Meeting two degree climate target means 80 percent of world's coal is "unburnable", study says

More than 80 per cent of the world's known coal reserves need to stay in the ground to avoid dangerous climate change, according to new research. Thirty per cent of known oil and 50 per cent of gas reserves are unburnable and drilling in the Arctic is out of the question if we're to stay below two degrees, the new research notes.

That vast amounts of fossil fuels must go unused if we're to keep warming in check isn't a new idea. What's novel about today's paper is that it pinpoints how much fuel is unburnable in specific regions of the world, from Canadian tar sands to the oil-rich Middle East.

Unburnable carbon

In its most recent report, the Intergovernmental Panel on Climate Change (IPCC) calculated how much carbon we can emit and still keep a decent chance of limiting warming to two degrees above pre-industrial levels. This is known as a carbon budget. Two degrees is the internationally-accepted point beyond which climate change risks become unacceptably high.

As of 2010, we could release a maximum of about  1000 billion more tonnes of carbon dioxide and still have a 50:50 chance of staying below two degrees, according to the  IPCC.

Today's paper compares this allowable carbon budget with scientists' best estimate of how much oil, gas and coal exist worldwide in economically recoverable form, known as "reserves".

Were we to burn all the world's known oil, gas and coal reserves, the greenhouse gases released would blow the budget for two degrees three times over, the paper finds.

Regional breakdown

The implication is that any fossil fuels that would take us over-budget will have to be left in the ground. Globally, this equates to 88 per cent of the world's known coal reserves, 52 per cent of gas and 35 per cent of oil, according to the new research.

The University College London team used a complex energy system model to investigate the fraction of "unburnable" fossil fuel reserves in 11 specific regions worldwide.

The results suggest the Middle East holds half of total global unburnable oil and gas reserves, with more than 260 billion barrels of oil and nearly 50 trillion cubic metres of gas needing to remain untouched if we're to stay within budget. This "unburnable" fraction equates to two thirds of the region's gas and 38 per cent of oil reserves. Russia accounts for another third of the world's total unburnable gas, as the map below shows.

Coal, the most polluting of the three types of fossil fuel, faces the most restrictive limits. Russia and the US could burn just five per cent or less of their coal reserves if we're to stay below two degrees. In Europe, keeping within budget means 89 per cent of known coal and 21 per cent of oil is unburnable.

Gas plays an important role in replacing coal in the electrical and industrial sectors in a two-degree world, the paper notes. According to the analysis, about 94 percent of Europe's gas reserves are still burnable in a two degree world. That's compared to about 30 per cent of the Middle Eastern gas reserves, which are much larger.

It's also worth noting the new model estimates the most cost-effective way to stay below the two degree budget, based on the production costs of the various types of fuel in each region.

So it's not the only way forward and is by no means a prescriptive solution, lead author Dr Christophe McGlade tells Carbon Brief. Instead, he said the research could feed into negotiations as a starting point for wider conversations about historical responsibility, equity and potential compensation mechanisms.

Unconventional gas

It's worth noting, the numbers above relate to known "reserves". These are fossil fuels that have already been discovered and have a high probability of being recovered under current economic conditions. This is different from fossil fuel "resources". These are all the fossil fuels thought to exist which are potentially recoverable irrespective of economic conditions.

Today's research suggests 25 per cent of Europe's unconventional gas resources could feasibly be exploited while still remaining below two degrees. This includes shale gas, tight gas and coal-bed methane. How much of this is economically viable to recover remains to be seen, however.

Should that change in the future, balancing the two degree budget would probably require that more readily available reserves elsewhere would have to go unburned, McGlade tells Carbon Brief.

Turning to the global picture, meeting our two-degree commitment in a cost-effective way means leaving all unconventional oil and 82 per cent of unconventional gas resources in the ground, according to the model. That means Canada's tar sands and the 100 billion barrels of oil estimated to exist in the Arctic would need to go untouched, the paper explains.

Capturing carbon

According to today's research, technology to capture greenhouse gas emissions before they reach the atmosphere would only have a limited impact on the proportion of fossil fuels that can be burned. Carbon Capture and Storage (CCS) would allow just six per cent more of the world's known coal reserves to be burned, with an even lower figure for oil and gas. The paper explains:

"Because of the expense of CCS, its relatively late date of introduction (2025), and the assumed maximum rate at which it can be built, CCS has a relatively modest effect on the overall levels of fossil fuel that can be produced before 2050 in a two-degree scenario".

The new research paints a stark picture of the compromises in fuel use necessary in a climate-constrained world. The researchers say it raises the question of how we divvy up the winners and losers, and that's one we should all now be asking of our policymakers.

McGlade, C & Ekins, P. (2015) The geographical distribution of fossil fuels unused when limiting global warming to 2C. Nature, http://dx.doi.org/10.1038/nature14016

This post by Roz Pidcock and Rosamund Pearce originally appeared on The Carbon Brief