Humanity’s climate ‘carbon budget’ dwindling fast

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The concept of a carbon budget is dead simple: figure out how much CO2 humanity can pump into the atmosphere without pushing Earth’s surface temperature beyond a dangerous threshold

The concept of a carbon budget is dead simple: figure out how much CO2 humanity can pump into the atmosphere without pushing Earth’s surface temperature beyond a dangerous threshold (AFP Photo/PATRIK STOLLARZ)

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Paris (AFP) – The concept of a carbon budget is dead simple: figure out how much CO2 humanity can pump into the atmosphere without pushing Earth’s surface temperature beyond a dangerous threshold.

The 2015 Paris climate treaty enjoins the world to set that bar at “well below” two degrees Celsius (3.6 Fahrenheit) in order to avoid an upsurge in killer heatwaves, droughts and superstorms made more destructive by rising seas.

Last year, the UN’s climate science body concluded this already hard-to-reach goal may not be ambitious enough.

Only a 1.5C cap above pre-industrial levels, for example, could prevent the total loss of coral reefs that anchor a quarter of marine life and coastal communities around the globe, the Intergovernmental Panel on Climate Change (IPCC) said in a landmark report.

But calculating exactly how much CO2 — produced mainly by burning fossil fuels but also deforestation — we can emit without busting through either of these limits has been deceptively hard to calculate.

Indeed, scientific estimates over the last few years have differed sharply, sometimes by a factor of two or three.

“The unexplained variations between published estimates have resulted in a lot of confusion,” Joeri Rogelj, a lecturer at Imperial College London, told AFP.

To help clear up that muddle, Rogelj and colleagues set out to solve the carbon budget puzzle — or at least make sure that everyone is reading from the same page.

This seemingly academic exercise, published Wednesday in the journal Nature, has huge real-world repercussions.

“The trillion-dollar question is how much of a carbon budget do we have left?”, Rogelj said.

– Wild cards –

About 580 billion tonnes, or gigatonnes (Gt), of CO2, if we’re willing to settle for a 50 percent chance of capping global warming at 1.5C, according to the October IPCC report, for which Rogelj was a coordinating lead author.

At current CO2 emission rates — 2018 saw a record 41.5 Gt — that budget would be exhausted in less than 14 years.

The CO2 allowance for a coin-toss chance of holding the rise in Earth’s temperature to 2C is more generous, about 1,500 Gt, and would last roughly 36 years.

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