See that black box over on the left-hand side of this blog? The one with the numbers counting down? That’s a little widget I assembled by rejigging one from trillionthtonne.org. The basic idea is that, if our climate can be expected to suffer severe disruption at a certain amount of global warming due to a certain amount of carbon emissions (since the beginning of the fossil-fuel era around 1850), then our best strategy should be to limit the cumulative carbon emissions to somewhere below that level, in this case 1 trillion tonnes of carbon.
But there’s plenty of uncertainty surrounding the estimate that a trillion tonnes of fossil-fuel emissions will lead to 2 °C of warming. What if the threshold is actually a lot lower? That, unfortunately, is the conclusion of a new paper in Geophysical Review Letters. Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases, by a team of Canadian climatologists led by Y.K. Arora of Environment Canada and the University of Victoria does not make for optimistic reading.
The results of this study suggest that limiting warming to roughly 2°C by the end of this century is unlikely since it requires an immediate ramp down of emissions followed by ongoing carbon sequestration in the second half of this century.
This is just one paper, but it’s an important one because it used an approach that will be incorporated in the next IPCC reports due out in a few years. The previous IPCC outlooks used 10-year-old simulation scenarios that were in bad need of an update. The author’s new computer runs are going to be hard to dismiss, because they do a good job of backcasting historical temperature increase. Their formulas say the Earth should have warmed by about 0.9 °C between 1850 and 2005, which is within the observed range of 0.76 ± 0.19.
The team looked at three scenarios involving low-, mid- and high-emissions. The latter two are too depressing to think about, so let’s just look at the low end, which calls for CO2 levels peaking at around 440 ppm (50 ppm above today’s levels) and then settling down at 420 ppm by the end of this century. They found that even such an ambitious target produces a warming of not 1.5 °C , not 2 °C, but 2.3 °C.
In terms of cumulative emissions, Arora et al. write that the trillionth tonne idea assumes that the cooling effects of aerosols and the warming effect of greenhouse gases other than CO2 cancel each other out, which they pretty much appear to do now. This makes the calculation easier, but it may not hold for much longer. The new paper’s simulations imply that the effects of the non-CO2 gases will overwhelm that of the aerosols in the coming decades, and so there will be much more warming for every tonne of emissions. Instead of having about half a trillion tonnes (500 Pg) left to emit before we kick in 2 °C of warming, we may have much less:
…our results suggest there is little room (∼160 ±80 Pg C) to limit the warming in 2100 to the 2.3°C associated with the RCP 2.6 concentration scenario.
Note that we’re emitting about 10 billion tonnes (10 Pg) of carbon annually now, meaning we have about 16 years left at the current rate, which assumes no one, not even China, builds any new coal plants at all. As for the old 2 °C target:
This implies that we have already surpassed the cumulative emission limit and so emissions must ramp down to zero immediately. The unprecedented reduction in fossil‐fuel emissions implied by either of these scenarios suggests that it is unlikely that
warming can be limited to the 2°C target agreed to in the 2009 Copenhagen Accord.
Curiously, soon after the advent of this paper was brought to my attention, I came across this missive from Mark Lynas, who has been following international climate negotiations underway in Bangkok. In it he bemoans the lack of interest in pursuing policies to keep global average warming to 1.5 °C. With the news that keeping things to even 2.3 may be unrealistic, let alone 2 degrees, I expect Lynas will be having a hard time elevating his chin.
Arora, V., Scinocca, J., Boer, G., Christian, J., Denman, K., Flato, G., Kharin, V., Lee, W., & Merryfield, W. (2011). Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases Geophysical Research Letters, 38 (5) DOI: 10.1029/2010GL046270