A re-analysis of twentieth century sea level rise suggests that waters rose more slowly than previously thought. This means that the rise over the last twenty-five years is even more out of kilter with what was happening before global warming kicked in.
We now know what's happening to ocean levels with an extraordinary degree of precision, thanks to satellite measurements. However, it wasn't always like that. Sea level is affected by a range of factors, including geological change, atmospheric pressure and ocean currents. Thus, a gauge in one location can produce a very different result from one somewhere else.
Global warming has caused sea levels to rise by melting polar and alpine ice, and by causing the water already in the oceans to expand. Since 1990, that has happened at a rate of three millimeters per year.
Between 1901 and 1990, estimates from multiple gauges have been at 1.5-1.8 millimeters per year. However, a study published by Harvard scientists Dr. Carling Hay and Dr. Eric Morrow in Nature have arrived at a figure of 1.2 millimeters per year. If they are right, it means that at least 60% of the current rise is a result of what humans are doing to the atmosphere.
Moreover, Hay and Morrow believe their work has serious implications for future projections. “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."
Even if we stopped releasing greenhouse gasses today, sea levels would keep rising. One way or another, our descendants will be coping with the effects of the collapse of major glacial systems. Nevertheless, there is a lot of uncertainty as to how much time they will have to confront these consequences.
Hay and Morrow say that past estimates have been plagued by data from unrepresentative locations. "Tide gauges are located along coasts, therefore large areas of the ocean aren't being included in these estimates. And the records that do exist commonly have large gaps,” said Hay.
By adding together the known sources of ice melt and the rate of thermal expansion, Morrow and Hay were able to estimate how fast sea levels were rising at different points in time.
"We expected that we would estimate the individual contributions, and that their sum would get us back to the 1.5 to 1.8 millimeters per year that other people had predicted," Hay said. "But the math doesn't work out that way. Unfortunately, our new lower rate of sea-level rise prior to 1990 means that the sea-level acceleration that resulted in higher rates over the last 20 years is really much larger than anyone thought."