Biospheric control over the climate system andthe global hydrological cycle F 2.3 223 ence of climatic conditions favourable to their existence. It does become problematic however for the whole Northern part ofthe Indian sub-continent, the North of China and Southern Siberia, the Australian steppes andthe savannah zones in Africa, the Western parts of North America andthe Andean region. • In the Global Forest scenario the forest regions increase dramatically andthe current core areas of desert and steppe retain their original form: the desert belt ofthe ancient world from the Maghreb andthe Sahara over the Middle East into portions ofthe Thar and Gobi deserts, Western Australia and Western South Africa, Patagonian, the Atacama andthe south western part of North America. There, no favourable climatic conditions can be established for forest. Changes cannot simply be maintained by afforestation and climatic conditions alone. Irrigation is a provisional option, but not a sustainable solution for these areas. F 2.3 Biospheric control over the climate system andthe global hydrological cycle These new simulations provide an initial evaluation ofthe maximum biospheric control over our climate system andthe global hydrological cycle. What was not considered is the exchange of many greenhouse gases such as carbon dioxide that are controlled by the biosphere. Also left unconsidered are the exchange processes between ocean and atmosphere that would influence many ofthe results. However, the general conclusion that may be derived is that the biosphere does indeed exercise an extensive control function over the climate system. This control is particularly strong in the global energy levels as a result ofthe cooling effect of vegetation and in the global hydrological cycle through increased evaporation, determined by evapotranspiration. Its influence is on the same scale to the changes that are anticipated as a result of doubling the CO 2 concentration (IPCC, 1996a). The geographic and temporal effect ofthe biosphere is seen above all in the fact that the biological influences are strongest during periods of maximum solar radiation, whereas the increased carbon dioxide concentration influences above all the winter period in the upper latitudes.
F 3 The biosphere in global transition F 3.1 Global human impacts on the biosphere Humankind began reshaping the Earth centuries ago; no ecosystem today is free of human influence (Vitousek et al, 1997).These interventions achieved a global dimension in the form of industrialization and, since that time, have changed biogeochemical and biogeophysical cycles and disrupted the regulatory function ofthe biosphere which is so essential to the Earth (Chapter C). In earlier times, too, humans intervened deep into the biosphere with their land use changes and reshaped whole swaths of land by deforestation (eg the Greeks and Romans in the Mediterranean,Aborigines in Australia).The climate reconstructions indicate for those periods climatic modifications by regions in terms of precipitation and temperature, but a constant CO 2 -concentration. On the other hand, the data analysis from ice cores extracted in Greenland show that even without human influence within just a few decades dramatic temperature fluctuations occurred and that the climate changed erratically. The new dimension of human intervention in the biosphere andthe climate is therefore less an issue of such modifications being unprecedented in Earth’s history, and more about the dramatic collision of a world globalizing at breakneck speed with highly divergent societies. On the one hand, there are the highly developed sensitive civilizations with high resource and energy consumption, on the other hand societies in poverty in which the generally prevalent high population growth increases the pressure on the environment.At the same time, the population ofthe Earth is larger than it has ever been before. It is also largely unknown whether humankind can respond appropriately to the changes it has itself caused and can adapt without major social upheaval. Against the background ofthe debate on climate policy, and specifically on the reduction of greenhouse gas emissions, the following section will discuss above all the role ofthe biosphere in the carbon cycle and consider the question to what extent intervention by civilization, andthe global modifications associated with such intervention, can be reduced to a sustainable level (IPCC, 1996b). F 3.1.1 Direct intervention in the biosphere: global trends The conversion of natural ecosystems into pasture and agricultural landandthe shift from agricultural to industrial land use are global trends of extraordinary quantitative dimensions. It is estimated that 40–50 per cent ofthe land surface has been converted or degraded by humankind so far (Leemans, 1999; Fig. F 3.1-1). Although humankind uses only approx 6 per cent of terrestrial net primary production (NPP) directly through harvesting plants to gain food, fuel or building materials or for paper manufacture, it is estimated that as a result of anthropogenic forest and steppe fires and air pollution, the potential natural global NPP is reduced by up to 25–40 per cent (Schlesinger, 1997). Since 1860 humankind has destroyed around 13 per cent ofthe pre-industrial biomass (Schlesinger, 1997) and thus amongst other things has caused the recorded increase in CO 2 in the atmosphere. Percentage of total land area [%] 100 75 50 25 Other Forest Pasture 0 Cropland 1700 1750 1800 1850 1900 1950 Year Figure F 3.1-1 Estimated changes in global terrestrial ecosystems between 1700 and 1995 (proportion of total land surface). Source: Leemans, 1999 Non-domesticated Domesticated