The IPCC Third Assessment: Science
The role of the Intergovernmental Panel on Climate Change
is to provide the international community with expert guidance
regarding scientific and technical aspects of the climate problem.
Here, we present the Panel's latest findings on the basic science of
Since 1990, the Intergovernmental Panel on Climate Change (IPCC)
has, at five-yearly intervals, assessed and reported on the current
state of knowledge and understanding of the climate issue.
These reports are intended to be used to provide policy makers with
an objective assessment and review of the information available on
The Third Assessment Report, completed this year, consists of three
- IPCC Working Group I reports on current understanding of the
basic science of climate change.
- Working Group II reports on impacts, adaptation measures and
vulnerability to the effects of climate change.
- Working Group III reports on the scientific, technical,
environmental and social aspects together with mitigation options to
Given the importance of the IPCC reports as a global scientific
consensus on our understanding of the climate issue, we consider it to
be appropriate and useful that Tiempo presents a summary of
each Working Group Report.
The IPCC Working Group I Report was approved in January 2001 as
delegates from 99 countries met in Shanghai, PR China. In this issue,
we present selected extracts from the Working Group I Summary for
Policymakers concerning the basic science of the climate issue. We
will be presenting excerpts from the other working group reports in
future issues of the bulletin.
The following text is taken verbatim from the Policymakers Summary.
All main conclusions are covered, though some explanatory detail has
In this summary, the following words have been used by the IPCC
where appropriate to indicate judgmental estimates of confidence:
- virtually certain (greater than 99% chance that a result is
- very likely (90-99% chance);
- likely (66-90%);
- medium likelihood (33-66% chance);
- unlikely (10-33% chance);
- very unlikely (1-10% chance);
- exceptionally unlikely (less than 1% chance).
Climate change in IPCC usage refers to any change in climate over
time, whether due to natural variability or as a result of human
activity. This usagediffers from that in the Framework Convention on
Climate Change, where climate change refers to a change of climate
that is attributed directly or indirectly to human activity that
alters the composition of the global atmosphere and that is in
addition to natural climate variability observed over comparable time
SUMMARY FOR POLICYMAKERS
The Third Assessment Report of Working Group I of the
Intergovernmental Panel on Climate Change builds upon past assessments
and incorporates new results from the past five years of research on
climate change. Many hundreds of scientists from many countries
participated in its preparation.
This Summary for Policymakers, which was approved by IPCC member
governments in Shanghai in January 2001, describes the current state
of understanding of the climate system and provides estimates of its
projected future evolution and their uncertainties.
An increasing body of observations gives a collective picture of
a warming world and other changes in the climate system.
Since the release of the Second Assessment Report (SAR), additional
data from new studies of current and palaeoclimates, improved analysis
of data sets, more rigorous evaluation of their quality, and
comparisons among data from different sources have led to greater
understanding of climate change.
The global average surface temperature has increased over the
20th century by about 0.6°C.
- The global average surface temperature... has increased since
1861. Over the 20th century the increase has been 0.6±0.2°C.
This value is about 0.15°C larger than that estimated by the
SAR for the period up to 1994, owing to the relatively high
temperatures of the additional years (1995 to 2000) and improved
methods of processing the data. These numbers take into account
various adjustments, including urban heat island effects. The record
shows a great deal of variability; for example, most of the warming
occurred during the 20th century, during two periods, 1910 to 1945
and 1976 to 2000.
- Globally, it is very likely that the 1990s was the warmest decade
and 1998 the warmest year in the instrumental record, since 1861.
- New analyses of proxy data for the Northern Hemisphere indicate
that the increase in temperature in the 20th century is likely to
have been the largest of any century during the past 1,000 years. It
is likely that, in the Northern Hemisphere, the 1990s was the
warmest decade and 1998 the warmest year. Because less data are
available, less is known about annual averages prior to 1,000 years
before present and for conditions prevailing in most of the Southern
Hemisphere prior to 1861.
Temperatures have risen during the past four decades in the
lowest 8 kilometres of the atmosphere.
- Since the late 1950s (the period of adequate observations from
weather balloons), the overall global temperature increases in the
lowest 8 kilometres of the atmosphere and in surface temperature
have been similar at 0.1°C per decade.
- Since the start of the satellite record in 1979, both satellite
and weather balloon measurements show that the global average
temperature of the lowest 8 kilometres of the atmosphere has changed
by +0.05±0.10°C per decade, but the global average surface
temperature has increased significantly by +0.15±0.05°C
per decade. The difference in the warming rate is statistically
significant. This difference occurs primarily over the tropical and
- The lowest 8 kilometres of the atmosphere and the surface are
influenced differently by factors such as stratospheric ozone
depletion, atmospheric aerosols, and the El Niño phenomenon.
Hence, it is physically plausible to expect that over a short time
period (for example, 20 years) there may be differences in
temperature trends. In addition, spatial sampling techniques can
also explain some of the differences in trends, but these
differences are not fully resolved.
Snow cover and ice extent have decreased.
- Satellite data show that there are very likely to have been
decreases of about 10% in the extent of snow cover since the late
1960s, and ground-based observations show that there is very likely
to have been a reduction of about two weeks in the annual duration
of lake and river ice cover in the mid- and high latitudes of the
Northern Hemisphere, over the 20th century.
- There has been a widespread retreat of mountain glaciers in
non-polar regions during the 20th century.
- Northern Hemisphere spring and summer sea-ice extent has
decreased by about 10 to 15% since the 1950s...
Global average sea level has risen and ocean heat content has
- Tide gauge data show that global average sea level rose between
0.1 and 0.2 metres during the 20th century.
- Global ocean heat content has increased since the late 1950s, the
period for which adequate observations of sub-surface ocean
temperatures have been available.
Changes have also occurred in other important aspects of
- It is very likely that precipitation has increased by 0.5 to 1%
per decade in the 20th century over most mid- and high latitudes of
the Northern Hemisphere continents, and it is likely that rainfall
has increased by 0.2 to 0.3% per decade over the tropical (10°N
to 10°S) land areas. Increases in the tropics are not evident
over the past few decades. It is also likely that rainfall has
decreased over much of the Northern Hemisphere sub-tropical (10°N
to 30°N) land areas during the 20th century by about 0.3% per
decade. In contrast to the Northern Hemisphere, no comparable
systematic changes have been detected in broad latitudinal averages
over the Southern Hemisphere...
- Warm episodes of the El-Niño-Southern Oscillation (ENSO)
phenomenon... have been more frequent, persistent and intense since
the mid-1970s, compared with the previous 100 years.
- In some regions, such as parts of Asia and Africa, the frequency
and intensity of droughts have been observed to increase in recent
Some important aspects of climate appear not to have changed.
- A few areas of the globe have not warmed in recent decades,
mainly over some parts of the Southern Hemisphere oceans and parts
- Changes globally in tropical and extra-tropical storm intensity
and frequency are dominated by inter-decadal to multi-decadal
variations, with no significant trends evident over the 20th
century. Conflicting analyses make it difficult to draw definitive
conclusions about changes in storm activity, especially in the
Emissions of greenhouse gases and aerosols due to human
activities continue to alter the atmosphere in ways that are expected
to affect the climate.
Changes in climate occur as a result of both internal variability
within the climate system and external factors (both natural and
anthropogenic). The influence of external factors on climate can be
broadly compared using the concept of radiative forcing. A positive
radiative forcing, such as that produced by increasing concentrations
of greenhouse gases, tends to warm the surface. A negative radiative
forcing, which can arise from an increase in some types of aerosols
(microscopic airborne particles) tends to cool the surface. Natural
factors, such as changes in solar output or explosive volcanic
activity, can also cause radiative forcing.
Concentrations of atmospheric greenhouse gases and their
radiative forcing have continued to increase as a result of human
- The atmospheric concentration of carbon dioxide (CO2)
has increased by 31% since 1750. The present CO2
concentration has not been exceeded during the past 420,000 years
and likely not during the past 20 million years. The current rate of
increase is unprecedented during at least the past 20,000 years.
- About three-quarters of the anthropogenic emissions of CO2
to the atmosphere during the past 20 years is due to fossil fuel
burning. The rest is predominantly due to land-use change,
- Currently the ocean and the land together are taking up about
half of the anthropogenic CO2 emissions. On land, the
uptake of anthropogenic CO2 very likely exceeded the
release of CO2 by deforestation during the 1990s.
- The atmospheric concentration of methane (CH4) has
increased by 1060 ppb (151%) since 1750 and continues to increase.
The present CH4 concentration has not been exceeded
during the past 420,000 years... Slightly more than half of current
CH4 emissions are anthropogenic (for example, use of
fossil fuels, cattle, rice agriculture and landfills). In addition,
carbon monoxide (CO) emissions have recently been identified as a
cause of increasing CH4 concentration.
- The atmospheric concentration of nitrous oxide (N2O)
has increased by 46 ppb (17%) since 1750 and continues to increase.
The present N2O concentration has not been exceeded
during at least the past thousand years. About a third of current N2O
emissions are anthropogenic (for example, agricultural soils, cattle
feed lots and chemical industry).
- Since 1995, the atmospheric concentrations of many of those
halocarbon gases that are both ozone-depleting and greenhouse gases
(for example, CFCl3 and CF2Cl2),
are either increasing more slowly or decreasing, both in response to
reduced emissions under the regulations of the Montreal Protocol and
- The observed depletion of the stratospheric ozone (O3)
layer from 1979 to 2000 is estimated to have caused a negative
radiative forcing (-0.15 Wm-2). Assuming full compliance
with current halocarbon regulations, the positive forcing of the
halocarbons will be reduced as will the magnitude of the negative
forcing from stratospheric ozone depletion as the ozone layer
recovers over the 21st century.
- The total amount of O3 in the troposphere is
estimated to have increased by 36% since 1750, due primarily to
anthropogenic emissions of several O3-forming gases...
Anthropogenic aerosols are short-lived and mostly
produce negative radiative forcing.
- The major sources of anthropogenic aerosols are fossil fuel and
biomass burning. These sources are also linked to degradation of air
quality and acid deposition.
- Since the SAR, significant progress has been achieved in better
characterising the direct radiative roles of different types of
aerosols. Direct radiative forcing is estimated to be -0.4 Wm-2
for sulphate, -0.2 Wm-2 for biomass burning aerosols,
-0.1 Wm-2 for fossil fuel organic carbon and +0.2 Wm-2
for fossil fuel black carbon aerosols. There is much less confidence
in the ability to quantify the total aerosol direct effect, and its
evolution over time, than that for the gases listed above. Aerosols
also vary considerably by region and respond quickly to changes in
- In addition to their direct radiative forcing, aerosols have an
indirect radiative forcing through their effects on clouds. There is
now more evidence for this indirect effect, which is negative,
although of very uncertain magnitude.
Natural factors have made small contributions to radiative
forcing over the past century.
- The radiative forcing due to changes in solar irradiance for the
period since 1750 is estimated to be about +0.3 Wm-2,
most of which occurred during the first half of the 20th century.
Since the late 1970s, satellite instruments have observed small
oscillations due to the 11-year solar cycle. Mechanisms for the
amplification of solar effects on climate have been proposed, but
currently lack a rigorous theoretical or observational basis.
- Stratospheric aerosols from explosive volcanic eruptions lead to
negative forcing, which lasts a few years. Several major eruptions
occurred in the periods 1880 to 1920 and 1960 to 1991.
- The combined change in radiative forcing of the two major natural
factors (solar variations and volcanic aerosols) is estimated to be
negative for the past two, and possibly the past four, decades.
Confidence in the ability of models to project future climate
Complex physically-based climate models are required to provide
detailed estimates of feedbacks and of regional features. Such models
cannot yet simulate all aspects of climate (for example, they still
cannot account fully for the observed trend in the surface-troposphere
temperature difference since 1979) and there are particular
uncertainties associated with clouds and their interaction with
radiation and aerosols. Nevertheless, confidence in the ability of
these models to provide useful projections of future climate has
improved due to the demonstrated performance on a range of space and
- Simulations that include estimates of natural and anthropogenic
forcing reproduce the observed large-scale changes in surface
temperature over the 20th century. However, contributions from some
additional processes and forcings may not have been included in the
models. Nevertheless, the large-scale consistency between models and
observations can be used to provide an independent check on
projected warming rates over the next few decades under a given
- Some aspects of model simulations of ENSO, monsoons and the North
Atlantic Oscillation, as well as selected periods of past climate,
There is new and stronger evidence that most of the warming
observed over the last 50 years is attributable to human activities.
The SAR concluded: The balance of evidence suggests a
discernible human influence on global climate. That report also
noted that the anthropogenic signal was still emerging from the
background of natural climate variability. Since the SAR, progress has
been made in reducing uncertainty, particularly with respect to
distinguishing and quantifying the magnitude of responses to different
external influences. Although many of the sources of uncertainty
identified in the SAR still remain to some degree, new evidence and
improved understanding support an updated conclusion.
- There is a longer and more closely scrutinized temperature record
and new model estimates of variability. The warming over the past
100 years is very unlikely to be due to internal variability alone,
as estimated by current models. Reconstructions of climate data for
the past 1,000 years also indicate that this warming was unusual and
is unlikely to be entirely natural in its origin.
- There are new estimates of the climate response to natural and
anthropogenic forcing, and new detection techniques have been
applied. Detection and attribution studies consistently find
evidence for an anthropogenic signal in the climate record of the
last 35 to 50 years.
- In the light of new evidence and taking into account the
remaining uncertainties, most of the observed warming over the last
50 years is likely to have been due to the increase in greenhouse
- Furthermore, it is very likely that the 20th century warming has
contributed significantly to the observed sea level rise, through
thermal expansion of sea water and widespread loss of land ice.
Within present uncertainties, observations and models are both
consistent with a lack of significant acceleration of sea level rise
during the 20th century.
Human influences will continue to change atmospheric composition
throughout the 21st century.
Models have been used to make projections of atmospheric
concentrations of greenhouse gases and aerosols, and hence of future
climate, based upon emissions scenarios
from the IPCC Special Report on Emissions Scenarios (SRES).
- Emissions of CO2 due to fossil fuel burning are
virtually certain to be the dominant influence on the trends in
atmospheric CO2 concentration during the 21st century.
- By 2100, carbon cycle models project atmospheric CO2
concentrations of 540 to 970 ppm for the illustrative SRES scenarios
(90 to 250% above the concentration of 280 ppm in the year 1750).
These projections include the land and ocean climate feedbacks.
Uncertainties, especially about the magnitude of the climate
feedback from the terrestrial biosphere, cause a variation of about
-10 to +30% around each scenario. The total range is 490 to 1260 ppm
(75 to 350% above the 1750 concentration).
- Reductions in greenhouse gas emissions and the gases that control
their concentration would be necessary to stabilize radiative
forcing. For example, for the most important anthropogenic
greenhouse gas, carbon cycle models indicate that stabilization of
atmospheric CO2 concentrations at 450, 650 or 1,000 ppm
would require global anthropogenic CO2 emissions to drop
below 1990 levels, within a few decades, about a century, or about
two centuries, respectively, and continue to decrease steadily
thereafter. Eventually CO2 emissions would need to
decline to a very small fraction of current emissions.
- The SRES scenarios include the possibility of either increases or
decreases in anthropogenic aerosols (for example, sulphate aerosols,
biomass aerosols, black and organic carbon aerosols) depending on
the extent of fossil fuel use and policies to abate polluting
emissions. In addition, natural aerosols (for example, sea salt,
dust and emissions leading to the production of sulphate and carbon
aerosols) are projected to increase as a result of changes in
- For the SRES illustrative scenarios, relative to the year 2000,
the global mean radiative forcing due to greenhouse gases continues
to increase through the 21st century, with the fraction due to CO2
projected to increase from slightly more than half to about three
quarters. The change in the direct plus indirect aerosol radiative
forcing is projected to be smaller in magnitude than that of CO2.
Global average temperature and sea level are projected to rise
under all IPCC SRES scenarios.
In order to make projections of future climate, models incorporate
past, as well as future emissions of greenhouse gases and aerosols.
Hence, they include estimates of warming to date and the commitment to
future warming from past emissions.
Figure 1: The range of projected
global-mean surface air temperature responses taking account of
key uncertainties in scientific understanding and in future
emissions. The results are derived from a simple climate model
tuned to a number of more complex models with a range of climate
sensitivities over the full range of 35 SRES greenhouse gas
emissions scenarios. Graphic amended from the Working Group I
Summary for Policymakers, courtesy of Sarah Raper and Mike Salmon.
- The globally averaged surface temperature is projected to
increase by 1.4 to 5.8°C over the period 1990 to 2100 (Figure
1). These results are for the full range of 35 SRES scenarios, based
on a number of climate models.
- The projected rate of warming is much larger than the observed
changes during the 20th century and is very likely to be without
precedent during at least the last 10,000 years, based on
- Based on recent global model simulations, it is very likely that
nearly all land areas will warm more rapidly than the global
average, particularly those at northern high latitudes in the cold
season. Most notable of these is the warming in the northern regions
of North America, and northern and central Asia, which exceeds
global mean warming in each model by more than 40%. In contrast, the
warming is less than the global mean change in south and southeast
Asia in summer and in southern South America in winter.
- Recent trends for surface temperature to become more El Niño-like
in the tropical Pacific, with the eastern tropical Pacific warming
more than the western tropical Pacific, with a corresponding
eastward shift of precipitation, are projected to continue in many
- Based on global model simulations and for a wide range of
scenarios, global average water vapour concentration and
precipitation are projected to increase during the 21st century. By
the second half of the 21st century, it is likely that precipitation
will have increased over northern mid- to high latitudes and
Antarctica in winter. At low latitudes there are both regional
increases and decreases over land areas. Larger year to year
variations in precipitation are very likely over most areas where an
increase in mean precipitation is projected.
Confidence in observed changes (latter half of
Changes in phenomenon
Confidence in projected changes (during the
Higher maximum temperatures and more hot days over
nearly all land areas
Higher minimum temperatures, fewer cold days and
frost days over nearly all land areas
Reduced diurnal temperature range over most land
Likely, over many areas
Increase of heat index over land areas
Very likely, over most areas
Likely, over many Northern Hemisphere mid- to high
latitude land areas
More intense precipitation events*
Very likely, over most areas
Likely, in a few areas
Increased summer continental drying and associated
risk of drought
Likely, over most mid-latitude continental
interiors. (Lack of consistent projections in other areas.)
Not observed in the few analyses available
Increase in tropical cyclone peak wind
Likely, over some areas
Insufficient data for assessment
Increase in tropical cyclone mean and peak
Likely, over some areas
Table 1: Estimates of confidence in observed and
projected changes in extreme weather and climate events. See
introduction for an explanation of terms likely
and very likely.
* For other areas, there are either insufficient data or
** Past and future changes in tropical cyclone location and
frequency are uncertain
- Table 1 depicts an assessment of confidence in observed changes
in extremes of weather and climate during the latter half of the
20th century (left column) and in projected changes during the 21st
century (right column). This assessment relies on observational and
modelling studies, as well as the physical plausibility of future
projections across all commonly-used scenarios and is based on
- For some other extreme phenomena, many of which may have
important impacts on the environment and society, there is currently
insufficient information to assess recent trends, and climate models
currently lack the spatial detail required to make confident
projections. For example, very small-scale phenomena, such as
thunderstorms, tornadoes, hail and lightning, are not simulated in
- Confidence in projections of changes in future frequency,
amplitude, and spatial pattern of El Niño events in the
tropical Pacific is tempered by some shortcomings in how well El Niño
is simulated in complex models. Current projections show little
change or a small increase in amplitude for El Niño events
over the next 100 years.
- Even with little or no change in El Niño amplitude, global
warming is likely to lead to greater extremes of drying and heavy
rainfall and increase the risk of droughts and floods that occur
with El Niño events in many different regions.
- It is likely that warming associated with increasing greenhouse
gas concentrations will cause an increase of Asian summer monsoon
precipitation variability. Changes in monsoon mean duration and
strength depend on the details of the emission scenario. The
confidence in such projections is also limited by how well the
climate models simulate the detailed seasonal evolution of the
- Most models show weakening of the ocean thermohaline circulation
which leads to a reduction of the heat transport into high latitudes
of the Northern Hemisphere. However, even in models where the
thermohaline circulation weakens, there is still a warming over
Europe due to increased greenhouse gases. The current projections
using climate models do not exhibit a complete shut-down of the
thermohaline circulation by 2100. Beyond 2100, the thermohaline
circulation could completely, and possibly irreversibly, shut-down
in either hemisphere if the change in radiative forcing is large
enough and applied long enough.
- Northern Hemisphere snow cover and sea-ice extent are projected
to decrease further.
- Glaciers and ice caps are projected to continue their widespread
retreat during the 21st century.
- The Antarctic ice sheet is likely to gain mass because of greater
precipitation, while the Greenland ice sheet is likely to lose mass
because the increase in runoff will exceed the precipitation
- Global mean sea level is projected to rise by 0.09 to 0.88 metres
between 1990 and 2100, for the full range of SRES scenarios. This is
due primarily to thermal expansion and loss of mass from glaciers
and ice caps. The range of sea level rise presented in the SAR was
0.13 to 0.94 metres based on the IS92 scenarios. Despite the higher
temperature change projections in this assessment, the sea level
projections are slightly lower, primarily due to the use of improved
models, which give a smaller contribution from glaciers and ice
Anthropogenic climate change will persist for many centuries.
- Emissions of long-lived greenhouse gases (that is, CO2,
N2O, PFCs, SF6) have a lasting effect on atmospheric
composition, radiative forcing and climate. For example, several
centuries after CO2 emissions occur, about a quarter of
the increase in CO2 concentration caused by these
emissions is still present in the atmosphere.
- After greenhouse gas concentrations have stabilized, global
average surface temperatures would rise at a rate of only a few
tenths of a degree per century rather than several degrees per
century as projected for the 21st century without stabilization. The
lower the level at which concentrations are stabilized, the smaller
the total temperature change.
- Global mean surface temperature increases and rising sea level
from thermal expansion of the ocean are projected to continue for
hundreds of years after stabilization of greenhouse gas
concentrations (even at present levels), owing to the long
timescales on which the deep ocean adjusts to climate change.
- Ice sheets will continue to react to climate warming and
contribute to sea level rise for thousands of years after climate
has been stabilized. Climate models indicate that the local warming
over Greenland is likely to be one to three times the global
average. Ice sheet models project that a local warming of larger
than 3°C, if sustained for millennia, would lead to virtually a
complete melting of the Greenland ice sheet with a resulting sea
level rise of about 7 metres. A local warming of 5.5°C, is
sustained for 1000 years, would be likely to result in a
contribution from Greenland of about 3 metres to sea level rise.
- Current ice dynamic models suggest that the West Antarctic ice
sheet could contribute up to 3 metres to sea level rise over the
next 1000 years, but such results are strongly dependent on model
assumptions regarding climate change scenarios, ice dynamics and
Further action is required to address the remaining gaps in
information and understanding.
Further research is required to improve the ability to detect,
attribute and understand climate change, to reduce uncertainties and
to project future climate changes. In particular, there is a need for
additional systematic and sustained observations, modelling and
process studies. A serious concern is the decline of observational
networks. The following are high priority areas for action.
Systematic observations and reconstructions:
- Reverse the decline of observational networks in many parts of
- Sustain and expand the observational foundation for climate
studies by providing accurate, long-term, consistent data including
implementation of a strategy for integrated global observations.
- Enhance the development of reconstructions of past climate
- Improve the observations of the spatial distribution of
greenhouse gases and aerosols.
Modelling and process studies:
- Improve understanding of the mechanisms and factors leading to
changes in radiative forcing.
- Understand and characterize the important unresolved processes
and feedbacks, both physical and biogeochemical, in the climate
- Improve methods to quantify uncertainties of climate projections
and scenarios, including long-term ensemble simulations using
- Improve the integrated hierarchy of global and regional climate
models with a focus on the simulation of climate variability,
regional climate changes and extreme events.
- Link more effectively models of the physical climate and the
biogeochemical system, and in turn improve coupling with
descriptions of human activities.
Cutting across these foci are crucial needs associated with
strengthening international cooperation and coordination in order to
better utilize scientific, computational and observational resources.
This should also promote the free exchange of data among scientists. A
special need is to increase the observational and research capacities
in many regions, particularly in developing countries. Finally, as is
the goal of this assessment, there is a continuing imperative to
communicate research advances in terms that are relevant to decision
The Working Group I Summary for Policymakers is
available on the IPCC web
site or can be obtained from the IPCC Secretariat, c/o World
Meteorological Organization, PO Box 2300, CH-1211 Geneva 2,
Switzerland. The full Working Group 1 report, Climate Change
2001: The Scientific Basis, is published by