This article was originally published at The Conversation. The publication contributed the article to Live Science's Expert Voices: Op-Ed & Insights.
A recent high-profile study led by US climatologist James Hansen has warned that sea levels could rise by several meters by the end of this century. How realistic is this scenario?
We can certainly say that sea levels are rising at an accelerating rate, after several millennia of relative stability. The question is how far and how fast they will go, compared with Earth's previous history of major sea-level changes.
Seas have already risen by more than 20 cm since 1880, affecting coastal environments around the world. Since 1993, sea level has been rising faster still (see chapter 3 here), at about 3 mm per year (30 cm per century).
One key to understanding future sea levels is to look to the past. The prehistoric record clearly shows that sea level was higher in past warmer climates. The best evidence comes from the most recent interglacial period (129,000 to 116,000 years ago), when sea level was 5-10 m higher than today, and high-latitude temperatures were at least 2℃ warmer than at present.
The two largest contributions to the observed rise since 1900 are thermal expansion of the oceans, and the loss of ice from glaciers. Water stored on land (in lakes, reservoirs and aquifers) has also made a small contribution. Satellite observations and models suggest that the amount of sea-level rise due to the Greenland and Antarctic ice sheets has increased since the early 1990s.
Before then, their contributions are not well known but they are unlikely to have contributed more than 20 percent of the observed rise.
Together, these contributions provide a reasonable explanation of the observed 20th-century sea-level rise.
Future rise
The Intergovernmental Panel on Climate Change (IPCC) projections (see chapter 13 here) forecast a sea-level rise of 52-98 cm by 2100 if greenhouse emissions continue to grow, or of 28-61 cm if emissions are strongly curbed.
The majority of this rise is likely to come from three sources: increased ocean expansion; glacier melt; and surface melting from the Greenland ice sheet. These factors will probably be offset to an extent by a small increase in snowfall over Antarctica.
With continued emissions growth, it is entirely possible that the overall rate of sea-level rise could reach 1 m per century by 2100 — a rate not seen since the last global ice-sheet melting event, roughly 10,000 years ago.
Beyond 2100, seas will continue to rise for many centuries, perhaps even millennia. With continued growth in emissions, the IPCC has projected a rise of as much as 7 m by 2500, but also warned that the available ice-sheet models may underestimate Antarctica's future contribution.
The joker in the pack is what could happen to the flow of ice from the Antarctic ice sheet directly into the ocean. The IPCC estimated that this could contribute about 20 cm of sea-level rise this century. But it also recognised the possibility of an additional rise of several tens of centimeters this century if the ice sheet became rapidly destabilised.
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