Every day tens of thousands of temperature measurements are taken all over the world, both on land and at sea. Averaged together they give us a snapshot measurement of the temperature at the Earth’s surface, and by using historical records we can see how that temperature has changed over the last 160 years.

For most of the last century surface temperatures have climbed steadily, but there have been occasional plateaus, including the decade-long pause we are experiencing right now. To understand better what’s happening, Matthew Palmer and Doug McNeall, from the Met Office Hadley Centre, UK, analysed a large number of climate simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The researchers investigated the relationships between the net radiation at the top of the atmosphere (TOA), global ocean heat content (OHC) and global average surface temperature (GST).

TOA is the sum of the incoming short-wave solar radiation, the reflected short-wave radiation (for example, from cloud tops and the Earth’s surface) and the outgoing long-wave radiation, re-emitted by the Earth. “My view is that net radiation (TOA) is the most fundamental measure of global warming since it directly represents the accumulation of excess solar energy in the Earth system,” explained Palmer.

The model simulations show substantial variability in surface temperature trends over a decade, which doesn’t correlate well with net radiation. “The lack of correlation tells us that surface temperature trends over the period of about 10 years are not a good indicator of how much energy is accumulating in the Earth system over that period,” Palmer told environmentalresearchweb. The simulations showed that large trends in surface temperature – around 0.3K per decade, compared with a contemporary surface warming rate of about 0.2K per decade – can arise from internal climate variability, and that these trends generally fail to correlate with net radiation over the same time period.

By contrast, ocean heat content (OHC) correlated well with net radiation, explaining 95% or more of the variance in net radiation for two-thirds of the simulations analysed. “This tells us that the ocean is the primary term in the Earth’s energy budget over decadal timescales,” said Palmer, whose findings are published in Environmental Research Letters (ERL).

A recent study published in the journal Nature Climate Change indicated that much of the current hiatus in surface temperature may be due to unusually strong trade winds in the Pacific Ocean, which have buried the surface heat deep underwater, reducing the amount of heat flowing back into the atmosphere. Once those trade winds relax again, the heat is likely to be released and the rapid rise of surface temperature could continue. That study is consistent with this model-based research by Palmer and McNeall, who make a clear case for ocean heat rearrangement in explaining decadal variations in surface temperature.

As Palmer and McNeall have shown, measuring surface temperature alone provides little indication of how much energy is accumulating in the Earth system. To quantify the increase in energy we must better monitor global ocean heat content because this represents the primary term in the Earth’s energy budget and is a reliable indicator of changes in net radiation.

However, measurements of ocean heat content and net radiation are not as easy to gather as those for surface temperature; their collection has only started relatively recently. Changes in net radiation have been monitored using satellite measurements from CERES (Clouds and Earth's Radiant Energy System) since 2001, while ocean heat content has been calculated for the upper 2 km of the open ocean since around 2005, using the Argo array of ocean profiling floats.

“For monitoring ongoing climate change it is imperative to improve our observations of ocean heat content, pushing further to observe the deep ocean below 2 km,” said Palmer. “It is also imperative to continue efforts to monitor changes in net radiation and its components from satellites.”

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