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E. Linacre and B. Geerts |
4/'98 |
The authoritative 1995 Second Report of the Intergovernmental Panel on Climate Change (1) predicts a global mean surface temperature rise of between 1.0-3.5 K by the end of the 21st century. In Australia, for instance, temperatures in parts of the north are expected to rise by 0.4-1.4 K by 2025, and in coastal areas by 0.3-1.0 K. There will be more hot days and fewer cold ones. Heating by 2070 may be 1.0 - 2.5 K. The already low winter rainfall in the north will decline by about 0-20% by 2030, and summer rainfall is likely to be more intense (2).
Any more precise estimation by means of computer modelling is prevented by our inability to properly simulate cloud microphysical processes. The cloudiness, and the characteristics of clouds (base height, depth, droplet distribution) has a large effect on climate.
Much of the warming is attributed to a build-up of carbon dioxide in the atmosphere world-wide. On the other hand, some cooling results from fine dust in the air. This stays airborne for only a few days, so it is concentrated around the source regions and the cooling effect is only local.
Along with the general warming there may be changes in the frequencies and intensities of extreme events like droughts, heat waves (Fig 1) and flooding rains, which are the main causes of weather-related casualties. Observations in the north-hemisphere mid latitudes are not consistent with regard to changes in variability. In some areas rainfall and/or temperature extremes are more common now than earlier in the 20th century, in other regions less.
In general, even a small rise of mean conditions leads to a large increase in the frequency of extreme values (Fig 1). A complication is that the curve shown in Fig 1 may itself change in shape, i.e. the standard deviation or skewness may change. For instance, there is a worldwide trend of rising daily minima and cold extremes but almost stationary maxima.
Fig 1. A small shift of the whole bell-shaped curve (i.e. of its mean value) results in a large increase in the number of events with maxima exceeding a threshold temperature, shown by the shaded area.
The rising of daily minimum temperatures reduces the frequency of frost. Thus, the frost-free season in the northeastern US now begins about 11 days earlier than in the 1940’s. The rise of minima may be due to increased cloudiness, which impedes nocturnal cooling.
Most of the warming is expected to occur in winter and at high latitudes. The consequently reduced difference between polar and low-latitude surface temperatures would weaken the midlatitude upper-level westerlies, and therefore reduce the incidence and/or strength of baroclinically-induced frontal disturbances (Note 13.B), and therefore the frequency and/or the intensity of precipitation. General circulation models (GCMs) do not confirm that the location or strength of the jet stream will be substantially altered.
Another consequence of a warmer world is increased precipitation overall, because of more evaporation from warmer ocean surfaces, notably in winter. However, such an increased rainfall will not be universal and may not generally compensate for the increased evaporation potential. GCMs suggest that a net soil moisture loss would occur in North America and southern Europe, leading to more droughts. However, soils in Russia would become more moist because an increase of cloudiness (which has been observed) would inhibit the evaporation. Once again, the number of factors involved make prediction more difficult. Some regions and seasons may see a decrease in precipitation, e.g. because of increased offshore upwelling or a change in the wind patterns at the surface or aloft. For instance, mediterranean climates (Section 16.1) are expected to have even drier summers, because of less soil moisture due to increased evaporation from it in a warmer spring. A trend has been observed towards less rain over the Sahel, but snowfall has increased in northern America. On the whole, the increased rainfall will lead to heavier rather than more frequent rains, as is already evident in the USA.
The possible impact of global warming on the frequency and intensity of tropical cyclones is poorly understood. The question is important in places like northern Australia, where they cause 20-50% of the annual rainfall, and in the USA where they are responsible for giant insurance claims. Hurricane Andrew in August 1992 killed 54 people, rendered 250,000 homeless and did 30 billion dollars of damage, mainly in Southern Florida. Unfortunately, a tropical cyclone eye is too small to feature on any GCM, so that it is not yet possible to estimate what the effect of global warming might be. Aircraft and satellite observations of the north Atlantic since the 1940’s indicate a reduction in the number of hurricanes, whereas the number of typhoons in the northwestern Pacific appears to have risen.
The authors conclude that, amongst many uncertainties, the evidence on balance shows a discernible influence of humanity on the global climate. More data and coupled atmosphere-ocean-land-sea ice GCMs will make prediction more accurate, but ‘the climate system is complex, and the chance remains that surprises will come about’. One uncertainty remains the causal factor for change, i.e. the quantity of greenhouse gases (and aerosols) that human society will emit.
The reality of global warming may be shown most convincingly by the retreat of all but a few glaciers on all continents (except Antarctica) and the migration of native plants upwards on mountains, e.g. in Switzerland, the US Sierra Nevada, Alaska and New Zealand (3). Mosquitoes carrying malaria and dengue fever are being reported at new heights in Latin America, central Africa and Asia.
Agricultural yields in the 21st century are likely to be reduced in low-latitude developing countries, where present temperatures are already near or above the optimum. That reduction will probably be offset by increased yields at mid latitudes, where longer frost-free periods and increased CO2 concentration may promote crop yield (5).
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