The Fifth Assessment Report is out from the IPCC.  Andy Revkin’s DotEarth blog has an excellent summary.  No real surprises — the Earth has warmed, is warming, and will warm, the oceans are acidifying and human beings are in fact responsible for these changes due primarily to carbon emissions from fossil fuels.

What is new is that the IPCC sets a carbon budget for planetary survival.  In order to have a 66% chance of avoiding catastrophic climate change in excess of 2 degrees C, the planet is limited to a total of one trillion tons total carbon emissions since the dawn of the industrial revolution:

Limiting the warming caused by anthropogenic CO2  emissions alone with a probability of >33%, >50%, and >66% to less than 2°C since the period 1861–1880, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1560 GtC, 0 and about 1210 GtC, and 0 and about 1000 GtC since that period respectively. These upper amounts are reduced to about 880 GtC, 840 GtC, and 800 GtC respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 531 [446 to 616] GtC, was already emitted by 2011.

Since we have already burned 531 gt of carbon, that trillion ton carbon budget leaves only 470 gt left for humanity to burn.  Ever.

So it turns out that Bill McKibben (who has been tirelessly popularizing the no-more-than 565 gt through 2050 figure from his Rolling Stone article) is too optimistic about how much carbon we can burn.  As I pointed out in a previous post, that 565 gt figure leads to a sustainable direct individual carbon footprint of just one ton per year — the equivalent of driving a Prius 5000 miles, taking one cross country flight, or a small fraction of the energy consumed by even the most energy efficient single family house in the US.  The Fifth IPCC Report suggests that one ton per capita per year globally is too much for planetary survival.


The IPCC points out that the globe is on track to pass the one trillion ton cumulative limit by 2040.  In terms of legal response, I think that the time for market incentives and regulatory approaches to limit carbon is rapidly slipping by.  Avoiding catastrophe is going to require an outright ban on burning fossil fuels.  Such bans are not unheard of at law, with varying degrees of success — the Endangered Species Act all but bans takings of endangered species and the Convention on International Trade in Endangered Species all but bans their trade.  Prohibition sought to ban all intoxicating liquor, but with limited success.  I am beginning to think the best analog for a response to global climate change is the abolition of slavery — it took two centuries and (domestically) a civil war, but slavery went from being a global economic norm to being nearly universally outlawed, condemned and banished globally.  We need to reach the same place as far as burning fossil fuels goes, and we have a scant two decades, not two centuries, to get there.