What is climate change? The common definition of climate refers to the average of weather, yet the definition of the climate system must reach out to the broader geophysical system that interacts with the atmosphere and our weather. The concept of climate change has acquired a number of different meanings in the scientific literature and in the media. Often, "climate change" denotes variations resulting from human interference, and "climate variability" refers to natural variations. Sometimes "climate change" designates variations longer than a certain period. Finally, "climate change" is often taken to mean climate fluctuations of a global nature, including effects from human activities such as the enhanced greenhouse effect and from natural causes such as volcanic aerosols.
For the purposes of the UNFCCC (and this report), the definition of climate change is: "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." This alteration of the global atmosphere includes changes in land use as well as anthropogenic emissions of greenhouse gases and particles. This FCCC definition thus introduces the concept of the difference between the effect of human activities (climate change) and climatic effects that would occur without such human interference (climate variability).
What drives changes in climate? The Earth absorbs radiation from the sun, mainly at the surface. This energy is then redistributed by atmospheric and oceanic circulations and radiated to space at longer ("terrestrial" or "infrared") wavelengths. On average, for the Earth as a whole, incoming solar energy is balanced by outgoing terrestrial radiation. Any factor that alters radiation received from the sun or lost to space or the redistribution of energy within the atmosphere and between atmosphere, land, and ocean can affect climate. A change in radiative energy available to the global Earth/atmosphere system is termed here, as in previous IPCC reports, radiative forcing (see Section 6.2 for more details). Radiative forcing (RF) is the global, annual average of radiative imbalance (W m-2) in net heating of the Earth's lower atmosphere as a result of human activities since the beginning of the industrial era almost 2 centuries ago.
Increases in the concentrations of greenhouse gases reduce the efficiency with which the surface of the Earth radiates heat to space: More outgoing terrestrial radiation from the surface is absorbed by the atmosphere and is emitted at higher altitudes and colder temperatures. This process results in positive radiative forcing, which tends to warm the lower atmosphere and the surface. This radiative forcing is the enhanced greenhouse effect-an enhancement of an effect that has operated in the Earth's atmosphere for billions of years as a result of naturally occurring greenhouse gases (i.e., water vapor, carbon dioxide, ozone, methane, and nitrous oxide). The amount of warming depends on the size of the increase in concentration of each greenhouse gas, the radiative properties of the gases involved, their geographical and vertical distribution, and the concentrations of other greenhouse gases already present in the atmosphere.
Anthropogenic aerosols (small particles and droplets) in the troposphere-derived mainly from the emission of sulfur dioxide from fossil fuel burning but also from biomass burning and aircraft-can absorb and reflect solar radiation. In addition, changes in aerosol concentrations may alter cloud amount and cloud reflectivity through their effect on cloud microphysical properties. Often, tropospheric aerosols tend to produce negative radiative forcing and thus to cool climate. They have a much shorter lifetime (days to weeks) than most greenhouse gases (which have lifetimes of decades to centuries), so their concentrations respond much more quickly to changes in emissions.
Other natural changes, such as major volcanic eruptions that produce extensive stratospheric aerosols or variations in the sun's energy output, also drive climate variation by altering the radiative balance of the planet. On time scales of tens of thousands of years, slow variations in the Earth's orbit, which are well understood, have led to changes in the seasonal and latitudinal distribution of solar radiation; these changes have played an important part in controlling variations of climate in the distant past, such as glacial cycles.
Any changes in the radiative balance of the Earth, including those resulting from an increase in greenhouse gases or aerosols, will tend to alter atmospheric and oceanic temperatures and associated circulation and weather patterns. These effects will be accompanied by changes in the hydrological cycle (for example, altered cloud distributions or changes in rainfall and evaporation regimes). Any human-induced changes in climate will also alter climatic variability that otherwise would have occurred. Such variability contains a wide range of space and time scales. Climate variations can also occur in the absence of a change in external forcing, as a result of complex interactions between components of the climate system such as the atmosphere and ocean. The El Ni�o-Southern Oscillation (ENSO) phenomenon is a prominent example of such natural "internal" variability.
In the observationally based record of global mean surface temperatures shown by the black line on Figure 6-2, both interannual variability and a positive trend are apparent. Year-to-year variations can be interpreted as resulting from internal variability; and the trend, as caused by external forcing mechanisms. For comparison, the yellow line on Figure 6-2 shows a control run from a coupled ocean-atmosphere general circulation model in which concentrations of greenhouse gases and aerosols are held fixed: This indicates that observed natural variability in global mean surface temperatures may be adequately simulated. The red and the blue lines in Figure 6-2 show the surface temperature simulated by two different general circulation models driven by increased greenhouse gas and sulfate aerosol concentrations. Both of the models simulate interannual variability and trends in surface temperature, but differences in model sensitivities (see Section 6.2) lead to differing temperature trends. Figure 6-2 also shows that estimates of the global mean temperature trend resulting from increased greenhouse gas concentrations alone (green line) leads to a larger temperature change than observed.
It is difficult to ascribe climate change to human activities and even harder to identify a particular change with a specific activity. The point at which change is detected in a climate variable is the point at which the observed global mean trend (signal) unambiguously rises above background natural climate variability (noise). Good observational records of climate and sufficiently accurate, reliable models are needed. To simulate climate change, the models require complete representation of all anthropogenic forcing mechanisms (i.e., changes in atmospheric composition). In practice, current climate change is just comparable to natural variability. Therefore, more sophisticated tools have been developed that use the spatial structure of specific climate variables expected to change, which is known as the "fingerprint" method of detection (e.g., Hasselmann, 1993; Santer et al., 1996).
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