Seas Are Rising at Fastest Rate in Last 28 Centuries
We have known for a long time that sea level is rising. The link to global warming has been both intuitive and evident.
Until now no study has extended the links all the way from burning fossil fuels, to global warming, to sea level rise, to coastal floods that were caused unambiguously by us.
The result is a complex mosaic with different rates of sea level rise in different places. Since 1950, for example, along the U.S. seaboard we’ve seen parts of Chesapeake Bay rise by a foot, half that in Boston, and half that again in Honolulu. Amidst all this noise, new research led by Robert Kopp of Rutgers University, has extracted an essential signal: the amount of global sea level rise that has come from us.
In our report, we took a simple approach. We subtracted yearly estimates for human-caused global sea level rise based on Kopp’s paper, from hourly water level records at 27 tide gauges around the United States. Then we compared how many days the water level exceeded the local threshold for nuisance flooding — with or without our subtractions.
Nuisance flooding is flooding that closes coastal area roads, overwhelms storm drains, and compromises infrastructure. It doesn’t wreck your home, but it could make it hard to get to work, or even to flush the toilet. The National Weather Service defines local nuisance flood thresholds based on decades of observing local impacts.
From 1950 through 2014, out of the 8,726 actual nuisance flood days that our analysis identified, 5,809 of them — two-thirds — would not have taken place if you remove Kopp et al.’s central estimate of human-caused global sea level rise. Even using a low estimate — one more than 95 percent likely to be too low — more than 3,500 of the flood days would not have taken place.
We present the first, to our knowledge, estimate of global sea-level (GSL) change over the last ∼3,000 years that is based upon statistical synthesis of a global database of regional sea-level reconstructions. GSL varied by ∼±8 cm over the pre-Industrial Common Era, with a notable decline over 1000–1400 CE coinciding with ∼0.2 °C of global cooling. The 20th century rise was extremely likely faster than during any of the 27 previous centuries. Semiempirical modeling indicates that, without global warming, GSL in the 20th century very likely would have risen by between −3 cm and +7 cm, rather than the ∼14 cm observed. Semiempirical 21st century projections largely reconcile differences between Intergovernmental Panel on Climate Change projections and semiempirical models.