In a recent post, I made the optimistic argument that, despite all the obstacles thrown up by rightwing denialism, the world is on track to reduce CO2 emissions to zero by 2050, on a trajectory that would hold atmospheric concentrations of greenhouse gases below 450 ppm. On current models, that gives us a 67 per cent chance of holding the long term increase in global temperatures below 2 degrees. Warming of 2 degrees would not be cataclysmic for humanity as a whole but it would be a disaster for many people and also for vulnerable ecosystems such as coral reefs. That’s why 350.org wants to reduce concentrations to 350 ppm from current levels above 400 ppm.
Is that even possible?
And, what would it mean for global warming?
In this post, I’ll argue that the answer to the first question is definitely yes. I’m going to start with the assumption (based on this post) that we can reduce emissions from fossil fuels to zero by 2050, and keep concentrations below 450 ppm at that point. What are the options to reduce concentrations over the following fifty years? In the absence of some new technological fix (not implausible, but there’s nothing in sight as of 2017), there are three main possibilities
* Reducing methane emissions and concentrations. Methane emissions arise mainly from agriculture (paddy rice and ruminants), with some possible addition from fracking. It appears feasible, though not trivial, to greatly reduce these sources at fairly low cost. And because methane has a short residence time, a reduction in emissions will lead fairly rapidly to a reduction in concentrations. The conversions are very tricky, but the radiative forcing associated with methane is currently about 0.5 watts, compared to 1.94 for CO2. So, if methane concentrations were reduced by 40 per cent, that would be equivalent to a 10 per cent reduction in CO2, or about 40 ppm.
* Natural absorption. Only around 50 per cent of the CO2 we emit (the so-called atmospheric fraction) ends up as increase in atmospheric concentrations, with the rest being absorbed by oceans. After that initial addition to sink, CO2 stays in the atmosphere for a long time. However, there is still some additional absorption by sinks. Yale Climate Connections suggests that around 50 per cent is absorbed in 50 years, and around 70 per cent in 100 years. So, by 2100, an additional 20 per cent or so of the CO2 emitted around now will have been absorbed by sinks. A rough estimate would be 0.2*(450-280) or 35 ppm, where 450 is the peak concentration and 280 the stable pre-industrial level. Of course,this is far from an ideal solution, since CO2 contributes to acidification of oceans and therefore to coral reef decline.
* Reforestation and other land use changes. Land use change is currently a big net contributor to global warming, but a systematic program of reforestation could turn this around. The potential has been estimated at 85 ppm.
Against these possibilities, there is currently a net cooling effect, equivalent to around 50 ppm, from aerosols associated with air pollution. Hopefully, pollution will be reduced over the coming century, but that makes the task of stabilizing the climate a bit more difficult.
A question I haven’t yet been able to find a good answer on is: how much warming would a trajectory peaking at 450 ppm and declining to 350 ppm ultimately produce? If anyone can point me to a good source, that would be great.
Finally, at least some of the pollutants we’ve emitted over the past century will, on our current understanding, stay there for hundreds or thousands of years, leading to long term problems of sea level rise. But if we can get to 2100 without destroying the planet through climate change or nuclear warfare, I’m sure our great-grandchildren will work out some way of cleaning up what’s left of our mess.