What would we have to do to limit global warming to 2°C, or better still to 1.5°C?
For the past decade or so, climate scientists have answered this vital question in terms of the total amount of carbon dioxide we can pump into the air. If we emit more than a certain total, temperatures will rise by more than 2°C, greatly increasing extreme weather and further accelerating the melting of ice sheets.
But the whole idea of a “carbon budget”, as this idea is known, has become embroiled in controversy and confusion. Now one prominent researcher says we should essentially abandon the idea.
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The basic concept is correct, says of the Center for International Climate Research in Norway. But it has proved impossibly confusing.
All over the place
Far from narrowing down what we need to do, recent studies have produced .
for 1.5°C could be 800 gigatonnes of carbon dioxide, giving us a couple of decades at current emission rates. Or it could be -100Gt CO2, meaning we have already emitted too much.
This is because different studies use different methods, assumptions, temperature records and even different definitions. That makes them virtually impossible to compare, Peters argues.
“The real power of the concept is to say emissions have to go to zero,” says Peters. Once our budget is used up, we cannot safely emit more. “I don’t know that people really get that.”
Carbon debt
Take the idea of a carbon budget for 1.5°C. You might think this means the maximum amount of CO2 we can emit and still be sure that average global surface temperatures never rise above 1.5°C.
But it’s already too late. “There is no scenario that looks at 1.5 without passing it first,” says Peters.
Instead, it’s assumed we emit too much CO2 at first but then, by some as-yet-unknown means, manage to remove huge quantities from the air. In these scenarios, the temperature typically passes 1.5°C around 2030, peaks around 1.7°C or 1.8°C by 2050, then slowly declines to below 1.5°C in the 2090s.
These budgets assume we can run up a “carbon debt” that will be left to future generations to repay, says of the Max Planck Institute for Meteorology in Hamburg, Germany. “We should not do what we do with fiscal budgets and go into debt.”
Accumulating carbon
Geden says we need to change how we interpret the carbon budget.
If policy makers are told that it is still possible to limit warming to 1.5°C if certain conditions are met – which is – there’s little pressure on them to do anything. Instead, he says, people should be told that it’s not possible unless certain conditions are met.
of the University of Oxford, UK was part of the team that published the first ever carbon budget in 2009, and co-authored a controversial 2017 study suggesting the budget for 1.5°C is much higher than thought. He defends the notion of a carbon budget.
“It’s the cumulative emissions that determine peak warming,” says Allen. “That’s just the way it is.”
Far from being abandoned, the carbon budget needs to be made part of climate policy, he says. “There should be a much clearer acknowledgement that, because of the finite carbon budget, meeting our climate goals will quite likely need taking carbon dioxide out of the atmosphere.” What’s more, today’s emitters should be made to pay for this, rather than leaving taxpayers to foot the bill, says Allen.
Nature Geoscience
Nature Geoscience
