Some climate scientists choose not to talk specifically about emissions reductions. Dr James Hansen of NASA’s Goddard Institute for Space Studies and Columbia University is one of them. Prof John Schellnhuber of the Potsdam Institute for Climate Impact Research is another. Instead they refer to a level of atmospheric CO2 (plus other greenhouse gases that together constitute a total forcing) that would be required to re-establish a ‘safe’ climate.
But surely targeting a safe level of CO2 is basically the same thing as aiming for an emissions cut of X% of some baseline year in Y years (e.g. 80% global reduction in emissions compared to 1990 levels by 2050)? Well, sort of, but not really. Both approaches certainly aim for a reduction in human-caused climate forcing, as the figure to the right indicates. Yet the difference between the two strategies is subtle but important.
To once again use an archery analogy (yes, I like these), aiming for a target CO2 concentration is like fixing your sights directly on the bullseye. That gold circle is what you want your arrow to hit, so you shoot for it. There’s a chance that you’ll miss, of course, but you use your past shooting experience, your training, and some good old ‘gut feeling’ to estimate where to aim – and are prepared to adjust your shooting hand up or down as the wind changes.
Alternatively, aiming for a given level of emissions reduction is like estimating the distance to the target butt, knowing the draw weight of the bow, the shaft stiffness of the arrow, and so on, and then using some ballistics theory to calculate the angle of launch you require in order for the arrow trajectory to rise and descend to the target according to the appropriate mathematical parabola. You don’t look at the target when you shoot, and have no real chance to adjust your aim should the wind direction or speed change.
Spot the difference? One is an explicit aim, the other is implicit. That difference might matter, it might not.
There is an article in the Guardian today which explains Schellnhuber’s position in some detail. To quote:
Roll back time to safeguard climate, expert warns
[A return to pre-industrial levels of carbon dioxide urged as the only way to prevent the worst impacts of global warming]
Scientists may have to turn back time and clean the atmosphere of all man-made carbon dioxide to prevent the worst impacts of global warming, one of Europe’s most senior climate scientists has warned.
Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany, told the Guardian that only a return to pre-industrial levels of CO2 would be enough to guarantee a safe future for the planet. He said that current political targets to slow the growth in emissions and stabilise carbon levels were insufficient, and that ways may have to be found to actively remove CO2 from the air.
Schellnhuber said: “We have to start pondering that it might not be enough to stabilise carbon levels. We should not rule out that it might be necessary to bring them down again.”