400 ppm of CO2: How will sea-level respond?

June 13, 2013

We are currently on the eve of a world with ~400 parts per million (ppm) of atmospheric carbon dioxide (398.35 ppm as of May 2nd, Mauna Loa Observatory). How global climate, sea-level and ecosystems will respond to this level of CO2 level is a key question for global change research. Recently, Foster and Rohling (2013) looked back into Earth’s geological history to explore the relationship between atmospheric CO2 and global sea-level.

CO2 and sea-level data for teh past 40 million years based on a variety of reconstruction methods show a strong sigmoidal relationship between the two variables. This strongly supports a key role for CO2 on Earth’s climate and sea level. Using this sigmoidal model, they predict that the “likely” (68 probability) sea-level response to a 400-450 ppm CO2 is about 9 meters. However, it’s important to note that geological and geochemical CO2 and sea-level reconstruction approaches have inherited errors. The regression error near 400 ppm of CO2 is particularly large, because most of the data are from the late Miocene to Pliocene (~14 to 3 million-years ago), a time period that is less well constrained. Moreover, geological records provide an “equilibrium” response of the Earth to changing greenhouse gas forcing — a condition that can take thousands of years to establish. 


G. L. Foster, E. J. Rohling, Relationship between sea level and climate forcing by CO2 on geological timescales. Proc. Natl. Acad. Sci. 110, 1209 (2013).