Prospects for the 21st Century 61 Scenarios for the 21st century

As outlined above, it is clear that human activities are fundamentally altering the global atmosphere, and in turn this alteration is affecting the terrestrial biosphere. This alteration is just beginning and is set to continue and probably accelerate for much of the coming century as human industrio-economic activity and population increases. Over the coming century, what will be the nature of the interaction between the biosphere and atmosphere, and how will we be able to measure and understand it?

The Intergovernmental Panel on Climate Change (IPCC) has published a range of scenarios for anthropogenic CO2 emissions over the 21st century. Their "business-as-usual scenario" projects that 1400 Pg C will released into the atmosphere over the coming century, but with a range of values varying from 2100 Pg C if humanity follows a fossil fuel intensive strategy to

800 Pg C if it follows is an environment-conscious, low emissions strategy. Whichever scenario is followed, a lot of CO2 is going to be pushed into the atmosphere.

How much of this CO2 remains in the atmosphere depends primarily on the behaviour of the global carbon cycle. Both ocean and terrestrial carbon sinks are likely to increase in magnitude over the coming decades, but that the rate of increase will slow, as the sinks begin to "saturate". The sink capacity in the oceans is chemically limited by biocarbonate-carbonate chemistry, whereas the CO2 fertilisation effect on land is probably limited by plant physiology and by structural considerations and ecosystem-level feedbacks that constrain how much biomass a forest can hold. Moreover, warming surface temperatures will reduce both the solubility of CO2 in ocean water and the residence time of carbon in soils.

A number of global carbon cycle models have been applied to this problem. Using a wide range of carbon cycle models and potential economic scenarios, IPCC [2001] predicts that by 2100 atmospheric CO2 concentration will have risen from its current value of 368 ppm to between 500 and 1000 ppm, and that global mean temperatures will rise by between 1.5° C and 5.8°C. A more recent uncertainty analysis [Stainforth et al. (2005)] suggest that the upper limit of possible change this century is about 10°C. Very approximately, these model outputs suggest that every 3 Pg of C emissions will result in a 1 ppm increase in atmospheric CO2 concentrations by 2100, which in turn will increase global mean temperatures by about 0.01°C. It is now inevitable that in this century we will face (and probably already are facing) a fundamentally altered atmosphere and climate.

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