One of the major effects of climate change is expected to be sea level rise (SLR). Figuring out how high the seas will rise has been a complicated challenge for scientists. At the same time, predicting this is important since many large and densely populated cities of the world are located along coasts and the implications of even a one metre rise could be devastating. SLR is caused by the >melting of glaciers and the expansion of oceans as their temperatures go up. Ocean warming further contributes to more intense storms and coastal flooding. According to the recently published Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), sea levels would rise by less than a metre by the end of the century.
Sujatha Byravan Antarctica and Greenland have the largest ice sheets and hold most of the freshwater ice on earth. The West Antarctic Ice Sheet (WAIS) rests mainly on bedrock under the sea and glaciologists have been noticing signs of decreasing mass. If some portion of the WAIS were to collapse, we should expect rapid SLR. The entire WAIS holds enough water to raise sea levels by 4.8 m. The East Antarctic Ice Sheet (EAIS) is the largest on the planet but has received less attention. The Totten glacier of East Antarctica is also melting more rapidly than expected and it holds enough water to raise sea levels by 3.4 m. Summer melts have also been increasing in the Greenland sheet, which, if completely melted, could raise sea levels by 6 m. Recent findings of a cold patch in the ocean just South of Greenland suggests accelerated melting of freshwater.
There was a 10 cm rise in sea levels between 2009 and 2010 along the East coast of the U.S. This surge was temporary, but has been attributed to warming oceans. Besides SLR, there are many other observed changes in the oceans: ocean acidification effects on snail shells, destruction of plankton, strange behaviour by sea creatures such as whales, salmon, and walruses. All in all, it is quite clear that there is something amiss in the oceans and that these symptoms are all tied to climate change.
Complex systems Modelling SLR has been difficult since factors responsible for determining the rates at which glaciers melt are interlinked with complex feedbacks which are themselves not well understood. There are various processes that could speed things up and the feedbacks can amplify effects. James Hansen, who retired from NASA and is now at Columbia University’s Earth Institute, is well known for being ahead of many scientists in making stark predictions on climate change. He has recently been expressing alarm over the ‘reticence’ of scientists on SLR.
According to Mr. Hansen, ice sheet models are quite primitive when compared to the rates of changes seen in paleo-climatic records. Scientists generally use climate models, ice sheet models and paleo-climatic data to make estimates. Paleo-climatic data studies examine the height of oceans in the pre-human past when temperatures were a few degrees warmer. Last month, Mr. Hansen and other colleagues published an article which concludes that a 2 degree Celsius rise in temperatures could be “highly dangerous” for the world. Since the IPCC had agreed that aiming to stay below 2 degrees Celsius might be a reasonable goal to avoid dangerous impacts from climate change, scientists have been keen to understand how high the seas were when the world was 2 degrees warmer. (The full paper can be accessed at http://bit.ly/1Oq8NTg).
“Our analysis is based on equal parts of information gleaned from paleo-climate studies, climate modelling, and modern observations of ongoing climate changes,” says Mr. Hansen. The scientists suggest that the time period over which ice melting could double could be just ten years, which means that non-linear processes are at work for SLR. The IPCC, on the other hand, has considered largely linear mechanisms for its predictions in the AR5.
According to Mr. Hansen, major ice melts will cause fresh water to pour into the oceans and change what is referred to as thermohaline circulation. Roughly put, this is a process in which warm waters flow towards the North and colder waters sink and flow back south. The disruption of the thermohaline circulation could lead to more intense storms. Studying the Eemian period (about 120,000 years back), which was probably only one to two degrees warmer than our world today, the scientists note that sea levels rose by 5 to 9 metres. If greenhouse gas emissions from human lifestyles and consumption are not abated, there will continue to be disastrous effects with increasing ice loss in Antarctica.
While many scientists considered these results as dire and possibly accurate, a few raised doubts over severity predicted. It is hard to tell if what is operating in these denials is scientific reticence, which Mr. Hansen has discussed in publications. This refers to the observation that scientists do not want to acknowledge or accept the severe SLR that data suggests. “My conclusion, based on the total information available, is that continued high emissions would result in multi-metre sea level rise this century and lock in continued ice sheet disintegration such that building cities or rebuilding cities on coast lines would become foolish,” says Mr. Hansen in his blog.
( Sujatha Byravan is Principal Scientist, Centre for Study of Science, Technology and Policy, Bangalore )
