Proposed carbon budgets for keeping global warming below 1.5°C above the pre-industrial surface temperature. A number of carbon budgets for 1.5°C have been published. The most frequently quoted case is the third listed (Table 2.2. of AR5 WG1 Synthesis report). The figure of 400 GtCO2 from 2011 is based on a 'threshold exceedance budget' or TEB. Graphic: David Spratt / Climate Code Red

By David Spratt
9 September 2016

(Climate Code Red) – The 2015 Paris climate talks put the 1.5°C temperature target firmly on the policy-making table, whilst also signing off on actions consistent with 3°C or more of warming. This has prompted more discussion in the climate movement about the emissions reduction task consistent with 1.5°C, and whether there is any “carbon budget” available.

Generally, in carbon-budget analysis, multiple climate model runs are used to provide a numerical estimate (or budget or quota) of the total allowable emissions consistent with not exceeding a specified temperature target. But these results must be taken in the context of model uncertainties and overestimation, and the risks of significantly overshooting the target associated with published carbon budgets.

Overshoot

Researchers have noted that there are “no scenarios that have a high probability of limiting warming to below the 1.5°C limit during the entire twenty-first century exist in the literature”. (1)

The published 1.5°C scenarios all involve “overshoot”, that is, exceeding the 1.5°C target and returning the temperature to 1.5°C by century’s end by use of “carbon drawdown” technologies to create “negative emissions”.

The overshoot scenarios mean temperatures may exceed 1.5°C for half a century and peak at around 1.8°C.

The increased risk and more damaging impacts, and the chances of triggering non-linear events, associated with a higher level of warming for several decades in these scenarios are often understated.  (It is an interesting turn of language that scenarios with a 50% chance of reaching 1.8°C and around a 40% chance of reaching 2°C are named “1.5C scenarios”.)

Some carbon drawdown techniques such as reforestation and afforestation are proven and safe, but limited in scale. Covering 3% of the world’s surface with forests would be equivalent to negating just 10% of the world’s current greenhouse gas emissions. Other carbon drawdown techniques include biochar, land management, accelerated weathering, bioenergy with carbon capture and sequestration, direct capture and sequestration, ocean fertilization, and seaweed and algal farming. Many of these are unproven, high cost at present, slow to implement, not currently deployable at the scale needed, and have implications for land use and the maintenance of food production and traditional land ownership, farming and biodiversity protection, because of the large spatial areas required.

The impact of carbon drawdown would be slow and “will not have an appreciable effect on global climate for decades” and hence does not provide an opportunity for rapid reductions of global temperature. (2)

It has been suggested that the use of 1.5°C overshoot scenarios with very large drawdown components has the effect of justifying a substantial continuing high use of fossil fuels for several decades "… rather than requiring that nations reduce emissions in the short-to-medium term”. (3) […]

Expert views

“We have no carbon budget left for the 1.5°C target and the opportunity for holding to 2°C is rapidly fading unless the world starts cutting emissions hard right now,” says Prof Michael Mann. (5) (If a proper “pre-industrial” starting point in the mid-18th century is used, rather than the conventional late-19th century baseline most commonly by scientists and policymakers, that 1.5°C figure is higher at 1.7°C.)

Prof. Stefan Rahmstorf of Germany’s Potsdam University considers that we are now “in a kind of climate emergency” and that at least 1.5°C is “locked in”. (6)
“Locked in” means that the warming will occur for the present level of emissions in the absence of large-scale carbon drawdown and/or solar radiation management.

Three other senior Australian scientists to whom I have spoken agree with the 1.5°C figure articulated by Mann and Rahmstorf. Some think it is likely to be somewhat higher.  [more]

Unravelling the myth of a "carbon budget" for 1.5C

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