International analysis narrows range of climate's sensitivity to CO2
Four years of research confidently narrows range of 1.5DEGC-4.5DEGC down
to 2.3DEGC-3.9DEGC

Date:
July 22, 2020
Source:
University of New South Wales
Summary:
The most advanced and comprehensive analysis of climate sensitivity
yet undertaken has revealed with more confidence than ever before
how sensitive the Earth's climate is to carbon dioxide. The new
research, revealed in a 165 page, peer-reviewed journal article
finds that the true climate sensitivity is unlikely to be in the
lowest part of the 1.5- 4.5DEGC range.



FULL STORY ==========================================================================
The most advanced and comprehensive analysis of climate sensitivity
yet undertaken has revealed with more confidence than ever before how
sensitive the Earth's climate is to carbon dioxide.


==========================================================================
For more than 40 years, the estimated likely range of the eventual global temperature response to a doubling of atmospheric carbon dioxide compared
to preindustrial levels has stubbornly remained at 1.5DEGC -- 4.5DEGC.

This new research, revealed in a 165 page, peer-reviewed journal article commissioned by the World Climate Research Programme (WCRP) written over
four years, finds that the true climate sensitivity is unlikely to be in
the lowest part of the 1.5-4.5DEGC range. The analysis indicates that if atmospheric carbon dioxide levels double from their pre-industrial levels
and are maintained, the world would probably experience eventual warming
from 2.3 - - 4.5DEGC. The researchers found there would be less than 5%
chance of staying below 2DEGC and a 6-18% chance of exceeding 4.5DEGC.

With the Earth's temperature already at around 1.2DEGC above preindustrial levels, if greenhouse gas emissions trajectories continue unabated
the world can expect to see a doubling of carbon dioxide in the next
60-80 years.

"Narrowing the range of climate sensitivity has been a major challenge
since the seminal US National Research Council paper came up with a 1.5 -- 4.5DEGC range in 1979 (Charney et al). That same range was still quoted
in the most recent IPCC report," said lead author Prof Steven Sherwood,
a University of New South Wales chief investigator with the ARC Centre
of Excellence for Climate Extremes.

The research was only made possible by bringing together an international
team of researchers from a wide range of climate disciplines. Using
temperature records since the industrial revolution, paleoclimate records
to estimate prehistoric temperatures, satellite observations and detailed models that examine the physics of interactions within the climate system
the team were able to combine more independent lines of evidence than
any previous study to get their results.



========================================================================== These lines of evidence were then combined in a statistically rigorous way allowing the team to find where the results overlapped. This allowed them
to converge on the best estimate of climate sensitivity. The team found
that, with new developments, the various lines of evidence corroborated
one another leading to more confidence in the result. The 2.3-4.5DEGC
range accounts cautiously for alternative views or assumptions and
"unknown unknowns," with a more straightforward calculation yielding an
even narrower 2.6-3.9DEGC likely range.

"This paper brings together what we know about climate sensitivity from measurements of atmospheric processes, the historical warming, and warm
and cold climates of the past. Statistically combined, these estimates
make it improbable that climate sensitivity is at the low end of the
IPCC range and confirm the upper range. This adds to the credibility of
climate model simulations of future climate," said co-author Gabi Hegerl
from the University of Edinburgh.

"An important part of the process was to ensure that the lines of evidence
were more or less independent," said Prof Sherwood. "You can think of
it as the mathematical version of trying to determine if a rumour you
hear separately from two people could have sprung from the same source;
or if one of two eyewitnesses to a crime has been influenced by hearing
the story of the other one." The researchers then went another step
further and identified the conditions that would be required for the
climate sensitivity to lie outside this most likely range.

The researchers show that low climate sensitivities previously thought
to be plausible, around 1.5-2C, could only occur if there were multiple unexpected and unconnected errors in the data analysis (for example
unexpected cloud behaviour and patterns of long-term ocean warming),
underlying their judgment that these low values are now extremely
unlikely.

A different set of circumstances make it unlikely that global temperatures would rise more than 4.5DEGC for a doubling of carbon dioxide from pre- industrial times, although these higher temperature responses are still
more likely than very low sensitivities.

Even with this qualification, the three-year long research process
initiated by the WCRP with double-checking at every step, a detailed examination of the physical processes and an understanding of the
conditions required for the estimate has finally consolidated an advance
on the 40-year-problem.

"These results are a testament to the importance of cross-disciplinary
research along with slow, careful science and perfectly highlight how international co- operation can unpick our most vexing problems," said co-author Prof Eelco Rohling from the Australian National University.

"If international policymakers can find the same focus and spirit of co- operation as these researchers then it will give us hope that we can
forestall the worst of global warming."

========================================================================== Story Source: Materials provided by University_of_New_South_Wales. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. S. Sherwood, M. J. Webb, J. D. Annan, K. C. Armour, P. M. Forster,
J. C.

Hargreaves, G. Hegerl, S. A. Klein, K. D. Marvel, E. J. Rohling, M.

Watanabe, T. Andrews, P. Braconnot, C. S. Bretherton, G. L. Foster,
Z.

Hausfather, A. S. von der Heydt, R. Knutti, T. Mauritsen,
J. R. Norris, C. Proistosescu, M. Rugenstein, G. A. Schmidt,
K. B. Tokarska, M. D.

Zelinka. An assessment of Earth's climate sensitivity using
multiple lines of evidence. Reviews of Geophysics, 2020; DOI:
10.1029/2019RG000678 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200722112648.htm

--- up 1 week, 1 hour, 55 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)