O U R S P E C I E S A N D I T S P L A N E T
The coming warmup: no time for complacency
By WALLACE S. BROECKER
AS A SCIENTIST WHO has spent all of his 40-plus professional years studying various aspects of the present-day climate system and its history, I have quite naturally developed a view about the ongoing buildup of greenhouse gases in our atmosphere. Let me put it this way: If humankind, for some reason or other, had decided that it was necessary to warm the planet, no other plan of action would have come anywhere near matching the potential offered by adding CO2, methane, and freons to the atmosphere. Further, no more clever inducement could be designed than hooking all of humanity onto fossil fuels and hamburgers.
I have no doubt that through this action the planet will be warmed. But the unanswered questions are, first, By how much? and second, What will be the consequences? In my view we are engaged in Russian roulette. The bullets range from benign to awesome, but fortunately none of them appears to carry the lethal punch capable of creating a planetary disaster.
Detractors point to the lack of a universally accepted warming signal as evidence that the warming will be moderate. Some go further and claim that the Earth's climate has a strong restoring character, which will squelch any warming. In my estimation, both arguments are false.
I view the failure of the Earth to respond at the pace predicted by the models to be the result of two interferences, one natural and one anthropogenic. Before the industrial revolution, the Earth experienced a Medieval warm and a Little Ice Age cold. As we don't understand what brought about these historic fluctuations, we can not eliminate the possibility that the greenhouse warming has been held back by a natural cooling. Further, it has been recently shown that the reflectivity of sulfate aerosols created from sulfur dioxide (SO2) emissions has nulled out much of the warming expected from carbon dioxide (CO2) and other heat-capturing gases. The important point is that neither of these compensations will long continue. The natural cooling must give way to a natural warming. Moreover, as SO2 and its products remain in the atmosphere for only tens of days while CO2 accumulates for centuries, the aerosol cooling will soon be eclipsed.
I find absolutely no support for the self-regulation concept. Clearly, by undergoing periods of intense glaciation riddled with abrupt shifts from one climate to another, the Earth has proved itself to be anything but a self-stabilizing system. Rather, the climate system has the bad habit of undergoing large and abrupt jumps from one mode of operation to another.
I believe that we should shift the argument from whether the Earth is warming to a discussion of what life will be like if the Earth warms at the pace predicted by models. In the United States some officials are planning to take actions designed to prevent our grandchildren from being saddled with debt. What about saddling them with an unacceptably warm climate? With the population scheduled to double by the middle of the 21st century, I see no possibility whatsoever that we can avoid at least doubling the CO2 content of our atmosphere. For such an increase, all the most advanced computer simulations predict a sizable warming. We must prepare for the consequences.
WALLACE S. BROECKER, Ph.D., is Newberry Professor of Geological Sciences at Columbia's Lamont-Doherty Earth Observatory. His work involving the role of oceans in global climate change has appeared in Nature, Scientific American, Oceanography, and other publications.
