Global warming

We started this chapter discussing temperature, moved through a number of other environmental conditions to pollutants, and now return to temperature because of the effects of those pollutants on global temperatures. It appears that the present air temperature at the land surface is 0.6 ± 0.2°C warmer than in preindustrial times (Figure 2.25), and temperatures are predicted to continue to rise by a further 1.4-5.8°C by 2100 (IPCC, 2001). Such changes will probably lead to a melting of the ice caps, a consequent rising of sea level and large changes in the pattern of global climates and the distribution of species. Predictions of the extent of global warming resulting from the enhanced greenhouse effect come from two sources: (i) predictions based on sophisticated computer models ('general circulation models') that simulate the world's climate; and (ii) trends detected in measured data sets, including the width of tree rings, sea-level records and measures of the rate of retreat of glaciers.

Not surprisingly, different global circulation models differ in their predictions of the rise in global temperature that will result from predicted increases in CO2. However, most model predictions vary only from 2.3 to 5.2°C (most of the variation is accounted for by the way in which the effects of cloud cover are modeled), and a projected rise of 3-4°C in the next 100 years seems a reasonable value from which to make projections of ecological effects (Figure 2.26).

But temperature regimes are, of course, only part of the set of conditions that determine which organisms live where. Unfortunately, we can place much less faith in computer projections of rainfall and evaporation because it is very hard to build good models of cloud behavior into a general model of climate. If we consider only temperature as a relevant variable, we would project a 3°C rise in temperature giving London (UK) the climate of Lisbon (Portugal) (with an appropriate vegetation of olives, vines, Bougainvillea and semiarid scrub). But with more reliable rain it would be nearly subtropical, and with a little less it might qualify for the status of an arid zone!

a 3-4°C rise in the next 100 years

Figure 2.25 Global annual surface temperature variations from 1860 to 1998. The bars show departures from the mean at the end of the 19 th century. The curve is a moving average obtained using a 21-year filter. Mean global temperatures are now higher than at any time since 1400. (After Saunders, 1999.)

Figure 2.25 Global annual surface temperature variations from 1860 to 1998. The bars show departures from the mean at the end of the 19 th century. The curve is a moving average obtained using a 21-year filter. Mean global temperatures are now higher than at any time since 1400. (After Saunders, 1999.)

Figure 2.26 The rise in global mean surface temperature projected by the global coupled model (i.e. both the oceans and the atmosphere are modeled) for climate variability and change in use at the Geophysical Fluid Dynamics Laboratory, Princeton, USA. Observed increases in greenhouse gases are used for the period 1865-1990 (and clearly the projections match closely the observed trend in temperature); thereafter, greenhouse gases are assumed to increase at 1% per year. Since the model simulates the global behavior of the oceans and atmosphere, the precise behavior depends on the initial state of the system. The three 'experiments' were started from different states. (After Delworth et al., 2002.)

Also, global warming is not evenly distributed over the surface of the earth. Figure 2.27 shows the measured global change in the trends of surface temperature over the 46 years from 1951 to 1997. Areas of North America (Alaska) and Asia experienced rises of 1.5-2°C in that period, and these places are predicted to continue experiencing the fastest warming in the first half of the present century. In some regions the temperature has apparently not changed (New York, for example) and should not change greatly in the next 50 years. There are also some areas, notably Greenland and the northern Pacific Ocean, where surface temperatures have fallen.

We have emphasized, too, that the distribution of many organisms is determined by occasional extremes rather than by average conditions. Computer modeled projections imply that the global distribution of climate change

Figure 2.27 Change in the surface temperature of the globe expressed as the linear trend over 46 years from 1951 to 1997. The bar below gives the temperatures in °C. (From Hansen et al., 1999.)

Figure 2.27 Change in the surface temperature of the globe expressed as the linear trend over 46 years from 1951 to 1997. The bar below gives the temperatures in °C. (From Hansen et al., 1999.)

global climatic change will also bring greater variance in temperature. Timmerman et al. (1999), for example, modeled the effect of greenhouse warming on the ENSO (see Section 2.4.1). They found that not only was the mean climate in the tropical Pacific region predicted to move towards that presently represented by the (warmer) El Niño state, but that interannual variability was also predicted to increase and that variability was predicted to be more skewed towards unusually cold events.

Global temperatures have changed naturally in the past, as we have seen. We are currently approaching the end of one of the warming periods that started around 20,000 years ago, during which global temperatures have risen by about 8°C. The greenhouse effect adds to global warming at a time when temperatures are already higher than they have been for 400,000 years. Buried pollen gives us evidence that North American forest boundaries have migrated north at rates of 100-500 m year-1 since the last ice age. However, this rate of advance has not been fast enough to keep pace with postglacial warming. The rate of warming forecast to result from the greenhouse effect is 50-100 times faster than postglacial warming. Thus, of all the types of environmental pollution caused by human activities, none may have such profound effects as global warming. We must expect latitudinal and altitudinal changes to species' distributions and widespread extinctions as floras and faunas fail to track and keep up with the rate of change in global temperatures (Hughes, 2000). What is more, large tracts of land over which vegetation might advance and retreat have been fragmented in the process of civilization, putting major barriers in the way of vegetational advance. It will be very surprising if many species do not get lost on the journey.

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