Assessment of global climate change

The IPCC has released a series of 'Assessment Reports' to provide an integrated view of climate change, updated to reflect developments in scientific knowledge and in policies about climate risk. The Fourth Assessment Report (AR4) was presented in 2007 (IPCC, 2007a). It included summaries of observed changes in climate indicators, projected future changes, projected future impacts, causes of climate change and discussion of options for action to manage the risk.

The AR4 considers that warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level (see Fig. 19.1). Furthermore, impacts of warming are being detected in most continents and oceans, although much of the observational evidence uses data collected since around 1970 and so it is not always possible to set measurements in their longer term context. Indeed, one of the difficulties in understanding evidence of climate change impacts, especially at regional (i.e. sub-continental) scales, is that any systematic change can be

Changes in temperature, sea level and Northern Hemisphere snow cover

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Changes in temperature, sea level and Northern Hemisphere snow cover

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Fig. 19.1 Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (blue) and satellite (red) data; and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period 1961-90. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties ((a) and (b)) and from the time series (c). (Fig.1.1, IPCC, 2007a.)

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Fig. 19.1 Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (blue) and satellite (red) data; and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period 1961-90. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties ((a) and (b)) and from the time series (c). (Fig.1.1, IPCC, 2007a.)

combined with natural variability and also obscured by adaptations or non-climate factors.

19.1.2 Attribution

When changes in climate are observed or suspected, there remains a question of attribution. Is the change a result of human activity or part of a cyclical or random natural process? In AR4 the IPCC considers that

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica).

The conclusion is supported by comparison of multiple climate model simulations driven either by natural forcing factors alone (e.g. solar activity) or with the addition of anthropogenic forcing, mainly greenhouse gas (GHG) emissions. This attribution depends on the accuracy of the climate model outputs, which are recognised as being uncertain. However, the evidence for anthropogenic increases in GHGs is considered compelling.

Modelling at smaller scales than the global or continental scale remains difficult. This has implications for hydrologists because most analysis of the impacts of climate change in hydrology will be required at smaller regional and catchment scales.

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