IPCC briefing on climate change

["Mark Jones" <jones118@xxxxxxxxxxxxxxx>]

http://www.ipcc.ch/pub/sarsum1.htm


Summary for Policymakers: The Science of Climate Change - IPCC Working Group
I
Contents

Greenhouse gas concentrations have continued to increase
Anthropogenic aerosols tend to produce negative radiative forcings
Climate has changed over the past century
The balance of evidence suggests a discernible human influence on global
climate
Climate is expected to continue to change in the future
There are still many uncertainties
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Considerable progress has been made in the understanding of climate change1
science since 1990 and new data and analyses have become available.

1. Greenhouse gas concentrations have continued to increase
Increases in greenhouse gas concentrations since pre­industrial times (i.e.,
since about 1750) have led to a positive radiative forcing2 of climate,
tending to warm the surface and to produce other changes of climate.

The atmospheric concentrations of greenhouse gases, inter alia, carbon
dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have grown
significantly: by about 30%, 145%, and 15%, respectively (values for 1992).
These trends can be attributed largely to human activities, mostly
fossil­fuel use, land­use change and agriculture.
The growth rates of CO2, CH4 and N2O concentrations were low during the
early 1990s. While this apparently natural variation is not yet fully
explained, recent data indicate that the growth rates are currently
comparable to those averaged over the 1980s.
The direct radiative forcing of the long­lived greenhouse gases (2.45 Wm­2)
is due primarily to increases in the concentrations of CO2 (1.56 Wm­2), CH4
(0.47 Wm­2) and N2O (0.14 Wm­2) (values for 1992).
Many greenhouse gases remain in the atmosphere for a long time (for CO2 and
N2O, many decades to centuries), hence they affect radiative forcing on long
time­scales.
The direct radiative forcing due to the CFCs and HCFCs combined is 0.25
Wm­2. However, their net radiative forcing is reduced by about 0.1 Wm­2
because they have caused stratospheric ozone depletion which gives rise to a
negative radiative forcing.
Growth in the concentration of CFCs, but not HCFCs, has slowed to about
zero. The concentrations of both CFCs and HCFCs, and their consequent ozone
depletion, are expected to decrease substantially by 2050 through
implementation of the Montreal Protocol and its Adjustments and Amendments.
At present, some long­lived greenhouse gases (particularly HFCs (a CFC
substitute), PFCs and SF6) contribute little to radiative forcing but their
projected growth could contribute several per cent to radiative forcing
during the 21st century.
If carbon dioxide emissions were maintained at near current (1994) levels,
they would lead to a nearly constant rate of increase in atmospheric
concentrations for at least two centuries, reaching about 500 ppmv
(approaching twice the pre­industrial concentration of 280 ppmv) by the end
of the 21st century.
A range of carbon cycle models indicates that stabilization of atmospheric
CO2 concentrations at 450, 650 or 1000 ppmv could be achieved only if global
anthropogenic CO2 emissions drop to 1990 levels by, respectively,
approximately 40, 140 or 240 years from now, and drop substantially below
1990 levels subsequently.
Any eventual stabilized concentration is governed more by the accumulated
anthropogenic CO2 emissions from now until the time of stabilization than by
the way those emissions change over the period. This means that, for a given
stabilized concentration value, higher emissions in early decades require
lower emissions later on. Among the range of stabilization cases studied,
for stabilization at 450, 650 or 1000 ppmv, accumulated anthropogenic
emissions over the period 1991 to 2100 are 630 GtC3, 1030 GtC and 1410 GtC,
respectively (approximately 15% in each case). For comparison the
corresponding accumulated emissions for IPCC IS92 emission scenarios range
from 770 to 2190 GtC.
Stabilization of CH4 and N2O concentrations at today's levels would involve
reductions in anthropogenic emissions of 8% and more than 50% respectively.
There is evidence that tropospheric ozone concentrations in the Northern
Hemisphere have increased since pre­industrial times because of human
activity and that this has resulted in a positive radiative forcing. This
forcing is not yet well characterized, but it is estimated to be about 0.4
Wm­2 (15% of that from the long­lived greenhouse gases). However, the
observations of the most recent decade show that the upward trend has slowed
significantly or stopped.

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2. Anthropogenic aerosols tend to produce negative radiative forcings
Tropospheric aerosols (microscopic airborne particles) resulting from
combustion of fossil fuels, biomass burning and other sources have led to a
negative direct forcing of about 0.5 Wm­2, as a global average, and possibly
also to a negative indirect forcing of a similar magnitude. While the
negative forcing is focused in particular regions and subcontinental areas,
it can have continental to hemispheric scale effects on climate patterns.
Locally, the aerosol forcing can be large enough to more than offset the
positive forcing due to greenhouse gases.
In contrast to the long­lived greenhouse gases, anthropogenic aerosols are
very short­lived in the atmosphere, hence their radiative forcing adjusts
rapidly to increases or decreases in emissions.

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3. Climate has changed over the past century
At any one location, year­to­year variations in weather can be large, but
analyses of meteorological and other data over large areas and over periods
of decades or more have provided evidence for some important systematic
changes.

Global mean surface air temperature has increased by between about 0.3 and
0.6°C since the late 19th century; the additional data available since 1990
and the re­analyses since then have not significantly changed this range of
estimated increase.
Recent years have been among the warmest since 1860, i.e., in the period of
instrumental record, despite the cooling effect of the 1991 Mt Pinatubo
volcanic eruption.
Night­time temperatures over land have generally increased more than daytime
temperatures.
Regional changes are also evident. For example, the recent warming has been
greatest over the mid­latitude continents in winter and spring, with a few
areas of cooling, such as the North Atlantic ocean. Precipitation has
increased over land in high latitudes of the Northern Hemisphere, especially
during the cold season.
Global sea level has risen by between 10 and 25 cm over the past 100 years
and much of the rise may be related to the increase in global mean
temperature.
There are inadequate data to determine whether consistent global changes in
climate variability or weather extremes have occurred over the 20th century.
On regional scales there is clear evidence of changes in some extremes and
climate variability indicators (e.g., fewer frosts in several widespread
areas; an increase in the proportion of rainfall from extreme events over
the contiguous states of the USA). Some of these changes have been toward
greater variability; some have been toward lower variability.
The 1990 to mid­1995 persistent warm­phase of the El Nino­Southern
Oscillation (which causes droughts and floods in many areas) was unusual in
the context of the last 120 years.

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4. The balance of evidence suggests a discernible human influence on global
climate
Any human­induced effect on climate will be superimposed on the background
"noise" of natural climate variability, which results both from internal
fluctuations and from external causes such as solar variability or volcanic
eruptions. Detection and attribution studies attempt to distinguish between
anthropogenic and natural influences. "Detection of change" is the process
of demonstrating that an observed change in climate is highly unusual in a
statistical sense, but does not provide a reason for the change.
"Attribution" is the process of establishing cause and effect relations,
including the testing of competing hypotheses.

Since the 1990 IPCC Report, considerable progress has been made in attempts
to distinguish between natural and anthropogenic influences on climate. This
progress has been achieved by including effects of sulphate aerosols in
addition to greenhouse gases, thus leading to more realistic estimates of
human­induced radiative forcing. These have then been used in climate models
to provide more complete simulations of the human­induced climate­change
"signal". In addition, new simulations with coupled atmosphere­ocean models
have provided important information about decade to century time­scale
natural internal climate variability. A further major area of progress is
the shift of focus from studies of global­mean changes to comparisons of
modelled and observed spatial and temporal patterns of climate change.

The most important results related to the issues of detection and
attribution are:

The limited available evidence from proxy climate indicators suggests that
the 20th century global mean temperature is at least as warm as any other
century since at least 1400 A.D. Data prior to 1400 are too sparse to allow
the reliable estimation of global mean temperature.
Assessments of the statistical significance of the observed global mean
surface air temperature trend over the last century have used a variety of
new estimates of natural internal and externally­forced variability. These
are derived from instrumental data, palaeodata, simple and complex climate
models, and statistical models fitted to observations. Most of these studies
have detected a significant change and show that the observed warming trend
is unlikely to be entirely natural in origin.
More convincing recent evidence for the attribution of a human effect on
climate is emerging from pattern­based studies, in which the modelled
climate response to combined forcing by greenhouse gases and anthropogenic
sulphate aerosols is compared with observed geographical, seasonal and
vertical patterns of atmospheric temperature change. These studies show that
such pattern correspondences increase with time, as one would expect, as an
anthropogenic signal increases in strength. Furthermore, the probability is
very low that these correspondences could occur by chance as a result of
natural internal variability only. The vertical patterns of change are also
inconsistent with those expected for solar and volcanic forcing.
Our ability to quantify the human influence on global climate is currently
limited because the expected signal is still emerging from the noise of
natural variability, and because there are uncertainties in key factors.
These include the magnitude and patterns of long­term natural variability
and the time­evolving pattern of forcing by, and response to, changes in
concentrations of greenhouse gases and aerosols, and land surface changes.
Nevertheless, the balance of evidence suggests that there is a discernible
human influence on global climate.

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5. Climate is expected to continue to change in the future
The IPCC has developed a range of scenarios, IS92a­f, of future greenhouse
gas and aerosol precursor emissions based on assumptions concerning
population and economic growth, land­use, technological changes, energy
availability and fuel mix during the period 1990 to 2100. Through
understanding of the global carbon cycle and of atmospheric chemistry, these
emissions can be used to project atmospheric concentrations of greenhouse
gases and aerosols and the perturbation of natural radiative forcing.
Climate models can then be used to develop projections of future climate.

The increasing realism of simulations of current and past climate by coupled
atmosphere­ocean climate models has increased our confidence in their use
for projection of future climate change. Important uncertainties remain, but
these have been taken into account in the full range of projections of
global mean temperature and sea­level change.
For the mid­range IPCC emission scenario, IS92a, assuming the "best
estimate" value of climate sensitivity4 and including the effects of future
increases in aerosol, models project an increase in global mean surface air
temperature relative to 1990 of about 2°C by 2100. This estimate is
approximately one­third lower than the "best estimate" in 1990. This is due
primarily to lower emission scenarios (particularly for CO2 and the CFCs),
the inclusion of the cooling effect of sulphate aerosols, and improvements
in the treatment of the carbon cycle. Combining the lowest IPCC emission
scenario (IS92c) with a "low" value of climate sensitivity and including the
effects of future changes in aerosol concentrations leads to a projected
increase of about 1°C by 2100. The corresponding projection for the highest
IPCC scenario (IS92e) combined with a "high" value of climate sensitivity
gives a warming of about 3.5°C. In all cases the average rate of warming
would probably be greater than any seen in the last 10,000 years, but the
actual annual to decadal changes would include considerable natural
variability. Regional temperature changes could differ substantially from
the global mean value. Because of the thermal inertia of the oceans, only
50­90% of the eventual equilibrium temperature change would have been
realized by 2100 and temperature would continue to increase beyond 2100,
even if concentrations of greenhouse gases were stabilized by that time.
Average sea level is expected to rise as a result of thermal expansion of
the oceans and melting of glaciers and ice­sheets. For the IS92a scenario,
assuming the "best estimate" values of climate sensitivity and of ice­melt
sensitivity to warming, and including the effects of future changes in
aerosol, models project an increase in sea level of about 50 cm from the
present to 2100. This estimate is approximately 25% lower than the "best
estimate" in 1990 due to the lower temperature projection, but also
reflecting improvements in the climate and ice­melt models. Combining the
lowest emission scenario (IS92c) with the "low" climate and ice­melt
sensitivities and including aerosol effects gives a projected sea­level rise
of about 15 cm from the present to 2100. The corresponding projection for
the highest emission scenario (IS92e) combined with "high" climate and
ice­melt sensitivities gives a sea­level rise of about 95 cm from the
present to 2100. Sea level would continue to rise at a similar rate in
future centuries beyond 2100, even if concentrations of greenhouse gases
were stabilized by that time, and would continue to do so even beyond the
time of stabilization of global mean temperature. Regional sea­level changes
may differ from the global mean value owing to land movement and ocean
current changes.
Confidence is higher in the hemispheric­to­continental scale projections of
coupled atmosphere­ocean climate models than in the regional projections,
where confidence remains low. There is more confidence in temperature
projections than hydrological changes.
All model simulations, whether they were forced with increased
concentrations of greenhouse gases and aerosols or with increased
concentrations of greenhouse gases alone, show the following features:
greater surface warming of the land than of the sea in winter; a maximum
surface warming in high northern latitudes in winter, little surface warming
over the Arctic in summer; an enhanced global mean hydrological cycle, and
increased precipitation and soil moisture in high latitudes in winter. All
these changes are associated with identifiable physical mechanisms.
In addition, most simulations show a reduction in the strength of the north
Atlantic thermohaline circulation and a widespread reduction in diurnal
range of temperature. These features too can be explained in terms of
identifiable physical mechanisms.
The direct and indirect effects of anthropogenic aerosols have an important
effect on the projections. Generally, the magnitudes of the temperature and
precipitation changes are smaller when aerosol effects are represented,
especially in northern mid­latitudes. Note that the cooling effect of
aerosols is not a simple offset to the warming effect of greenhouse gases,
but significantly affects some of the continental scale patterns of climate
change, most noticeably in the summer hemisphere. For example, models that
consider only the effects of greenhouse gases generally project an increase
in precipitation and soil moisture in the Asian summer monsoon region,
whereas models that include, in addition, some of the effects of aerosols
suggest that monsoon precipitation may decrease. The spatial and temporal
distribution of aerosols greatly influences regional projections, which are
therefore more uncertain.
A general warming is expected to lead to an increase in the occurrence of
extremely hot days and a decrease in the occurrence of extremely cold days.
Warmer temperatures will lead to a more vigorous hydrological cycle; this
translates into prospects for more severe droughts and/or floods in some
places and less severe droughts and/or floods in other places. Several
models indicate an increase in precipitation intensity, suggesting a
possibility for more extreme rainfall events. Knowledge is currently
insufficient to say whether there will be any changes in the occurrence or
geographical distribution of severe storms, e.g., tropical cyclones.
Sustained rapid climate change could shift the competitive balance among
species and even lead to forest dieback, altering the terrestrial uptake and
release of carbon. The magnitude is uncertain, but could be between zero and
200 GtC over the next one to two centuries, depending on the rate of climate
change.

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6. There are still many uncertainties
Many factors currently limit our ability to project and detect future
climate change. In particular, to reduce uncertainties further work is
needed on the following priority topics:

Estimation of future emissions and biogeochemical cycling (including sources
and sinks) of greenhouse gases, aerosols and aerosol precursors and
projections of future concentrations and radiative properties.
Representation of climate processes in models, especially feedbacks
associated with clouds, oceans, sea ice and vegetation, in order to improve
projections of rates and regional patterns of climate change.
Systematic collection of long­term instrumental and proxy observations of
climate system variables (e.g., solar output, atmospheric energy balance
components, hydrological cycles, ocean characteristics and ecosystem
changes) for the purposes of model testing, assessment of temporal and
regional variability, and for detection and attribution studies.
Future unexpected, large and rapid climate system changes (as have occurred
in the past) are, by their nature, difficult to predict. This implies that
future climate changes may also involve "surprises". In particular, these
arise from the non­linear nature of the climate system. When rapidly forced,
non­linear systems are especially subject to unexpected behaviour. Progress
can be made by investigating non­linear processes and sub­components of the
climatic system. Examples of such non­linear behaviour include rapid
circulation changes in the North Atlantic and feedbacks associated with
terrestrial ecosystem changes.


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Footnotes:

1 Climate change in IPCC Working Group I usage refers to any change in
climate over time whether due to natural variability or as a result of human
activity. This differs from the usage in the UN Framework Convention on
Climate Change where "climate change" refers to a change of climate which is
attributed directly or indirectly to human activity that alters the
composition of the global atmosphere and which is in addition to natural
climate variability observed over comparable time periods.
2 A simple measure of the importance of a potential climate change
mechanism. Radiative forcing is the perturbation to the energy balance of
the Earth­atmosphere system (in Watts per square metre [Wm­2]).
3 1 GtC = 1 billion tonnes of carbon.
4 In IPCC reports, climate sensitivity usually refers to the long­term
(equilibrium) change in global mean surface temperature following a doubling
of atmospheric equivalent CO2 concentration. More generally, it refers to
the equilibrium change in surface air temperature following a unit change in
radiative forcing (oC/Wm­2).


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