Are oceans warming or cooling?

Are the oceans warming or are the oceans cooling? Is there simply no change? The present traditional view of the scientific community is that oceans have been warming and that the warming of the oceans is related to climate change and that the in some cases you can link specific effects of ocean warming to manmade excess greenhouse gas.

When I wrote recently about climate change deniers I received a number of comments to the effect that the oceans are not warming, but in fact are cooling and the comments were put forward to show that the climate was not warming but cooling.

Most scientific opinion agrees that between 1961 and 2003 ocean temperature has increased by 0.1 degree Celsius from the surface to a depth of 700 metres. This temperature increase is based upon many millions of historical measurements. It therefore that the oceans are gradually warming but that is not conclusive, it is persuasive.

In June 2008 a team of US and Australian researchers published their ocean observation is Nature. Their observations were such that they concluded that the IPCC work on ocean heat was wrong – the IPCC had, in the views of the researchers based on their data underestimated the rate of ocean warming by 50% for the last forty years of the 20th Century.

Now the oceans are a vast body of water; it is not possible to measure all but a tiny fraction of it. Any data that you get from observations is bound to be flawed in some way and will not reflect the big picture. It is also hard to accurately sample the heat measurements of the ocean, because of all the factors involved; there are currents, very deep ocean areas, a moving amorphous surface and storms as well as many other factors which may provide false readings or misunderstood readings at a given time.

It is also important to understand that the oceans act as a heat buffer. They store 90% of the heat in the earth’s climate system and release some of it and suck other heat in.

The US and Australian researchers used expendable probes which measured the upper 700 metres of some of the ocean; they then related these measurements with the known effect of the thermal expansion of water. You may remember that water has its maximum density at 3.98 degrees Celsius; below this temperature it expands as it forms into ice and above this temperature it expands as it warms.

That is one reason why sea levels are rising – so the scientists who hold that the oceans are warming say.

Now measuring the temperatures of oceans may be relatively straightforward but there are natural systemic biases in the results (or in at least 70% of the results) which means that you have to constantly cross check the data you get with other data. By systemic bias I mean that there is an error in the measurements obtained which arises from the complexity of the differing factors, rather than any flaw in the measuring instruments or computations.

There are systemic biases in all climate change computations. This means that you can argue falsely from data that has not be corrected for its systemic bias and this usually results making false claims about a set of data.

A year before the US/Australian researchers published their findings two other scientists, equally respectable, D E Harrison and Mark Carson published a paper that looked at trends in ocean temperatures at depths of 100 metres, 300 metres and 500 metres. They concluded that some parts of the ocean had cooled by 3 degrees Celsius and other parts had warmed by “the same magnitude.

They suggested (as scientists do) that “additional attention is given to uncertainty estimates of basin average and World Ocean thermal trends”. It is back to the thorny issue of systemic bias; we do not know how to correct it, and we need to concentrate more on correcting it. Nevertheless Messrs Harrison and Carson’s work is often mis-cited as “evidence” on the climate change deniers blogosphere that some in the scientific community believe that the oceans have cooled by 3 degrees Celsius.

Like all scientists trying to understand phenomena that cannot be proved or explained empirically, Harrison and Carson urge caution; they do not proclaim that the oceans are not getting warmer, or that they are getting cooler, or even that they are staying at the same temperature. Their work is one part of a very large body of work already undertaken, which in itself will only be part of the work that will deliver a final irrefutable answer on ocean warming and ocean cooling.

I shall return to the other issues about ocean warming and ocean cooling later. In the meantime we should all bear in mind that climate change models, climate data and theory are all far from perfect. We are working with circumstantial evidence. But some circumstantial evidence is very strong, such as when you find a trout in the milk … or perhaps when you see the sea level rising.