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Conservation Science for a Healthy Planet

Potential Global Warming From Doubling of CO2 Reduced from 4.5C to 2.8C; critical for guiding efforts to stay under 2C increase since pre-industrial times

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If one is the loneliest number, two is the most terrifying. Humanity must not pass a rise of 2 degrees Celsius in global temperature from pre-industrial levels, so says the Paris climate agreement. Cross that line and the global effects of climate change start looking less like a grave situation and more like a catastrophe.

…today in the journal Nature, researchers claim they’ve reduced the uncertainty in a key metric of climate change by 60 percent, narrowing a range of potential warming from 3°C to 1.2°C.

…The metric is called equilibrium climate sensitivity, but don’t let the name scare you. “It’s essentially the amount of global warming we would predict if we just doubled the atmospheric carbon dioxide and let the atmosphere and climate come to equilibrium with the carbon dioxide,” says lead author Peter Cox, who studies climate system dynamics at the University of Exeter….

….the researchers say this means the probability of the ECS being less than 1.5°C—the Paris Climate Agreement’s super optimistic goal beyond the 2°C goal—is less than 3 percent. The upside, though, is they say this new estimate means the probability of the ECS passing 4.5°C is less than 1 percent.

…It’s just that global climate change is an exceedingly complex problem. There’s no way any scientist can dig down into all the granular details—changes in vegetation, small-scale hydrology, every single weather event like a hurricane or tornado. So what scientists do is find simplified descriptions of these small-scale events….

Peter M. Cox, Chris Huntingford & Mark S. Williamson. Emergent constraint on equilibrium climate sensitivity from global temperature variability. Nature 553, 319–322 (18 January 2018). doi:10.1038/nature25450

ABSTRACT: Equilibrium climate sensitivity (ECS) remains one of the most important unknowns in climate change science. ECS is defined as the global mean warming that would occur if the atmospheric carbon dioxide (CO2) concentration were instantly doubled and the climate were then brought to equilibrium with that new level of CO2. Despite its rather idealized definition, ECS has continuing relevance for international climate change agreements, which are often framed in terms of stabilization of global warming relative to the pre-industrial climate. However, the ‘likely’ range of ECS as stated by the Intergovernmental Panel on Climate Change (IPCC) has remained at 1.5–4.5 degrees Celsius for more than 25 years1. The possibility of a value of ECS towards the upper end of this range reduces the feasibility of avoiding 2 degrees Celsius of global warming, as required by the Paris Agreement. Here we present a new emergent constraint on ECS that yields a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius. Our approach is to focus on the variability of temperature about long-term historical warming, rather than on the warming trend itself. We use an ensemble of climate models to define an emergent relationship2 between ECS and a theoretically informed metric of global temperature variability. This metric of variability can also be calculated from observational records of global warming3, which enables tighter constraints to be placed on ECS, reducing the probability of ECS being less than 1.5 degrees Celsius to less than 3 per cent, and the probability of ECS exceeding 4.5 degrees Celsius to less than 1 per cent.

And for more perspective on this see:

A ‘new’ measurement of climate sensitivity?



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