Two degrees is out of reach

This is the worrying news coming from the latest-generation of climate modelling efforts, just published in Geophysical Research Letters (abstract) by a Canadian team of scientists. The team use an Earth system model, fired up with a new generation of carbon emissions scenarios, to conclude that even limiting the 2100 temperature rise to 2.3C above pre-industrial would require “an immediate and rapid ramp down of emissions, followed by negative emissions (sequestration) in the later half of this century”.

As for the goal of holding “the increase in global average temperature below 2C above pre-industrial levels”, well, to get onto that trajectory we would have to take global emissions “down to zero immediately”, because the allowable cumulative carbon budget associated with that temperature target has already been emitted. Whoops! This is unfortunate because keeping below 2 degrees is now the world’s ‘official’ goal, which was agreed at Cancun amidst great celebrations (pdf here – see para 4). Now, in the cold light of day, it looks like we’re already too late.

This new piece of rather pessimistic science makes a particular mockery of one of the big wins from Cancun for small island and other vulnerable countries – the promise of a negotiating process between 2013-15 to review the adequacy of the global goal and to consider strengthening it towards limiting the global temperature rise to 1.5C. It looks like this latter target is already in fantasy land, and will be even more so in four years time.

One of the ideas of the ‘review’ is to take into account input from the IPCC’s fifth assessment report, due out early in 2015. Although this latest study is based on results from just one model (CanESM2), it is driven by the new ‘representative concentration pathways’ which will be a key aspect of the next IPCC report. These replace the old ‘SRES’ storyline emissions scenarios which were first developed back in 2001 and fed into the Fourth and Third Assessment reports. This result is especially important because it is based on the outputs of a full Earth system model, which takes into account land-ocean and and other carbon-cycle feedbacks not fully represented in earlier climate models – and hints that the 2015 IPCC report will be more pessimistic about achieving temperature goals than earlier reports have been.

It would be nice if this dose of real-world cold water could increase the sense of urgency in the climate negotiations currently underway in Bangkok. Judging from today’s opening plenary, this seems unlikely – parties were wrangling over the mere content of the agenda for the negotiations over the year ahead, and the ‘global goal’ issue is barely even up for consideration amidst the more technical issues of what a future treaty might look like and how it might work.

Even so, there is little point in giving in to despair and torpor, even with the glacial pace at which the negotiations are proceeding. Sooner or later, the world’s big emitters are going to have to come to an agreement which limits the temperature rise we can expect over the century ahead… and 2.3 degrees is still a lot better than the 4.7 degrees predicted by the model if emissions go on rising past mid-century because we all give up and do nothing. We will have to come back to this, year after year after year, until a deal is done.

Update, 6 April, 10.30am (Bangkok time):

I’ve just had a conversation with Malte Meinshausen, who contributed to the  ‘Representative Concentration Pathways’ (greenhouse forcing scenarios) that the new Canadian climate model results are based on. He emphasised that it is important to bear in mind that analyses of many different models will be feeding into the IPCC’s Fifth Assessment Report, and that some or many of these may give a less pessimistic picture than CanESM2 does. It does appear that CanESM2 has a rather high representation of 20th-century temperature rise (of 0.9C rather than the observed 0.76, as reported in the paper), so may be on the warm side regarding climate sensitivity. Like everyone here in Bangkok, he is very busy, but if I can persuade him to write a few summary paragraphs for this blog, I will post them straight up here.

© Mark Lynas
« · »