Scientists studying the geological record have determined that at slightly above current temperatures we are about 20 metres below what the sea level equilibrium should be. Sea levels are increasing and forecast to rise at least a metre this century (although there is a low probability they could be higher than this), much of the change in sea level will occur over several hundred or thousands of years.
Scientists looking at the Pliocene period from 2.7 to 3.2 million years ago, the last time temperatures were at a similar range of 2 degrees C above average, estimated peak sea level was 22 ± 10 m higher than modern levels (extreme likelihood). This rise in sea level would require the equivalent of the Greenland and West Antarctic ice sheets disintegrating, and some volume loss from the East Antarctic Ice Sheet. We are already seeing substantial mass loss from Greenland and West Antarctic Ice sheets.
There is enormous inertia involved in Sea level rise. Changes in global temperature that help precipitate changes in sea level can take centuries to millennia to result in new sea level equilibrium. Sea Levels have been stable for several thousand years, but have started increasing over the last 100 years in response to the increase in carbon emissions driving the increasing global temperatures of global warming.
Sea level changes work something like a big rubber band - it is relatively stable with the equilibrium stretching and stretching until sea level rise eventually accelerates through the non-linear process of ice sheet melting and collapse to catch up and form a new equilibrium in keeping with the atmospheric carbon and climate temperatures.
So even if we manage to cap the global warming temperature rise to 2 degrees C, we are probably in store for something like a 20 metre rise in sea level to match it's new equilibrium, but this will probably take several hundred years. If we pass the 2 degrees C level and end up at 4 - 6 degrees C, which is becoming more likely with the current level of inaction and business as usual, then sea level equilibrium could be even higher.
"The difference in water volume released is the equivalent of melting the entire Greenland and West Antarctic Ice Sheets, as well as some of the marine margin of the East Antarctic Ice Sheet," said H. Richard Lane, program director of the National Science Foundation's Division of Earth Sciences, which funded the work of the latest study that was published in the journal Geology. "Such a rise of the modern oceans would swamp the world's coasts and affect as much as 70 percent of the world's population."
"You don't need to sell your beach real estate yet, because melting of these large ice sheets will take from centuries to a few thousand years," Miller said. "The current trajectory for the 21st century global rise of sea level is 2 to 3 feet (0.8 to 1 meter) due to warming of the oceans, partial melting of mountain glaciers, and partial melting of Greenland and Antarctica."
Miller said, however, that this research highlights the sensitivity of the earth's great ice sheets to temperature change, suggesting that even a modest rise in temperature results in a large sea-level rise. "The natural state of the earth with present carbon dioxide levels is one with sea levels about 20 meters higher than at present," he said.
The study - High tide of the warm Pliocene: Implications of global sea level for Antarctic deglaciation (abstract) was based upon field from rock and soil cores in Virginia, Eniwetok Atoll in the Pacific and New Zealand. It was led by Kenneth G. Miller, Professor of earth and planetary sciences in the School of Arts and Sciences at Rutgers University.
Some of the key research for this study was conducted in New Zealand. "Whanganui holds one of the world’s best geological archives of global sea-level during the warm climate of the Pliocene and is a key data set in this new study," said Professor Naish from Victoria University of Wellington, who has been conducting research there for the last 20 years.
Professor Naish's research has also involved leading an international team to Antarctica as part of the ANDRILL Project to drill beneath the floor of the Ross Sea in 2006 and discovered that the Antarctic ice sheets retreated significantly during the Pliocene epoch. "What we’re seeing is that the evidence of Antarctic ice sheet collapse is consistent with evidence for sea-level rise in this new study," said Professor Naish.
Abrupt sea level rise
There was an intersting media conference at last years American Geophysical Union conference in San Fransisco at which climate scientists James Hansen, Eelco Rohling, and Ken Caldeira explained that the climate sensitivity may be greater than previously thought based upon the geological record. "The paleoclimate record reveals a more sensitive climate than thought, even as of a few years ago. Limiting human-caused warming to 2 degrees is not sufficient," said NASA climatologist James Hansen.
Eelco Rohling, Professor of Ocean and Climate Change at Southampton University, Southampton, United Kingdom, presented on Ice Sheet and sea level response. Rohling spoke on the sea level disequilibrium and ice volume adjustment, and how fast we can expect some of these adjustments to take place.
"We have this relationship, what is the basic underlying natural relationship between sea level and climate forcing and if we use that, then at 1.6 watts per square metre forcing where we are at the moment, then we would expect the equilibrium sea level in the natural state to be 25 metres (±3-5 metres) above the present. So this is essentially the elastic band of climate that we are stretching. We are creating a disequilibrium. Now the problem is we are not only stretching but we're stretching it really quickly so the system cannot keep up at all. ...The adjustment is similar to yanking an elastic band really quickly - there could be snaps in there - and we could have very abrupt adjustments happening. And these abrupt adjustments we can say something about if we go to the last interglacial."
Rohling's figures for the level of sea level disequilibrium is similar to this latest study.
The big question is how fast can the ice sheets collapse? We know the melting of ice sheets is a non-linear process. NASA climatologist James Hansen explained on ABC Television program, The 7.30 Report in 2007 to 7.30 Report anchorman Kerry O'Brien that:
"the problem is that the climate system in general has a lot of inertia and that means that it takes time for the changes to begin to occur but then, once they do get under way, it becomes very difficult to stop them and that is true in spades for the ice sheets. If we once begin to disintegrate it will become very difficult, if not impossible, to stop them and we are beginning to see now on both Greenland and west Antarctica disintegration of those ice sheets. They're both losing ice at a rate of about 150 cubic kilometres per year and that's still not a huge sea level rise."
"Sea level rise is now going up about 3.5 centimetres per decade. So that's more than double what it was 50 years ago. But it's still not disastrous; it's a problem, but it's not disastrous. But the potential is for a much larger sea level rise. If we get warming of two or three degrees Celsius, then I would expect that both West Antarctica and parts of Greenland would end up in the ocean, and the last time we had an ice sheet disintegrate, sea level went up at a rate of 5 metres in a century, or one metre every 20 years. That is a real disaster, and that's what we have to avoid."
So, with rapid ice sheet disintegration we get strong pulses of sea level rise of several metres per century. This has happened in the geological past, even with a much slower rate of atmospheric climate change. We don't know when we might trigger the first of these pulses. But we are changing the climate much much more rapidly than has ever happened on the geological time scale. What may have happened naturally over several thousands years in geological time, we are doing in a brief 150 years.
So Greenland Ice sheet is melting in 2011 with near record mass loss. There have been warnings about Greenland ice caps to raise sea levels going back to at least 2004 and earlier.
Polar scientists are working feverishly trying to understand the dynamics of ice sheets, with some progress, but the science is difficult and complex in a harsh cold environment. The rate of mass loss can accelerate as disintegration is a non-linear process with many contributory factors.
where are we with global carbon mitigation action?
The Durban conference put in place the framework for a new global climate treaty coming into effect in 2020, 8 years hence. No substantive action for 8 years, while greenhouse gas emissions continue to increase.
The latest proposals and pledges from Durban put the world on the path of 4.3°C of warming by the end of the century. A statement released by Climate Action Tracker on December 11, 2011 stated:
"Without new pledges for emissions reduction on the table, our Climate Scoreboard analysis projects future global temperature increases far above the global goal of 2°C (3.6 °F) , pointing towards temperature increase of 4.3°C (2.6 - 6.9°C) or 7.7°F (4.6 - 12.3°F) by the end of the century."
Co-author of the sea-level study, New Zealand Professor Tim Naish comented, "If the present levels of carbon dioxide in the atmosphere are not abated, and humans were to disappear from the planet and return in 2,000 years, they would find a world where the oceans have risen 20 metres," said Professor Naish in a university media release.
- Kenneth J Miller et al, High tide of the warm Pliocene: Implications of global sea level for Antarctic deglaciation (abstract), Geology, March 19, 2012
- Eurekalert media release, Global sea level likely to rise as much as 70 feet for future generations
- Victoria University of Wellington Media Release, March 20, 2012 - Future generations could face sea levels 20 metres higher
- Climate Citizen, March 17, 2007 - NASA Climatologist Predicts Disastrous Sea Level Rise
- Climate Citizen, October 2011 - Greenland Ice sheet is melting in 2011 with near record mass loss
- Climate Citizen, November 2011 - Thwaites and Pine Island Glaciers are accelerating with the West Antarctic Ice sheet losing mass
- Climate Citizen - December 17, 2011 - Climate sensitivity - paleoclimate record points towards potential rapid climate change
- Image - Regions Vulnerable to Sea Level Rise from Globalwarmingart.com Copyright Robert A. Rohde (GFDL / CC-BY-SA)