They found that sea level was relatively stable from 200 B.C. to 1,000 A.D. During a warm climate period beginning in the 11th century referred to as both the Medieval Climate Anomaly and the Medieval Warming Period, sea level rose by about half a millimeter per year for 400 years. There was then a second period of stable sea level associated with a cooler period, called the Little Ice Age, which persisted until the late 19th century. Since then, average sea level has risen by more than 2 millimeters per year, the steepest rate for more than 2,100 years.
"Prior to the past few decades there was no obvious contribution from melting ice sheets," said Michael Mann, professor of meteorology at Penn State. "It is only over the past five years or so that we have clear evidence that the ice sheets are losing mass. Prior to that they appear to have been stable as far back as the end of the last ice age."
To reconstruct sea level, the research team used microfossils called foraminifera preserved in sediment cores from coastal salt marshes in North Carolina (see Salt marsh sediments help gauge climate-change-induced sea level rise for details on that aspect) . The age of these cores was estimated using radiocarbon dating and several complementary techniques.
To ensure the validity of their approach, the team members confirmed their reconstructions against tide-gauge measurements from North Carolina for the past 80 years and global tide-gauge records for the past 300 years. A second reconstruction from Massachusetts confirmed their findings. The records were also corrected for contributions to sea-level rise made by vertical land movements.
The team's research shows that the reconstructed changes in sea level during the past millennium are consistent with past global temperatures and can be described using a model relating the rate of sea-level rise to global temperature.
"The data from the past help to calibrate our model and will improve sea-level rise projections under scenarios of future temperature rise," research team member Stefan Rahmstorf said.
"One of the largest uncertainties in projecting the impacts of climate change involve predicting the amount and rate of future sea level rise," said Mann. "The societal ramifications are as great as any climate change impact, but, because the uncertainties are particularly large due to limitations in the representations of some key processes, such as ice sheet collapse, in existing models, we still do not know how sea level will rise."
Citation: Andrew C. Kemp, Benjamin P. Horton, Jeffrey P. Donnelly, Michael E. Mann, Martin Vermeer, and Stefan Rahmstorf, 'Climate related sea-level variations over the past two millennia', PNAS 2011 ; published ahead of print June 20, 2011, doi:10.1073/pnas.1015619108 (FREE to read)
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