February 25, 2016

Study: 20th century sea level rise fastest in millennia

Sea level rise is a readily apparent and physically understandable impact of global warming. The oceans warm and expand. Glaciers and ice sheets melt and add water to the oceans. So there is a basic physical intuition that says sea level rise accompanies warming periods. As much as human greenhouse gases increase temperatures, they will also contribute to sea level rise.

We often see skepticism about the historical novelty of sea level rise and the level to which human activities contribute to the observed rise (sea levels have been rising since the last ice age after all…). A new paper by Kopp et al. shows scientists’ refining understanding of both of these questions using new statistical tools to develop a long term (~3000 year) record of global mean sea level from a variety of data sources with a rigorous treatment of uncertainty.


Figure 1: New history of global mean sea level  (Image Source: Bob Kopp)

Analyzing this new product, the authors conclude with a high degree of certainty:

  • The 20th century witnessed the fastest mean sea level rise in the last 27 centuries.
  • At most, known non-human variability and temperature trends could explain half of the sea level rise observed in the 20th
  • Future sea levels will depend on CO2 emissions this century; A lower end scenario (the potentially achievable IPCC RCP4.5 scenario) would see 13 inches to 2 feet 9 inches (90 percent chance) of global sea level rise by 2100, while the highest scenario would see 20 inches to 4 feet (RCP8.5).

For the projections of the future, present energy policies will lead to CO2 concentrations somewhere between those two IPCC scenarios. Those projections also rely on a constant relationship between temperature and sea level rise, which would be upset by the increasing melting of ice shelves in Antarctica and Greenland. The study’s lead author includes a discussion of how decision makers might consider such risks in his excellent blog post on the new paper, looking at both the best available probability distribution for future increases and the best available physically motivated distributions for the ice sheet melt.

The above conclusions are not particularly surprising, given previous assessments of historical and predicted sea level rise. But they do show with increased confidence the difference between the 20th century and the few millennia that predated it. It also reveals how difficult it is to reconcile these increases in sea level with the proposition that humans have little to do with the trend.

This science is not going to go away. Conservatives should cease with the waffling and look at policy solutions.