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.”
Carbon levels have fluctuated over the last few hundred thousand years, but have rarely gone much beyond 280 parts per million (ppm), which is commonly referred to as the pre-industrial concentration. Over the last few centuries, human emissions of greenhouse gases have forced that concentration up as high as 387ppm, and it is rising at more than 2ppm each year.
World governments are currently trying to agree a deal that would restrict emissions and stabilise carbon levels at 450ppm, in an effort to limit global temperatures to 2C warmer than pre-industrial times.
Schellnhuber, who has advised the German government and European Commission on climate, said: “It is a compromise between ambition and feasibility. A rise of 2C could avoid some of the big environmental disasters, but it is still only a compromise.”
He said even a small increase in temperature could trigger one of several climatic tipping points, such as methane released from melting permafrost, and bring much more severe global warming.
“It is a very sweeping argument, but nobody can say for sure that 330ppm is safe,” he said. “Perhaps it will not matter whether we have 270ppm or 320ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms.”
To read the full article, click here.
A recent study in the final throes of peer review, by Hansen and 9 co-authors, undertakes a detailed analysis of past climate responses to greenhouse gas forcing to show that the safe ‘target CO2′ to aim at is between 300 and 350 ppm CO2. Now given that we are already at ~387ppm in 2008 and rising at 2ppm per year, it’s clear on this basis that we’ve already overshot. All logic says that it’s time to nock another arrow to the bowstring and shoot again at the target – as quickly as possible – and be prepared to shoot again if we miss.
But instead, we’ve turned our back on the archery range and are running in the opposite direction…