New paper finds sea level changes since 1950 have been due to natural variability

A paper published today in Climate of the Past finds that sea level changes in the tropical Pacific during the period 1950-2009 are explained due to natural changes in the cycle of El Ninos and La Ninas [called "internal variability"]. The authors state, "While our analysis cannot rule out any influence of anthropogenic forcing [man-made 'greenhouse' gases], it concludes that the latter effect in that particular region is still hardly detectable." The paper finds no evidence of accelerating sea level rise and instead shows the running 17 year average rise has decelerated over the past ~59 years.

Middle graph shows rate of sea level change [running 17 year average] has declined over the past 59 years and is highly correlated to the natural variability of El Nino and La Nina cycles.

Clim. Past, 8, 787-802, 2012
www.clim-past.net/8/787/2012/
doi:10.5194/cp-8-787-2012
Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?

B. Meyssignac^1,2, D. Salas y Melia⁴, M. Becker^1,3,*, W. Llovel⁵, and A. Cazenave^1,2
1Université de Toulouse, UPS (OMP-PCA), LEGOS, UMR5566, 14 Av Edouard Belin, 31400 Toulouse, France
²CNES, LEGOS, UMR5566, 18 avenue Edouard Belin 31 401 Toulouse, France
³CNRS, LEGOS, UMR5566, 14 av Edouard Belin, 31400 Toulouse, France
⁴Météo-France CNRM/GMGEC CNRS/GAME, 31000 Toulouse, France
⁵JPL, California Institute of Technology, Pasadena, California, USA
^*now at: ESPACE-DEV/UAG, UMR228, Cayenne, French Guiana, France


Abstract. In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is still hardly detectable.

Final Revised Paper (PDF, 17519 KB)   Discussion Paper (CPD)