Climate uncertainty is a sign of good science


By Mark Buchanan

Bloomberg View

[]Scientists can’t say for sure that humans are the cause of global climate change. They’re still a little uncertain, and that’s a good thing. It means the science is working the way it should.

A well-educated friend of mine, a climate-change skeptic, once told me that he didn’t believe anything coming out of the big computer models that scientists use to reason about the complex nonlinear feedbacks driving the Earth’s climate system. He has a point: Researchers are doing the best they can in the midst of great complication and uncertainty.

My surprise came a few minutes later, when my friend announced that it is, in any event, obvious that recent global warming couldn’t possibly be caused by humans. Somehow, the forbidding complexity of climate physics didn’t prevent his intuition from finding its way to rock-solid conclusions, without the aid of any models at all.

I recalled my friend’s odd logic amid the reaction to the most recent report from the Intergovernmental Panel on Climate Change. The report — its contents were leaked in June, and it will be officially released this week — concludes that it’s now 95 percent certain that human activity lies behind at least half the warming seen in the past half-century. Skeptics savaged the report for revising slightly downward earlier estimates of the warming likely to be seen in the next two decades — as if trying to be accurate was an offense.

The scientists behind the IPCC report actually deserve credit for acknowledging their uncertainty. We all have some lessons to learn about how poor a guide intuition can be in understanding the workings of the climate system.

Take the matter of sea-level rise. Intuitively, there are two factors that should affect it. Warmer weather causes water to expand, making the sea surface rise. And rising temperatures can also melt glaciers, increasing the amount of water in the oceans. Measure how much temperatures have changed, and how much glaciers have melted, and what you find should tell you how much the seas have risen.

Not so fast. About five years ago, scientists estimated that this physics-based model could account for only about one-fourth of the observed rise in the sea level. So other researchers tried a mathematical approach: They correlated historical records of sea-level rise and global temperatures and found they could explain recent trends quite accurately.

The result was highly controversial: When extrapolated into the future, the mathematical method predicted twice as much sea-level rise over the next century. In the worst case, flooding would swamp 200 million homes globally.

It now appears that the physics-based model suffered from a failure of intuition. It overlooked factors such as geophysical sources of heat entering the oceans and human groundwater use, which adds to the outflow of rivers into the oceans. Taking these factors into account, recent studies point again to the more extreme estimates of the mathematical model.

Still, everything about climate science remains messy. If sea levels are rising, how can it be that scientists have observed water levels in some parts of Alaska falling by more than an inch a year? More evidence of a climate hoax? Nope: On further study, it turns out that the land in the area, which had been weighed down by immense glacial sheets, is now rebounding upward, making the sea appear to fall.

Aside from weird land movements, you would at least expect sea levels to rise equally around the world, right? After all, water spreads out. No again. The local rate of sea-level rise actually fluctuates by a factor of about 10 around the planet. Winds and ocean currents can make water pile up in some places more than others. Since 1950, for example, waters off the coast of Cape Hatteras in North Carolina have risen about three to four times faster than the global average, in part because the Gulf Stream and other ocean currents that used to push those waters up against the coast have weakened. They no longer push as hard as they did.

This is the kind of complexity we have to live with. Unfortunately, too many people see uncertainty as evidence of bad science or worse. In reality, scientists are at their best when they acknowledge it.

(Mark Buchanan, a theoretical physicist and the author of “Forecast: What Physics, Meteorology and the Natural Sciences Can Teach Us About Economics,” is a Bloomberg View columnist.)



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