On 27 September 2013 the 5th Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) will be released.

One part of this report will address the so-called “warming hiatus”. This is the argument that warming has stopped, with the further assertion in some quarters that we therefore have nothing to worry about in the future.

It is a fact, based on observations of air temperature, that the rate of global warming measured as surface air temperature has slowed over the past 15 years. The last decade is still the warmest in the past 150 years.

If you measure global heat content then global warming has not slowed. If you measure other indices including sea level rise or ocean temperatures or sea ice cover global warming has not slowed.

However, the warming trend in air temperatures has slowed over the last 15 years. There is a great deal of interest in this “hiatus” in the sense of whether it points to some fundamental error in climate science.

The 5th Assessment Report by the IPCC explains the slowing in the rate of global warming in roughly equal terms as the consequence of reduced radiative forcing (the difference between radiative energy that hits the earth and energy radiated back to space), increased heat uptake by the oceans and natural variability.

The reduced radiative forcing (the amount of energy available to drive the climate system) is due to the recent solar minimum (a period of low solar activity), and volcanic and anthropogenic aerosols (these are particles such as sulphur and soot, which block some radiation from hitting the earth).

The slowing in the rate of warming over the last 15 years is not in the least surprising. We have seen a combination of the solar minimum, anthropogenic aerosol emissions and back-to-back La Niñas.

What is surprising – and what is deeply concerning to me and almost entirely missed in the media commentary – is that we have not cooled dramatically over the last 15 years.

To the right is the global surface temperature graph – this comes from a NASA site but any other reputable temperature reconstruction makes similar points. Note that there were periods through the 20th century where combinations of aerosols from volcanoes and human sources, solar variability and natural variability led to very significant cooling.