As a regular of Bitsofscience.org you may recall a publication on ocean warming [between 2003-2010] in Geophysical Research Letters from last July.

That one was little-noticed by the world’s media, but now its findings may receive more attention, as an independent study by NCAR, published yesterday in Nature Climate Change, has investigated the same subject and reaches a confirming conclusion: in recent years atmospheric warming has been delayed due to increased heat transport to the deeper ocean.

What exactly did we miss?

Although this century kicked off with the hottest decade on record, 2010 was the hottest year and in 2011 the Arctic may have broken both the summer and the winter melting record, there has still been heat missing: the rise in global temperatures is smaller than what one would expect from the rise in greenhouse gas concentrations, which – despite UNFCCC attempts to tackle climate change since Kyoto 1997 and Copenhagen 2009 – has even accelerated.

In other words: by breaking of temperature records and melting of ice, climate change has shown itself – but over the last decade or so not to the extent that it ‘should’ have. Part of global warming has -apparently- been offset by some global cooling.

Global cooling explanations

And part of that cooling we may owe to Chinese sulfur emissions, Boston University climate scientists said in July. But their PNAS publication also referred to natural climate cycles, superimposed on the trend line, like ENSO and solar variability, both of which have been net contributors to global cooling over 1998-2008 [so climate skeptics cannot – as they still do – point to either the Sun or El Niño to explain the world’s temperature graph over that period of time].

Sulfur aerosols in the atmosphere reflect sunlight. It is a process climate satellites can witness – and measure. The NCAR scientists now state these satellites still see there is more energy radiation entering the upper atmosphere than leaving it, in whatever part of the spectrum.

According to their publication we’re left with a “net energy flux into the climate system of about 1 W m⁻².“ This is missing heat that aerosols cannot explain.

What doesn’t go up must have gone down

If the heat did not escape to space and it could not be traced in the air, it must have radiated away in the opposite direction: down. As land warming is proportional to atmospheric warming, that can be no answer. There were now basically two more options. Either a big chunk of ice has been melting extraordinarily fast – which would cool the surrounding air – or somehow ocean currents would have changed in a way that favoured more rapid warming of deep water.

Although Greenland is melting fast – it isn’t melting fast enough to explain the graph, neither is Antarctica.

So theoretically we’re now stuck with one option. We’d have to do a lot of scuba diving to actually measure it, but the excess heat would have to be stored in the deeper ocean, which NCAR scientists define as deeper than 300 meters.

Wait and see what happens in an El Niño-dominated decade

Fortunately their Community Climate System Model gave a better impression of how. It is thought capable of reproducing complex interactions between the atmosphere, the oceans, land masses and ice masses. In many simulation runs of continued warming hiatus periods arose of at max a decade. These could quite easily be linked to La Niña dominance in the ENSO cycle, which is what happened over the last decade.

Small changes in ocean heat transport have a bigger effect for the atmosphere than for the deep ocean, because there is much more water beneath 300 meters, than water and air mass above it.

This means – as the NCAR model shows – that during hiatus periods the deep ocean could warm 18 percent more, simultaneous with 60 percent less warming in the upper ocean.

Of course the opposite is also true. Inevitably we’ll enter a period in which ocean currents [through dominant El Niño episodes and perhaps a relatively strong Atlantic Meridional Overturning Circulation] favour less heat storage in the deep water – and more around the water surface, where warming could suddenly increase by 60 percent. And then we’d really be in for some new climate records.

© Rolf Schuttenhelm | www.bitsofscience.org