"Ten Years"

There's another part of Al Gore's performance that bothers me--this notion that we have "ten years" in which to turn the global warming picture around." I think he said it in his movie, and here he says it in the Rolling Stone interview:

Rolling Stone: And that has to be done within ten years?

Gore: No, we don't have to do all of it in ten years -- that would be impossible. What the scientists are saying when they give this dark warning is that we may have as little as ten years before we cross a tipping point, beyond which there's an irretrievable process of degradation.

Where does this 10-year figure come from? Throwing in a cool term like "tipping point" doesn't give it any more heft. Maybe it comes from James Hansen. In his recent book review in the NY Review of Books, he writes:

...we have at most ten years—not ten years to decide upon action, but ten years to alter fundamentally the trajectory of global greenhouse emissions.

This in turn comes from a statement he makes earlier:

Any responsible assessment of environmental impact must conclude that further global warming exceeding two degrees Fahrenheit will be dangerous. Yet because of the global warming already bound to take place as a result of the continuing long-term effects of greenhouse gases and the energy systems now in use, the two-degree Fahrenheit limit will be exceeded unless a change in direction can begin during the current decade. Unless this fact is widely communicated, and decision-makers are responsive, it will soon be impossible to avoid climate change with far-ranging undesirable consequences. We have reached a critical tipping point.

I've never seen this 10-year anywhere in the scientific literature, or this two degree figure. Carbon emissions are a continuum that rise by 1-2% a year. Nothing spectacular is going to happen to them 10 years from now--most likely they'll just be rising 1-2%/yr as they are now. Ice melts in a continuum. It melts basically the same at +1.5°C as it melts at +3.0°C. Species shift in a continuum. Sea level rises in a smooth and continuous fashion.

Yes, there can be nonlinearities in the climate system that result in abrupt shifts, like the Younger Dryas, and we may be at risk for them in the future. But I've never seen anything scientific that suggests that an abrupt change is going to happen in ten years if we continue on our current path for ten more years. That's not what Gore and Hansen are talking about, anyway. What "tips" in ten years? What quantity, what physical phenomena? In ten years we're just ten years deeper in debt, ten years warmer.

I think people are making this number up, for political reasons, to impart a sense of urgency in their listeners and viewers and readers. There's nothing scientific about it. And I think that's misleading when you're talking about a scientific answer to a scientific problem. I think it's better to just be honest: Look, this problem is only going to get worse, gradually worse, year-by-year-by-year. There's a possiblity that certain phenomenon, like Northern Atlantic currents, will shift abruptly in a short period of time and shut-down, but we don't really know when, or if, this is going to happen. But it might. It could. Prudence demands that we begin acting now.

What's wrong with that?