There is ample evidence of major sea level changes in Earth’s history. For example, during the warm period before the start of the last ice age, about 120,000 years ago, the global mean temperature in was slightly warmer than it is today. The mean sea level at that time was about 5 or 6 m higher than today.
When the ice sheet was at its maximum towards the end of the Ice age about 18,000 years ago, sea levels were about 100 m lower than today, enough for Britain, for example, to join the European continent.
It is certainly true that the main reason for sea level decline 18,000 years ago was the amount of water trapped in the vast expanses of the polar ice caps. It must also be true that the main reason sea levels were 5 or 6 m higher during the last warm interglacial period was a reduction in the Antarctic ice sheets or Greenland. But changes on shorter timescales are largely driven by other factors that combine to produce a significant effect on mean sea level.
During the 20th century, observations showed that the mean sea levels rose between 10 and 20 cm. Most of the contribution to this rise (about a third) comes from the thermal expansion of seawater; As the oceans warm, the water expands and sea levels rise. Other significant contributions come from melting glaciers and from long-term adjustments that are still occurring. The contributions from the Greenland and Antarctic ice sheets are thought to be small.
Another contribution to the sea-level change of uncertain magnitude comes from changes in terrestrial water storage, such as the growth of reservoirs or irrigation.
Much of sea level rise is due to thermal ocean expansion.The exact calculation of the dilation is complex because it depends decisively on the temperature of the water. Another complication is that the entire ocean is not changing temperature at the same rate.
However, modeling the effects of climate change on the behavior of glaciers is complex. The growth or deterioration of a glacier depends on the balance between the amount of snow that falls on it, especially in winter, and the amount that melts in summer. Both winter snowfall and mean summer temperature are important, and both must be accounted for in future projections of glacier melt rate.
The 21st century mean sea level rise was calculated for each of the Special Emission Scenario Reports (SRES), by summing the different contributions. Those due to thermal expansion (typically about sixty percent of the total) and changes in land ice (typically about twenty-five percent of the total) were calculated using a simple climate model that was calibrated separately for each AOGCMs
Added to this were the relatively small contributions of permafrost changes, the effects of sediment deposition, and the long-term adaptation of the ice sheets to past climate changes. In addition to the uncertainties inherent in the emission scenarios, there is also the uncertainty of the actual temperature increase (and thus the contribution of thermal expansion) depending on the chosen value of climate sensitivity. Different models also provide significantly different estimates of the extent of sea-level rise due to melting glaciers and small ice caps. The total range for 2100 is about 10 cm to 90 cm.
The projections for the next 100 years. During that time, only a small portion of the ocean will have warmed significantly due to the slow mixing that occurs in much of the ocean. Therefore, sea level rise due to global warming will lag behind the change in surface temperature. As the rest of the oceans gradually warm over the next few centuries, sea levels will continue to rise at about the same rate, even as average surface temperatures stabilize. However, sea level rise will not be uniform across the globe. The effects of thermal expansion in the oceans will vary significantly by location.
In addition, movements of the earth that occur for natural reasons, such as tectonic movements or human activity (such as ocean rise will continue at about the same rate as the rest of the oceans for the next several centuries.
An any given location, all of these factors must be taken into account to determine the likely value of future sea level rise. Interestingly, and perhaps surprisingly, the expected net contribution from changes in the Antarctic and Greenland ice sheets is small. In a warmer world, there is more water vapor in the atmosphere, leading to more snowfall. But there is also more ablation (melt erosion) of ice at the boundaries of the ice sheets, where ice melt and iceberg calving occur during the summer months.
For Greenland, excision is greater than buildup.For both together, the net effect is about zero under current conditions, although this estimate is subject to significant uncertainties. However, if we look further into the future, major ice sheet changes could occur. vulnerable; its complete melting will lead to a sea level rise of about 7 m. Investigations with ice sheet models show that if the temperature rises by more than 3 ◦C, ablation clearly outweighs accumulation and the melting of the ice sheet begins.
A warming of 5.5 ◦C if sustained over 1,000 years is likely to cause sea levels to rise by about 3 m. The part of the Antarctic ice sheet of most concern is West Antarctica (about 90° W longitude); its decay would cause sea levels to rise by about 6 m. Because much of it is on land well below sea level, it has been suggested that rapid iron discharge could occur as the surrounding ice shelves weaken. Although the studies are far from conclusive, current models of ice dynamics do not indicate that rapid breakup is likely and suggest that the West Antarctic Ice Sheet’s contribution to sea level rise over the next millennium will be less than 3 m.