The acceleration in global sea level from the 20th century to the last two decades has been significantly larger than scientists previously thought, according to a new Harvard study.
The study, co-authored by Carling Hay, a post-doctoral fellow in the Department of Earth and Planetary Sciences (EPS), and Eric Morrow, a recent PhD graduate of EPS, shows that previous estimates of global sea-level rise from 1900-1990 had been over-estimated by as much as 30 percent.
The report, however, confirms previous estimates of sea-level change since 1990, suggesting that the rate of sea-level change is increasing more quickly than previously believed.
“What this paper shows is that sea-level acceleration over the past century has been greater than had been estimated by others,” Morrow said. “It’s a larger problem than we initially thought.”
“Scientists now believe that most of the world’s ice sheets and mountain glaciers are melting in response to rising temperatures.” Hay added. “Melting ice sheets cause global mean sea level to rise. Understanding this contribution is critical in a warming world.”
Previous estimates had placed sea-level rise at between 1.5 and 1.8 millimeters annually over the 20th century. Hay and Morrow, however, suggest that from 1901 until 1990, the figure was closer to 1.2 millimeters per year. But everyone agrees that global sea level has risen by about 3 millimeters annually since that time, and so the new study points to a larger acceleration in global sea level.
“Another concern with this is that many efforts to project sea-level change into the future use estimates of sea level over the time period from 1900 to 1990,” Morrow said. “If we’ve been over-estimating the sea-level change during that period, it means that these models are not calibrated appropriately, and that calls into question the accuracy of projections out to the end of the 21st century.”
To obtain their improved estimate of 20th century global sea level, Hay and Morrow approached the challenge of estimating sea-level rise from a completely new perspective.
Typically, Hay said, estimates of sea-level rise are created by dividing the world’s oceans into sub-regions, and gathering records from tide gauges – essentially yard-sticks used to measure ocean tides – from each area. Using records that contain the most complete data, researchers average them together to create estimates of sea level for each region, then average those rates together to create a global estimate.
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Sea level rise image via Shutterstock.