Sea-level rise by 2100.

IN HIS NEWS AND ANALYSIS PIECE REPORTING ON THE NEWLY RELEASED FIFTH ASSESSMENT report (AR5) by Working Group I (WGI) of the Intergovernmental Panel on Climate Change (IPCC) (“A Stronger IPCC Report,” 4 October, p. 23), R. A. Kerr highlights three fundamental conclusions about climate change that were assessed with equal or greater confi dence than in previous IPCC reports. He also points to three “contentious points” on which he states that the AR5 “took a moderate line.” Kerr includes sea-level projections among these points, and reports “a rise of 40 to 60 centimeters by late in the century and a worst case of 1 meter by 2100, [which is] higher than in 2007 but far below the meter or two of sea-level rise that some expect.” As the authors of the IPCC WGI AR5 chapter on “Sea-Level Change,” we wish to clarify that for the highest emission scenario considered (RCP8.5), the AR5 reported a “likely” range of 0.45 to 0.82 m for sea-level projections for the late 21st century (average over 2081 to 2100) and of 0.52 to 0.98 m by 2100. The difference in sea level between these two periods is large because in 2081 to 2100, the “likely” rate of rise is 8 to 16 mm per year, which is up to about 10 times the average rate of rise during the 20th century. In the calibrated uncertainty language of the IPCC, this assessed likelihood means that there is roughly a one-third probability that sea-level rise by 2100 may lie outside the “likely” range. That is, the AR5 did not exclude the possibility of higher sea levels. However, we concluded that sea levels substantially higher than the “likely” range would only occur in the 21st century if the sections of the Antarctic ice sheet that have bases below sea level were to collapse. We determined with medium confi dence that “this additional contribution would not exceed several 10ths of a meter of sea-level rise during the 21st century.” We could not defi ne this possible contribution more precisely because “there is currently insuffi cient evidence to evaluate the probability of specifi c levels above the assessed ‘likely’ range.” The upper boundary of the AR5 “likely” range should not be misconstrued as a worst-case upper limit, as was done in Kerr’s story as well as elsewhere in the media and blogosphere. For policy and planning purposes, it may be necessary to adopt particular numbers as an upper limit, but according to our assessment, the current state of scientifi c knowledge cannot give a precise guide. JOHN A. CHURCH,* PETER U. CLARK, ANNY CAZENAVE, JONATHAN M. GREGORY, SVETLANA JEVREJEVA, ANDERS LEVERMANN, MARK A. MERRIFIELD, GLENN A. MILNE, R. STEVEN NEREM, PATRICK D. NUNN, ANTONY J. PAYNE, W. TAD PFEFFER, DETLEF STAMMER, ALAKKAT S. UNNIKRISHNAN CSIRO Marine and Atmospheric Research, Hobart, TAS, 7001, Australia. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA. Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, 31400, Toulouse, France. Met Offi ce Hadley Centre, Exeter, EX1 3PB, UK. National Oceanography Centre, Liverpool, L3 5DA, UK. Potsdam Institute for Climate Impact Research, 14473, Potsdam, Germany. Department of Oceanography, University of Hawaii at Manoa, Honolulu, HI 96822, USA. Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada. Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309, USA. School of Behavioural, Cognitive, and Social Sciences, University of New England, Armidale, NSW 2351, Australia. School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, UK. Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO 80309, USA. Oceanography and Remote Sensing of the Earth System, University of Hamburg, 20146, Hamburg, Germany. National Institute of Oceanography, Goa, 403004, India.