The Remaining Emissions Budget

On April 30th, Nature finally published something \we’ve wanted to see for a long, long time – a peer-reviewed paper that integrates the latest science towards the very pragmatic goal of defining a entirely citable, global emergency emissions reduction trajectory. The paper is called Greenhouse-gas emission targets for limiting global warming to 2C (download it here) and it’s written by a team led by Malte Meinshausen, of the Potsdam Institute for Climate Impact Research, which has in recent years been the source for much of best scientific work on precautionary emissions trajectories. Also on the author’s list is William Hare (better known in as Bill Hare, particularly within the climate movement) – both Bill and Malte have long been key members of the Greenpeace International climate team.

What Meinshausen et. al. have done is define a comprehensive probabilistic framework which calculates the total amount of CO2 that can be emitted between 2000 and 2050, relative to any given chance of meeting, but not overshooting, a particular temperature target. If you’re interested in high chance of meeting a safe target – or at least the most widely supported of plausibly manageable targets – which would hold the global average temperature increase to 2C above pre-industrial levels target, that budget is extremely small. More precisely, the 2C target corresponds to total 2000-50 emissions of about 1000 Gigatons of CO2, a number to be compared to the approximately 300 Gt that were emitted between 2000 and the end of 2008. (At which point the annual emissions rate was about 36 Gt CO2 per year).

Here’s the essential text from the paper’s executive summary:

“More than 100 countries have adopted a global warming limit of 2C or below (relative to pre-industrial levels) as a guiding principle for mitigation efforts to reduce climate change risks, impacts and damages. However, the greenhouse gas (GHG) emissions corresponding to a specified maximum warming are poorly known owing to uncertainties in the carbon cycle and the climate response. Here we provide a comprehensive probabilistic analysis aimed at quantifying GHG emission budgets for the 2000-50 period that would limit warming throughout the twenty-first century to below 2C, based on a combination of published distributions of climate system properties and observational constraints. We show that, for the chosen class of emission scenarios, both cumulative emissions up to 2050 and emission levels in 2050 are robust indicators of the probability that twenty-first century warming will not exceed 2C relative to pre-industrial temperatures. Limiting cumulative CO2 emissions over 2000-50 to 1,000 Gt CO2 yields a 25% probability of warming exceeding 2 C-and a limit of 1,440 Gt CO2 yields a 50% probability-given a representative estimate of the distribution of climate system properties. As known 2000-06 CO2 emissions were approx234 Gt CO2, less than half the proven economically recoverable oil, gas and coal reserves can still be emitted up to 2050 to achieve such a goal.”

This is important stuff, and worth the time that it will take to read and understand. Here, we only want to make a few key points.

First, the “2C emergency trajectory” that we’ve long been using in our own work is based on very similar work by Paul Baer, EcoEquity’s Research Director. (See High Stakes: Designing emissions pathways to reduce the risk of dangerous climate change for Paul’s original paper and, of course, the Greenhouse Development Rights framework for its principal application). This is notable because, despite the copious criticism that we’ve gotten for defending a depressingly stringent target pathway, the Meinshausen et. al. numbers are even more severe than ours. Which is to say that our analyses have typically allowed 1140 Gt of CO2 emissions between 2000 and 2050, whereas the Meinshausen work gives us only 1000 Gt.

Second, the 350 ppm CO2 stabilization goal that has, recently, gained significant momentum as a more honestly precautionary alternative to the more “realistic” (but unfortunately quite dangerous) 2C target – see http://www.350.org – is even more daunting still in its demands. In particular, a Meinshausen-compliant 350 trajectory requires that total 2000 – 2050 CO2 emissions are held to a maximum of 750 Gt, when we have already (2000 through 2008) emitted about 300 Gt. In the informal Q & A document that Meinshausen and Hare put together to explain their research, they address this directly:

“Q23: How do these results relate to the 350 ppm CO2 concentration goal proposed by James Hansen and others

A: In the recent work by Hansen and co-authors, Target Atmospheric CO2: Where Should Humanity Aim, cumulative net emissions to 2050 are approximately 750 Gt CO2, and thus give according to our results a higher than 75% chance of staying below 2C.

Background: James Hansen and co-authors have proposed an initial climate protection goal of returning atmospheric CO2 to 350 ppm – back from its present levels of over 385 ppm. The fossil fuel reserves are quite consistent with our estimates. In the lower Hansen et al scenario [as shown in figure 6 of the Hansen et al. paper noted just above] about 900 Gt CO2 are emitted from fossil fuel use in the period 2000-2050, however the scenario also assumed that about 150 Gt CO2 are taken up by the increasing sink in the forests: thus the cumulative net emissions to 2050 are approximately 750 Gt CO2, and thus give according to our results a higher than 75% chance of staying below 2C.”

Third, the situation is actually worse than the Meinshausen et. al. paper suggests, because it is based on a dataset in which 2005 to 2009 emissions are estimated on the basis of the now rather aged and out-of-date IPCC SRES scenarios. The bad news here is that, in reality, global emissions during that period were much higher than predicted in even the most pessimistic of the SRES cases, and the Meinshausen et.al. paper did not take this into account. A revision is, it seems, forthcoming, but in the meanwhile, it pays to be well advised.

Even in a world of emergencies, this is an emergency situation.

Finally, note that the next update to the Greenhouse Development Rights modeling will, among much else, define two new emergency emissions-reduction trajectories, both of which are compliant with the Meinshausen et. al. methodology. The first will be defined to give us a 75% of holding the 2C line, and the second will be defined with respect to the 350 CO2 Concentration target. Both will endeavor to take the historical emissions record fully into account.