The climate system

So far the forecasting of detailed weather over a few days and of average weather for a month or so, up to perhaps a season ahead, has been described in order to introduce the science and technology of modelling, and also because some of the scientific confidence in the more elaborate climate models arises from their ability to describe and forecast the processes involved in day-to-day weather.

Forecasting for the African Sahel region

The Sahel region of Africa forms a band about 500 km wide along the southern edge of the Sahara Desert that gets most of its rainfall during northern hemisphere summer (particularly July to September). Rainfall has decreased in this region since the 1960s. Particularly pronounced periods of drought occurred during the 1970s and 1980s with devastating impact on the local economy (Figure 5.12). Variations in Sahel rainfall are linked to fluctuations in patterns of sea surface temperature (SST), a connection that has provided since the 1980s a basis for generating seasonal forecasts of rainfall for the region.8 The advent of climate models (see next sections) has brought substantial improvements in such forecasting (Figure 5.12) which is now limited by the accuracy of model predictions of sea surface temperature - possible for months ahead but not, as yet, for years.9 Such forecasts can also benefit by including within the models variations in the characteristics of land surface vegetation and soil that also infl uence local rainfall.10

Figure 5.11 Observed Sahel July-September rainfall for each year (orange) compared to an ensemble mean of ten simulations with a climate model (GFDL-CM2.0) forced with observed sea surface temperatures (red). Both model and observations are normalised to unit mean over 1950-2000. The pink band represents ±1 standard deviation of intra-ensemble variability.

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Climate is concerned with substantially longer periods of time, from a few years to perhaps a decade or longer. A description of the climate over a period involves the averages of appropriate components of the weather (for example, temperature and rainfall) over that period together with the statistical

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1997

Oct Jan Apr Jul

1998

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1999

Source: ECMWF

Figure 5.12 Plots of forecasts of El Niño events showing the forecast sea surface temperature in the Niño-3 region from various start times throughout the large 1997-8 El Niño. Different lines of the same colour indicate different ensemble members. The background indicates the location of Niño-3.

Oct Jan Apr Jul

1997

Oct Jan Apr Jul

1998

Oct Jan Apr

1999

Source: ECMWF

Figure 5.12 Plots of forecasts of El Niño events showing the forecast sea surface temperature in the Niño-3 region from various start times throughout the large 1997-8 El Niño. Different lines of the same colour indicate different ensemble members. The background indicates the location of Niño-3.

variations of those components. In considering the effect of human activities such as the burning of fossil fuels, changes in climate over periods of decades up to a century or two ahead must be predicted.

Since we live in the atmosphere the variables commonly used to describe climate are mainly concerned with the atmosphere. But climate cannot be described in terms of atmosphere alone. Atmospheric processes are strongly coupled to the oceans (see above); they are also coupled to the land surface. There is also strong coupling to those parts of the Earth covered with ice (the cryosphere) and to the vegetation and other living systems on the land and in the ocean (the biosphere). These five components - atmosphere, ocean, land, ice and biosphere - together make up the climate system (Figure 5.13).

Chapter 2 considered the rise in global average temperature which would result from the doubling of the concentration of atmospheric carbon dioxide assuming that no other changes occurred apart from the increased temperature at the surface and in the lower atmosphere. The rise in temperature was found to be 1.2 °C and results from what is often called temperature feedback (see box following this section). However, it was also established that, because of other feedbacks (which may be positive or negative) associated with the temperature

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