Projections of Climate Change

Since greenhouse gas emissions are uncertain we cannot get climate change forecasts or predictions. We can only project climate change for a given emission scenario as a plausible future. Many climate research centres have used the above and other emission scenarios and have projected future climate, typically until 2100 but some are also going beyond that date. All these projections have been assessed by IPCC. Here results from one of the contributing centres, the Max Planck Institute for Meteorology in Hamburg, Germany, will be presented. As all the scenarios described and prescribed for the model did not contain a dedicated climate protection policy, the results cannot contain a feedback between climate change realized at a certain time and the emissions thereafter, be it due to economic pressure or through planned emission management. Hence, the scenario results are mere input for climate policy development within countries or the global community under the UNFCCC umbrella.

Figure 3.4 shows a "shocking" result: The 21st century would see a global mean temperature change of 3 to 4 K, nearly the same as the temperature difference between the last glacial maximum (18,000 years ago) and the present interglacial (Holocene), which was 4 to 5 K, but now pressed into one century. Only scenario B1 stays well below with about 2.5 K warming until 2100. We have clearly entered the anthropocene. Further inspection of figure 3.4 points to extreme warming in the Arctic and lowest warming in areas with a deep mixing ocean (Southern Ocean, northern North Atlantic). Continents warm generally stronger than ocean areas, except the Arctic Ocean where sea ice loss leads to more than double the average warming.

Figure 3.4 Near surface air temperature change in the 21st century for scenarios A1B and B1. The values were calculated by comparing the 30 year period from 1961-1990 with the 2071-2100 period (MPI, 2006b)
Figure 3.5 Relative precipitation changes in percent for scenario A1B in January and July (comparison between 2071-2100 and 1961-1990). Changes can surmount 50 percent in both directions.

For many areas precipitation changes are more important than temperature changes. Therefore figure 3.5 is of high interest. A general shift of rain belts emerges: more precipitation in the inner tropics and most high latitudes of both hemispheres but much less in the outer tropics and sub-tropics. Slightly exaggerated: where it rains already enough it will become more and less for those having often not enough, although overall precipitation increases by 5 (scenario B1) to 7 percent (scenarios A2 and A1B) until 2100.

How would sea level change for the above scenarios? One has to take into account three processes, namely density changes of sea water (mainly thermal expansion), melting of ice on land (glaciers and ice sheets) and changes in ocean circulation. The largest contribution in the 21st century will come from sea water density changes (+21 cm for B1, +26 and 28 cm for A1B and A2, respectively) and regionally from rearranged ocean circulation. Melting of ice on Greenland would contribute +13 cm (A1B) while for the Antarctic ice sheet the model calculates -5 cm. Figure 3.6 clearly shows the strong redistribution of sea level rise through ocean circulation changes. Sea level rise for example in the Arctic is about 0.5 m for scenario A1B due to density changes of sea water (~ + 25 cm) and intensified westerly flow (+ 20 cm) as well as melting of land ice (+ 0.08 m).

Figure 3.6 Sea level change (mostly rise) due to sea water density changes (mainly thermal expansion) and changed atmospheric as well as oceanic circulation for scenario A1B. Please note the high values in the Arctic Ocean where less saline water leads to lower density and thus higher levels despite modest temperature changes in the ocean interior (MPI, 2006b)

Figure 3.6 Sea level change (mostly rise) due to sea water density changes (mainly thermal expansion) and changed atmospheric as well as oceanic circulation for scenario A1B. Please note the high values in the Arctic Ocean where less saline water leads to lower density and thus higher levels despite modest temperature changes in the ocean interior (MPI, 2006b)

As it became clear from figure 3.4 for the air temperature changes already, the Arctic experiences the strongest warming due to the positive ice/snow albedo-temperature feedback (see section 2.5). Scenarios A1B and A2 lead to an ice free Arctic Ocean at the end of summer, which certainly would have a major impact on marine and terrestrial ecosystems there and in adjacent areas. Figure 3.7 points to these dramatic changes both for sea-ice and snow cover on land.

Snow cover in September, nowadays common in northern Siberia, Canada and Alaska will be largely gone.

Climate Model Snow
Figure 3.7 Sea ice and snow cover on land in March and September for present climate (model result as well) and for scenarios B1 and A1B in 2100

Frequently also a breakdown of the so-called thermohaline circulation in the North Atlantic is discussed and used for guard rails in climate policy development. This thermohaline circulation transports warm ocean waters from the tropics to high latitudes and cold water masses in the deep North Atlantic southward. A reduced sea water density in near surface layers of the high latitude ocean either through warming or more precipitation (both phenomena are forecast) would disturb this overturning circulation or in extreme cases stop it. What do the most comprehensive models to date project for the emission scenarios? Yes, the overturning circulation diminishes by about 30 percent until 2100 leading to less warming in parts of the North Atlantic but with a rather modest influence on Europe. Further model experiments with additional melt water fluxes from Greenland also did not lead to a breakdown. However, in model runs lasting millennia such a breakdown was simulated for the time after the year 2300 in 3 of 5 realisations of scenario A1B and always in scenario A2.

Can the model results with such a low resolution of about 200 km at the equator also point to extreme weather event changes? As we know from statistics of damages by such events, published by the Munich Re-insurance Company, the frequency of and the damages by such catastrophes have dramatically increased, however, mainly through local misbehaviour (e.g. buildings in flood-prone areas). A certain category of such extreme events can be modelled already: those happening over larger spatial and temporal scales like maximum precipitation over 5 days causing large river floods or the length of dry spells leading to droughts. The coupled model results for the 20th century show an increase in the length of heat waves, a reduction in frost days as well as an increase in maximum 5-day precipitation. Are these trends continuing? As figure 3.8 for scenario A1B underlines the 5-day maximum precipitation amount in a year increases nearly on global scale except for North Africa and some Mediterranean climate areas. In other words: the frequency and intensity of major floods will increase in most regions. It is also clear from figure 3.8 that tropical areas will experience the strongest absolute precipitation increase.

Figure 3.8 Changes of 5-day extreme precipitation per year (in percent) for scenario A1B (2071-2100) in comparison to observed values from 1961-1990 (MPI, 2006a)

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