For those who really grok the precautionary principle, aiming for a lower, and therefore inherently safer, maximum atmospheric carbon-dioxide concentration is the logical choice. Civilization arose over the last 10,000 years in a world in which CO2 represented just 280 of every million atoms we and every other respiring organism inhaled. Given the uncertainty over what level of the trace gas leads to dramatic changes in the climate — we know there’s a relationship but haven’t been able to nail down the tipping point — the closer to pre-industrial levels the better.
But there’s a problem.
That problem is we’ve already crossed the 350 ppm threshold that some climatologists say is the highest safe level. Such speculation, put forward by NASA’s James Hansen and a growing number of others, is based on paleoclimatic data that links the freezing and thawing of the planet’s ice caps with such a number, give or take 100 ppm. When CO2 levels fall below that range, the ice caps grow, and so the inference is that letting them rise above that range will cause them to melt.
There isn’t anything like a universal scientific consensus on whether 350 or something higher, around 450, make more sense as a global target. One of the problems is that “safe” is a tricky term to define. Just how much risk are we willing to live with? Everyone has different estimates of just what the probability is that we can hold global average temperatures to no more than 2 °C above pre-industrial levels for a given concentration of CO2.
Terms like “Herculean” are typically attached to the goal of keeping CO2 levels below 450 before we have gotten off the business-the-usual train. Reducing them from today’s 387 to 350 seems like a fantasy, no?
Not to activists like Bill McKibben, who was behind yesterday’s global 350 Action Day, in which lots of concerned citizens arranged themselves in those three numerals for the benefit of aerial cameras. His logic, as relayed to the New York Times Andy Revkin goes like this:
“Three-fifty is the number that says wartime footing, let’s see how fast we can possibly move, and let’s hope against hope that it’s fast enough.”
But it isn’t just passionate activists who believe that 350 is doable. There’s Tufts University economist Frank Ackerman, for one. At Yale’s 360 magazine he argues that most economics studies of the cost of climate change mitigation show it’s entirely within the real of practicality to cut emissions of fossil fuels dramatically without causing another global financial meltdown.
At first glance, there is a bewildering range of estimates of the costs of climate protection. Look more closely, however, and there are just a few projections of economic disaster, out in right field by themselves. Other estimates range from modest costs to small net economic gains.
The outliers are the handful of private consultant studies funded by partisan lobbying groups such as the U.S. Chamber of Commerce and the National Association of Manufacturers.
These projections of economic ruin have not been reproduced by any major academic or non-profit research group. Many economic models find that the modest steps called for in recent U.S. proposals would have very small costs and virtually undetectable effects on total employment
And of four studies that looked specifically at getting emissions down enough to manage a reduction to the magic number:
One group starts from the (realistic) assumption of high unemployment, and finds that long-run employment and economic growth would be increased by a program of public investment in green technology and emissions reduction that leads to 350 ppm. The other three groups adopt the common assumption that short-run unemployment can be ignored in long-run models. They generally find that the needed emissions reductions will cost an average of 1 to 3 percent of world economic output, for some years to come.
There are some other challenges associated with focusing exclusively on 350 ppm. For one thing, just as important as the target is the speed with which we get there. We can only afford to pour so much more carbon into the biosphere (see the trillionth tonne thing), so we can’t afford to wait very long to stop cutting emissions. The longer we wait, the steeper the slope of the graph that charts the reductions necessary to keep the cumulative carbon emissions below the critical value.
It may very well be that 350 is the best long-term target. But no one, or at least, no one worth blogging about, is demanding we bring down emissions to 350 ppm directly from current levels. It is widely recognized that our best efforts will probably see CO2 levels top 400 ppm before they can begin falling. Short-term targets are at least as important. And it can be argued that a simple “350-or-bust” message (which I just made up) could do more harm if it doesn’t acknowledge that there’s no easy way to get there within a century. Even Hansen says he isn’t expecting action on a shorter schedule.
And if we need a short-term CO2 target, we need short-term emissions reductions goals. Ambitious ones, too, on the order of 40% cuts by 2020 in the developed world. Sounds like a lot, but the alternative is to give up or pretend that the world’s climatologists are wrong.