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

Contents 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.

2. Anthropogenic aerosols tend to produce negative radiative forcings

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.

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:

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.

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: 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.


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).