August 21, 2014

expert reaction to new research on ocean heat sinks and global average temperatures

A report in the journal Science investigated a link between a hiatus in global average surface temperatures and heat storage within oceans.

Dr Emily Shuckburgh, Head of Open Oceans at the British Antarctic Survey and on secondment to the Department of Energy and Climate Change, said:

“There is a conundrum as to why global-average surface temperature has changed little in recent years while greenhouse gas emissions have increased.  Observations indicate that more energy is coming into the top of the atmosphere than is going out, implying the atmosphere-ocean system as a whole must be accumulating heat away from the surface, and the open questions are ‘where?’ and ‘how?’

“This paper describes a natural cycle where heat is shifted into and out of the deep ocean in the Atlantic and Southern oceans, with the switch between storage and release happening after about 20 to 35 years. The suggestion is that we are presently in a “storage” phase of this cycle, but in due course we will switch to a “release” phase and it is expected that we will then experience another episode of accelerated surface warming.

“The recent hiatus in surface temperature increase is leading us to gain fascinating insight into how the natural cycles of the atmosphere and ocean work and how these cycles interact with the trends driven by greenhouse gas emissions.”

Prof Richard Allan, Professor of Climate Science at the University of Reading, said:

“The research by Tung and Chen re-emphasises the role of gradual fluctuations in our vast oceans in explaining the slowing in surface warming since around 2000, but it shifts the focus from the Pacific Ocean to the Atlantic. Although it is human nature to seek a single cause for notable events, in reality the complexity of the climate system means that there is not one simple explanation for a decade of unusual climatic conditions.

“Over the 21st century the many natural influences will in some periods reduce surface warming from rising greenhouse gas concentrations, as in the recent 15 year period; but in other decades they will compound the heating effect resulting in periods of rapid climate change.”

Prof Joanna Haigh FRS, Professor of Atmospheric Physics at Imperial College London, said:

“This careful analysis offers new insight into the distribution within the oceans of the extra heat trapped in the climate system by human-produced greenhouse gases. It demonstrates that global mean surface air temperature is bound to meander on multi-decadal timescales around its inevitable upward trend.”

Prof Andrew Watson FRS, Royal Society Research Professor at the University of Exeter, said:

“On first reading, my impression is that it is a credible study based on the best available data, However, all these analyses of ocean heat content are interpreting small changes in ocean temperature, and it will need to be picked over and repeated by others before being fully accepted.

“It will be very interesting to see whether their finding that during the last decade the heat has penetrated to depth mostly in the Southern and Atlantic Oceans stands up. It does make oceanographic sense however, because we know these are the major sites for deeper water formation — water from the surface Pacific does not penetrate nearly so deeply into the ocean.”

Prof Piers Forster, Professor of Climate Change at the University of Leeds, said:

“The hiatus has a really interesting morphology. This paper suggests that heat disappearing into the depths of the Atlantic and Southern oceans are the dominant cause. Their ideas seem fine but I’m also convinced there is more going on: the El Nino and relative cooler European and Asian winters remain important aspects to understand.

“Most importantly, this paper is another nail in the coffin of the idea that the hiatus is evidence that our projections of long term climate change need revising down. Variability in the ocean will not affect long-term climate trends but may mean we have a period of accelerated warming to look forward to.”

Prof David Lee, Director of the Centre for Aviation, Transport, and the Environment (CATE) at Manchester Metropolitan University, said:

“To me, the simple conclusion is that in a sense, the ‘surprise’ is ‘no surprise’.  Climate is a complex system, and while we know the basics of it, our understanding continues to be refined. However, the evidence presented by this paper is that the temperature hiatus is not a justification for complacency on CO2 emissions; rather the opposite, bearing out the simpler projections of increased temperatures by the end of the century on current emission trajectories.”

‘Varying planetary heat sink led to global-warming slowdown and acceleration’ by Xianyao Chen and Ka-Kit Tung published in Science at 6pm UK time on Thursday 21st August.

All our previous output on this subject can be seen at this weblink: http://www.sciencemediacentre.org/?s=slowdown&cat=

Declared interests

Prof Lee’s research (largely on aviation/climate) is funding by the UK Department for Transport, and the European Commission. He represents the UK in technical bodies that advise the International Civil Aviation Organization