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The Real Uncertainties of Climate Change

by Michael D. Lemonick

Climate science is full of uncertainties — but not the uncertainties you might think. There's essentially no uncertainty, for example, about the fact that the Earth has warmed over the past century. There's essentially none about the fact that human activity has contributed to that warming, through emissions of greenhouse gases including carbon dioxide, methane, nitrous oxides and more.

Where things do get uncertain, unsurprisingly, is when you start talking about the future. It's one thing to measure what's been happening; projecting what will happen requires certain assumptions — for example, about exactly how the Earth will respond to warmer temperatures.

Climate scientists don't try to hide these uncertainties, no matter what impression you might get from some news reports. They talk about them all the time, and incorporate them into their projections. That's why they invariably cite a range of likely temperatures and sea levels for the end of the century — because citing a single number would imply certainty, and it's just not there.

That's also why a story in the news section of the current Nature, titled The Real Holes in Climate Science is especially useful. It highlights four major areas where climate scientists would love to have a better understanding, so they could make better projections about the future.  They are:

  • Regional climate projection — that is, the downscaling of global models to try and anticipate what will happen on a regional basis. Especially in areas where the topography is complex—where mountains, for example, can impede airflow — it’s tricky to make the transition from global to local. The good news is that downscaling, just like global modeling, continues to improve.
  • Precipitation. Different climate models disagree on how rain and snow patterns will change. One ominous note: the best data from the real world seem to show that all the models underestimate the magnitude of the change.
  • Aerosols. These are particles and droplets floating in the air, not gases. Some kinds enhance warming, some inhibit it.
  • Tree rings. They’re used to gauge past temperatures, but it’s not clear that they’re reliable gauges in all circumstances. Even without tree-ring data, many other lines of evidence show that the warming over the past half-century is unprecedented. Still, uncertainties in the tree-ring record make it harder to narrow down the overall uncertainty in estimating ancient temperature

How to explain these uncertainties to the public is another problem, as an editorial in the same issue points out. Emphasizing the knowledge gaps risks undermining the greater message. But de-emphasizing them runs another risk: that those whose goal is to undermine that greater message will accuse the climate science community of trying to hide the truth.

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