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 if current climate trends continue.
Findings published in the journal Science showed that the seas rose in response to melting ice sheets in Greenland and Antarctica, said lead author Andrea Dutton, a University of Florida geochemist.
“This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate,” she said.
Dutton and an international team of scientists assessed evidence of higher sea levels during several periods to understand how polar ice sheets respond to warming. Combining computer models and observations from the geologic record, 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.
“As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond,” she said. “While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades.”
The researchers 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.
The researchers also sought to understand how quickly sea level rose and which ice sheets may be most susceptible. They 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.
From the AAAS website:
Sea-level rise due to polar ice-sheet mass loss during past warm periods
A. Dutton1,*,A. E. Carlson2, A. J. Long3, G. A. Milne4, P. U. Clark2, R. DeConto5, B. P. Horton6,7, S. Rahmstorf8, M. E. Raymo9
Although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level (GMSL) rise over the last century, mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions to GMSL rise under future warming. To better constrain polar ice-sheet response to warmer temperatures, we draw on evidence from interglacial periods in the geologic record that experienced warmer polar temperatures and higher GMSLs than present. Coastal records of sea level from these previous warm periods demonstrate geographic variability because of the influence of several geophysical processes that operate across a range of magnitudes and time scales. Inferring GMSL and ice-volume changes from these reconstructions is nontrivial and generally requires the use of geophysical models.
Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise. Advances in our understanding of polar ice-sheet response to warmer climates have been made through an increase in the number and geographic distribution of sea-level reconstructions, better ice-sheet constraints, and the recognition that several geophysical processes cause spatially complex patterns in sea level. In particular, accounting for glacial isostatic processes helps to decipher spatial variability in coastal sea-level records and has reconciled a number of site-specific sea-level reconstructions for warm periods that have occurred within the past several hundred thousand years. This enables us to infer that during recent interglacial periods, small increases in global mean temperature and just a few degrees of polar warming relative to the preindustrial period resulted in ≥6 m of GMSL rise. Mantle-driven dynamic topography introduces large uncertainties on longer time scales, affecting reconstructions for time periods such as the Pliocene (~3 million years ago), when atmospheric CO2 was ~400 parts per million (ppm), similar to that of the present. Both modeling and field evidence suggest that polar ice sheets were smaller during this time period, but because dynamic topography can cause tens of meters of vertical displacement at Earth’s surface on million-year time scales and uncertainty in model predictions of this signal are large, it is currently not possible to make a precise estimate of peak GMSL during the Pliocene.
Our present climate is warming to a level associated with significant polar ice-sheet loss in the past, but a number of challenges remain to further constrain ice-sheet sensitivity to climate change using paleo–sea level records. Improving our understanding of rates of GMSL rise due to polar ice-mass loss is perhaps the most societally relevant information the paleorecord can provide, yet robust estimates of rates of GMSL rise associated with polar ice-sheet retreat and/or collapse remain a weakness in existing sea-level reconstructions. Improving existing magnitudes, rates, and sources of GMSL rise will require a better (global) distribution of sea-level reconstructions with high temporal resolution and precise elevations and should include sites close to present and former ice sheets. Translating such sea-level data into a robust GMSL signal demands integration with geophysical models, which in turn can be tested through improved spatial and temporal sampling of coastal records.
Further development is needed to refine estimates of past sea level from geochemical proxies. In particular, paired oxygen isotope and Mg/Ca data are currently unable to provide confident, quantitative estimates of peak sea level during these past warm periods. In some GMSL reconstructions, polar ice-sheet retreat is inferred from the total GMSL budget, but identifying the specific ice-sheet sources is currently hindered by limited field evidence at high latitudes. Given the paucity of such data, emerging geochemical and geophysical techniques show promise for identifying the sectors of the ice sheets that were most vulnerable to collapse in the past and perhaps will be again in the future.