Expert reaction to Nature paper on sea surface temperature

SMC UK: New research on the cooling of the Earth’s surface that occurred in the 1970s suggests that the culprit might in fact have been the rapid cooling oceans in the Northern Hemisphere rather than the increase in sulphate aerosols from pollution and volcanic eruptions which have traditionally been blamed for this hiatus in global warming.

Our colleagues at the Science Media Centre in London gathered reaction to the Nature paper on sea surface temperature*

Professor Gabi Hegerl, Professor of Climate System Science at the University of Edinburgh, said:

“This research draws attention to the interesting variability in the North Atlantic. The findings show that climate variability can cause quite rapid changes, and that the ocean may be involved.

“In my opinion, this research does not question our current interpretation of the overall causes of 20th century warming – the assessment that greenhouse gas forcing contributed likely more than half of the warming since 1950 accounts for climate variability such as seen here.

“Therefore, estimates of aerosol forcing from the changes observed also account for the variability in climate, enhancing or mimicking part of the pattern expected from aerosols.

“The event described in the paper has a really interesting parallel to warming seen in the early 20th century. Then, as in the event discussed in the paper, we probably had North Atlantic SST changes and a contribution from external drivers.  Both are important in explaining the findings.”

Professor Mark Maslin, Director of the Environment Institute at University College London, said:

“David Thompson and colleagues have published an important paper which adds to our understanding of global warming but provides a stark warning.  They confirm yet again the long term warming trend over the last 100 years, which we know is due to increased greenhouse gases in the atmosphere.

“What they have done is added a deeper understanding of the brief cooling phase that occurred in the early 1970s.  Most of the cooling observed in the global temperature records, it seems, is driven by a drop in ocean temperatures.  Though all the oceans seem to have cooled it is most pronounced in the North Atlantic Ocean.  So this early 1970s cooling, instead of being due to increased sulphate aerosols or decadal cycles in ocean circulation, was probably caused by the rapid freshening of the North Atlantic during the so called ‘great salinity anomaly’.

“This paper is important in that it shows how sensitive the climate of the Northern hemisphere is to the salinity balance in the North Atlantic ocean, providing a stark warning of what may happen in the future with larger releases of freshwater into the North Atlantic.  Thompson and colleagues also show that these events may briefly interrupt the century-long warming trend; but they have had no long term effect on the continued rise in global temperatures.”

Professor Andrew Watkinson, Director of the Living With Environmental Change programme, said:

“Whilst the human fingerprint on climate warming over the 20th century is clear, there is still much to learn about reasons for global and regional variations in temperature.  The reasons for short term changes on the scale of less than a decade are, in particular, often difficult to attribute to specific causes.

“Here we see new methodologies being used to show that mid-century differences in temperature change between the northern and southern hemispheres are due to a discrete cooling event in the northern oceans.   This highlights the importance of understanding short-term, regional fluctuations in sea-surface temperature, as ocean/atmosphere interactions may drive rapid change.”

* An abrupt drop in Northern Hemisphere sea surface temperature around 1970, Nature 467:7314, September 23 2010