August 28, 2013

expert reaction to global surface temperatures and Pacific cooling

A study published in Nature  suggested the current hiatus in global warming is part of natural climate variability, due to recent cooling of tropical Pacific sea surface temperatures. 

Professor Piers Forster, Professor of Climate Change, University of Leeds, said:

“Their work provides further evidence that the rearrangement of heat in ocean is implicated in the slowdown.  It nicely brings together earlier work. However, it still does not provide an overall cause, as by giving their model the observed Pacific temperatures they are in some ways forcing it to have the right response. What they find is that the interaction of El Nino with the seasonal cycle partly explains the observed Northern Hemisphere winter cooling, and it is this cooling that is the main reason for the weaker global trend. We still need to look for the original cause though – and I’m sure there is one!”

Dr Richard Allan, Reader in Climate Science, Department of Meteorology, University of Reading, said:

“This new study adds further evidence that the recent slowdown in the rate of global warming at the Earth’s surface is explained by natural fluctuations in the ocean and is therefore likely to be a temporary respite from warming in response to rising concentrations of greenhouse gases. This is important since it adds to a great body of evidence in continuing to confirm the realism of projected dangerous warming in the future due to human activities such as burning fossil fuels.

“In their experiment, Kosaka and Xie artificially alter the flow of heat from the East Pacific ocean to the atmosphere above so that sea surface temperatures in this region are forced to agree with observations. This results in the realistic simulation of the recent surface warming slowdown globally and some unusual weather patterns such as drought experienced in the southern USA.

“Although in their simulation the East Pacific is being given the correct ocean temperature already, this region only covers about 8 percent of the globe. Their experiment highlights the importance of this region in determining natural climate variability from one decade to the next.

“The experiment does not show why the recent hiatus in global warming occurs, merely that this type of ocean change can explain the recent slowdown in global surface temperature rise. Free-running climate simulations (used to make future projections of climate change) are able to produce this type of variability but are not designed to predict the exact timings of these changes.

“The study is also the first to bring together key pieces of information crucial in understanding the warming hiatus of the last 15 years:

• The hiatus affects northern hemisphere winter most strongly (e.g. Cohen et al. 2012 GRL) and this is explained by the interplay between El Niño (a natural oscillation of the oceans affecting atmospheric rainfall patterns globally) and the seasonal cycle (the transfer of heat from the warm tropics to the cold polar regions is altered by El Niño and it’s adversary La Niña but mostly in winter months).
• The observed recent increases in the tropical Pacific trade winds and associated rainfall patterns also known as the Walker circulation (e.g. Sohn et al. 2013 Climate Dynamics; L’Heureux et al. 2013 Nature Climate Change). Colder waters in the East Pacific during La Niña cause stronger winds to the west which alter rainfall patterns. Observations also show that this has caused water to pile up in the western Pacific increasing sea level here slightly (e.g. Merrifield 2010 J. Climate).
• Some regional climate anomalies are reproduced by the simulations (e.g. drought in the southern USA but generally not weather patterns over Europe or Russia which are affected to a lesser degree by the tropical Pacific).

“Rearrangement of heating in the ocean is not dealt with in this study but this work provides more evidence that the recent slowdown in global surface warming is linked to changes in the Pacific, particularly the upper few hundred metres rather than the deep ocean below 1000m. This is the subject of much research, for example the DEEP-C project funded by the UK Natural Environment Research Council.

“Observations show that the planet is continuing to heat up beneath the ocean surface (e.g. Loeb et al. 2012, Nature Geosciences) at a rate equivalent to continuous heating by over 150 billion 2 kilo-Watt kettles. The recent slowdown in global surface warming is explained by natural fluctuations in the Pacific ocean which have caused the additional heating from rising concentrations of greenhouse gases to temporarily affect the layers below the ocean surface rather than warming surface temperatures.”

More information on the DEEP-C website.

SMC briefing notes on recent slowdown in global temperature rise.

‘Recent global-warming hiatus tied to equatorial Pacific surface cooling’ by Yu Kosaka & Shang-Ping Xie, published in Nature on Wednesday 28 August 2013.