Last year, I wrote about the challenges of finding current data on greenhouse gas emissions. Lately, I've been looking into the numbers on the global carbon budget. My conclusion: while there are many good pieces written, most of the graphics are poor, and, while most generally agree on the numbers (+/- 30%), there is a lot of confusion because people don't use consistent units. Some people use GtC (giga tonnes of carbon) or a million, million metric tonnes of carbon. Others use GtCO2. Sometimes people will use GtCO2eq. And there is the not-so-occasional use of simply Gt (presumably we're supposed to just know whether its carbon or carbon dioxide).
Here is my graphic, depicting what I understand to be a fairly widely held view of the numbers.
The next picture gets into a little more detail. In the IPCC's 2014 AR5 report, they used 2011 data and, assuming a 66% chance of staying under 2 degrees Celsius, noted we had a remaining carbon budget of about 1000 GtCO2. As of 2014 we've used about 150 GtCO2 of this, the remainder is 850 GtCO2 or about 230 GtC.
Another well-known writing on this topic is Bill McKibben's piece in Rolling Stone from 2012. He also used 2011 numbers, but took the numbers from Carbon Tracker, which used an 80% confidence level of staying under 2 degrees Celsius. His number was 565 GtCO2 or 155 GtC. Again, since 2011 we've used about 25% of this remaining budget, leaving only about 440 GtCO2 or 120GtC.
There are other numbers in other places. Rob Jackson and colleagues in The Bridge came up with a remaining budget of 900 GtCO2 or 245 GtC with a 66% certainty level of staying below 2 degrees Celsius.
Of course all these numbers depend on our understanding of the climate system, what policies we come up with to mitigate land-use drivers, and the accuracy of the emissions data.
But the general story told is that, if we think about the Earth's fossil fuels, we've used about 40% of what we know to be recoverable, and we can use perhaps one-sixth to one-third of of what remains, depending on how much we are willing to risk going over two degrees Celsius. Of course, two degree Celsius is not a magic number. There's no way to know if there are tipping points below two degrees Celsius.
directs research at the Energy Trans Lab