Expert judgement provides better understanding of the effect of melting ice sheets

[OKIsItJustMe]

https://www.eurekalert.org/pub_releases/2019-05/uob-ejp052019.php

News Release 20-May-2019
Expert judgement provides better understanding of the effect of melting ice sheets

University of Bristol

Melting ice sheets in Greenland and the Antarctic, and subsequent sea level rise (SLR) this will cause, is widely recognised as posing a significant threat to coastal communities and ecosystems.

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Their findings are published this week in the journal Proceedings of the National Academy of Sciences of the United States of America.

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"Projections of total global SLR using this method yielded a small but meaningful probability of SLR exceeding two metres by the year 2100 under the high temperature scenario, roughly equivalent to 'business as usual', well above the 'likely' upper limit presented in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)."

The findings suggest that coastal communities should therefore not rule out the possibility of 21st-century SLR in excess of two metres when developing adaptation strategies.

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https://www.pnas.org/cgi/doi/10.1073/pnas.1817205116

[OKIsItJustMe]

1. Scientific reticence and sea level rise

http://dx.doi.org/10.1088/1748-9326/2/2/024002

Scientific reticence and sea level rise

J E Hansen

NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA E-mail: [email protected]
Received 23 March 2007
Accepted for publication 3 May 2007 Published 24 May 2007
Online at stacks.iop.org/ERL/2/024002

Abstract
I suggest that a ‘scientific reticence’ is inhibiting the communication of a threat of a potentially large sea level rise. Delay is dangerous because of system inertias that could create a situation with future sea level changes out of our control. I argue for calling together a panel of scientific leaders to hear evidence and issue a prompt plain-written report on current understanding of the sea level change issue.

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Under BAU forcing in the 21st century, the sea level rise surely will be dominated by a third term: (3) ice sheet disintegration. This third term was small until the past few years, but it is has at least doubled in the past decade and is now close to 1 mm/year, based on the gravity satellite measurements discussed above. As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005–15 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields a sea level rise of the order of 5 m this century. Of course I cannot prove that my choice of a ten-year doubling time for nonlinear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU forcing.

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The nonlinearity of the ice sheet problem makes it impossible to accurately predict the sea level change on a specific date. However, as a physicist, I find it almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale. The threat of a large sea level change is a principal element in our argument (Hansen et al 2006a, 2006b, 2007) that the global community must aim to keep additional global warming less than 1 ◦C above the 2000 temperature, and even 1 ◦C may be too great. In turn, this implies a CO2 limit of about 450 ppm, or less. Such scenarios are dramatically different than BAU, requiring almost immediate changes to get on a fundamentally different energy and greenhouse gas emissions path.

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[nitpicker]

2. Link to the PNAS report

https://www.pnas.org/content/early/2019/05/14/1817205116

[nitpicker]

3. And from the BBC

https://www.bbc.com/news/science-environment-48337629

Rise in global sea levels could have 'profound consequences'

By Matt McGrath
Environment correspondent

8 hours ago

Scientists believe that global sea levels could rise far more than predicted, due to accelerating melting in Greenland and Antarctica. The long-held view has been that the world's seas would rise by a maximum of just under a metre by 2100. This new study, based on expert opinions, projects that the real level may be around double that figure. This could lead to the displacement of hundreds of millions of people, the authors say.
(snip)

To try to get a clearer picture, some of the leading researchers in the field carried out what is termed a structured expert judgement study, where the scientists make predictions based on their knowledge and understanding of what is happening in Greenland, West and East Antarctica.

In the researchers' view, if emissions continue on the current trajectory then the world's seas would be very likely to rise by between 62cm and 238cm by 2100. This would be in a world that had warmed by around 5C - one of the worst-case scenarios for global warming.
(snip)

For expected temperature rises up to 2C, Greenland's ice sheet remains the single biggest contributor to sea-level rise. However, as temperatures go beyond this, the much larger Antarctic ice sheets start to come into play.

"When you start to look at these lower likelihood but still plausible values, then the experts believe that there is a small but statistically significant probability that West Antarctica will transition to a very unstable state and parts of East Antarctica will start contributing as well," said Prof Bamber. "But it's only at these higher probabilities for 5C that we see those type of behaviours kicking in."
(snip)

Much of the land losses would be in important food growing areas such as the delta of the Nile. Large swathes of Bangladesh would be very difficult for people to continue to live in. Major global cities, including London, New York and Shanghai would be under threat.

"To put this into perspective, the Syrian refugee crisis resulted in about a million refugees coming into Europe," said Prof Bamber. "That is about 200 times smaller than the number of people who would be displaced in a 2m sea-level rise."
(snip)