Burning all the fossil fuels we know to exist on Earth could push global temperature an average of 8C above preindustrial levels, according to new research. The Arctic would bear the brunt of the warming, with temperatures potentially rising 17C, say the authors.
The new paper, published today in Nature Climate Change, looks at would happen over the next 300 years or so if the world continues to burn coal, oil and gas with no efforts to limit emissions.
Dr. Malte Meinshausen from the Potsdam Institute for Climate Impact Research, who was not involved in the research, tells Carbon Brief:
The paper is in stark contrast to the tone of conversation in Bonn, where climate talks are taking place on how to meet the Paris Agreement‘s goal of keeping warming “well below” 2C, or even 1.5C.
Five trillion tonnes
The starting point for the new study is a world in which there are no efforts to curb emissions. Under this scenario, CO2 stabilises at roughly 2,000 parts per million (ppm) in 2300.
For context, this is more than five times higher than today’s level (~399ppm) and seven times what it was before humans started industrialising (~280ppm).
Another way to express this is the total amount of carbon released since the beginning of the industrial period, known as cumulative emissions. For this scenario of unmitigated fossil fuel burning, a total of 5tn tonnes would have found its way to the atmosphere by 2300 in the form of carbon dioxide, the paper explains.
This scenario effectively assumes the Paris Agreement – adopted last December – fails to gain any traction. Kasia Tokarska, a PhD student at the University of Victoria in Canada and lead author on the paper, tells Carbon Brief:
“Sixth extinction event”
Five trillion tonnes of carbon would raise global temperatures by 6.4-9.5C, relative to preindustrial times, according to the study. The Arctic, which is already warming faster than the rest of the world, would see temperatures rise at least 14.7, even as high as 19.5C.
As well as a double figure temperature rise, burning all the world’s fossil fuels would result in a factor of four increase in rainfall in the tropical Pacific, say the authors.
As the map below from the paper shows, rainfall would decrease in other places, including Australia, the Mediterranean, southern Africa, the Amazon, central America and North Africa.
The consequences of such changes would be huge for polar and tropical rainforest ecosystems, as well as for mountain species, the Tundra and coral reefs, says Prof Camille Parmesan, an expert in marine life at Plymouth University. She tells Carbon Brief:
Since model projections already show the loss of whole ecosystems with 4C or 6C, it follows that a 8-10C rise could trigger the loss of more common ecosystems as well. Parmesan says:
The temperatures that today’s study talks about are higher than scientists have predicted in the past for unmitigated fossil fuel burning.
But whereas past studies have used relatively simple models, today’s uses four complex Earth System Models (ESMs). These include a more advanced, more realistic simulation of the various interrelating elements of the climate, Tokarska tells Carbon Brief:
What difference does the extra model complexity make?
Until now, simple models have suggested that global temperature rises approximately in proportion with cumulative carbon emissions up to about 2tn tonnes but that after that point, warming slows down.
Today’s study suggests that is not the case. The ESMs find the relationship holds up to the full 5tn tonnes, leading to the paper’s conclusion that:
Whether or not the linear relationship holds for higher emissions may well be interesting from a scientific perspective. But if CO2 was ever to reach this high, it would be a bit of a moot point, Allen tells Carbon Brief:
Meinshausen echoes this point, telling Carbon Brief:
So, how likely is it that we will get close to the 5tn tonnes figure? Is it purely hypothetical or a realistic worst-case scenario?
To give some perspective, the authors equate 5tn tonnes of carbon “approximately to the unregulated exploitation of the fossil fuel resource”.
In other words, it is about equivalent to how much fossil fuels is thought to exist deep in earth’s crust, should we be able to burn it all, they say.
It’s worth a quick technical note here on fossil fuel “resources” and “reserves”. The authors point to a 2013 report by the International Energy Agency, which explains the difference as follows:
But today’s study uses the term “fossil fuel resource” to mean the sum total of both reserves plus resources, Tokarska tells Carbon Brief.
Since this total includes fossil fuels that are not currently economically recoverable – and, arguably, may never be – it could be suggested this is a hypothetical rather than realistic extreme scenario. Indeed, Tokarska acknowledges:
But the distinction between reserves and resources is probably less relevant when looking so far into the future, says Allen. He tells Carbon Brief:
Either way – taking reserves or resources – the temperatures are huge.
It’s also worth noting that 5tn tonnes is at the low end of estimates of the total resource, Tokarska says. For example, the latest report from the Intergovernmental Panel on Climate Change (IPCC) puts the figure for fossil fuel resources in 2011 in the region of 8-13tn tonnes of carbon (31-50tn tonnes of CO2).
This is not the first time scientists have done this kind of “thought experiment”.
Meinshausen’s 2011 paper described what would happen if you take the IPCC’s high emissions scenario (RCP8.5) and continue it to 2300. The paper also contained a figure of ~5tn tonnes for the total cumulative carbon emissions under this zero-mitigation scenario.
Today’s study essentially repeats the exercise with more complex models. Doing so highlights the importance for projections of carbon cycle feedbacks – knock on effects that can speed up or slow down the pace of warming. Prof Richard Allan from the University of Reading tells Carbon Brief, for example:
But while the authors of today’s study consider 5tn tonnes of carbon to be a reasonable estimate of where we could end up without any mitigation, the reality is likely to be different. As Allan puts it:
This article was originally published on Carbon Brief. Re-produced with permission.