Sea level rise is now a globally observed, accepted and burning issue. Global sea level is not only rising, but the rate of increase also shows an upward trend in recent decades. There is a wealth of quantitative data, though with variations, collected from numerous land based or remote sensing based sources. As per climate scientists, we are already facing a threat of sea level rise of approximately 2.3 m for each degree Celsius of temperature rise within the next 2000 years (Levermann, et al., 2013). Our planet is getting hotter and global warming is causing the oceans to absorb a lot of extra heat (up to 90 per cent). In the last 100 years the Earth’s climate has warmed about 1°C. The loss of Arctic sea ice exposes the sea water, darkens its surface, resulting in absorption of heat. The warming north polar region and the warming Arctic sea heats up the overlying air, changing atmospheric circulation, with consequences well beyond the Arctic Region.
Global warming is directly related to the amount of CO2 present in the atmosphere. The ice mass that melts will vary according to the amount of global warming. The same is true of thermal expansion. Human activities which contributed to the rise of the earth’s surface temperature over the past century will also play a role in the future. A small rise in sea levels can have adverse effects on coastal areas. The rising sea level will then cause erosion, flooding of wetlands, aquifer and soil contamination and habitat loss for fish, birds and plants. Storms can surge over the seawalls, flooding the coastal cities with sea water. Higher sea levels usually mean more destructive weather events as storm surges get bigger, more powerful and destructive. Coastal cities and ports will be flooded and will force millions of people out of their homes.
Based on satellite data, the National Aeronautics and Space Administration (NASA), shows that sea level continued to rise at the rate of about 3.2 mm/year and till February 2018 the global sea level had recorded a rise of about 87 mm since 1993 (NASA, 2018). The coastal tide gauge records of Commonwealth Scientific and Industrial Research Organisation (CSIRO) from 1870 to 2000 indicates sea level rises of about 200 mm.
Sea level rise is the long term response of the changing climate system and its rise at specific locations may vary from the global average due to local factors. The Intergovernmental Panel on Climate Change (IPCC 2013) has predicted that in the present scenario the global average sea level could rise nearly 1 m by 2100 from the baseline of 1986-2005 which is well within the broad range of 0.3 m to 2.5 m given by National Oceanic and Atmospheric Administration (NOAA) for the
As per the IPCC report (2013) the global average sea level rise at a rate of 3.2 mm per year is consistent with the sum of contributions from thermal ocean expansion due to rising temperatures (1.1 mm per year), glacier melt (0.76 mm per year), Greenland ice sheet melt (0.33 mm per year), Antarctic ice sheet melt (0.27 mm per year) and changes to land water storage (0.38 mm per year).
Although the present sea level measurements do not match with the predictions, there is no doubt about sea level rise. In a recently published article, only about 0.8 per cent of tidal gauges show the sea level rise near IPCC’s predictions. The article explained the discrepancy between the measurements of sea level rise (Gosselin, 2018). The predictions of sea level rise came from two types of measurement, satellite and tidal gauges. The satellite measurement was not adequately accounted for in multi-decadal variations two to three decades back. Also, tidal gauges are placed in inhabited areas along coastlines. Thus, a discrepancy arises between prediction and measurement.
Defining Sea Level Rise
Sea level rise means an increase of water volume in the world’s oceans. Increase in water volume can be mainly caused by melting of ice sheets and glaciers on land and thermal expansion of water in the ocean. In general approximation, about 8 mm of ocean water contributes to the ice sheets in the form of snowfall and the same amount of water is returned to the ocean after melting of ice.
The results of a study published in the journal Science in November 2012, conducted by a team of 47 researchers from 26 laboratories and supported by NASA and the European Space Agency (ESA), reveal that the melt rate of Greenland and Antarctica ice sheets has increased in the last 20 years (Shepherd et al., 2012). Together, these ice sheets are losing more than three times as much ice each year (equivalent to sea level rise of 0.95 mm) as they were in the 1990s (equivalent to sea level rise of 0.27 mm). About two-thirds of the ice loss is coming from Greenland, with the rest from Antarctica (ibid.). Most of the Antarctic continent ice is considered as stable ice but has started melting due to global warming. It is estimated that the Antarctic continent is losing the ice about 220 gigatons per year since 2012 (Sheperd et. al, 2012).
Sea level in the past
Sea level has changed over the earth’s history and presently is very close to the lowest ever recorded about 250 million years ago (Permian-Triassic period) (Muller et al, 2008). It is estimated that about 100 million years ago (late Cretaceous) sea level was 50-100 m higher than the present sea level (Miller, 2005). Scientists have found that during the Pliocene (about 5 to 3 million years ago), atmospheric carbon dioxide was at a level of 400 ppm—same as at present and the sea level was about more than 9 m higher than it is at presently (Rahmstorf, 2012). During the Permian interglacial period about 130,000 and 115,000 years ago, sea level was 6 to 9 m higher than today, when the global temperature was not much warmer than our current era. Similarly, about 20,000 years ago (Last Glacial Maxima) the world’s sea level was about 130 m lower than it is today (Clark and Tarasov, 2014). In the 11,400 years since the end of the last Ice Age, sea level has risen at an average of about 1.2 m/century. This rapid sea level rise was of the result of melting of land-based ice in both the higher latitudes and altitudes. About 10,000 years back most of such ice had melted with the result that sea level now is about 130 m higher than the last glacial maximum (20000 years).
Sea level is closely linked with the earth’s climate—global warming is the leading cause of sea level rise. A warming climate promotes sea level rise with the melting of the Greenland and Antarctica ice sheets, as well as that of mountain glaciers world over. About 1 per cent loss of ice from these three sources would produce a likely increase in sea levels of around 76 cm. The thermal expansion of ocean water because of global warming further adds to this.
Taking these factors into consideration, the oceans would actually rise much more than is anticipated. Researchers have found that algae growing on the surface of the Greenland ice sheet may be accelerating meltdown by darkening the surface (Ryan et. al, 2018). While the snow and ice surfaces reflect back the sun’s radiating heat (albedo), the dark surfaces and ice free areas absorb more heat coming from the sun’s radiation.
The mechanisms that drive global changes in sea level are complex. Many factors contribute to sea level rise and can be recognised once they begin to manifest—but not all of them have necessarily been predicted. They are affected by seasons, astronomical tides, storm surges, ocean currents and density, among other influences.
The land based ice sheets of Antarctic, Arctic and Greenland will play a leading role in sea level rise if the melting rate increases. The Antarctic continent has about 90 per cent of the world’s ice with an average thickness of 2,133 m. It is estimated that ice in the west Antarctica can raise the world sea level by about 8 m (IPCC, 2001; IPCC, 2007). On the other hand, melting of ice sheet in Antarctica can raise the world sea level by more than 70 m. In the Arctic region, the temperature is higher than Antarctica and ice is likely to melt sooner. The Greenland ice sheet can significantly contribute about 7 m to the sea level rise. As the volume of ice is more than water, therefore floating icebergs and Arctic sea ice melting contribute to only a small sea level rise. Apart from the Polar Regions all other ice caps, ice field and glaciers can raise the world sea level by about 45 cm. At 4 oC water is densest and with rising temperatures water naturally expands and the same weight of water occupies more volume. As per a report published by IPCC in 2007, the thermal expansion will contribute about 15-30 cm to the 21st Century sea level rise. Therefore, in all there is potential for the global sea level rise of about 80-90 m if the ice over all the regions melt.
The West Antarctic is sensitive and vulnerable to ice melting. Scientists observed that vast glaciers are rooted below sea level and lie on downward sloping sea beds. The warm water can melt them from below and as the glaciers retreat, more and more ice will be exposed to melting.
The ability of societies to adapt to 21st Century sea level rise is more likely to be dependent upon social factors, including finance, than technical capabilities. In a recent prediction, if carbon dioxide emissions continue unabated, sea levels could rise nearly twice at the end of this century than what was previously predicted (DeConto and Pollard, 2016). This could devastate coastal communities around the globe. If high levels of greenhouse gas emissions continue at the present rates, the rise in ocean levels could be much faster. The sea level rise threatens low lying countries such as Bangladesh, Maldives and some island nations where inundation will force inhabitants to migrate. The critical question is—what is the extent of our adaptation. Coastal societies have been adapting to environmental changes and local sea level rise because coasts are amongst the most dynamic environments on earth. The adaption to sea level rise is not whether we can technically adapt but the primary challenge is how much this will cost and whether societies are ready to bear these costs. Sea level rise and cost for adaption of technology is also likely to lead to socio-economic problem and conflicts.
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