Warming or Cooling?

As if the politics of addressing global climate change weren't already daunting enough, a new paper published in Nature this week suggests that the Northern Hemisphere, where most of humanity lives, could be due for a cooling trend, thanks to shifting ocean currents. Coming at a time when the idea of global cooling has been making the rounds on the internet, the prospect of a break in the observable warming trend greatly complicates the task of policy makers who are answerable to their electorates. It would be much harder to contemplate jarring changes to the economy to reduce greenhouse gas emissions, if the polar ice were to stop retreating and glaciers stabilized, or even began to grow again.

The new prediction is based on modeling work described in today's New York Times. It also prompts comparisons to predictions that global warming might trigger even more dramatic cooling, by altering the strength and path of the Atlantic thermohaline current, or "salt conveyor". Once again, we are reminded that global warming has always been a highly imprecise term for the complex processes now at work. That's why I prefer "climate change"--not as a euphemism, but as a more accurate description of the outcomes we face. Some even prefer "global weirding."

But while environmentalists may embrace this new scientific view of climate change as more volatile than the steady warming many have expected, climate skeptics will see it as a glaring inconsistency, particularly if the global rise in temperature stalls. That matters because it seems unlikely that the public's growing worries about climate change result from having absorbed the scientific consensus embodied in the reports of the Intergovernmental Panel on Climate Change (IPCC), rather than from media coverage of melting icecaps, shrinking glaciers, unusual heat waves and droughts, and the other evidence that fits a pattern of warming. If the visible evidence began seriously to diverge from that trend, I am skeptical that faith in science would sustain the public concern that must underpin any serious regulatory efforts, whether we're talking about emissions cap and trade, a carbon tax, or even the milder sector-specific targets that the President recently proposed.

Those who approach climate change with a quasi-religious fervor are likely to become apoplectic at any suggestion that a few cooler months or years might derail the growing policy momentum to institute the means of dramatically reducing emissions. But while they might be comfortable dismissing out of hand a winter that was 0.5 degrees Celsius colder than the previous one--punctuated by a sharp rebound in March--the rest of us might prefer a polite and practical conversation about how such variability could still be consistent with the overall trendline. Ultimately, our understanding of climate change must always remain incomplete, and so we must remain flexible enough to incorporate the new knowledge we will inevitably turn up along the way. Isn't that the essence of science?