Sea levels are set to rise in relation to global warming. Factors contributing to SLR include decreased global land ice volume and thermal expansion of warming oceans. The Intergovernmental Panel on Climate Change (2013) projected sea levels would likely reach 0.28 to 0.98 m above present by the end of the 21st century. Since the publication of the IPCC report, it has become apparent that especially contributions of the Antarctic to sea level rise have been underestimated, but as of 2017 no consensus has been reached as to by how much. A previous IPCC consensus report (2007) estimated sea level rise to be substantially less, namely 18 to 59 cm. The primary difference between the two estimates is a better modelling of land-ice, but also advances in the understanding of ice calving have contributed.
The current rate of global mean sea-level rise (~3 mm/year; SLR) has accelerated compared to the mean of the 20th century (~2 mm/year), and is expected to rise further under all but the most ambitious mitigation scenario. Sea-level rise will not be uniform over the globe and the specific contributions of sea live rise determine which regions will get more sea level rise compared to others. This is important for regional adaptation to rising seas. Important questions remain regarding the melt rate of ice in West Antarctica and southern Greenland. Also unknown are the actual levels of natural climate variability and greenhouse gas accumulation that will be reached this century. However, even if atmospheric composition were stabilized today, global warming and SLR would continue. Avoiding these changes requires a reduction in emissions to substantially below present levels.
Sea-level rise presents challenges to coastal communities and ecosystems, and planners are engaged in assessing management options. Accordingly, it is desirable to have an estimate of SLR this century to properly design mitigation and adaptation strategies. An approximation of SLR by the end of the century will allow estimates of coastal erosion and changes in vulnerability to coastal hazards, assessments of threats to coastal ecosystems and development of climate risk management policies.
Video Future sea level
Global warming
Heat-trapping gases such as carbon dioxide, methane, and others have increased in atmospheric abundance due to human industrial activities driving a global trend of atmospheric warming (, Figure 1). Measurements indicate that the amount of carbon dioxide and other greenhouse gases in the atmosphere have increased over the past century and exceed levels of the past million years as measured by ice cores from Antarctica. Measurements also indicate that global atmospheric temperature has risen 0.85 ± 0.18 °C between 1880 and 2012. In the same time space, sea levels have risen 210 mm globally.
Further increases in greenhouse gas abundance will lead to further atmospheric temperature increases. When making projections of sea level rise, assumptions have to be made on human behavior. Most studies into global sea level rise compute sea levels for different set of assumptions on greenhouse gas emissions.
Maps Future sea level
Global ice volume
Heat in the atmosphere leads to melting of glaciers and sea ice, a decrease in the extent of snow cover, and shifts from snowfall to rainfall. Melting of glacier ice and snow contribute to SLR, whereas melting sea ice does not directly contribute to sea-level rise because it already displaces its own mass in the oceans. However, as sea ice retreats, more dark open water is exposed to absorb heat from the atmosphere and from the Sun, contributing to heating of the Arctic and Antarctic and further encouraging SLR.
Most researchers anticipate the disappearance of the summer ice pack before mid-century, but now they speculate that losing summer ice cover by 2020 is not unreasonable. This is a situation which is called an "albedo flip." Albedo is a measure of the reflectivity of Earth's surface. That is, the former heat-reflecting ice surface will become a heat-absorbing body of water.
Global ice volume is expected to decrease in three major categories. The first is the Greenland ice sheet. If melted completely, this ice sheet could raise global sea level by approximately six meters. The second major contribution is the Antarctic ice sheet, where especially the West Antarctic Ice Sheet is quite vulnerable to rising temperatures. This area only could theoretically contribute 3.3 m of sea level rise. Thirdly, mountain glaciers around the world are melting in response to global warming.
Ocean heating
In terms of heat content, it is the world ocean that dominates atmospheric climate. The oceans store more than 90% of the heat in Earth's climate system and act as a buffer against the effects of climate change. For instance, an average temperature increase of the entire world ocean by 0.01 °C may seem small, but in fact it represents a very large increase in heat content. If all the heat associated with this anomaly was instantaneously transferred to the entire global atmosphere it would increase the average temperature of the atmosphere by approximately 10 °C. Thus, a small change in the mean temperature of the ocean represents a very large change in the total heat content of the climate system. Of course, when the ocean gains heat the water expands and this represents a component of global sea-level rise.
The thermal expansion of water increases with temperature and pressure of the water. So cold Arctic ocean water will expand less for a given increase in temperature compared to warm tropical water. Because different climate models have slightly different patterns of ocean heating, they do not agree fully on the predictions for the contribution of ocean heating on sea level rise.
Sea level at the end of the 21st century
The Intergovernmental Panel on Climate Change (2013) has made consensus estimates of sea level changes to the year 2100, using the available scientific literature. There projections are based on a combination of all the well-understood contributors to sea level rise, but do exclude some processes that are less well understood. In the case of rapid cuts in emission (the so-called RCP2.6 scenario) they deem it likely that sea level will rise with 26-55 cm. Likely in the terminology of the IPCC means that this is the 67% confidence interval. The higher value should thus not be read as an upper limit, which can be substantially higher. For a scenario with very high emissions they project the sea level to rise with 52-98 cm.
There are broadly two ways of modelling sea level rise. On the one hand, there is process-based modelling, where all relevant and well-understood physical processes are included in a physical model. A disadvantage of this method is that not all relevant processes are understood to a sufficient level. Alternatively, some scientist use semi-empirical techniques that use geological data from the past to determine likely sea level responses to a warming world in addition to some basic physical modelling. This type of modelling was partially motivated by the fact that in the 2007 IPCC report, most physical models underestimated the amount of sea level rise compared to observations. The consensus estimate of sea level rise in their 2007 report was 18 to 59 cm. These numbers also reflect different emission scenarios.
An expert elicitation in 2014 among 90 active researchers in global sea level changes, provided a probabilistic assessment of sea level rise by the year 2100 and 2300. They gave estimates of sea level rise for two different scenarios, one in which humans did not emit much greenhouse gases, and the other in which emissions would go up significantly throughout the 21st century. For this low emission scenario their best estimate was 0.5 m sea level rise for 2100 (likely range 0.4-0.6 m), while the high emission scenario yielded a best estimate of 1.0 m sea level rise (likely range 0.7-1.2 m). Their estimates for year 2300 ranged from 0.7-1.2 m for low emissions to 2-3 m for high emissions.
A January 2017 report by the National Oceanic and Atmospheric Administration indicates a range of GMSL rise of 0.3 - 2.5 m possible during the 21st century.
Upper limit
The IPCC likely estimate of 0.28 - 0.98 m sea level rise till 2100 does not include physical processes that are still poorly understood. These physical processes can further increase the estimate of global mean sea level rise. A separate report from the same period estimates that SLR will be between 0.2 and 2.0 m.. A study in 2016 by James Hansen et al. showed that freshwater release can lead to a set of positive feedback loops, which can further increase ice melt in Greenland and Antarctica.
Far future
If greenhouse gas concentrations were stabilized today, sea level would nonetheless continue to rise for hundreds of years. After 500 years, sea-level rise from thermal expansion alone may have reached only half of its eventual level, which models suggest may lie within ranges of 0.5 to 2 m. Glacier retreat will continue and the loss of a substantial fraction of Earth's total glacier mass is likely. Areas that are currently marginally glaciated are likely to become ice-free. But it is unlikely that greenhouse gases will be stabilized soon, so we can probably count on additional atmospheric heating - and sea-level rise.
The Greenland ice sheet might be lost completely if there is a sustained temperature increase of a certain threshold value. This value lies somewhere between 2°C and 4°C warming compared to pre-industrial period.
Research suggests that Earth's warming temperatures may be on track to melt the Greenland and Antarctic ice sheets sooner than previously thought and ultimately lead to a global sea-level rise of at least 6 m. Researchers found that if the current warming trends continue, by 2100 Earth will likely be at least 4 °C warmer than present, with the Arctic at least as warm as it was nearly 130,000 years ago when the Greenland ice sheet was a mere fragment of its present size. Study leader Jonathan T. Overpeck of the University of Arizona in Tucson said, "The last time the Arctic was significantly warmer than present day, the Greenland Ice Sheet melted back the equivalent of about 2 to 3 m of sea level." The research also suggests the Antarctic ice sheet melted substantially, contributing another 2 to 3 m of sea-level rise. The new research suggests melting could accelerate, thereby raising sea level as fast as, or faster than, 1 m per century. A 2013 study of past sea levels estimated that "we are committed to a sea-level rise of approximately 2.3 meters per °C within the next 2,000 years."
See also
- Current sea level rise
- Climate change
- Climate change in the Arctic
- Climate of Antarctica
- Retreat of glaciers since 1850
- West Antarctic Ice Sheet
Notes
Source of article : Wikipedia
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