At the Climate Futures Forum in Wellington a couple of weeks ago, David Karoly discussed the idea of considering carbon emissions as a “stock” problem, not a “flow” problem. If we want to give ourselves a 75 percent chance of coming in below a 2ºC rise in the global average temperature, then we (as in all humanity) can emit around one trillion tonnes of CO2 (for more see Meinshausen et al here, discussed in the context of emissions targets at HT in this post). It doesn’t much matter when we do the emitting, because CO2 hangs around in the atmosphere for a long time, but stick to that limit we must if we’re serious about avoiding damaging warming. I like that way of thinking about the issue, as I noted in my report on the Forum, but it seems that I may have been rather optimistic about the height of the ceiling we’re living under, and our chances of hitting a 2ºC target. A new study by a team of Canadian climate modellers, Arora et al, Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases in Geophysical Research Letters, 38 (5) DOI: 10.1029/2010GL046270 (pdf here), suggests 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.
The paper (based on modelling runs for the next IPCC report, using new emissions scenarios) notes that previous estimates of the carbon budget have assumed that the cooling effects of future aerosol emissions will cancel out the warming effects of non-CO2 greenhouse gases. This, the authors suggest, is unlikely, and future carbon emissions will effectively warm the planet more:
[...] 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. It would require an immediate and rapid ramp down of emissions, followed by negative emissions (sequestration) in the later half of this century.
Hitting a 2ºC target means using up all that headroom. We therefore have to move to zero emissions more or less immediately. If that sounds eminently unfeasible, then that’s probably because it is. The world is therefore heading for something a lot worse than a mere two degrees of warming. Through their failure to confront this stark reality our politicians are conspiring to condemn future generations to a living hell. If you’re reading this, Nick Smith, I would welcome your considered reply…
Hat tip: James Hrynyshyn at Class: M