The Safety of a Two-degree Tipping Point is a Political Dream
The dream is that we can with safety and confidence allow the average temperature of the earth to rise by two degrees C above the long-term mean. The hypothesis is that (1) we can stop there; and (2) that the changes in the earth will be tolerable, if not minor. I do not know where the two-degree hypothesis was invented but it is a political wish and has no basis at all in biophysical reality.
To think about the issue realistically one must realize that solar energy is received differentially by the earth with the greatest intensity in the tropics and the least in the polar regions. In the tropics most of the energy that is absorbed and not reflected back into space is dissipated into the atmosphere in the latent heat of vaporization: it is absorbed in the evaporation of water, which is abundant in the tropics, and the tropics are not warmed greatly as the heat-trapping gases accumulate in the global atmosphere. The moisture vapor enters the normal circulation patterns of the atmosphere and is carried to the higher latitudes and to higher elevations. Cooled, the vapor condenses and produces rain. In condensing the latent heat of vaporization is released and the region is warmed.
The warming is especially great in the mid- and higher latitudes, 45 degrees north and above. In the northern hemisphere there is much land at these latitudes with much forest and tundra with large stores of carbon in plants, soil and in the peat deposits of the tundra. The change in temperature in these regions is two to four times the average for the earth as a whole. That rise in temperature is confirmed in various ways not the least of which is the melting of the sea ice of the Arctic Ocean now at its lowest level in history.
The result on land is already apparent in the increased frequency of fires and insect damage in the largest forested region in the world, the boreal forest of North America, Europe and Asia. It also appears in the north as the melting of once-permanently frozen ground and the release of carbon as carbon dioxide and methane from such soils and from the accelerated decay of tundra peat.
These transitions in the northern regions are all in the direction of releasing additional carbon from large carbon reservoirs into the atmosphere: positive feedbacks, factors that make the problem worse. Stopping a warming, once underway, becomes increasingly difficult. The assumption that a warming that reaches an average for the earth as a whole of 2 degrees with 6 or more degrees in some sections of the high latitudes is either safe or potentially reversible is a very large and risky assumption, almost certainly incorrect. Taking that risk is little short of foolhardy.
The time to experiment with easy or painless transitions to a carbon-free energy system was forty years past when it became clear that the world was headed for severe climatic disruption. Safety now lies in an abrupt change, an overt abandonment of fossil fuels as the primary energy source for industrialization. We can meet the requirements of the agreement signed and universally ratified in 1992 to stabilize the heat-trapping gas content of the atmosphere substantially immediately and then move to reduce it over the next decades.
The alternative is the potential for an environmental, political, and economic chaos whose seeds are being planted at the moment as persistent devastating droughts on all continents including Australia.
George M. Woodwell, Founder and Director Emeritus
The Woods Hole Research Center
Woods Hole, Massachusetts 02543
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