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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How much the climate warms up depends on the total amount of CO2 emissions, not just the CO2 in a year, or a decade. The rate of heating is usually measured over long periods because short term measurements may be misleading.

Why doesn’t the temperature rise at the same rate CO2 increases?

The amount of CO2 is increasing all the time – we just passed a landmark 400 parts per million concentration of atmospheric CO2, up from around 270ppm before the industrial revolution. That’s a 48% increase.

A tiny amount of CO2 and other greenhouse gases, like methane and water vapour, keep the Earth’s surface 30 degrees warmer than it would be without them. We have added 50% more CO2 but that doesn't mean the temperature will go up by 50% too.

There are several reasons why. Doubling the amount of CO2’s only increases the temperature by a small amount. The ways the climate reacts is also complex, and it is difficult to separating the effects of natural changes from man-made ones over short periods of time.

As you increase the amount of CO2, temperatures do not rise in direct proportion to the increase (in the jargon, the rise is not linear, but logarithmic). In fact, although estimates vary – it’s a hot topic in climate science, if you’ll forgive the pun – the last IPCC report (AR4) described the likely range as between 2 and 4.5 degrees C, for a doubling of CO2 from pre-industrial levels.

When we consider the temperature change to date, we are looking for relatively small temperature increases, and indeed the change so far in average global temperatures is around 0.8 degrees C.

“According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)… the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade.

Because this increase is small, it can be hard to detect over short periods of time, when it can be masked by other aspects of the climate and changes that take place all the time. Such changes are referred to as ‘natural variability’. For example, cycles of warming and cooling in the oceans cause temperature changes across the surface of the globe, and their fluctuations are hard to separate from small changes in temperature caused by CO2 emissions during the same period. Particulate emissions from countries burning a lot of coal or wood are also under investigation, because they may be having a cooling effect. In fact, one reason that scientists prefer to measure changes over long time frames (30 years is a standard period used to determine a reliable trend) is that the effects of natural variation – thought of as short-term noise – can more easily be separated from the signal scientists are trying to measure: the effects of man-made CO2 on temperature.

Yet the physical properties of CO2 and other greenhouse gases cannot change. The same heat they were re-radiating back to Earth during previous decades must be evident now, subject only to changes in the amount of heat arriving from the sun – and we know that has changed very little. But if that’s true, where is this heat going?

The answer is into the deep oceans. Here is a graphic showing where the heat is currently going:

The mechanisms for this transfer of heat into the deep oceans are not well understood, but recent improvements in measurement techniques, equipment and modelling have allowed scientists to more accurately gauge the amount of heat the oceans are absorbing. Recent research (Balmaseda et. al, 2013) has found a surprising amount of heat below 700 metres:

“In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution”.

When deep oceans warm, the heat is being transported from the surface layers. This leads to cooling at the surface, a reduction in humidity (water vapour is a powerful greenhouse gas), and of course a lowering of atmospheric temperatures.

The Earth’s climate is a complex system, with all the parts interacting with each other, sometimes in ways we can’t predict. The energy that anthropogenic CO2 is adding to the climate system is not currently appearing as surface warming, but it is still there. However, since the effects are complex, and natural variability will always play a part, the rate at which surface temperatures increase will not always be in direct proportion to the amount of CO2 humans are adding to the atmosphere.

Updated on 2013-06-22 by gpwayne.

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