Section: ENVIRONMENT
Why are climatologists so highly confident that human activities are dangerously warming the earth?
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
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 reflected 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 confident 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 arc-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
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. 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 identifies
several additional significant human-induced forcing
mechanisms--microscopic particles called aerosols, stratospheric and
tropospheric ozone, surface albedo (reflectivity) and aircraft
contrails--although the influences of these mechanisms are much less
certain than those of greenhouse gases.
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, reflecting sunlight back to space.
Another source of uncertainty comes from the direct effect of aerosols
from human origins: How much do they reflect 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 firs 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--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.
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
different 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 flow 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 reflect 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 sea level 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 decades, 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 influence. 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 confidently 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 significantly further, upping this to very likely (more than 90
percent probable).
The source of the extra confidence comes from
a multitude of separate advances. For a start, observational records
are now roughly five 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 considerable fidelity.
Finally, some important apparent inconsistencies noted in the
observational 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 significant 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 resolved 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 he the ideal way to test for the
cause of climate change, hut 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 confidence in the use of models for both
attribution and projection of climate changes. The first 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 flow 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 first 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. Models using only natural
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
influence. 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
atmospheric layers. The observed contrast, however, is just that
expected from the combination of greenhouse 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
difficult. This problem arises because natural small-scale temperature
variations are less "averaged out" and thus more readily mask the
change signal. Nevertheless, continued warming means the signal is
emerging on smaller scales. The report has found that human activity is
likely to have influenced temperature significantly 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 human-induced 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 influenced 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 confidence 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 efficient 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 significant 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 difficult 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.
MORE THE EXPLORE
All IPCC reports and summaries are available at www.ipcc.ch
More information from the authors is available at www.SciAm.com/ontheweb
INFLUENCES ON CLIMATE
A tug-of-war between positive forcings
(influences that cause the climate to grow warmer) and negative
forcings (those that cause it to grow cooler) is a hands-down "victory"
for the predominantly human-induced forces that lead to warming (left
graph). The dominant human-induced forcings are from the long-lived
greenhouse gases in the atmosphere, whose concentrations have soared in
the past 200 years or so {right graphs).
Estimates for global averages of radiative
forcing in 2005 are shown for the major mechanisms. The black error
bars indicate the level of certainty associated with each forcing: it
is 90 percent likely that values lie within the error bars. The
radiative forcing of the greenhouse gases, for example, is quite
certain, as opposed to the uncertainty associated with the aerosol
effects. (Volcanic aerosols are not included in the graph because of
their episodic nature.)
Greenhouse Gases: The Major Forcings
Carbon dioxide, methane and nitrous oxide
concentrations of the past were derived from ice cores; those for
recent times come from samples of the atmosphere. Large recent
increases can be attributed to human activities.
OBSERVED EVIDENCE
Observations of global average surface
temperature, sea level and snow cover for the Northern Hemisphere in
March and April document increased warming. Red lines represent values
averaged over a decade, with the blue shading indicating the range of
uncertainty; blue dots show yearly values. All measures are relative to
1961-1990.
HUMAN-INDUCED TEMPERATURE CHANGE
Models using only natural forcings (blue) do
not reflect the actual increases in temperature. When both natural and
human-induced forcings (orange) are included, however, the models
reproduce the real-world rise in temperature, both on a global scale
and on a continental scale. Changes are shown relative to the average
for 1901-1950.
PROJECTED TEMPERATURE CHANGES
Projected changes in surface temperature
(relative to 1980-1999), based on 22 models from 17 different programs,
were calculated for three socioeconomic scenarios. All three scenarios
are based on studies made before 2000 and assume no additional climate
policy; in other words, they are not mitigation scenarios.
GRAPH: Radiative Forcing: The Overview
GRAPH: CARBON DIOXIDE (parts per million)
GRAPH: METHANE (parts per million)
GRAPH: NITROUS OXIDE (parts per million)
GRAPH: CHANGES IN TEMPERATURE (° Celsius)
GRAPH: CHANGES IN GLOBAL SEA LEVEL (millimeters)
GRAPH: CHANGES IN NORTHERN HEMISPHERE SNOW COVER (millions of square kilometers)
GRAPH: GLOBAL CHANGE, TOTAL
GRAPH: GLOBAL LAND CHANGE
GRAPH: GLOBAL OCEAN CHANGE
GRAPH: NORTH AMERICA
GRAPH: SOUTH AMERICA
GRAPH: EUROPE
GRAPH: ASIA
GRAPH: AFRICA
GRAPH: AUSTRALIA
MAP: SCENARIO 1 Low emissions, 2020-2029 & 2090-2099
MAP: SCENARIO 2 Moderate emissions, 2020-2029 & 2090-2099
MAP: SCENARIO 3 High emissions, 2020-2029 & 2090-2099
PHOTO (COLOR): Arctic sea ice, 1979
PHOTO (COLOR): Arctic sea ice, 2005
PHOTO (COLOR)
~~~~~~~~ By William Collins; Robert Colman; James Haywood; Martin R. Manning and Philip Mote
The authors were participants in Working
Group I of the 2007 IPCC assessment. William Collins is a professor in
residence in the department of earth and planetary science at the
University of California, Berkeley, and a senior scientist at Lawrence
Berkeley National Laboratory and the National Center for Atmospheric
Research in Boulder, Colo. Robert Colman is a senior research scientist
in the Climate Dynamics Group at the Australian Bureau of Meteorology
Research Center in Melbourne. James Haywood is the manager of aerosol
research in the Observational Based Research Group and the Chemistry,
Climate and Ecosystem Group at the Met Office in Exeter, England.
Martin R. Manning is director of the IPCC WG I Support Unit at the NOAA
Earth System Research Laboratory in Boulder, Colo. Philip Mote is the
Washington State climatologist, a research scientist in the Climate
Impacts Group at the University of Washington, and an affiliate
professor in the department of atmospheric sciences.
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