The loss of ice from mountain glaciers, Greenland, and Antarctica is contributing significantly to sea level rise. Global mean sea levels have risen by approximately 20 cm since 1900, with glacier contributions making up roughly half of this total. Thermal expansion as the oceans warm is the other main driver of sea-level rise. In the first decade of the 21st century, losses in glacier ice and the two continental ice sheets account for about two-thirds of the ca. 33 mm of sea level rise.
Of the world's 197 sovereign nations, 150 (76%) border on an ocean and are therefore directly vulnerable to the gradual swelling of the world's oceans. Sea level rise affects an even larger proportion of global infrastructure and society, as urban centers and populations are concentrated near the world's coastlines. A survey of global cities with populations exceeding 2 million reveals 98 such cities in contact with the ocean, representing a population of about 664 million people at the time of writing; millions more reside in smaller coastal settlements. The process of sea level rise is gradual, so adaptation in developed countries is broadly feasible, but at high costs. Effects are expected to be more severe in low-lying developing countries, where vulnerability to tropical cyclones is high and where populations are likely to be displaced in the coming decades.
Despite recent progress in monitoring and evaluating the rates and sources of sea level rise, future changes in sea level are notoriously difficult to forecast. A number of critical ice-dynamical processes and climate feedbacks are absent or weakly parameterized in current ice sheet models, giving them a conservative bias; they understate the range and extent of potential ice sheet response to climate change. It is therefore difficult to provide an informed estimate of the most likely or most severe sea level rise that can be expected this century. As the century progresses, this may prove to be one of the most critical aspects of cryospheric change.
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