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 study is yet another reminder about the profoundly different planet we would create by burning all fossil fuels…It is hard to imagine a single ecosystem that would remain untouched.”

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, CO 2 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:

“It is relevant to know what would happen if we do not take actions to mitigate climate change – for example, if we do not ever implement the Paris Agreement…From a scientific perspective, it is interesting to study how the climate system would respond under such high forcing levels.”

“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:

“The temperature and precipitation changes [the authors] project…are way out of bounds for several ecosystems. This is no big surprise, since even what is viewed as ‘moderate’ warming will cause loss of Arctic sea ice, and hence the entire ecosystem adapted to sea ice.”

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:

“Grasslands didn’t evolve until CO 2 was low enough that grasses could out-compete trees. At least one research group has predicted loss of grasslands at very high CO 2 …[Overall, it is] likely these types of extreme climate changes would lead to a 6th mass extinction event.”

Profound changes

The temperatures that today’s study talks about are higher than scientists have predicted in the past for unmitigated fossil fuel burning.

For example, a 2009 study led by Prof Myles Allen, professor of geosystem science at the University of Oxford, projects a 4-5C relative to preindustrial for ~5tn tonnes of carbon.

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:

“These models have a much more complex representation of the climate system and include dynamic carbon cycle feedbacks and dynamic atmosphere, for example, which may be not represented well in the simpler models.”

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:

“[T]he unregulated exploitation of the fossil fuel resource could ultimately result in considerably more profound climate changes than previously suggested.”

Whether or not the linear relationship holds for higher emissions may well be interesting from a scientific perspective. But if CO 2 was ever to reach this high, it would be a bit of a moot point, Allen tells Carbon Brief:

“I very much hope we don’t find out if it is linear out to 5 trillion tonnes in the real world, because frankly, we’ll be worried about a lot more than the linearity of the temperature/cumulative-carbon relationship if we do.”

Meinshausen echoes this point, telling Carbon Brief:

“Whether the increase in temperatures flattens off or – as this study shows – is likely to continue linearly with cumulative emissions, is secondary, as it is hopefully a world that we will never see.”

Reality check

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:

“Resources are those volumes that have yet to be fully characterised, or that present technical difficulties or are costly to extract…Reserves are those volumes that are expected to be produced economically using today’s technology.”

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:

“Using an estimate of proven reserves would result in a lower warming estimate.”

But the distinction between reserves and resources is probably less relevant when looking so far into the future, says Allen. He tells Carbon Brief:

“Many of today’s fossil carbon reserves would have been deemed resources 50 years ago, before it occurred to anyone we would develop technologies to extract oil from the deep ocean, for example, or frack methane from rock. If the resource is there, and we don’t get a grip on climate policy, it will get used someday.”

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 CO 2 ).

Thought experiment

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:

“The simulations show that the ability of the land to take up some of the carbon emissions and the deep ocean to take up the heat trapped by rising greenhouse gases begins to wane by the end of this century, which exacerbates warming.”

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 is a useful ‘what if’ study that exercises computer simulations to their limits. But, in reality, the damage to societies and ecosystems by such severe climate change would cripple economies to such an extent that it would be practically impossible to burn all the fossil fuel reserves.”

Main image: On July 12, 2011, crew from the U.S. Coast Guard Cutter Healy retrieved a canister dropped by parachute from a C-130, which brought supplies for some mid-mission fixes.



Tokarska, K. B. et al., (2016). The climate response to five trillion tonnes of carbon. Nature Climate Change DOI: doi:10.1038/nature08019