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Urgent emission reductions needed to achieve 1.5°C warming limit; “not yet geophysically impossible” but requires ambitious emissions reductions

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  • limiting the increase in global average temperatures above pre-industrial levels to 1.5°C is not yet geophysically impossible, but likely requires more ambitious emission reductions than those pledged so far
  • Contrary to many other studies, this finds we have more than 700 billion tons left to emit to keep warming within 1.5 degrees Celsius, with a two-thirds probability of success. “That’s about 20 years at present-day emissions
  • From their study: Emission budgets and pathways consistent with limiting warming to 1.5°C [abstract below]
    • Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0°C above the mid-nineteenth century.
    • limiting warming to 1.5°C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation
September 18, 2017 University of Oxford read full ScienceDaily article here

[Scientists] investigated the geophysical likelihood of limiting global warming to “well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C.”  …the paper concludes that limiting the increase in global average temperatures above pre-industrial levels to 1.5°C, the goal of the Paris Agreement on Climate Change, is not yet geophysically impossible, but likely requires more ambitious emission reductions than those pledged so far….

….’This paper shows that the Paris goals are within reach, but clarifies what the commitment to ‘pursue efforts to limit the temperature increase to 1.5°C’ really implies. Starting with the global review due next year, countries have to get out of coal and strengthen their existing targets so as to keep open the window to the Paris goals. The sooner global emissions start to fall, the lower the risk not only of major climatic disruption, but also of the economic disruption that could otherwise arise from the need for subsequent reductions at historically unprecedented rates, should near-term action remain inadequate.’…

A group of prominent scientists on Monday created a potential whiplash moment for climate policy, suggesting that humanity could have considerably more time than previously thought to avoid a “dangerous” level of global warming. The upward revision to the planet’s influential “carbon budget” was published by a number of researchers who have been deeply involved in studying the concept, making it all the more unexpected. But other outside researchers raised questions about the work, leaving it unclear whether the new analysis — which, if correct, would have very large implications — will stick.In a study published in the journal Nature Geoscience, a team of 10 researchers, led by Richard Millar of the University of Oxford, recalculated the carbon budget for limiting the Earth’s warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above temperatures seen in the late 19th century. It had been widely assumed that this stringent target would prove unachievable — but the new study would appear to give us much more time to get our act together if we want to stay below it.

What this paper means is that keeping warming to 1.5 degrees C still remains a geophysical possibility, contrary to quite widespread belief,” Millar said in a news briefing…..

It is very hard to see how we could still have a substantial CO2 emissions budget left for 1.5 °C, given we’re already at 1 °C, thermal inertia means we’ll catch up with some more warming even without increased radiative forcing, and any CO2 emissions reductions inevitably comes with reduced aerosol load as well, the latter reduction causing some further warming,” Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany said by email.

…In 2013, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) calculated that humanity could emit about 1,000 more gigatons, or billion tons, of carbon dioxide from 2011 onward if it wanted a good chance of limiting warming to 2 degrees C — launching the highly influential concept of the “carbon budget.”

The allowable emissions or budget for 1.5 degrees C would, naturally, be lower. One 2015 study found they were 200 billion to 400 billion tons. And we currently emit about 41 billion tons per year, so every three years, more than 100 billion tons are gone. No wonder a recent study put the chance of limiting warming to 1.5 degrees C at 1 percent. Peters said that according to the prior paradigm, we basically would have used up the carbon budget for 1.5 degrees Celsius by the year 2022.

That’s what makes the new result so surprising: It finds that we have more than 700 billion tons left to emit to keep warming within 1.5 degrees Celsius, with a two-thirds probability of success. “That’s about 20 years at present-day emissions,” Millar said at the news briefing….

Richard J. Millar, Jan S. Fuglestvedt, Pierre Friedlingstein, Joeri Rogelj, Michael J. Grubb, H. Damon Matthews, Ragnhild B. Skeie, Piers M. Forster, David J. Frame & Myles R. Allen. Emission budgets and pathways consistent with limiting warming to 1.5°C. Nature Geoscience Published online doi:10.1038/ngeo3031

ABSTRACT: The Paris Agreement has opened debate on whether limiting warming to 1.5°C is compatible with current emission pledges and warming of about 0.9°C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200GtC would limit post-2015 warming to less than 0.6°C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240GtC with ambitious non-CO2 mitigation. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0°C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5°C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250GtC and unlikely greater than 540GtC. Hence, limiting warming to 1.5°C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

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