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Tens of millions of measurements of ocean temperature and saltiness across the globe have been collected by buoys, ships and the Argo profiling floats since 1970 (Source: School of Environment/University of Washington)

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Global warming: oceans hide the heat

Tuesday 30th September 2014 1:10 pm

Welcome back to my epic miniseries on the so-called pause in global warming.

So what is this ‘pause’? Did the relentless rise in the Earth’s temperature come to a halt? No. Was there an easing off in the rate at which the Earth’s surface temperature is increasing? Yes, but unfortunately, only slightly.

I previously mentioned two causes for this. One was that the observation window (only nine years) was much too short to see a long-term trend. Plus, the temperatures in the Arctic, which is the fastest warming part of the planet, were excluded — for various technical reasons.

Let’s combine these two causes with a whole bunch of cooling factors. These cooling factors include volcanic dust and increased pollution from industrial processes (which reflect the Sun’s incoming solar energy back into space), La Niña events in the Pacific Ocean, slightly reduced power output from the Sun and so on. All of what I have just mentioned accounts for roughly half of the pause.

The other half of the reason for the pause is that less heat is staying at the surface, and instead, is going into the oceans.

Now this is a little complicated, so let me introduce you to a technical term that is very relevant — heat capacity. Heat capacity is a measure of how much heat energy a substance can store. Mathematically, it’s the ratio between how much heat energy enters a material, and the resulting temperature change.

You might have noticed that when you unload the dishwasher, the ceramic plates are dry, while the plastic plates are still wet with a myriad of droplets.

At the end of the washing cycle, a heater switches on and heats everything inside the dishwasher. Plastic has a lower heat capacity, and can’t store much heat energy. So there’s not enough heat energy to evaporate off the water droplets. But ceramic has a much greater heat capacity. So when the ceramic plates get heated to the same temperature as the plastic plates, they can absorb and store much more heat energy. This extra heat energy can then heat up, and evaporate away, the water droplets. So the ceramic plates come out perfectly dry, while the plastic plates are sparkling with water droplets.

It turns out that, volume for volume, water has over 3000 times the heat capacity of air. If you shove the same amount of heat energy into the same volumes of water and air, the air will get a lot hotter than the water. The air’s temperature will increase over 3000 times more than will the temperature of the water. Water has a massive heat capacity, as compared to air.

Now about 93 per cent of all the extra heat of global warming goes into the oceans, and only about one to two per cent remains to heat up the air. So you need only a tiny increase in the percentage of heat energy going into the oceans to slow down the rate of increase in air temperature.

The Earth’s oceans form a very complex system. They cover some 70 per cent of the Earth’s surface, and are hard to access because they have an average depth of some 3.7 kilometres.

Even so, we have taken tens of millions of measurements of ocean temperature and saltiness across the globe since 1970. These have been collected by buoys, ships and the Argo profiling floats. As a result, we have very good evidence that a few conveyor belt of various ocean currents have pushed the warmer surface waters into the depths — below 700 metres.

Recently, a very small part of the eastern Pacific, amounting to just 8.2 per cent of the Earth’s entire surface, has become unusually cool. It has been sending warm surface waters into the deep Pacific Ocean. It turns out that there are roughly 30-year natural cycles in the Pacific Ocean involving the movement of heat energy – the so-called Pacific Decadal Oscillation.

Since around the late 1990s, this cycle has been in a phase where the surface water in this patch of the central and eastern tropical Pacific is cooler. This has caused a rapid build-up and storage of heat in the deeper ocean below 700 metres.

A similar pattern, with the same result, has been happening on the other side of the Americas, in the Atlantic Ocean.

Over the last 50 years, the oceans have stored 250 zetajoules of energy. That’s 500 times the total annual energy generation capacity of the human race.

Much of this heat will be transferred back into the atmosphere over the next few decades. Next time, I’ll finish off by talking about what we can do about this …

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This blog first appeared on Dr Karl's Great Moments in Science

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