The Myths Of 1.5 C Carbon Budgets - The Experts Chime In
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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.
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So what are some “expert elicitations” on the 1.5°C budget issue? “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. Warming above the 1880-1900 baseline is now around 1°C, according to the World Meteorological Organisation. If fossil fuel use stopped today, the loss of aerosol cooling would quickly add around 0.5°C of warming. It is on this basis that the “locked in” figure of 1.5°C can be derived in a simple manner.
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http://www.climatecodered.org/2016/09/unravelling-myth-of-carbon-budget-for.html
