Ice sheets in Greenland and Antarctica, whose melt contributed to the last rise, are melting again

Last Time it was This Warm, Sea Levels were 20 Feet Higher, Study says

Antony Long, Photo. Icebergs near Timiamuit, in southeastern Greenland.
Rutgers Today

When past temperatures were similar to or slightly higher than the present global average, sea levels rose at least 20 feet, suggesting a similar outcome could be in store century unless there is a substantial reduction in greenhouse gas emissions.

Findings published in the journal Science showed that the seas rose in response to melting ice sheets in Greenland and Antarctica

“Present climate is warming to a level associated with significant polar ice-sheet loss in the past” said Rutgers oceanographer Benjamin Horton, one of study’s authors.

In the new study, researchers assessed evidence of higher sea levels the past 3 million years to understand how polar ice sheets respond to warming. Using observations from the geologic record, supported by computer modeling, they found that during past periods with average temperatures 1 to 3 °C (1.8 to 5.4 °F) warmer than preindustrial levels, sea level peaked at least 20 feet higher than today.

“This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate,” said lead author Andrea Dutton, a University of Florida geochemist.

Dutton, Horton and an international team of scientists concluded that sea levels rose 20 to 30 feet higher than present about 125,000 years ago, when global average temperature was 1°C higher than preindustrial levels (similar to today’s average). Sea level peaked somewhere between 20 and 40 feet above present during an earlier warm period about 400,000 years ago, when global average temperatures are less certain, but estimated to be about 1 to 2°C warmer than the preindustrial average. During those times, atmospheric carbon dioxide levels peaked around 280 parts per million, but today’s levels are around 400 ppm and rising. The team of researchers looked at the last time period when carbon dioxide was this high – about 3 million years ago – but couldn’t get a confident estimate on sea-level rise, in part due to land motion that has distorted the position of past shorelines.

“We are beginning to understand the magnitude that sea level rose in the past and which ice sheets may be responsible” said Horton, a professor of marine and coastal sciences in the School of Environmental and Biological Sciences. The researchers acknowledged that the rate of sea-level rise associated with polar ice sheet retreat is not well known, and that this is an important target for future research. Developing a better sense of which ice sheet sectors were most susceptible in the past, as well as how quickly this process occurs, could inform how policymakers plan for and mitigate sea-level change.

“Scenarios of future rise are dependent upon understanding the response of sea level to climate changes. Accurate estimates of past sea-level variability provide a context for such projections,” said Horton.


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