Climate sceptics confuse the public by focusing on short-term fluctuations

Climate sceptics confuse the public by focusing onshort-term fluctuations

Stefan Rahmstorf: Bj?rn Lomborg denies data that sea levelsare rising faster than expected with no sign of slowing down

As a lead author of the last IPCCreport, I find it gratifying that Bj?rn Lomborg sings the praise of the 'careful work' of the 'hugelyrespected' IPCC. However, Lomborg misrepresents what we wrotein the report. It did not conclude that sea level will stay within the boundsof 18-59 cm by 2100. Rather, effects of sliding ice will come on top of this,which are too hard to predict to give an upper limit. So the IPCC forecast is18-59 cm plus an unknown extra rise.

The IPCC report also found that during 1961-2003, sea levelhas risen 50% faster (1.8 mm/year) than projected by models (1.2 mm/year). Thisis not a 'minority view' but the IPCC's. In a separate study in Science wefurther compared projections (which started in 1990) of the previous IPCCreport with observed data (up to 2006) and got a similar result.

Graph showing sea level rises


We have updated our study with the latest sea level data upto the end of 2008 (see graph above) — sea level continues to rise faster thanexpected with no sign of slowing down.

The red curve shows annual sea level since 1970 based ontide gauges (the thick red line is a non-linear trend). The blue curve showssatellite sea level from 1993 to the end of 2008; the thick blue line is thelinear trend (3.2 mm/year) over this period. For comparison, the projections ofthe 2001 IPCC report are shown (dashed lines and grey uncertainty range).Lomborg, maintains that this rise is 'better than expected', pointingto the brief downward blip in 2006 and claiming to see a slowdown since 2005. (Figureupdated from Rahmstorf et al, Science 2007)

Lomborg argues that 18 years could be too short for a robusttrend comparison because of decadal variations in trend – but the 42-yearperiod analysed by IPCC yields the same result. And it is telling that he thengoes on to draw an 'inescapable' conclusion about a slow-down of sealevel rise from just four years of data. This is another well-worn debatingtrick: confuse the public about the underlying trend by focusing on short-termfluctuations. It's like claiming spring won'tcome if there is a brief cold snap in April.

Why does Lomborg cite the trend since 2005? Last October,he cited that of the previous two years. Why now four years? Becausethe trend of the past two years (2007-2008) is now + 3.7 mm/year? It is evenworse. The trend since the beginning of anyyear of the data series varies between 1.6 mm/year and 9.0 mm/year,depending on the start year chosen. Using 2005, Lomborg cherry-picked the byfar lowest. He's done this before, see for example his recentclaim that the globe is cooling.

Finally, Lomborg says that the Copenhagen Climate Congress this week is'an effort by a group of scientists and activists to cast aside theconsensus view of thousands of scientists from the IPCC'. Let's do a factcheck. The Congress is organised by a 'star alliance' of globaluniversities: Copenhagen, Yale, Berkeley, Oxford, Cambridge, Tokyo, Peking – toname some. On the opening day tomorrow, RajendraPachauri, chairman of the IPCC will give a plenary lecture. Sciencehas progressed since the cut-off date for research to be included in the lastIPCC report. We have three years of new results to discuss, including newmeasurements on ice flow and sea level. In total, over 1,600 research papersand posters will be presented by scientists including leading IPCC authors fromover 70 countries - the list reads like a 'Who's Who' of climatescience.

That Lomborg tries to pass off the biggest climate sciencecongress of the year as 'political activism of a few' is hilarious.

• Stefan Rahmstorf is a climate scientist and oceanographerat the Potsdam Institute for Climate ImpactResearch. He has just published the popular science paperback Our Threatened Oceans, together with marine biologist Katherine Richardson and willpresent latest data on sea level rise at the Copenhagen Climate Congress. This articleis a response to a comment pieceby Bj?rn Lomborg.