Climate Prediction

There are three categories of events that may affect the climate: (1) events that occur outside the earth, (2) natural events on the surface of or within the earth, and (3) human activities.

The source of virtually all the energy that drives the atmosphere is the sun. Thus, any changes in the intensity of the solar radiation arriving at the earth will clearly have a significant effect on the weather and on the climate. The intensity of the radiation arriving at the top of the atmosphere will depend on variations in the intensity of the radiation emitted by the sun, changes in the transmission properties of space between the sun and the earth, and changes in the sun-earth distance. Further details can be found in Cracknell (1994a).

The natural events on the surface of the earth that have an effect on the climate include plate tectonics, variations in the polar ice caps, volcanic eruptions, and ocean circulation. These factors have various time scales associated with them (for further details see, e.g., Cracknell, 1994a).

Human activities that may affect the climate include: (1) increase in the concentration of carbon dioxide in the atmosphere, largely from the con sumption of fossil fuels, (2) increase in the concentration of other greenhouse gases in the atmosphere, (3) depletion of the ozone layer in the stratosphere, (4) development of land areas, and (5) other human activities, including rain making, irrigation, and the creation of artificial lakes and reservoirs, change of land use associated with urbanization and above-ground nuclear explosions.

The operational use of numerical models in weather forecasting began in the 1960s. Although their usefulness was originally limited, there have been rapid developments since then, and within 10 years the models were able to provide better forecasts of the basic motion field than could be achieved by an unaided human forecaster. These improvements have come about for two reasons. First, the power of computing facilities available has increased enormously. Second, the amount of data available to describe the present atmospheric conditions has greatly increased in recent years. In addition to conventional surface measurements and radiosonde measurements, there are also satellite observations, including satellite soundings and satellite-derived wind speeds obtained from cloud tracking from geostationary satellite images.

Climate Change

In looking at climate change we are looking at long-term changes compared to averages. Viewed over a century or a millennium, we see that climatic parameters (temperature, rainfall, etc.) are basically stable and vary only slowly. It is the nature of these slow, long-term variations that are of concern in climate studies after local, short-term fluctuations have been smoothed out. It is the stability of the long-term components that makes climate prediction possible.

Sometimes some people (usually politicians) would like to claim, for whatever reason, that there is no evidence for human-induced global warming and that, therefore, there is no need to restrain our behavior in terms of curtailing emissions of greenhouse gases into the atmosphere. Differences among the various models arise at various stages: modeling of the physical processes, choice of grid spacing, boundary conditions and their parameterization, availability of computing power, which increases very rapidly with increasing spatial resolution, and assumptions made about the future.

It is important to realize that many processes occur on a scale that is quite small compared with the grid point spacing. One can therefore never expect to obtain predictions out of model calculations that relate to these processes at these scales. It would therefore be rather risky to rely on absolute predictions made with one particular model.

Intergovernmental Panel on Climate Change

To reach a consensus out of the rapidly expanding mass of conflicting or confusing results obtained from various climate models, the World Mete orological Organization (WMO; chapter 31) and the United Nations Environment Program (UNEP) set up the Intergovernmental Panel on Climate Change (IPCC) in 1988. The IPCC attempted to establish a consensus among the results of calculations from 20-30 different climate models. The IPCC has, over a decade, become more confident about the importance of the role of human activity vis-à-vis the natural variations in the climate. It also warns, however, that regional or local changes may not necessarily all be in the same direction as the general trend. The IPCC's main conclusions can be summarized (Houghton et al., 2001; McCarthy et al., 2001) as follows:

1. Global mean near-surface air temperature increased by about 0.6°C over the 20th century.

2. Temperatures in the lowest 8 km of the atmosphere have risen during 1960-2000 at about 0.1°C per decade.

3. Snow cover and ice extent have decreased.

4. Global mean sea level has risen and ocean heat content has increased.

5. There have been changes in precipitation, with increases in some areas but decreases in some other areas.

6. Most of the warming observed over the last 50 years is attributable to human activities; any contribution from natural factors is small.

7. Concentrations of atmospheric greenhouse gases, including tro-pospheric ozone, have continued to increase as a result of human activities.

8. Stratospheric ozone depletion and anthropogenic aerosols have a cooling effect, or negative greenhouse effect.

9. Human influences are expected to change the atmospheric composition throughout the 21st century.

10. Projected changes in atmospheric composition arising from human activities, based on various assumptions, will lead to further anticipated temperature and sea level rises throughout the 21st century.

11. There is likely to be an increase in various extreme events. Climate Change and Agriculture

Temperature and rainfall are the key factors in making decisions about what crops to grow. Thus, agriculture will need to adapt to changes in climate. In nondesert areas droughts arise as fluctuations within the local weather pattern. They cannot be prevented or eliminated, but making use of climate models to obtain reliable predictions of their expected frequency of occurrence can contribute toward wise planning of agriculture to minimize drought losses.

It is useful to consider the global estimates of the area that potentially can be endangered by drought. If from the total global land area we elimi-

nate the territories now unsuitable for agriculture (deserts, tropical woods, polar areas and regions with complex topography and also the territories, where the difference between the annual precipitation and potential evaporation is > 200 mm), the remaining areas may be regarded as agricultural ones. According to the agroclimatological estimates, more than 50% of agricultural land can be endangered by drought.

One of the most important consequences of global warming is degradation of drylands (chapter 33) in different regions of the world. It is important to determine the principal consequences of expected regional climate changes caused by modern global warming in agriculture of different regions and whether they will result in increasing the occurrence and severity of droughts or whether droughts will become less frequent. In the subsequent sections of this chapter we present some preliminary answers to this question.

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