What climate changes are likely

In terms of key environmental parameters, the Earth system has recently moved well outside the range of natural variability exhibited over at least the last half million years. The nature of changes now occurring simultaneously in the Earth System, their magnitude and rates of change are unprecedented and unsustainable.

Paul Crutzen (Nobel Laureate) and Will Steffen (International Geosphere-Biosphere Programme, Executive Director), 2003.

The complexity of the climate system makes it difficult to predict some aspects of human-induced climate change: exactly how fast it will occur, exactly how much it will change, and exactly where those changes will take place. In contrast, scientists are confident of other predictions. Mid-continent warming will be greater than over the oceans, and there will be greater warming at higher latitudes. Some polar and glacial ice will melt, and the oceans will warm; both effects will contribute to higher sea levels. The hydrologic cycle will change and intensify, leading to changes in water supply as well as flood and drought patterns.

American Geophysical Union, Position Statement, December 2003.

Human-induced climate change is only an issue if it is large enough and rapid enough to create real problems for natural ecosystems and for human societies. In this and the following chapter we will look at the magnitude and rate of climate change, including sea-level rise and changes in extreme events, that are likely to result from human-induced emissions of greenhouse gases, and at what the effects might be on nature and society.

Given the acknowledged uncertainties, the Intergovernmental Panel on Climate Change (IPCC) tried in successive reports to state what it was confident about, and what was more or less likely or possible, but still rather uncertain. The quote from the American Geophysical Union - the non-government association of American geophysical scientists - does the same in a very summary form. The IPCC, in its report in 2001 extended this process to a treatment of possible sudden or irreversible changes in the climate system which might be catastrophic, but about which we know relatively little regarding likelihood, timing, magnitude and impacts.

Complete surprises are possible. A prime example is the sudden appearance of the 'ozone hole', which first occurred without warning over Antarctica during the 1970s in the Southern Hemisphere's spring. The ozone hole now appears far more rapidly and is far more long lasting than anyone anticipated in the early 1970s. At the time, scientists like me were worrying about possible gradual destruction of ozone in the upper atmosphere; we were taken by surprise when it happened in a few years over Antarctica, far from the sources of the chemicals thought to be threatening the ozone layer. 'Repairing' the ozone hole is likely to take the best part of a century, despite strong international agreements on doing so (see Chapter 9 for a more detailed discussion of the ozone problem). And it could have been far worse. As Paul Crutzen stated, on receipt of the Nobel Prize for his work on ozone:

... if the chemical industry had developed organobromine compounds [which contain bromine] instead of the CFCs [which contain chlorine] ... then without any preparedness, we would have been faced with a catastrophic ozone hole everywhere and in all seasons during the 1970s ... Noting that nobody had given any thought to the atmospheric consequences of the release of Cl or Br [chlorine or bromine] before 1974,1 can only conclude that mankind has been extremely lucky.

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