Kerry Emanuel

Despite uncertainties, need to confront climate change is clear

By Kerry Emanuel
February 15, 2010

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OUTSIDE SCIENTIFIC forums, contemporary discussions of the phenomenon of global warming are now so heated that one wonders whether they are contributing to the phenomenon itself.

With all the interest in alleged misdeeds of the Intergovernmental Panel on Climate Change and hacked email exchanges among climate scientists, it is easy to lose track of the compelling strands of scientific evidence that have led almost all climate scientists to conclude that mankind is altering climate in potentially dangerous ways. Recent suggestions by gubernatorial candidate Charles Baker that the scientific community is split on this issue have unfortunately added fuel to this largely manufactured debate.

A few essential points are undisputed among climate scientists. First, the surface temperature of the Earth is roughly 60 F higher than it would otherwise be thanks to a few greenhouse gasses that collectively make up only about 3 percent of the mass of our atmosphere.

Second, the concentrations of the two most important long-lived greenhouse gases, carbon dioxide and methane, have been increasing since the dawn of the industrial era; carbon dioxide alone has increased by about 40 percent. These increases have been brought about by fossil fuel combustion and changes in land use.

Third, in the absence of any feedbacks except for temperature itself, doubling carbon dioxide would increase the global average surface temperature by about 1.8 F. And fourth, global temperatures have been rising for roughly the past century and have so far increased by about 1.4 F. The rate of rise of surface temperature is consistent with predictions of human-caused global warming that date back to the 19th century and is larger than any natural change we have been able to discern for at least the past 1,000 years.

Disputes within climate science concern the nature and magnitude of feedback processes involving clouds and water vapor, uncertainties about the rate at which the oceans take up heat and carbon dioxide, the effects of air pollution, and the nature and importance of climate change effects such as rising sea level, increasing acidity of the ocean, and the incidence of weather hazards such as floods, droughts, storms, and heat waves. These uncertainties are reflected in divergent predictions of climate change made by computer models. For example, current models predict that a doubling of carbon dioxide should result in global mean temperature increases of anywhere from 2.5 to 7.5 F.

The uncertainties in the models, theory, and observations of climate change and associated risks and the sheer complexity of the problem provide many rounds of ammunition for the agenda-driven, be they apocalyptic or denialist. For the lawyerly, with the ability and will to cherry-pick the evidence, there is much ripe fruit to hurl in the increasingly heated climate wars of our generation.

But when the dust settles, what we are left with is the evidence. And, in spite of all its complexity and uncertainties, we should not lose track of the simple fact that theory, actual observations of the planet, and complex models - however imperfect each is in isolation - all point to ongoing, potentially dangerous human alteration of climate.

It is easy to be critical of the models that are used to make such predictions - and we are - but they represent our best efforts to objectively predict climate; everything else is mere opinion and speculation. That they are uncertain cuts both ways; things might not turn out as badly as the models now suggest, but with equal probability, they could turn out worse. Science cannot now and probably never will be able to do better than to assign probabilities to various outcomes of the uncontrolled experiment we are now performing, and the time lag between emissions and the response of the climate to increasing greenhouse gas concentrations forces us to make decisions sooner than we would like. We do not have the luxury of waiting for scientific certainty, which will never come, nor does it do anyone any good to assassinate science, the messenger.

We have never before dealt with a problem that threatens not us, but our distant descendants. The philosophical, scientific, and political issues are unquestionably tough. We might begin by mustering the courage to confront the problem of climate change in an honest and open way.

Kerry Emanuel is director of the Program in Atmospheres, Oceans, and Climate at Massachusetts Institute of Technology.

Clarification: An earlier version of this op-ed bore the headline "Climate changes are proven fact," which did not reflect the view of the author.

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