[font face=Serif][font size=5]Scientific reticence and sea level rise[/font]

J E Hansen
NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA
Email: jhansen@giss.nasa.gov

Received 23 March 2007
Accepted 3 May 2007
Published 24 May 2007

[font size=3]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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[font size=4]4. Urgency: this problem is nonlinear![/font]

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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.

An important point is that the nonlinear response could easily run out of control, because of positive feedbacks and system inertias. Ocean warming and thus melting of ice shelves will continue after growth of the forcing stops, because the ocean response time is long and the temperature at depth is far from equilibrium for current forcing. Ice sheets also have inertia and are far from equilibrium: and as ice sheets disintegrate their surface moves lower, where it is warmer, subjecting the ice to additional melt. There is also inertia in energy systems: even if it is decided that changes must be made, it may require decades to replace infrastructure.

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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[font face=Serif][font size=5]Study: Global sea levels have risen six meters or more with just slight global warming[/font]

07/09/2015

[font size=3]CORVALLIS, Ore. – A new review analyzing three decades of research on the historic effects of melting polar ice sheets found that global sea levels have risen at least six meters, or about 20 feet, above present levels on multiple occasions over the past three million years.

What is most concerning, scientists say, is that amount of melting was caused by an increase of only 1-2 degrees (Celsius) in global mean temperatures.

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“Studies have shown that both the Greenland and Antarctic ice sheets contributed significantly to this sea level rise above modern levels,” said Anders Carlson, an Oregon State University glacial geologist and paleoclimatologist, and co-author on the study. “Modern atmospheric carbon dioxide levels are today equivalent to those about three million years ago, when sea level was at least six meters higher because the ice sheets were greatly reduced.

“It takes time for the warming to whittle down the ice sheets,” added Carlson, who is in OSU’s College of Earth, Ocean and Atmospheric Sciences, “but it doesn’t take forever. There is evidence that we are likely seeing that transformation begin to take place now.”

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[font face=Serif][font size=5]Integrating Past Warm Climate Data, Scientists Hone Sea-Level Rise Predictions[/font]

[font size=4]UMass Amherst climate scientist, international team usher in new era[/font]

July 9, 2015



[font size=3]AMHERST, Mass. – In a recent review of the various methods used to reconstruct the relationship between past sea-level rise and climate change, climate scientists including Robert DeConto of the University of Massachusetts Amherst say we are on the verge of understanding how quickly the Greenland and Antarctic ice sheets may respond to warming, and what the rates of sea-level change might be.

Writing in the current issue of Science, DeConto and colleagues at several U.S. and European universities examine how understanding ice sheet response during past warm periods is evolving as researchers in different disciplines integrate their findings. In particular, they consider evidence of ancient sea level and ice sheet reconstructions in various models and review the state of knowledge of the magnitudes, rates and sources of sea-level rise during several of the most prominent interglacial peaks of the last 3 million years.

“Improving our understanding of individual polar ice sheet contributions to global mean sea level is a key challenge,” the authors point out. “An important uncertainty for future projections of the Greenland ice sheet is the threshold temperature beyond which it undergoes irreversible retreat, with current estimates ranging from 1 to 4 degrees C. above pre-industrial temperatures.”

Because they show that this threshold is similar to observed temperatures in the past, they add, “improved estimates of Greenland ice sheet loss for a given local or global temperature increase during past warm periods will thus provide a critical constraint on this threshold.”

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[font face=Serif][font size=5]Evidence from past suggests climate trends could yield 20-foot sea-level rise[/font]

Published: July 9 2015

[font size=3]When past temperatures were similar to or slightly higher than the present global average, sea levels rose at least 20 feet, suggesting a similar outcome could be in store if current climate trends continue.

Findings published in the journal Science showed that the seas rose in response to melting ice sheets in Greenland and Antarctica, said lead author Andrea Dutton, a University of Florida geochemist.

“This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate,” she said.

Dutton and an international team of scientists assessed evidence of higher sea levels during several periods to understand how polar ice sheets respond to warming. Combining computer models and observations from the geologic record, they found that during past periods with average temperatures 1 to 3 °C (1.8 to 5.4 °F) warmer than preindustrial levels, sea level peaked at least 20 feet higher than today.

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