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Tropospheric aerosols cene (Dickens 1999). During the warming after the last ice age no such methane release occurred.

Independent of the indications of warming of the lower atmosphere with increasing C02 concentration, there are several uncertainties making predictions very difficult (see also Veizer et al. 2000). These uncertainties are in meteorology as well as in oceanography:

• Meteorological uncertainties

Aerosols: Radiation balance not only depends on outgoing radiation, but also on incoming radiation. Incoming radiation is reduced by the increased reflection of clouds and aerosols. This compensates for the effect of reduced re-radiation of long-wave heat radiation and therefore cooling caused by increased C02. The influence of gases and aerosols (Fig, 5.3.3) was assessed by IPCC-WG I (1996): Despite all uncertainties the conclusion remains that the predominant effect of trace gases is to cause global warming. The influence of incoming radiation had also been seriously discussed in a different connection, namely the atomic winter (Ehrlich et al. 1983; Turco et al. 1983). According to calculations, an atomic war would lead to an enormous development of soot and aerosols from fires in large cities, and this would decrease the incoming radiation to such an extent that within 1-2 weeks the earth's mean temperature would decrease to -15 to -25 °C for >1 year and thus survival of life on earth would be unlikely. This applies particularly to a start of such a war during the growing season in the northern hemisphere. This knowledge led to the realisation that an atomic war would jeopardise the sur vival of both sides, and it thus influenced negotiations on nuclear disarmament.

• Oceanographic uncertainties about global marine currents

Conveyor belt: One ocean current is particularly important for Europe, the oceanic conveyor belt, which connects the oceans (Fig. 5.3.4 A; Commission of Inquiry of Global Climate Change, Federal Parliament of Germany 1992). Warm surface water flows from the south Atlantic to the north Atlantic as the Gulf Stream. Because of evaporation, and the stream of salt-rich water from the Mediterranean, this water becomes increasingly more saline and therefore does not freeze when it meets the cold Arctic water and the front of sea ice off Greenland. The salt-rich water cools and sinks into the depth of the north Atlantic and flows back as a cold, salt-containing deep sea current. It has long been known that the heat released during the cooling and sinking of the surface stream causes the mild, moderate climate in Europe (Dris-coll and Haug 1998). This oceanic conveyor belt reacts sensitively to the salt content of water in the north Atlantic. Salt-depleted water would not sink to the depth of the oceans. Thus heat would not be produced in the north Atlantic and Europe would cool down. This occurred during the post-glacial period when the large inland lake of North America, Agassiz, which formed from the melting ice in the region of Alberta, Saskatchewan and Montana, flowed into the north Atlantic. Consequently, the Younger Dryas ice age occurred in Europe. Melting ice in the Arctic or a decrease of water flow from the

g Year

| Fig. 5.3.4. A Schematic presentation of the oceanic "conveyer belt" The warm upper currents spread from the Pacific to the Atlantic. This water becomes saltier and does not freeze when it meets the cold water and ice front of the Arctic. The surface water sinks near the coast of Norway and north of Iceland and forms deep, salty water that flows back to the Pacific (after the Commission of Enquiry of Global Climate Change of the Federal Republic of Germany; Ramstorf et al. 1999). B Comparison of models of deep-water formation in the north Atlantic. All models agree in predicting that, with further global warming, the strength of the Gulf Stream will be reduced. This has potentially serious consequences for the European climate. (After Ramstorf et al. 1999)

g Year

| Fig. 5.3.4. A Schematic presentation of the oceanic "conveyer belt" The warm upper currents spread from the Pacific to the Atlantic. This water becomes saltier and does not freeze when it meets the cold water and ice front of the Arctic. The surface water sinks near the coast of Norway and north of Iceland and forms deep, salty water that flows back to the Pacific (after the Commission of Enquiry of Global Climate Change of the Federal Republic of Germany; Ramstorf et al. 1999). B Comparison of models of deep-water formation in the north Atlantic. All models agree in predicting that, with further global warming, the strength of the Gulf Stream will be reduced. This has potentially serious consequences for the European climate. (After Ramstorf et al. 1999)

Siberian rivers could have a similar effect to a cooling of the Gulf Stream, as the Arctic ice masses are derived from the fresh water of the Siberian rivers. Redirection of the Siberian rivers for irrigation of Asiatic steppes, planned in the 1960s, would have had enormous climatic consequences for Europe. Even a weakened Gulf Stream could cause a cooling of western Europe and this would result in an enormous increase of atmospheric C02, as the C02-rich water would no longer sink into the deep sea and would produce a significant gradient of precipitation. A comparison of coupled ocean-atmosphere models shows a weakening of the Gulf Stream with continuing warming (Fig. 5.3.4 B; Broeker 1997; Rahmstorf 1999).

El Niño: There is another marine stream which affects climate, the so-called El Niño. Periodically, the surface water in the eastern Pacific becomes significantly warmer, so that the regular currents transporting water from east to west no longer occur. This has enormous effects on the weather conditions in such a year (Fig. 5.3.5; Bengtsson 1997). Catastrophic rainfall occurs in South America, and drought in Australia and parts of Indonesia. The harvest in South Africa is affected. In addition to changes in the distribution of precipitation, the warming of the Pacific leads to

0° 30°0 60°0 90°0 120°0 150°0 180° 150°W 120°W 90°W 60°W 30°W 0° | Fig. 5.3.5. Influence of the El Nino effect on the regional climates In Asia, Africa and America. (After Bengtsson 1997)

release of large amounts of C02 in the nearby continents. About 1 year later Europe notices the effects of El Niño, i.e. Europe's climate depends on events occurring in the Pacific (Taylor et al. 1998; Rodwell et al. 1999). North Atlantic oscillation (NAO): This is a current in the north Atlantic corresponding to El Niño which determines the position of a high-pressure area in the Azores and thus the air pressure gradient between the Azores and the low-pressure areas over Iceland. The position of the Azores high pressure decides whether rain falls over the Mediterranean region in summer or over Scandinavia. NAO is positive, with a strong pressure gradient between Iceland and the Azores, a sign of a strong thermo-haline circulation (THC) and thus for a warm winter in northern Europe. Despite the prediction that the THC would decrease with global warming, at the moment an increased THC (possibly transient) is observed (Hurrell et al. 2001).

The Intergovernmental Panel on Climate Change (IPCC) discussed the effects that various scenarios of future increased C02 concentration would have on climate. The conclusion drawn, in 1996, from a wide scientific forum (Fig. 5.3.6 A; Azar and Rodhe 1997), was that a short-term increase in C02 concentration to about 400 ppm, but a decrease to 350 ppm within the next 100 years, would increase global temperature by 0.5 K compared to the last 1000 years. A temperature increase of 1 K is regarded as a critical limit. This limit is exceeded at 450 ppm. In the year 2000 the global C02 concentration was ca. 375 ppm and thus 95 ppm above the pre-indus-trial C02 concentration of 280 ppm. In 1998 the temperature increase was 0.75 K, near the critical limit of 1 K. At the moment, a doubling or tripling of the pre-industrial C02 concentration is predicted, leading to a calculated temperature increase of 3-4 K. This is confirmed in the third report of the IPCC.

Because of the complicated interaction between oceans and atmosphere, and because weather determines irregularities in marine currents, the validity of the prediction by the IPCC, that man was able to affect global climate, were regarded sceptically for a long time. However, the predictions of the IPCC have been confirmed over time. In 1998 the average global temperature differed significantly from the trend of the last 1000 years (Fig. 5.3.6 B; Kerr 2000). The temperature increased by 0.75 K, exceeding the temperature maxima of the last 1000 years. The delay between the increase in C02 and temperature is caused by the heat capacity of oceans (they act as a heat sink; Levitus et al. 2000) and the melting of the polar ice caps (energy required to supply the latent heat required for melting; Ro-bock et al. 1999). The observed global increase in temperature since 1960 is confirmed by worldwide analysis of the annual growth rings of trees (see, e.g., LaMarche et al. 1984; Vaganov et al. 1999). It has been shown that the warming of oceans since 1950 was caused by trace gases produced by human activity (Barnett et al. 2001).

Predicted temperature change AT (°C)

Predicted temperature change AT (°C)

B Year

Fig. 5.3.6. A Model predicting the influence of future scenarios of higher C02 concentration on the average earth temperature. Shown are the scenarios used (5 350 to 5 1000) and on the right the expected changes in the average global temperature. The vertical lines are the average pre-industrial temperature and the critical boundary at which significant effects are expected (1 K temperature rise). Average values are shown with the uncertainties in the predictions (vertical error bars) as well as measured values in 1998. B Global changes in temperature since the year 1000. Long-term trends from 1000 to 1900 (dotted line), the running averages for 30 years (bold line) and 5 years (thin black line), and the yearly extremes (thin red line) and 2-sigma uncertainty in the past (yellow thin line) are given. (After Kerr 2000)

B Year

Fig. 5.3.6. A Model predicting the influence of future scenarios of higher C02 concentration on the average earth temperature. Shown are the scenarios used (5 350 to 5 1000) and on the right the expected changes in the average global temperature. The vertical lines are the average pre-industrial temperature and the critical boundary at which significant effects are expected (1 K temperature rise). Average values are shown with the uncertainties in the predictions (vertical error bars) as well as measured values in 1998. B Global changes in temperature since the year 1000. Long-term trends from 1000 to 1900 (dotted line), the running averages for 30 years (bold line) and 5 years (thin black line), and the yearly extremes (thin red line) and 2-sigma uncertainty in the past (yellow thin line) are given. (After Kerr 2000)

The temperature trend of the last 1000 years shows a slight but continuous decrease up to 1900. The question is whether slight, but natural, fluctuations are noticed at all by man. There are several historic events that correlate with the temperature minima. Around 1200 the last major period of forest clearance started in Europe. In the second half of the nineteenth century large numbers of people emigrated to America after the catastrophic failure of harvests. The minimum at the turn of the century followed the eruption of Krakatau in 1883. The year 1880 is often regarded as the base year of pre-indus-trial times.

There are indications that anthropogenic sources are no longer the sole cause, but that there is a positive feedback of the temperature increase on respiration (degradation of organic matter; Stott et al. 2000). Schimel et al. (1994) calculated from the Century Model that the global C pool in soils decreases by 11 Gt per K temperature increase (this corresponds to a decrease of soil C by about 0.5%).

The evidence that anthropogenic trace-gas emissions may lead to climate change was the basis for the Framework Convention on Climate Change (FCCC) (Benedick 2001) and the commitment to reduce C02 emissions in the Kyoto Protocol (WBGU 1998); see also Chapter 5.4.2.

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