Have we blown our carbon budget?

A recent study in "Nature Climate Change" made headlines with the warning that the global carbon budget - which refers to the amount of carbon we can release into the atmosphere before we exceed climate change targets - might be a lot less than we thought.

That's because projections for temperature rise tend to work from the baseline "pre-industrial" temperature of the late 19th century.

The study's authors say human activity had begun warming the planet much earlier - meaning we might have to reduce our emissions by up to 40 percent more than planned.

Yet as dramatic as that sounds, lead author Andrew Schurer says the real message of the study is that if we're going to have rigorous targets - like the 2 degrees Celsius (3.6 degrees Fahrenheit) enshrined in the Paris Agreement - these need to be properly defined.

"If we really want the targets defined relative to a pre-industrial baseline then it is likely that we may need tougher mitigation than we previously thought," he told DW.

Nice round numbers

Climate change is an immensely complex process. To grasp how urgent the situation is and plan action, huge volumes of data and analysis are boiled down to clear boundaries that we cannot cross: like a limit in increase of the average temperature of our planet by more than 2 degrees Celsius.

It's a target designed to balance what we can bear in terms of climate impacts with what we can realistically aim to achieve in terms of emissions cuts.

The concept of the carbon budget indicates how much carbon we can release into the atmosphere before global warming reaches that 2-degree rise in global temperatures. This has been estimated at around 3,000 gigatons - of which we have already squandered about two-thirds.

Clear messages for decisive action

Luke Sussams of Carbon Tracker Initiative says his organization seized on the carbon budget as a useful - and increasingly popular - tool to communicate the scale and urgency of the problem.

"It is the most simple way, and therefore the most effective way, to frame the problem," Sussams told DW.

The main appeal of the carbon budget is that it allows experts to calculate how much of the world's fossil fuel resources we have left. Carbon Tracker estimates that anywhere between two-thirds and four-fifths of remaining coal, oil and gas reserves are "unburnable."

Given that a hefty share of those reserves is already factored into the balance sheets of fossil fuel companies, the need for urgent action becomes starkly clear.

And that's helped spur concrete action - like the movement to divest from companies counting on profits that can only be realized if the global aim of keeping emissions in check is tossed aside.

But the carbon budget has its limitations.

Politically impossible

"Politically, [carbon budget] is perhaps not that useful," Sussams admits. "Because if you are going to actually use the carbon budget to then go across the 196 countries and hold them to 2 degrees - how would you divide it up?"

The Global Carbon Project has had a go. Its Global Carbon Atlas visualizes different ways to dole out fossil fuel resources, taking into account factors like population size and inertia (the idea that you cannot realistically expect carbon-heavy economies to suddenly go cold turkey).

Josep Canadell of the Global Carbon Project said even their estimates present problems. "Now matter how you cut the pie, you have the developed world - Germany, and anyone else in Europe, the United States, Australia and Japan - eating more than their share," he told DW.

These political complications are why the United Nations climate framework has long since abandoned carbon budget as any basis for negotiations over national contributions to climate action.

"You can just imagine the sort of pushback, and the uproar, if a country thought they weren't given enough of a carbon budget," Sussams said. 

Carbon overdraft

In fact, Canadell said, the UNFCCC's crowning achievement inherently accepts the impossibility of keeping within the carbon budget.

"The reality is that by signing the Paris Agreement, we actually have signed into allowing the carbon budget to be overblown," Canadell said. "We have signed to go beyond the carbon budget in the first part of the century - and in the second part of the century, we do negative emissions."

"Negative emissions" means that not only do we - the humans who will still be around after 2050 - have to stop emitting carbon; we will have to actually start removing it. 

This would rely on large-scale and affordable carbon capture and storage (CCS) - which has so far been elusive. Nuclear energy is also often cited as a bridging solution - although this certainly carries it own risks

In any case, Canadell believes delaying action is a slippery slope. 

How soon is now

"The risk is that is if we are going to overblow the budget, you can always say: Well, let's blow it away a little more so we can relax a bit. We'll fix it later - of course by a different generation than our own," he says.

Yet simplifying things into an easily-grasped concept - the carbon budget - brings its own uncertainties.

Carbon dioxide is the biggest climate change culprit. But the carbon budget makes assumptions on a myriad of other factors - such as how quickly we will be able to remove aerosols from the atmosphere and reduce emissions of other greenhouse gases like methane - that are little better than guesswork.

In reference to Schurer's study on changing the baseline for "pre-industrial," Canadell said: "We had plenty of uncertainty - now we have more."

"But the main message doesn't change," he added.

Which is: Fending off climate catastrophe will be an uphill battle, wherever you set the limit.