There is one silver lining to the recession. The global economic engine has slowed so much that it's pumping out less greenhouse gases. For the first time in decades, annual carbon-dioxide emissions are expected to drop this year.
Instead of growing at 3 percent or so, emissions are expected to shrink by 3 percent. The savings is roughly what you would get by taking all the cars in the U.S. off the road. Or shutting the whole planet down for three weeks.
Scientists have lots of ways of tracking global conditions, so I got to wondering if the financial crisis would leave its mark in the physical environment, maybe get encoded in some tree ring, or in a bubble of gas trapped in a layer of antarctic ice - something an alien civilization might discover in the distant future, point to and wonder what had happened.
It turns out there is a record of the Great Depression in ice cores because lead pollution declined.
But I emailed several climate scientists to ask if the current carbon dioxide reductions would leave a trace. The answer seems to be no....
Eric Steig, University of Washington:
You would need a very high accumulation rate site to see this. The problem is that the air as it is trapped in the ice continues to mix with the atmosphere for a while, so you get a very smoothed out record, and short-lived events will not be visible. I am positive that somewhere in Antarctica — where you get the very very highest accumulation rates, over 2 m a year of ice (that is 5 m or more of snow), with very precise measurements one could retrospectively see an event like this. But it's unlikely anyone would ever do it. With snowfall rates that high, you don't get very far back in time after a lot of drilling, so it isn't very appealing.
Richard Alley, Penn State University:
Great idea, but my suspicion is that the answer is "no". The emissions are believed to have dropped, so the rate of growth of CO2 will be slightly lower this year than last. (Growth in the concentration will continue, because emissions are still well above the rate of removal from the air into the ocean or wherever else the CO2 is going.) So, if we had a really really good record of the CO2 with sufficiently high time resolution, we would see a bump.
The ice-core records that "work"—cold enough and clean enough ice to avoid artifacts linked to meltwater dissolving CO2 or dead flies in the ice decaying and giving off CO2—have the bubble-trapping depth a few tens of meters down below cold snow being squeezed to ice by the weight of more snowfall on top. If you release a tracer (say, someone had the beans last night for dinner...) in the air above the snow, it takes about 5 years for diffusion in the tiny spaces over 70 m or so to transfer a notable amount of the tracer to the bubble-trapping depth, and along the way a sharp, short-lived spike of tracer will be smoothed out into a bell-curve-like shape.
For ice-core analyses, the uncertainty in the measurement is just under 1 part per million. (With larger samples, measurements in the free air are more accurate.) We've been raising CO2 something like 1-2 ppm per year, and the effect of dropping our emissions by a few percent will be a few percent of 1 ppm, so is not detectable in the ice-core data with modern technology even before it is smoothed out, which it will be."