No, the lesson is not that Katrina was caused by or made worse by global warming. There is, at present, no evidence that Katrina was meteorological payback for our ongoing emissions of greenhouse gases.

Rather, the lesson of Katrina is about risk.

The possibility of a large hurricane wreaking havoc on the Louisiana coast has been known for years. Everything from infrastructure damage to long-term flooding of New Orleans to the enormous refugee problem was foreseen in excruciatingly accurate detail.

We also knew the things we could do to reduce the impact of a killer hurricane. We could shore up the levees, for example, or work to recover the disappearing wetlands and barrier islands that shield New Orleans from storms. But these were deemed “too expensive” and postponed. We rolled the dice.

Now, our country is going to spend hundreds of billions of dollars to rebuild New Orleans and surrounding areas — at least ten times more than the cost of mitigating the catastrophe in the first place.

What does this have to do with global warming?

Global warming is also a risk. Because of limitations in our scientific knowledge of the climate, as well as our inability to predict how technology and the world’s economies will evolve over the next century, we cannot precisely predict how our future climate will evolve and what the impacts of the warming will be.

A 2001 assessment [PDF] by the scientific community suggested that, by the year 2100, the Earth’s average temperature will be 1.5°C to 5.8°C warmer than today. The bottom end of this range is likely manageable by adapting to the changing climate. But the top end of the range, a warming of 5.8°C, is impossible to adapt to; it would be an ecological and humanitarian disaster of unimaginable proportions. At present, we simply have no idea which of these futures we’re heading for.

Early in his first term, President Bush often cited scientific uncertainty as a reason to defer action on climate change. As that argument progressed over the last few years from being merely wrong to patently ridiculous, the president has quietly shifted to the argument that addressing global warming is too expensive.

Would it be expensive? Possibly. But it’s also worth noting that opponents of environmental regulations always make this argument, and it almost always turns out to be wrong. For example, prior to the phaseout of chlorofluorocarbons, which deplete the ozone layer, opponents proffered all forms of apocalyptic predictions about the consequences. In the end, the phaseout was essentially unnoticeable.

President Bush’s wait-and-see policy on global warming is a titanic roll of the dice. He’s betting that future climate change will be modest, and that if it’s not, we’ll have enough time to reduce emissions enough to head off the impacts.

Make no mistake, he might be right. Katrina, however, showed us what happens when policymakers are wrong. Sometimes it’s better to spend money to head off an uncertain risk than to wait for the risk to materialize. Worst-case scenarios sometimes do come true.

For global climate change, wait and see is particularly risky. The cheapest way to reduce greenhouse gas emissions is to start soon and do it slowly over the coming century. If we wait a few decades and then begin a crash course to reduce greenhouse gas emissions rapidly, the cost will be vastly greater and our ability to head off global warming will be reduced. Reducing emissions of greenhouse gases right now might be expensive, but delaying might be much more expensive.