August 24, 2006 Opinion
Facing the prospect of rapid warming
WILLIAM H. SCHLESINGER
DURHAM – Fossil trees in Antarctica show us that climate has changed greatly in the past. Suppose the current warming is just part of a longer trend that we can’t do anything about?
It is worth looking at what we know about global climate and atmospheric carbon dioxide levels for the past 10,000 years — the Holocene. That period includes the entire history of organized human society — culture, trade, language, money, agriculture and cities. If we care about what happens to humans, we should care about changes that are unusual to our history.
During the Holocene, the Earth’s climate and atmosphere, while not invariant, have been remarkably stable. Scientists have analyzed bubbles of gas trapped in the Greenland and Antarctic ice packs as a measure of past atmospheric composition. All these studies show that for past 10,000 years, the atmospheric carbon dioxide does not appear to have varied by more than about 5 percent from a long-term mean of about 270 parts per million (ppm). In the past 150 years alone, we know that it has risen a dramatic 30 percent to 380 ppm, largely due to the burning of fossil fuels.
Independent studies of tree rings and ice cores show that despite some unusual periods, such as the Little Ice Age, 1550 to 1850 A.D., the Earth’s mean temperature does not appear to have been more than about 1 degree Fahrenheit warmer or cooler than its long-term average of about 59 degrees. But projections of global warming indicate a rise in North Carolina’s temperature of 5 to 9 degrees by the end of this century.
Long-term perspective is important. The Earth’s conditions have varied significantly in the distant geologic past due to natural causes. Studies of marine sediments indicate that the atmospheric carbon dioxide was as high as today’s level, and the Earth was unusually warm, in the Miocene epoch, more than 20 million years ago.
The Earth’s carbon dioxide level and temperature fell slowly to low levels as we entered the last glacial period, which reached its peak 21,000 years ago. The planet warmed rapidly at the end of the period, rising about 10 degrees in a few decades. There were major disruptions of vegetation at the end of the glacial epoch, but we know little about the effects of such warming on the nomadic primitive human culture of that time. There were many fewer people, and they were independent of the expensive societal infrastructure that sustains human populations today.
During the coming 50 years the carbon dioxide in the atmosphere will rise to more than 550 ppm — unprecedented in human history. Over the next century the Earth will also warm to a level it has never reached in human history. Scientists who warn us about climate change are not oblivious to past change; they are worried about the magnitude of current change compared to what humans have experienced in the past.
Having evolved in a period of relative constancy, it is easiest for us to think about a favorable and sustainable future if the Earth’s climate remains relatively constant. Some change is inevitable and humans are smart enough to be flexible, but given a choice, we should always choose a path of slower change that allows us to adapt.
Slow change will allow us to breed new crop plants that can do well in a warmer, drier climate, to develop treatments for new diseases and, as sea level rises, to move inland. Slow change will also allow adjustments of nature — shifts in the timing of flowering, the migration of birds and blooms of phytoplankton that support the ocean’s food chain.
If we do not alter our current profligate use of fossil fuels, large changes in climate will be new, costly and potentially dangerous to humans. Our choice is to pay the cost now and avoid a trip into uncharted climate regimes, or to ask future generations to pay it later, when the consequences are unknown but well beyond our evolutionary history.
(William H. Schlesinger is dean of the Nicholas School of the Environment and Earth Sciences at Duke University.)