A widely reported "pause" in global warming may be an artefact of scientists looking at the wrong data, says a climate scientist at the European Space Agency.
Global average sea surface temperatures rose rapidly from the 1970s but have been relatively flat for the past 15 years. This has prompted speculation from some quarters that global warming has stalled.
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".
"It is like looking at the last hair on the tail of a dog and trying to decide what breed it is," he said on Friday at the Royal Society in London.
Climate scientists have been arguing for some time that the lack of warming of the sea surface is due to most of the extra heat being taken up by the deep ocean. A better measure, he said, was to look at the average rise in sea levels. The oceans store the vast majority of the climate's heat energy. Increases in this stored energy translate into sea level rises.
"The sea level shows us the engine of global climate not one of the consequences," said Briggs.
In the past 50 years, sea levels indicated that the stored energy had increased by 250 zetajoules, he said. A zetajoule is 1021 joules. For comparison, mankind generates 0.5 zetajoules of energy every year in its power stations.
Since 1993, satellites have measured sea levels rising by an average of 3mm per year. Unlike the surface temperature, this rise continued throughout the supposed pause in global warming.
Christopher Merchant at the University of Reading has been working to understand why the increase in the stored energy has not translated into an increase in sea surface temperature.
"There are a number of contributions and a picture is emerging," he says. Those contributions include the cooling effect of aerosols from Asian industrialisation, natural variability in the climate system and solar variability.
In March, climate scientists identified another potentially important contribution. The trade winds across the Pacific have strengthened in the past decade, which could be helping to drive a deep circulation of water that traps heat in the depths of the ocean, leaving the surface relatively unaffected for now.
Scientists are now trying to simulate the behaviour using computer models. This is difficult because the behaviour of the deep ocean is too poorly known to be reliably included.
"The models don't have the skill we thought they had. That's the problem," said Peter Jan van Leeuwen, director of the National Centre of Earth Observation at the University of Reading.
Building reliable computer models is depdendent on knowing which parameters are important to the climate, and over the past decade scientists have homed in on the key parameters of the Earth's system.
They call these the Essential Climate Variables or ECVs. There are 50 of them and they include measurable quantities such as air pressure and temperature, sea ice cover, ocean colour, ozone and carbon dioxide content.
Another problem in building computer models has been knowing how to compare measurements taken by different satellites. Instruments and techniques change and each has its own quirks. So, even if two instruments have measured the same property, it can be like comparing apples with oranges.
The Esa Climate Change Initiative (CCI) is a €75m programme, active since 2009, to address this and produce a trustworthy set of ECV data that can be accessed by all. It has transformed the entire 30-year-old data archives of Esa and its member states' Earth observation satellites into a single, calibrated data set.
Ruth Boumphrey, head of Earth observation at the UK Space Agency, said, "CCI allows us to put trust into our climate observations. This is a paradigm shift for how we look at our planet. Even if we don't agree on the interpretation of the data, we can now all agree on what has been measured."
Smartphone apps for scientists and the general public will be released in the coming weeks to allow them to access this data. In addition, a workshop for app developers is taking place at Esa's Esrin centre, Italy, in September to develop new ideas and concepts for other apps that use Esa's satellite Earth observation data.
"The data from CCI will really help us build better models of the Earth's climate," said van Leeuwen.
Esa operates a fleet of Earth observation satellites. The latest, Sentinel-1, was launched on 4 April 2014 and is the first satellite in the Copernicus Earth observation programme. To foster trust and transparency in climate science, all data will be made accessible to anyone who wants it through the CCI initiative.
Original headline: Apparent pause in global warming blamed on 'lousy' data
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