Hot topics                    

Friday 30 May 2008

RAPID ROUNDUP: Climate change correction: Nature - experts respond

Scientists have spotted a large discontinuity in the record of twentieth-century global-mean surface temperature. A study in Nature this week suggests that an abrupt temperature drop in 1945 — previously interpreted to be part of a larger cooling trend — is actually due to uncorrected instrumental biases introduced when measuring temperatures at sea. The discovery solves a long-standing mystery in climate change research and will have a significant impact on the historical record of temperatures from the mid-twentieth century.

An Australian expert and several British experts comment on the research below. Feel free to use the comments below in your stories. If you would like to speak to an expert, or need a copy of the paper, please don’t hesitate to contact us on (08) 8207 7415 or by email.

Professor Neville Nicholls is in the School of Geography & Environmental Science at Monash University, and was a lead author on the most recent IPCC Synthesis Report.

"This is an important paper that reconciles a problem in the observed warming of global temperatures during the 20th century. The paper demonstrates that the very warm global ocean temperatures in the 1940s are an artefact of a change in the way ocean temperature measurements were made during WWII. This solves a problem that has bugged climate modellers for years - climate models when driven by natural and anthropogenic factors can reproduce the variations in global and regional temperature through the 20th century very well, except for warm ocean temperatures during the 1940s (e.g., see the IPCC Synthesis Report Figure SPM4). The land temperatures during the 1940s were not as warm as the ocean temperatures, and the models can simulate the land temperatures quite well at that time.

This is exciting new evidence that shows that the models are very good at climate prediction - the one period of data that they didn't seem to be doing a good job with now turns out to be an artefact."

Professor Mike Hulme, School of Environmental Sciences, UEA, Norwich, UK:

“The paper in Nature by Thompson et al. (29 May 2008) is a good example of the value of continuous and critical re-evaluation and testing of ideas, theories and data in science. The authors discover an anomalous and rapid drop in late 1945 of the global-mean sea surface temperature record and ask why was this so? They provide a convincing explanation related to the way sea surface temperature measurements were made at the time and later corrected.

“The paper should also raise the question why such an important discontinuity in this iconic index of global warming was not spotted earlier and appropriate corrections made. The answer to this question lies in observing and understanding the practices of scientific knowledge creation and the social and institutional contexts in which science operates. This new study by Thompson et al. doesn’t erode our belief that the world is getting warmer. It does tell us, however, that science loses its unique claims to valid knowledge if challenge, scrutiny and dissent are ever lost from its operating norms in favour of a cosy and unquestioning consensus.”

Dr Tim Palmer, Head of Research Division, European Centre for Medium-Range Weather Forecasts, Reading, UK.

"Our Earth has been warming over the last hundred years, and scientists believe that the principal cause is anthropogenic emissions of greenhouse gases. However, the observed global-mean temperature record does not show uniform warming. Part of the year to year variability in temperature can be linked to natural climate fluctuations such as associated with the El Nino event. Others, particularly associated with the cooling in immediate post-WW2 years may arise from pollution associated with sulphate aerosols. However, there has always been some uncertainty in whether the aerosol explanation can account for all of the post-WW2 temperature drop, much of which was indeed quite sudden.

"The authors have identified an additional factor for this sudden drop in global temperature around 1945. It is totally artificial and arises because the fraction of US to UK- based ships taking sea temperature measurements changed suddenly after the war. In those days, the method by which sea temperature was measured depended on the country of origin of the ship.

"The paper shows how difficult it is to produce fully reliable historical estimates of global mean surface temperature. Tiny biases due to the measurement method need to be identified and taken into account. Nevertheless, by identifying a new source of bias, the paper gives further support that main trends in 20th Century temperature are anthropogenic in origin."