Distr.
GENERAL
FCCC/CP/1996/5/Add.1
FCCC/SBSTA/1996/7/Add.1/Rev.1
17 May 1996
ORIGINAL: ENGLISH
CONFERENCE OF THE PARTIES
Second session
Geneva, 8-19 July 1996
Item 5 of the provisional agenda
SUBSIDIARY BODY FOR SCIENTIFIC AND TECHNOLOGICAL ADVICE
Third session
Geneva, 9-16 July 1996
Item 3 of the provisional agenda
Paragraphs Page
I. INTRODUCTION 1 - 8 3
II. MAIN FINDINGS 9 - 10 4
GE.96-
Paragraphs Page
III. TECHNICAL SUMMARY AND SUPPORTING
CHAPTERS 11 - 16 5
A. Introduction 11 - 15 5
B. Observations 16 6
I. Main findings of IPCC Working Group I 7
II. IPCC Working Group I - Tables of contents of the Technical
Summary and Supporting Chapters 11
III. IPCC Working Group I - Glossary of terms 16
1. The Working Group on Scientific Assessment (WG I) of the
Intergovernmental Panel on Climate Change (IPCC) was established in
1988 and re-established in 1992 to assess available information on
the science of climate change, in particular that arising from human
activities. The most important aspects were:
(a) Developments in the scientific understanding of past and
present climate, of climate variability, of climate predictability
and of climate change including feedbacks from climate
impacts;
(b) Progress in the modelling and prediction of global and
regional change of climate and sealevel change;
(c) Observations of climate, including past climates, and
assessment of trends and anomalies;
(d) Gaps and uncertainties in current knowledge.
2. The first IPCC Scientific Assessment completed in 1990 as part of the First Assessment Report (IPCC (1990)) concluded that the increase in atmospheric concentrations of greenhouse gases since pre-industrial times had altered the energy balance of the Earth
and that global warming would result.
3. A primary conclusion identified by the 1990 report was the
expected continued increase in greenhouse gas concentrations as a
result of human activity, leading to significant climate change in
the coming century. The projected changes in temperature,
precipitation and soil moisture were not uniform over the globe.
Anthropogenic aerosols were recognized as a possible source of
regional cooling but no quantitative estimates of their effects were
available. The 1992 Supplementary Report of Working Group I
confirmed, or found no reason to alter, the major conclusions of the
1990 Assessment. It presented a new range of global mean temperature
projections based on a new set of IPCC emission scenarios (IS92 a to
f) and reported progress in quantifying the effects of anthropogenic
aerosols.
4. The 1994 Working Group I report on Radiative Forcing of Climate
Change provided a detailed assessment of the global carbon cycle and
of aspects of atmospheric chemistry governing the abundance of
non-CO2 greenhouse gases. Some pathways that would
stabilize atmospheric greenhouse gas concentrations were examined,
and new or revised calculations of the global warming potential for
38 species were presented.
5. The IPCC Second Assessment Report on the Science of Climate
Change presents a comprehensive assessment of climate change science
as at 1995, including updates of relevant material in all three
preceding reports. Key issues examined in the Second Assessment
Report concern the relative magnitude of human and natural factors in
driving climate change, including the role of aerosols; the
estimation of future climate and sealevel change on both global and
continental scales; and whether any human influence on present-day
climate can be detected.
6. An important distinction should be noted with respect to the
term "climate change". In the Second Assessment Report the term
refers to change arising from any source, human or natural. Within
the United Nations Framework Convention on Climate Change, this term
however, refers exclusively to change brought about by human
activities (see annex III to the present note). In many instances the
two uses will in effect be the same, and this is particularly true
for projections of climate change over the next century.
7. As indicated in document FCCC/SBSTA/1996/7/Rev.1, the
contribution of Working Group I will constitute one of the four
volumes that make up the IPCC Second Assessment Report. It comprises
a Summary for Policymakers and a Technical Summary supported by 11
chapters on relevant scientific issues prepared by teams of
scientists with expert knowledge in the respective
fields.
8. The purpose of this addendum is to make the material included in the Working
Group I contribution more accessible to delegates and to highlight
some of the findings. As noted in document FCCC/SBSTA/1996/7/Rev.1,
paragraph 18, this is not intended to provide an interpretation of
the findings or to serve as replacement for the IPCC text, but as an
invitation to consult the Second Assessment Report.
9. The main findings of the IPCC Working Group I, as adopted by
the IPCC at its plenary session in Rome in December 1995, are
presented in the Summary for Policymakers of the contribution of
Working Group I to the IPCC Second Assessment Report. Copies of the
Summary will be available to members of the SBSTA and the Conference
of the Parties (COP) in all official languages of the United
Nations.
10. To assist members of the Subsidiary Body for Scientific and
Technological Advice (SBSTA) and the COP, the secretariat has
prepared a summary of the main findings of the IPCC, which
constitutes annex I to this addendum. In drawing up this summary the
secretariat was aware of the difficulties of selecting the findings
and presenting them outside the full context of the carefully worded
text agreed in the Summary for Policymakers. The presentation in
annex I, therefore, is intended primarily to assist delegations that
may not have received the Summary in their working
languages.
11. The Earth absorbs radiation from the Sun, mainly at the
surface. The energy is then redistributed by the atmospheric and
oceanic circulation and radiated out to space at longer
("terrestrial" or "infrared") wavelengths. On average, for the Earth
as a whole, the incoming solar energy is balanced by outgoing
terrestrial radiation.
12. Increases in the concentration of greenhouse gases will reduce
the efficiency with which the Earth loses energy to space. More of
the outgoing terrestrial radiation from the surface is absorbed by
the atmosphere and emitted at higher altitudes and colder
temperatures. This results in a positive radiative forcing which will
tend to warm the lower atmosphere and surface. This is the enhanced
greenhouse effect. The amount of warming depends on the size of the
increase in concentration of each greenhouse gas, the radiative
properties of the gases involved, and the concentration of other
greenhouse gases already present in the atmosphere.
13. Any changes in the radiative balance of the Earth, including
those due to an increase in greenhouse gases or in aerosols, will
tend to alter atmospheric and oceanic temperatures and the associated
circulation and weather patterns. These will be accompanied by
changes in the hydrological cycle (for example, altered cloud
distributions or changes in rainfall and evaporation
regimes).
14. Any human-induced changes in climate will be superimposed on a
background of natural climatic variations which occur on a whole
range of space- and time-scales. To distinguish anthropogenic climate
changes from natural variations, it is necessary to identify the
anthropogenic "signal" against the background "noise" of natural
climate variability.
15. The IPCC Working Group I has conducted a full assessment of
scientific and technical knowledge and understanding of possible
climate change due to anthropogenic emissions into the atmosphere.
The data for this assessment, which includes conflicting views, have
been collated by teams of lead authors, who are eminent in their
respective fields and are drawn from developing and developed
countries. The data are presented in a Technical Summary and the 11
chapters supporting the Summary for Policymakers and constituted the
basis for the IPCC findings, the more important of which are
presented in annex I to this addendum.
16. Annex II provides tables of contents and some observations on
the contents of each of the 11 supporting chapters as well as on the
preceding Technical Summary. Since the Technical Summary and the 11
supporting chapters may not be available to the SBSTA and the COP at
the forthcoming sessions, members are encouraged to contact their
national IPCC focal points for appropriate briefing and advice, as
necessary, and to consult the corresponding texts.
As indicated in the Summary for Policymakers of the contribution
of Working Group I to the Second Assessment Report, the main
conclusions of the IPCC, in the light of new data and analyses since
1990, are as follows:
(a) Greenhouse gas concentrations have continued to
increase
Since pre-industrial times (around 1750), the atmospheric
concentrations of carbon dioxide (CO2), methane
(CH4) and nitrous oxide (N2O) have increased by
30 per cent, 145 per cent and 15 per cent respectively (1992 values),
largely owing to human activities, mostly fossil fuel use, land-use
change and agriculture;
The direct radiative forcing(1) of
the long-lived greenhouse gases is due primarily to increases in the
concentrations of CO2, CH4 and
N2O;
At present, some long-lived 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 are maintained at near current (1994)
levels, they could lead to a nearly constant rate of increase in
atmospheric concentrations for at least two centuries, reaching about
500 ppmv (approaching twice the pre-industrial 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, 110
or 240 years from now, and drop substantially below 1990 levels
thereafter;
For a given stabilized concentration value, higher emissions in
early decades require lower emissions later on;
Stabilization of CH4 and N2O concentrations at today's levels would involve reductions in anthropogenic emissions of 8 per cent and more than 50 per cent respectively.
(b) Anthropogenic aerosols tend to produce negative
radiative forcing
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 Wm-2 as
a global average, and possibly also to a negative indirect forcing of
a similar magnitude;
Locally, the aerosol forcing can be large enough to more than
offset the positive forcing due to greenhouse gases;
In contrast to the long-lived greenhouse gases, anthropogenic
aerosols are very short-lived in the atmosphere and therefore do not
spread far beyond their place of origin.
(c) Climate has changed over the past
century
Global mean surface temperature has increased by between about
0.3oC and 0.6oC since the late 19th
century;
Recent years have been among the warmest since 1860, despite the
cooling effect of the 1991 Mt. Pinatubo volcanic
eruption;
Regional changes are evident;
Global sealevel 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.
(d) The balance of evidence suggests a discernable
human influence on global climate
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 long-term
natural variability and the time-evolving 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.
(e) Climate is expected to continue to change in the
future
The increasing realism of simulations of current and past climate
by coupled atmosphere-ocean climate models has increased confidence
in their use for projection of future climate change;
For the mid-range IPCC emission scenario, IS92a, assuming the
"best estimate" value of climate
sensitivity(2) and including the
effects of future increases in aerosols, models project an increase
in global mean surface temperature relative to 1990 of about
2oC by 2100. This estimate is approximately one third
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 1oC
by 2100. The corresponding projection for the highest IPCC scenario
(IS92e), combined with a "high" value of climate sensitivity, gives a
warming of about 3.5oC. 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. Because of the thermal inertia of
the oceans, only 50-90 per cent of the eventual equilibrium
temperature change would have been realized by 2100, and temperature
would continue to increase beyond 2100;
Average sealevel is expected to rise as a result of thermal
expansion of the oceans and melting of glaciers and ice sheets.
Models project an increase in sealevel of about 50 cm from the
present to 2100. This estimate is approximately 25 per cent lower
than the "best estimate" in 1990 owing to the lower temperature
projection, but also reflecting improvements in the climate and ice
melt models. Regional sealevel changes may differ from the global
mean value owing to land movement and ocean current
change;
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. Knowledge is currently insufficient to predict whether there
will be any changes in the occurrence or geographical distribution of
severe storms, for example, 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.
(f) There are still many
uncertainties
Many factors currently limit 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 long-term instrumental and proxy
observation of climate system variables (for example, solar output,
atmospheric energy balance components, hydrological cycles, ocean
characteristics and ecosystem changes) for the purposes of models
testing, assessment of temporal and regional variability and
detection and attribution studies.
Technical Summary
1. Introduction
2. Greenhouse gases, aerosols and their radiative forcing
3. Observed trends and patterns in climate and sealevel
4. Modelling climate and climate change
5. Detection of climate change and attribution of causes
6. The prospects for future climate change
7. Advancing our understanding
This provides an extended but concise Technical Summary of the
detailed information provided in the supporting chapters. A useful
attachment to this Summary is a glossary of terms used in the Working
Group I contribution to the Second Assessment Report. This glossary
is contained in annex III.
Chapter 1 - The Climate System: An Overview
1.1. Climate and climate system
1.2. The driving forces of climate
1.3. Anthropogenic climate change
1.4. Climate response
1.5. Observed climate change
1.6. Prediction and modelling of climate change
This chapter provides a general overview of the climate change
problem from interference with the global energy balance through an
enhanced greenhouse effect due to anthropogenic causes, climate
response and the effects of the land and oceans, climate
predictability and climate projection. The text includes five figures
and four references.
Chapter 2 - Radiative Forcing of Climate
Change
2.1. CO2 and the carbon cycle
2.2. Other trace gases and atmospheric chemistry
2.3. Aerosols
2.4. Radiative forcing
2.5. Trace gas radiative forcing indices
This chapter represents an update of the IPCC Working Group I
report on Radiative Forcing of Climate Change published in 1994. Most
of the main findings are already reflected in annex I above. The text
includes 16 figures and some 240 references.
Chapter 3 - Observed Climate Variability and
Change
3.1. Introduction
3.2. Has the climate warmed?
3.3. Has the climate become wetter?
3.4. Has the atmospheric/oceanic circulation changed?
3.5. Has the climate become more variable or extreme?
3.6. Is the 20th century warming unusual?
3.7. Are the observed trends internally consistent?
The findings of this chapter, which poses questions concerning
changes in temperature, rainfall and atmospheric circulation, are
reflected in annex I above. The text includes 23 figures and some 380
references.
Chapter 4 - Climate Process
4.1. Introduction to climate processes
4.2. Atmospheric processes
4.3. Oceanic processes
4.4. Land surface processes
This chapter assesses the processes in the climate system that are
believed to be the major contributors to the uncertainties in current
projections of greenhouse warming. Many of these processes involve
the coupling of the atmosphere, ocean and land through the
hydrological cycle. Continued progress in climate modelling will
depend on the development of comprehensive data sets and their
application to improving important parametrizations. The text
contains 9 figures and some 200 references.
Chapter 5 - Climate Models - Evaluation
5.1. What is model evaluation and why is it important?
5.2. How well do coupled models reproduce current climate?
5.3. How well do the component atmosphere, land surface, ocean and sea-ice models perform?
5.4. How well do models perform under other conditions?
5.5. How well do we understand model sensitivity?
5.6. How can our confidence in models be increased?
This chapter considers and evaluates the models currently in use
to simulate and predict the climate system. It reviews performance
under different conditions and considers how confidence in models can
be increased. The text contains 34 figures and some 260
references.
Chapter 6 - Climate Models - Projection of Future
Climate
6.1. Introduction
6.2. Mean changes in climate simulated by three-dimensional climate models
6.3. Global mean temperature change for the IPCC (1992) emission scenarios
6.4. Simulated changes of variability induced by increased greenhouse gas concentration
6.5. Changes in extreme events
6.6. Simulation of regional climate change
6.7. Reducing uncertainties, future model capabilities and
improved climate change estimates
This chapter focuses on the estimation of the effects on future
climate of changes in atmospheric composition due to human
activities. An important development since the IPCC First Assessment
Report (1990) is the improved quantification of some radiative
effects of aerosols, and climate projections presented include, in
addition to the effects of increasing greenhouse gas concentrations,
some potential effects of anthropogenic aerosols. The text includes
38 figures and some 260 references.
Chapter 7 - Changes in Sealevel
7.1. Introduction
7.2. How has sealevel changed over the last 100 years?
7.3. Factors contributing to sealevel change
7.4. Can sealevel change during the last 100 years be explained?
7.5. How might sealevel change in the future?
7.6. Spatial and temporal variability
7.7. Major uncertainties and how to reduce them
This chapter assesses the current state of knowledge regarding
climate and sealevel change, with special emphasis on scientific
developments since the 1990 IPCC Report. The main focus is on changes
that occur on the time-scale of a century. Evidence of sealevel
changes during the last 100 years are identified and examined for
factors that could be responsible for such changes. Possible changes
in sealevel which could occur in the next 100 years as a result of
global warming are then considered. The text contains 15 figures and
some 250 references.
Chapter 8 - Detection of Climate Change and Attribution of
Causes
8.1. Introduction
8.2. Uncertainties in model projections of anthropogenic emissions
8.3. Uncertainties in estimating natural variability
8.4. Evaluation of recent studies to detect and attribute climate change
8.5. Qualitative consistency between model predictions and observations
8.6. When will an anthropogenic effect on climate be
identified?
This chapter considers progress in attempts, since the 1990 IPCC
Report, to identify an anthropogenic effect on climate. The first
area of significant advance is that model experiments are now
starting to incorporate the possible climatic effects of
human-induced changes in sulphate aerosols and stratospheric ozone.
The inclusion of these factors has modified in important ways the
picture of how climate might respond to human influences. Thus, the
potential climate change "signal" due to human activities is better
defined, although important signal uncertainties still remain. The
text contains 12 figures and some 130 references.
Chapter 9 - Terrestrial Ecosystems : Biotic Feedbacks to
Climate
9.1. Introduction
9.2. Land-atmosphere CO2 exchange and the global carbon balance : the present
9.3. Possible effects of climate change and atmospheric carbon dioxide increases on ecosystem structure
9.4. Effects of climate change and carbon dioxide increases on regional and global carbon storage : transient and equilibrium analyses
9.5. Methane : effects of climate change and increase in atmospheric CO2 on methane flux and carbon balance in wetlands
9.6. Nitrous oxide
9.7. Global-scale biogeophysical feedbacks: changes in ecosystem
structure and function affect climate
This chapter considers the closely coupled effects of terrestrial
ecosystems. Changes in climate and the CO2 concentration
in the atmosphere cause changes in the structure and function of
terrestrial ecosystems. In turn, changes in the structure and
function of terrestrial ecosystems influence the climatic system
through biogeochemical processes that involve the land-atmosphere
exchanges of radiatively-active gases such as CO2,
CH4 and N2O, and changes in biogeophysical
processes that involve water and energy exchanges. The combined
consequence of these effects and feedbacks are taken into account in
evaluating the future state of the atmosphere or of terrestrial
ecosystems. The text contains 7 figures and some 300
references.
Chapter 10 - Marine Biotic Responses and Feedbacks to Climate
Change
10.1. Introduction
10.2. Ocean processes - biogeochemical responses
10.3. Feedbacks : Influence of marine biota on climate change
10.4. The state of biogeochemical ocean modelling
This chapter considers the responses of marine biogeochemical
processes to and influence on climate. Atmospheric CO2 is
the most important greenhouse gas increasing rapidly owing to human
activities. The oceans contain about 40,000 GtC in dissolved
particulate, and living forms. By contrast, land biota, soils and
detritus total about 2,200 GtC. It is imperative therefore to
understand the contribution of biogeochemical processes in
maintaining the steady state functioning of the ocean carbon cycle.
The chapter contains 7 figures and some 200 references.
Chapter 11 - Advancing our Understanding
11.1. Introduction
11.2. Framework for analysis
11.3. Anthropogenic emissions
11.4. Atmospheric concentrations
11.5. Radiative forcing
11.6. Response of the climate system
11.7. Natural climate variations and detection and attribution of climate change
11.8. Impacts of climate change
11.9. Cross-cutting issues
11.10. International programmes
11.11. Research priorities
This chapter looks at future activities required to advance
understanding of climate change. The findings are reflected in annex
I.
Term Definition
Aerosols Airborne particles. The term has also come to be
associated, erroneously, with the propellant used in "aerosol
sprays".
Climate change (UNFCCC usage) 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.
Climate change (IPCC usage) Climate change as referred to in the
observational record of climate occurs because of internal changes
within the climate system or in the interaction between its
components, or because of changes in external forcing either for
natural reasons or because of human activities. It is generally not
possible to make attribution clearly between these causes.
Projections of future climate change reported by IPCC generally
consider the influence on climate or anthropogenic increases in
greenhouse gases and other human-related factors.
Climate sensitivity In IPCC reports, climate sensitivity usually
refers to the long-term (equilibrium) change in global mean surface
temperature following a doubling of atmospheric CO2
concentration. More generally, it refers to the equilibrium change in
surface air temperature following a unit change in radiative forcing
(oC/Wm-2).
Diurnal temperature range The difference between maximum and
minimum temperatures over a period of 24 hours.
Equilibrium climate experiment An experiment where a step change
is applied to the forcing of a climate model and the model is then
allowed to reach a new equilibrium. Such experiments provide
information on the difference between the initial and final states of
the model, but not on the time-dependent response.
Equivalent CO2 The concentration of CO2 that
would cause the same amount of radiative forcing as the given mixture
of CO2 and other greenhouse gases.
Evapotranspiration The combined process of evaporation from the
Earth's surface and transpiration from vegetation.
Greenhouse gas A gas that absorbs radiation at specific
wavelengths within the spectrum of radiation (infrared radiation)
emitted by the Earth's surface and by clouds. The gas in turn emits
infrared radiation from a level where the temperature is colder than
the surface. The net effect is a local trapping of part of the
absorbed energy and a tendency to warm the planetary surface. Water
vapour (H2O), carbon dioxide (CO2), nitrous
oxide (N2O), methane (CH4) and ozone
(O3) are the primary greenhouse gases in the Earth's
atmosphere.
Ice-cap A dome-shaped glacier usually covering a highland near a
water divide.
Ice sheet A glacier more than 50,000 km2 in area
forming a continuous cover over a land surface or resting on a
continental shelf.
Radiative forcing A simple measure of the importance of a
potential climate change mechanism. Radiative forcing is the
perturbation to the energy balance of the earth-atmosphere system (in
Wm-2) following, for example, a change in the
concentrations of carbon dioxide or a change in the output of the
sun; the climate system responds to the radiative forcing so as to
re-establish the energy balance. A positive radiative forcing tends
to warm the surface and a negative radiative forcing tends to cool
the surface. The radiative forcing is normally quoted as a global and
annual mean value. A more precise definition of radiative forcing, as
used in IPCC reports, is the perturbation of the energy balance of
the surface-troposphere system, after allowing for the stratosphere
to re-adjust to a state of global-mean radiative equilibrium (see
chapter 4 of the 1994 IPCC Report). Sometimes called "climate
forcing".
Spatial scales continental 10-100 million square kilometres
(km2)
regional 100,000 - 10 million km2
local less than 100,000 km2
Soil moisture Water stored in or at the continental surface and
available for evaporation. In the First Assessment Report of 1990 a
single store (or "bucket") was commonly used in climate models.
Today's models, which incorporate canopy and soil processes, view
soil moisture as the amount held in excess of plant 'wilting
point'.
Stratosphere The highly stratified and stable region of the
atmosphere above the troposphere extending from about 10 km to about
50 km.
Thermohaline circulation Large-scale density-driven circulation in
the oceans, driven by changes in temperature and
salinity.
Transient climate experiment An experiment in which the
time-dependent response of a climate model is analysed in response to
a time-varying change of forcing.
Troposphere The lowest part of the atmosphere from the surface of
the Earth to about 10 km is the altitude in mid-latitudes (ranging
from 9 km in high latitudes to 16 km in the tropics on average) where
clouds and "weather" phenomena occur. The troposphere is defined as
the region where temperature generally decreases with
height.
1. A simple measure of the importance of a potential climate change mechanism. Radiative forcing is the perturbation to the energy balance of the Earth-Atmosphere system (in watts per square metre [Wm-2]).
2. In IPCC reports, climate sensitivity usually refers to the long-term (equilibrium) change in global mean surface temperature following a doubling of atmospheric equivalent C02 concentration. More generally, it refers to the equilibrium change in surface air temperature following a unit change in radiative forcing (0C/Wm-2).