding or climate-model experiment. Data
get checked and rechecked, ideas tested over
and over again. Do the observations fit the predicted
changes? Could there be some alternative
explanation? Good climate scientists, like
all good scientists, want to ensure that the highest
standards of proof apply to everything they
discover.
And the evidence of change has mounted as
climate records have grown longer, as our understanding
of the climate system has improved
and as climate models have become ever more
reliable. Over the past 20 years, evidence that
humans are affecting the climate has accumulated
inexorably, and with it has come ever greater
certainty across the scientific community in
the reality of recent climate change and the potential
for much greater change in the future.
This increased certainty is starkly refl ected in
the latest report of the Intergovernmental Panel
on Climate Change (IPCC), the fourth in a series
of assessments of the state of knowledge on
the topic, written and reviewed by hundreds of
scientists worldwide
The panel released a condensed version of the
first part of the report, on the physical science
basis of climate change, in February. Called the
"Summary for Policymakers," it delivered to
policymakers and ordinary people alike an unambiguous
message: scientists are more confi -
dent than ever that humans have interfered with
the climate and that further human-induced climate
change is on the way. Although the report
finds that some of these further changes are
now inevitable, its analysis also confirms that
the future, particularly in the longer term, remains
largely in our hands-the magnitude of
expected change depends on what humans
choose to do about greenhouse gas emissions.
The physical science assessment focuses on
four topics: drivers of climate change, changes
observed in the climate system, understanding
cause-and-effect relationships, and projection of
future changes. Important advances in research
into all these areas have occurred since the IPCC
assessment in 2001. In the pages that follow, we
lay out the key findings that document the extent
of change and that point to the unavoidable conclusion
that human activity is driving it.
Drivers of Climate Change
KEY CONCEPTS
Scientists are confident that
humans have interfered with
the climate and that further
human-induced climate change
is on the way.
The principal driver of recent
climate change is greenhouse
gas emissions from human
activities, primarily the burning
of fossil fuels.
The report of the Intergovernmental
Panel on Climate Change
places the probability that
global warming has been
caused by human activities at
greater than 90 percent. The
previous report, published in
2001, put the probability at
higher than 66 percent.
Although further changes in the
world's climate are now inevitable,
the future, particularly in the
longer term, remains largely in
our hands-the magnitude of
expected change depends on
what humans choose to do about
greenhouse gas emissions.
-The Editors
Atmospheric concentrations of many gases-
primarily carbon dioxide, methane, nitrous
oxide and halocarbons (gases once used widely
as refrigerants and spray propellants)-have
increased because of human activities. Such
gases trap thermal energy (heat) within the
atmosphere by means of the well-known greenhouse
effect, leading to global warming.
The atmospheric concentrations of carbon dioxide,
methane and nitrous oxide remained roughly
stable for nearly 10,000 years, before the
abrupt and rapidly accelerating increases of the
past 200 years [see right illustrations in box on
page 67]. Growth rates for concentrations of
carbon dioxide have been faster in the past 10
years than over any 10-year period since continuous
atmospheric monitoring began in the
1950s, with concentrations now roughly 35
percent above preindustrial levels (which can
be determined from air bubbles trapped in ice
cores). Methane levels are roughly two and a
half times preindustrial levels, and nitrous
oxide levels are around 20 percent higher.
How can we be sure that humans are responsible
for these increases? Some greenhouse gases
(most of the halocarbons, for example) have no
natural source. For other gases, two important
observations demonstrate human influence.
First, the geographic differences in concentrations
reveal that sources occur predominantly
over land in the more heavily populated Northern
Hemisphere. Second, analysis of isotopes,
which can distinguish among sources of emissions,
demonstrates that the majority of the increase
in carbon dioxide comes from combustion
of fossil fuels (coal, oil and natural gas).
Methane and nitrous oxide increases derive
from agricultural practices and the burning of
fossil fuels.
Climate scientists use a concept called radiative
forcing to quantify the effect of these increased
concentrations on climate. Radiative
forcing is the change that is caused in the global
energy balance of the earth relative to preindustrial
times. (Forcing is usually expressed as watts
per square meter.) A positive forcing induces
warming; a negative forcing induces cooling.
We can determine the radiative forcing associated
with the long-lived greenhouse gases fairly
precisely, because we know their atmospheric
concentrations, their spatial distribution and
the physics of their interaction with radiation.
Climate change is not driven just by increased
greenhouse gas concentrations; other mechanisms-
both natural and human-induced-also
play a part. Natural drivers include changes in
solar activity and large volcanic eruptions. The
report identifi es several additional signifi cant
human-induced forcing mechanisms-microscopic
particles called aerosols, stratospheric
and tropospheric ozone, surface albedo (refl ectivity)
and aircraft contrails-although the in-
fl uences of these mechanisms are much less certain
than those of greenhouse gases [see left illustration
in box on opposite page].
Investigators are least certain of the climatic
influence of something called the aerosol cloud
albedo effect, in which aerosols from human origins
interact with clouds in complex ways and
make the clouds brighter, refl ecting sunlight
back to space. Another source of uncertainty
comes from the direct effect of aerosols from human
origins: How much do they refl ect and absorb
sunlight directly as particles? Overall these
aerosol effects promote cooling that could offset
the warming effect of long-lived greenhouse gases
to some extent. But by how much? Could it
overwhelm the warming? Among the advances
achieved since the 2001 IPCC report is that scientists
have quantified the uncertainties associated
with each individual forcing mechanism
through a combination of many modeling and
observational studies. Consequently, we can
now confidently estimate the total human induced
component. Our best estimate is some
10 times larger than the best estimate of the natural
radiative forcing caused by changes in solar
activity.
This increased certainty of a net positive radiative
forcing fits well with the observational evidence
of warming discussed next. These forcings
can be visualized as a tug-of-war, with positive
forcings pulling the earth to a warmer climate
and negative ones pulling it to a cooler state. The
result is a no contest; we know the strength of
the competitors better than ever before. The
earth is being pulled to a warmer climate and
will be pulled increasingly in this direction as
the "anchorman" of greenhouse warming continues
to grow stronger and stronger.
Observed Climate Changes
The many new or improved observational data
sets that became available in time for the 2007
IPCC report allowed a more comprehensive
assessment of changes than was possible in earlier
reports. Observational records indicate that
11 of the past 12 years are the warmest since
reliable records began around 1850. The odds of
such warm years happening in sequence purely
by chance are exceedingly small. Changes in
three important quantities-global temperature,
sea level and snow cover in the Northern Hemisphere
[see box on page 68]-all show evidence
of warming, although the details vary. The previous
IPCC assessment reported a warming
trend of 0.6 ± 0.2 degree Celsius over the period
1901 to 2000. Because of the strong recent
warming, the updated trend over 1906 to 2005
is now 0.74 ± 0.18 degree C. Note that the 1956
to 2005 trend alone is 0.65 ± 0.15 degree C,
emphasizing that the majority of 20th-century
warming occurred in the past 50 years. The climate,
of course, continues to vary around the
increased averages, and extremes have changed
consistently with these averages-frost days and
cold days and nights have become less common,
while heat waves and warm days and nights have
become more common.
JARGON BUSTER
RADIATIVE FORCING, as used
in the box on the opposite page, is
the change in the energy balance of
the earth from preindustrial times
to the present.
LONG-LIVED GREENHOUSE
GASES include carbon dioxide,
methane, nitrous oxide and halocarbons.
The observed increases
in these gases are the result of
human activity.
OZONE is a gas that occurs both in
the earth's upper atmosphere and at
ground level. At ground level ozone
is an air pollutant. In the upper
atmosphere, an ozone layer protects
life on the earth from the sun's
harmful ultraviolet rays.
SURFACE ALBEDO is the refl ectivity
of the earth's surface: a lighter
surface, such as snow cover, refl ects
more solar radiation than a darker
surface does.
AEROSOLS are airborne particles
that come from both natural (dust
storms, forest fi res, volcanic eruptions)
and man-made sources, such
as the burning of fossil fuels.
CONTRAILS, or vapor trails,
are condensation trails and artifi cial
clouds made by the exhaust of
aircraft engines.
TROPOSPHERE is the layer of the
atmosphere close to the earth. It rises
from sea level up to about 12 kilometers
(7.5 miles).
STRATOSPHERE lies just above
the troposphere and extends upward
about 50 kilometers.
TOM
The properties of the climate system include
not just familiar concepts of averages of temperature,
precipitation, and so on but also the state
of the ocean and the cryosphere (sea ice, the
great ice sheets in Greenland and Antarctica,
glaciers, snow, frozen ground, and ice on lakes
and rivers). Complex interactions among differ-
ent parts of the climate system are a fundamental
part of climate change-for example, reduction
in sea ice increases the absorption of heat by
the ocean and the heat fl ow between the ocean
and the atmosphere, which can also affect cloudiness
and precipitation.
A large number of additional observations
are broadly consistent with the observed warming
and refl ect a flow of heat from the atmosphere
into other components of the climate system.
Spring snow cover, which decreases in concert
with rising spring temperatures in northern
midlatitudes, dropped abruptly around 1988
and has remained low since. This drop is of concern
because snow cover is important to soil
moisture and water resources in many regions.
In the ocean, we clearly see warming trends,
which decrease with depth, as expected. These
changes indicate that the ocean has absorbed
more than 80 percent of the heat added to the climate
system: this heating is a major contributor
to sea-level rise. Sea level rises because water expands
as it is warmed and because water from
melting glaciers and ice sheets is added to the
oceans. Since 1993 satellite observations have
permitted more precise calculations of global sealevel
rise, now estimated to be 3.1 ± 0.7 millimeters
per year over the period 1993 to 2003. Some
previous decades displayed similarly fast rates,
and longer satellite records will be needed to determine
unambiguously whether sea-level rise is
accelerating.
Substantial reductions in the extent
of Arctic sea ice since 1978 (2.7 ± 0.6 percent
per decade in the annual average, 7.4 ± 2.4
percent per decade for summer), increases in
permafrost temperatures and reductions in glacial
extent globally and in Greenland and Antarctic
ice sheets have also been observed in recent
decades. Unfortunately, many of these quantities
were not well monitored until recent de-
cades, so the starting points of their records vary.
Hydrological changes are broadly consistent
with warming as well. Water vapor is the strongest
greenhouse gas; unlike other greenhouse
gases, it is controlled principally by temperature.
It has generally increased since at least the
1980s. Precipitation is very variable locally but
has increased in several large regions of the
world, including eastern North and South
America, northern Europe, and northern and
central Asia. Drying has been observed in the
Sahel, the Mediterranean, southern Africa and
parts of southern Asia. Ocean salinity can act
as a massive rain gauge. Near-surface waters of
the oceans have generally freshened in middle
and high latitudes, while they have become saltier
in lower latitudes, consistent with changes in
large-scale patterns of precipitation.
Reconstructions of past climate-paleoclimate-
from tree rings and other proxies provide
important additional insights into the
workings of the climate system with and without
human infl uence. They indicate that the
warmth of the past half a century is unusual in
at least the previous 1,300 years. The warmest
period between A.D. 700 and 1950 was probably
A.D. 950 to 1100, which was several tenths
of a degree C cooler than the average temperature
since 1980.
Attribution of Observed Changes
Although confidence is high both that human
activities have caused a positive radiative forcing
and that the climate has actually changed,
can we confi dently link the two? This is the
question of attribution: Are human activities
primarily responsible for observed climate
changes, or is it possible they result from some
other cause, such as some natural forcing or
simply spontaneous variability within the climate
system?
The 2001 IPCC report concluded
it was likely (more than 66 percent probable)
that most of the warming since the mid-20th
century was attributable to humans. The 2007
report goes signifi cantly further, upping this to
very likely (more than 90 percent probable).
The source of the extra confi dence comes
from a multitude of separate advances. For a
start, observational records are now roughly fi ve
years longer, and the global temperature increase
over this period has been largely consistent with
IPCC projections of greenhouse gas-driven
warming made in previous reports dating back
to 1990.
In addition, changes in more aspects of
the climate have been considered, such as those
in atmospheric circulation or in temperatures
within the ocean. Such changes paint a consistent
and now broadened picture of human intervention.
Climate models, which are central to
attribution studies, have also improved and are
able to represent the current climate and that of
the recent past with consid erable fi delity. Finally,
some important apparent inconsistencies noted
in the observ ational record have been largely
resolved since the last report.
The most important of these was an apparent
mismatch between the instrumental surface
temperature record (which showed signifi cant
warming over recent decades, consistent with a
human impact) and the balloon and satellite atmospheric
records (which showed little of the
expected warming). Several new studies of the
satellite and balloon data have now largely re-
solved this discrepancy-with consistent warming
found at the surface and in the atmosphere.
An experiment with the real world that
duplicated the climate of the 20th century with
constant (rather than increasing) greenhouse
gases would be the ideal way to test for the cause
of climate change, but such an experiment is of
course impossible. So scientists do the next best
thing: they simulate the past with climate
models.
Two important advances since the last IPCC
assessment have increased confi dence in the use
of models for both attribution and projection of
climate changes. The fi rst is the development of
a comprehensive, closely coordinated ensemble
of simulations from 18 modeling groups around
the world for the historical and future evolution
of the earth's climate. Using many models helps
to quantify the effects of uncertainties in various
climate processes on the range of model
simulations. Although some processes are well
understood and well represented by physical
equations (the fl ow of the atmosphere and ocean
or the propagation of sunlight and heat, for example),
some of the most critical components of
the climate system are less well understood,
such as clouds, ocean eddies and transpiration
by vegetation. Modelers approximate these
components using simplified representations
called parameterizations.
The principal reason to develop a multimodel ensemble for
the IPCC assessments is to understand how this lack of
certainty affects attribution and prediction of
climate change. The ensemble for the latest assessment
is unprecedented in the number of
models and experiments performed.
The second advance is the incorporation of
more realistic representations of climate processes
in the models. These processes include
the behavior of atmospheric aerosols, the dynamics
(movement) of sea ice, and the exchange
of water and energy between the land and the
atmosphere. More models now include the major
types of aerosols and the interactions between
aerosols and clouds.
When scientists use climate models for
attribution studies, they fi rst run simulations
with estimates of only "natural" climate
influences over the past 100 years, such as
changes in solar output and major volcanic
eruptions. They then run models that include
human-induced increases in greenhouse gases
and aerosols. The results of such experiments
are striking [see box below]. Models using only
natur al forcings are unable to explain the
observed global warming since the mid-20th
century, whereas they can do so when they
include anthropogenic factors in addition to
natural ones. Large-scale patterns of temperature
change are also most consistent between
models and observations when all forcings are
included.
Two patterns provide a fingerprint of human
infl uence. The first is greater warming over
land than ocean and greater warming at the
surface of the sea than in the deeper layers.
This pattern is consistent with greenhouse gas-
induced warming by the overlying atmosphere:
the ocean warms more slowly because of its
large thermal inertia. The warming also indicates
that a large amount of heat is being taken
up by the ocean, demonstrating that the planet's
energy budget has been pushed out of balance.
A second pattern of change is that while
the troposphere (the lower region of the
atmosphere) has warmed, the stratosphere, just
above it, has cooled. If solar changes provided
the dominant forcing, warming would be
expected in both atmos pher ic layers. The
from the combination of green house gas
increases and stratospheric ozone decreases.
This collective evidence, when subjected to
careful statistical analyses, provides much of
the basis for the increased confidence that
human influences are behind the observed
global warming. Suggestions that cosmic rays
could affect clouds, and thereby climate, have
been based on correlations using limited records;
they have generally not stood up when
tested with additional data, and their physical
mechanisms remain speculative.
What about at smaller scales? As spatial and
temporal scales decrease, attribution of climate
change becomes more diffi cult. This problem
arises because natural small-scale temperature
variations are less "averaged out" and thus more
readily mask the change signal. Never the less,
continued warming means the signal is emerging
on smaller scales. The report has found that human
activity is likely to have infl uenced temperature
signifi cantly down to the continental scale
for all continents except Antarctica.
Human influence is discernible also in some
extreme events such as unusually hot and cold
nights and the incidence of heat waves. This does
not mean, of course, that individual extreme
events (such as the 2003 European heat wave)
can be said to be simply "caused" by humaninduced
climate change-usually such events are
complex, with many causes. But it does mean
that human activities have, more likely than not,
affected the chances of such events occurring.
Projections of Future Changes
How will climate change over the 21st century?
This critical question is addressed using simulations
from climate models based on projections
of future emissions of greenhouse gases and
aerosols. The simulations suggest that, for
greenhouse gas emissions at or above current
rates, changes in climate will very likely be larger
than the changes already observed during the
20th century. Even if emissions were immediately
reduced enough to stabilize greenhouse
gas concentrations at current levels, climate
change would continue for centuries. This inertia
in the climate results from a combination of
factors. They include the heat capacity of the
world's oceans and the millennial timescales
needed for the circulation to mix heat and carbon
dioxide throughout the deep ocean and
thereby come into equilibrium with the new
conditions.
To be more specific, the models project that
over the next 20 years, for a range of plausible
emissions, the global temperature will increase
at an average rate of about 0.2 degree C per decade,
close to the observed rate over the past 30
years. About half of this near-term warming
represents a "commitment" to future climate
change arising from the inertia of the climate
system response to current atmospheric concentrations
of greenhouse gases.
The long-term warming over the 21st century,
however, is strongly infl uenced by the future rate
of emissions, and the projections cover a wide
variety of scenarios, ranging from very rapid to
more modest economic growth and from more
to less dependence on fossil fuels. The best estimates
of the increase in global temperatures
range from 1.8 to 4.0 degrees C for the various
emission scenarios, with higher emissions leading
to higher temperatures. As for regional impacts,
projections indicate with more confi dence
than ever before that these will mirror the patterns
of change observed over the past 50 years
(greater warming over land than ocean, for example)
but that the size of the changes will be
larger than they have been so far.
The simulations also suggest that the removal
of excess carbon dioxide from the atmosphere
by natural processes on land and in the ocean
will become less effi cient as the planet warms.
This change leads to a higher percentage of
emitted carbon dioxide remaining in the atmosphere,
which then further accelerates global
warming. This is an important positive feedback
on the carbon cycle (the exchange of carbon
compounds throughout the climate system).
Although models agree that carbon-cycle
changes represent a positive feedback, the range
of their responses remains very large, depending,
among other things, on poorly understood
changes in vegetation or soil uptake of carbon
as the climate warms. Such processes are an important
topic of ongoing research.
The models also predict that climate change
will affect the physical and chemical characteristics
of the ocean. The estimates of the rise in
sea level during the 21st century range from
about 30 to 40 centimeters, again depending on
emissions. More than 60 percent of this rise is
caused by the thermal expansion of the ocean.
Yet these model-based estimates do not include
the possible acceleration of recently observed
increases in ice loss from the Greenland and
Antarctic ice sheets.
Although scientific understanding of such effects is
very limited, they could add an additional 10 to 20
centimeters to sea-level rises, and the possibility of
significantly larger rises cannot be excluded. The
chemistry of the ocean is also affected, as the increased
concentrations of atmospheric carbon dioxide
will cause the ocean to become more acidic.
Some of the largest changes are predicted for
polar regions. These include signifi cant increases
in high-latitude land temperatures and in the
depth of thawing in permafrost regions and
sharp reductions in the extent of summer sea ice
in the Arctic basin. Lower latitudes will likely
experience more heat waves, heavier precipitation,
and stronger (but perhaps less frequent)
hurricanes and typhoons. The extent to which
hurricanes and typhoons may strengthen is uncertain
and is a subject of much new research.
Some important uncertainties remain, of
course. For example, the precise way in which
clouds will respond as temperatures increase is
a critical factor governing the overall size of the
projected warming. The complexity of clouds,
however, means that their response has been
frustratingly diffi cult to pin down, and, again,
much research remains to be done in this area.
We are now living in an era in which both humans
and nature affect the future evolution of
the earth and its inhabitants. Unfortunately, the
crystal ball provided by our climate models becomes
cloudier for predictions out beyond a
century or so. Our limited knowledge of the response
of both natural systems and human society
to the growing impacts of climate change
compounds our uncertainty. One result of global
warming is certain, however. Plants, animals
and humans will be living with the consequences
of climate change for at least the next thousand
years.
FACING OUR FUTURE: Notes from the Editors
The human race can respond to climate change in two ways:
adaptation and mitigation. Adaptation means learning how to
survive and prosper in a warmer world. Mitigation means
limiting the extent of future warming by reducing the net
release of greenhouse gases to the atmosphere.
Given that rising temperatures are already encroaching on
us and that an unstopped increase would be overwhelming,
a strong combination of both adaptation and mitigation will
be essential.
Unfortunately, disagreements over the feasibility, costs and
necessity of mitigation have notoriously bogged down global
responses to date. To project mitigation strategies for the looming
problems-and their costs-Working Group III of the IPCC considered
various estimates of economic expansion, population growth and
fossil-fuel use for its 2007 report. The six resulting scenarios predict
atmospheric concentrations of carbon dioxide equivalents
(that is, greenhouse gases and aerosols equivalent
to carbon dioxide) ranging from 445 parts per million to 1,130 ppm,
with corresponding increases in temperatures from 2.0 to as
much as 6.1 degrees C (approximately 3.6 to 11 degrees F)
over preindustrial levels.
To keep the temperature increase to the lowest of those projections,
the group estimates that the world must stabilize atmospheric
greenhouse gases at 445 ppm by 2015. (Current concentrations are
approaching 400 ppm.) The scientists believe that any higher
temperatures might trigger severe flooding in some places
and severe drought in others, wipe out species and cause
economic havoc.
The group's report looks in detail at the most promising
technologies and policies for holding the gases at 445 ppm.
It emphasizes the importance of improving energy efficiency
in buildings and vehicles, shifting to renewable energy sources
and saving forests as "carbon sinks." Policies include setting
a target for global emissions, emissions trading schemes,
caps, taxes and incentives. But the IPCC scientists made
their assessment before a study published online this past
April in the Proceedings of the National Academy of Sciences
USA reported that worldwide carbon dioxide emissions
between 2000 and 2004 increased at three times the rate
of the 1990s- from 1.1 to 3.2 percent a year. In other words,
the actual global emissions since 2000 grew faster than those
projected in the highest of the scenarios developed by the IPCC.
That research indicates that the situation
is more dire than even the bleak IPCC assessment forecasts.
Global warming is real and, as Working Group I
of the IPCC stated in its January-February
2007 report, "very likely" to be largely the
result of human activities for at least the past
half a century. But is that warming signifi cant
enough to pose real problems? That
determination fell to Working Group II, a
similarly international assembly of scientists
who focused on the vulnerability of natural
and human environments to climate change.
In the April 2007 summary of its fi ndings,
Working Group II concluded that human induced
warming over the past three and a half
decades has indeed had a discernible infl uence
on many physical and biological systems.
Observational evidence from all continents and
most oceans shows that many natural systems
are being affected by regional climate changes,
particularly temperature increases. The ground
in permafrost regions is becoming increasingly
unstable, rock avalanches in mountainous
areas are more frequent, trees are coming into
leaf earlier, and some animals and plants are
moving to higher latitudes or elevations.
Looking to the future, the group also projected
that ongoing shifts in climate would
affect the health and welfare of millions of
people around the world. The severity of the
effects would depend on precisely how much
warming occurred. Among the most probable
consequences:
More frequent heat waves, droughts, fires,
coastal flooding and storms will raise the toll
of deaths, injuries and related diseases.
Some infectious diseases, such as malaria,
will spread to new regions.
High concentrations of ground-level ozone
will exacerbate heart and respiratory
ailments.
By the 2080s, rising sea levels will flood the
homes and property of millions of people,
especially in the large deltas of Asia and
Africa and on small islands.
The harm from these changes will be most
severe for impoverished communities. The poor
are generally more dependent on climate-sensitive
resources such as local water and food,
and by defi nition their adaptive capacities are
economically limited.
The effects of global warming would not be
universally bad, particularly for the next few
decades. For example, whereas higher temperatures
would hurt the growth of important cereals
in equatorial nations fairly quickly, they would
for a time raise productivity on farms in mid- to
high-latitude countries, such as the U.S. But
once the temperature increase exceeded three
degrees Celsius (5.4 degrees Fahrenheit), agricultural
declines would set in even there, barring
widespread adaptive changes.
THE CONSEQUENCES OF ONGOING WARMING
WHAT NEEDS TO BE DONE
Central and South America
Gradual replacement of tropical forest
by savanna in eastern Amazonia
Replacement of semiarid vegetation
by arid-land vegetation
Species extinctions in many
tropical areas
Reduced water availability
Loss of arable land in drier areas
Decreased yields of some
important crops
Reduced livestock productivity
The Regional Picture
North America
In the western mountains, decreased snowpack, more
winter flooding and reduced summer flows
An extended period of high fi re risk and large increases
in area burned
Increased intensity, duration and number of heat waves
in cities historically prone to them
In coastal areas, increased stress on people and property
as climate interacts with development and pollution
Europe
Increased risk of inland flash floods
In the south, more health-threatening heat waves and
wildfi res, reduced water availability and hydropower
potential, endangered crop production and reduced
summer tourism
In the central and eastern areas, more health-threatening
heat waves and peatland fi res and reduced summer rainfall
and forest productivity
In the north, negative impacts eventually outweigh such
initial benefi ts as reduced heating demand and increased
crop yields and forest growth
Small islands
Threats to vital infrastructure,
settlements and facilities because
of sea-level rise
Reduced water resources in many places
by midcentury
Beach erosion, coral bleaching and other
deteriorating coastal conditions, leading
to harmed fi sheries and reduced value as
tourist destinations
Invasion by nonnative species, especially
on mid- and high-latitude islands
Polar regions
Thinning and shrinking of glaciers and
ice sheets
Changes in the extent of Arctic sea ice
and permafrost
Deeper seasonal thawing of permafrost
Asia
Increased flooding, rock avalanches
and water resource disruptions
as Himalayan glaciers melt
Ongoing risk of hunger in several
developing regions because of
crop productivity declines combined
with rapid population
growth and urbanization
Australia and New Zealand
Intensifi ed water security
problems in southern
and eastern Australia
and parts of NewZealand by 2030
Further loss of biodiversity
in ecologically richsites by 2020
Increased storm severity
and frequency inseveral places
Africa
Decreased water availability by 2020 for
75 million to 250 million people
Loss of arable land, reduced growing
seasons and reduced yields in some areas
Decreased fish stocks in large lakes
The lists here indicate just some of the disturbing effects,
beyond those enumerated in the discussion at the left,
that Working Group II foresees in various parts of the world over
the coming century. The group made most of these predictions
with high or very high confi dence. Find more details at
www.ucar.edu/news/features/climatechange/ 
