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Tuesday, July 16, 2013

Longterm sea level rise estimated at 2.3 metres for every degree Celsius of global warming

"Continuous sea-level rise is something we cannot avoid unless global temperatures go down again," said climate scientist Anders Levermann from the Postdam Institute for Climate Impact Research. "Thus we can be absolutely certain that we need to adapt. Sea-level rise might be slow on time scales on which we elect governments, but it is inevitable and therefore highly relevant for almost everything we build along our coastlines, for many generations to come."

Anders Levermann was taking about the implications of a new study he is a co-author of - The multi-millennial sea-level commitment of global warming. The study is published in the Proceedings of the National Academy of Sciences online during July 2013. It is one of the first studies to combine analyses of four major contributors to potential sea level rise into a collective estimate, and compare it with evidence of past sea-level responses to global temperature changes.

The result is a longterm estimate that global sea levels will rise about 2.3 meters, or more than seven feet, over the next two thousand years for every degree (Celsius) the planet warms. "the total sea-level commitment after 2,000 y is quasi-linear, with a sensitivity of 2.3 m °C" reports the study.

"The study did not seek to estimate how much the planet will warm, or how rapidly sea levels will rise," noted Professor Peter Clark, an Oregon State University paleo-climatologist and co-author on the PNAS article. "Instead, we were trying to pin down the 'sea-level commitment' of global warming on a multi-millennial time scale. In other words, how much would sea levels rise over long periods of time for each degree the planet warms and holds that warmth?"

"The simulations of future scenarios we ran from physical models were fairly consistent with evidence of sea-level rise from the past," Clark added. "Some 120,000 years ago, for example, it was 1-2 degrees warmer than it is now and sea levels were about five to nine meters higher. This is consistent with what our models say may happen in the future."

Our civilisations have been established based upon a stable sea level. We have built our ports, and many of our cities on the coastal plains adjacent to the oceans of the world. And now they will all be under threat by the encroachment of rising seas. Initially it will be damage by storm surges on the rising oceans. Houses with coastal views, like the ones pictured at Cabbage Tree Bay on the central coast north of Sydney, Australia, may find themselves slipping into the ocean as the cliffs behind the beach erodes.

Sea level rise will have a personal cost to those whose homes are affected, but it will also have a huge social and economic cost to all economies as they struggle to adapt the built environment to the inexorable rising sea levels. AfterHurricane Sandy, New York has already committed a US$20 billion plan to adapt New York City to defend it against rising seas and future storms. But even this costly defensive adaptation will prove only temporary and useful this century.

Primary contributors to sea level rise: Thermal Expansion, Mountain glaciers, Greenland and Antarctic Ice sheets

Sea level rise occurs through four major contributions. Thermal expansion of global oceans and melting of mountain glaciers are the most prominent factors contributing to sea level rise at the moment. As global warming continues this will change to the Greenland and Antarctic ice sheets becoming the dominant contributors. We are already seeing accelerating ice loss from Greenland while Antarctica is presently contributing less than 10 percent. Eventually half the contribution to sea level rise may come from Antarctica.

The mining of ground water over the last decades for drinking, and industrial scale mining and agricultural use has also been shown to be a small, but significant contribution to sea level rise. This is not likely to be significant in the very long term affecting the results of this study.

"CO2, once emitted by burning fossil fuels, stays an awful long time in the atmosphere," said Anders Levermann, lead author of the study and research domain co-chair at the Potsdam Institute for Climate Impact Research. "Consequently, the warming it causes also persists."

There is tremendous inertia in the oceans and ice sheets resulting in a slow initial response to changing temperatures and climate. "The problem is: once heated out of balance, they simply don't stop," said Levermann. "We're confident that our estimate is robust because of the combination of physics and data that we use."

The international team of scientists used data from sediments from the bottom of the sea and ancient raised shorelines found on various coastlines around the world. Computer simulation models were calibrated against observational paleo-climate data.

The hundreds of simulations across the four major contributors to sea level rise produced mostly linear results with sea level rise commensurate with the amount of warming. Greenland produced an exception with results indicating a threshold where response to warming is amplified. Indeed. A study published in March 2012 warned that Global Warming threshold for Greenland Ice Sheet collapse reduced to 1.6 degrees C (Robinson et al 2012)

Fig. 1. Sea-level commitment per degree of warming as obtained from physical model simulations of (
A) ocean warming, (B) mountain glaciers and ice caps, and (C) the Greenland and (D) the Antarctic Ice Sheets. (E) The corresponding total sea-level commitment, which is consistent with paleo-estimates from past warm periods (PI, pre-industrial, Plio, mid-Pliocene). Temperatures are relative to pre-industrial. Dashed lines and large dots provide linear approximations: (A) sea-level rise for
a spatially homogeneous increase in ocean temperature; (A,D,E)constant slopes of 0.42, 1.2, and 1.8 and 2.3 m/°C. Shading as well as boxes represent the uncertainty range as discussed in the text. (A – C
) Thin lines provide the individual simulation results from different models (A and B) or different parameter combinations (C). The small black dots in D represent 1,000-y averages of the 5-million-year simulation of Antarctica. Source: Levermann, Clark et al, 2013

"As the ice sheet in Greenland melts over thousands of years and becomes lower, the temperature will increase because of the elevation loss," Clark said. "For every 1,000 meters of elevation loss, it warms about six degrees (Celsius). That elevation loss would accelerate the melting of the Greenland ice sheet."

Antarctic geography provides a different response. The Antarctic ice sheet is so cold, elevation loss won't affect it the same way. Continuing research shows that Antarctic ice sheet disintegration comes primarily from warming southern ocean waters melting ice shelves, resulting in retreat of grounding lines and increase in the flow of ice stream and glaciers to discharge ice bergs. West Antarctica is particularly unstable as much of the ice sheet is over a deep depression below sea level.

"The Antarctic computer simulations were able to simulate the past five million years of ice history, and the other two ice models were directly calibrated against observational data - which in combination makes the scientists confident that these models are correctly estimating the future evolution of long-term sea-level rise," said co-author professor Peter Clark from Oregon State University.

Confusing Rate of Sea level rise with the sensitivity of sea level to temperature

While this study says the long term rate of sea level rise is likely to be 2.3 metres per degree celsius over perhaps several centuries, it explicitly doesn't tell us what short term rates of sea level rise might be.

According to an initial report by Reuters botched reporting of this study by warning of rapid sea level rise. The story headline as published in the Sydney Morning Herald on July 14 said "Models point to rapid sea-level rise from climate change", which is at odds with the content of the story and the press releases issued by Potsdam Institute for Climate Impact Research and Oregon State University.

Global Sea Level is rising 60% faster than IPCC projections according a 2012 study comparing the actual rise in CO2 concentration, global temperature and sea level with past projections done by the IPCC. (Stefan Rahmstorf et al 2012)

The researchers said "the observed rate of sea-level rise on multi-decadal timescales over the past 130 years shows a highly significant correlation with global temperature (Vermeer and Rahmstorf 2009) by which the increase in rate over the past three decades is linked to the warming since 1980, which is very unlikely to be a chance coincidence." Rahmstorf stressed that "the new findings highlight that the IPCC is far from being alarmist and in fact in some cases rather underestimates possible risks."

Other studies have also investigated projected rates of Sea level rise. James Hansen predicted in an 2007 interview on the ABC 7.30 Report that the earth will pass a tipping point resulting in Sea Level Rise of up to a metre every 20 years based upon previous rates of paleoclimate sea level rise. Hansen posits in a December 2012 discussion paper that an exponential rate of Ice sheet Mass Loss, and multi-metre sea level rise is possible later this century.

Professor Elco Rohling was co-author of a study published November 2012 which identified a Climate change connection between Global temperatures, ice volume and sea level. According to the study sea level rise reached speeds of "at least 1.2 metres per century during all major episodes of ice-volume reduction" in the last 150,000 years. (KM Grant et al 2012)

Some scientists think we may have a 20 metre sea level rise already in the pipeline based upon studying the geological record, the current level of warming (0.8 degrees C) and the warming inertia already built into the system (at least 1 degree C more) without considering our present emissions trajectory. There have been scientific projections done for sea level rise for next 500 years which says that "Most rise is expected after stabilization of forcing, due to the long response time of sea level. For all scenarios the rate of sea level rise would be positive for many centuries, requiring 200-400 years to drop to the 1.8 mm/yr 20th century average."

Many of these studies identified a relationship between global temperature rise and sea level. The current study demonstrates a linear relationship between rate of global warming and rate of sea level rise averaged out over thousands of years, and supported by observational data on past temperature change and sea level rise.

Last word comes from Professor Clark who says "Keep in mind that the sea level rise projected by these models of 2.3 meters per degree of warming is over thousands of years. If it warms a degree in the next two years, sea levels won't necessarily rise immediately. The Earth has to warm and hold that increased temperature over time."

"However, carbon dioxide has a very long time scale and the amounts we've emitted into the atmosphere will stay up there for thousands of years," he added. "Even if we were to reduce emissions, the sea-level commitment of global warming will be significant."


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