The warming pause

The Herald reports:

A slowdown in warming that has provided fuel for climate sceptics is one of the thorniest issues in a report to be issued by United Nations experts on Friday.

Over the past 15 years, the world’s average surface temperature rose far slower than many climate models have predicted.

According to projections, global warming should go in lockstep with the ever-rising curve of heat-trapping carbon emissions. But in recent years, warming has lagged. So, where has the missing heat gone?

For climate sceptics, the answer is clear. Either the computer models used to project temperature rise are flawed, or man-made global warming is just a green scam, they say.

I don’t buy into any nonsense that there is some global conspiracy involving thousands of scientists. But I think it is clear that the models to project future temperature increases are imperfect. This is no surprise. The global ecosystem is hugely complex and there are many factors which will take decades or even longer to fully comprehend. We may never fully understand how all the different aspects interact.

But that is not to say it is in the too hard basket. First of all there clearly is still warming over the medium term. The direct impact of increased levels of greenhouse gases in the atmosphere is relatively simple to calculate, and there is no real scientific dispute over the direct impact.

Where we have uncertainty is how the rest of the climate ecosystem responds to the warming caused by greenhouse gases. The models in the past have projected a multiplying impact, where temperatures increase quite rapidly. It may turn out to be that in fact the rest of the ecosystem will actually mitigate the impact of greenhouse gases. Note mitigate does not mean reverse.

Over the past 50 years, the mean global temperature rise was 0.12C per decade, slowing to an average 0.05C per decade over the past 15 years.

Half of the slowdown could be attributed to volcanic eruptions, whose particles reflect sunlight, and a bigger-than-expected drop in heat from the sun’s changing activity cycle, said a summary of the report.

The other half is attributed to a “cooling contribution from internal variability”.

Laurent Terray with the French computer modelling agency Cerfacs said the term is used to explain a shift in the way heat is distributed between land, sea and air.

Still unclear is what causes the variation or determines its duration.

“We know that this kind of episode, of a decadal length or thereabouts, can occur once or twice a century,” said Terray. “If it continues for two more decades, we may start to think that the computer models are underestimating internal variability.”

New research by Britain’s Met Office suggests the “missing” heat, or some of it, is being transferred from the ocean surface to the deeps.

Temperatures at depths below 3000m have been rising since the 1990s, implying a source of heat-trapping today will contribute to warming tomorrow.

As one can see the ecosystem is very complex, and there are multiple ways different parts can interact.

I am looking forward to seeing the IPCC update when it is released.