Monday, 16 June 2014

Did ESA say our data are "lousy"? [Answer: No]

Talking last Friday at the Royal Society (London) about ESA's Climate Change Initiative programme was an interesting experience. I gave a presentation on the title of "Ocean Warming". The idea of the talk was to argue that signals in data from the CCI teams are consistent with an "oceanic heat burial" hypothesis published in March this year.  This is the idea that surface temperatures during the 2000s have been fairly level (see Figure 1 below), but at the same time, the ocean overall has continued to gain heat because of greenhouse gas forcing of climate. This apparent contradiction is resolved by changes in tropical circulation that have 'hidden' heat below the ocean surface over the past decade. Patterns consistent with the associated circulation changes can be seen in sea surface temperature, ocean colour and sea level data from CCI.

Figure 1. The measurements from space (red line) are SST CCI data, which show the same year-to-year picture of sea surface temperature changes as the in situ only data from the Hadley Centre in blue. Within the next 3 years of the project, we aim to extend this time series at both ends.

There were other talks, too, including a very positive introduction by the Rt Hon David Willetts (Minister for Universities and Science), one on the cryosphere as seen in CCI data by Andrew Shepherd, and talks by representatives of ESA. 

So, it is interesting what the press picked up. The Guardian have published an article following the meeting with the headline Apparent pause in global warming blamed on 'lousy' data. Within the article it says:

Now, Stephen Briggs from the European Space Agency's Directorate of Earth Observation says that sea surface temperature data is the worst indicator of global climate that can be used, describing it as "lousy".

If you read it quickly, you might think that ESA meant our SST data are "lousy"!

In fact, the point being made was that the energy required to warm just the ocean surface and the surface air temperature is a tiny part of the total energy that the Earth is gaining because of greenhouse gas forcing of climate (see Figure 2). Turning it round the other way, this means that variations in the rate of surface temperature change do not necessarily imply that the Earth has stopped gaining heat. The heat can still be going into other, much more dominant, components of the climate. This is scientifically correct, and indeed, my talk showed a specific example of that. (There will inevitably be year-to-year, decade-to-decade variability in surface temperatures -- weather doesn't stop because of global warming.) 

So, Stephen Briggs' words did not mean he thought our SST data were terrible, despite the impression given by the headline!

In my view, there are many compelling reasons to use surface temperature as an indicator to describe global climate. People experience temperature directly (albeit, subjectively), it is relevant to human comfort and health, temperature (with wind) drives evaporation, it is relevant to agriculture (on land) and fisheries (at sea), we have instrumental records of temperature going back over one hundred and fifty years, etc.

In contrast, the total energy gain in the climate system would seem rather remote to most people, I expect -- although it would be great if everyone understood physics and climate science sufficiently well to grasp its significance.

Figure 2. Analysis of heat content of climate system, from IPCC AR5 WG1 Ch3 Box 3.1.

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