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| Last Updated:29/05/2020

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Sea level likely to rise by 0.45 to 0.82 metres in late 21st century

PANAJI: The sea level is likely to rise in the range of 0.45 to 0.82 metre towards the late 21st century, as per projections for the highest emission scenario, defined as representative concentration pathways (RCP) 8.5.

This has been stated in the synthesis report for policy makers (SPM) of the fifth assessment report (AR5) drawn up by the Intergovernmental Panel on Climate Change (IPCC).

IPCC through its report (AR5) has not ruled out the possibility of higher sea levels in the 21st century if sections of the Antarctic ice sheet that have bases below sea level were to collapse.

"This should not be misconstrued as the worst case scenario, and it may be necessary to adopt particular numbers as upper limit for planning purposes," stated A S Unnikrishnan, chief scientist, CSIR- 
National Institute of Oceanography (NIO), a co-author of the paper and lead author of AR5.

This was also clarified in a recent paper 'Sea-Level Rise by 2100', published in the journal, Science, by the lead authors of chapter "Sea Level Change", Chapter 13 of the Working Group I report.

"We could not define this possible contribution more precisely because there is currently insufficient evidence to evaluate the probability of specific levels above the assessed 'likely' range," the authors have stated.

These ranges have been obtained from climate models, known as coupled model inter-comparison project (CMIP5), and an assessment of the rate of melting of glacier and ice sheets. Global warming causes sea level to rise mainly through the thermal (volume) expansion of the ocean, melting of ice-sheets and land ice.

Unnikrishnan explained that the projections are based on the present level of understanding and knowledge of various processes that cause sea level to rise. Observations using past tide gauge records indicate that global sea level rose by 0.19 metre during 1901 to 2010.

Future projections towards late 21st century (average over 2081 to 2100), provide a range of 0.45 to 0.82 m and 0.52 to 0.98 metre for 2100 for RCP8.5.

The SPM was approved by various governments in September 2013. The individual chapters that consist of the Working Group I report are currently in the final stage of printing and publishing.

RCPs are climate scenarios defined based on the radiative forcing in year 2100 relative to year 1750. Radiative forcing in Wm-2 is a way of quantifying the net radiation at the top of the lower atmosphere between that coming from the sun and that emitted from the earth's surface.

The climate model simulations including earth system models were made for different emission rates of carbon dioxide (CO2) in the atmosphere. In the current IPCC, four RCPs are defined. When the concentrations of CO2 attain 421ppm (RCP2.6), 538ppm (RCP4.5), 670ppm (RCP 6.0) and 936ppm (RCP 8.5) by the year 2100.

The projections in AR5 are higher than those indicated in the 2007 report, but below a metre or two of sea level rise that some expect, the scientists have stated.

The assessment report was initiated in 2010 after IPCC for working group 1 chose 259 scientists from 39 countries, including ten from India.

The Times of India (27-12-2013)