21 September 2013

Is global warming in a hiatus? Not if you measure global heat content

by Prof. Andy Pitman, via The Conversation

Prof. Andy Pitman, Univ. of NSW
On September 27 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.
Below 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.
Figure 1: Global surface temperature. NASA

Between about 1880 and 1890, temperatures cooled by about 0.4C. Between 1900 and 1910 temperatures cooled close to 0.3C. Between 1945 and 1950 temperatures cooled about 0.35C. Between 1962 and 1965 temperatures cooled about 0.3C. There are other examples, but these were decade-scale cooling of 0.3C to 0.4C.

The most recent period of similar relevance starts with the extremely hot year, 1998. Since 1998, through to 2012, the temperatures cooled by 0.03C. However you choose to view the figure you simply have to conclude that natural variability, aerosols and solar variability have caused global cooling in the past of a scale that dwarfs anything that has occurred in the last 15 years.

So, here is what I think we should be genuinely concerned about.

Given the double-dip La Niña, coupled with the solar minimum and coupled with the high aerosol output from some developing nations, the question in the minds of some climate scientists is not “why has it cooled?”, because it has not cooled in any significant sense and the climatologically significant trends (calculated over 30 years) remain upwards.
Indeed, despite a suite of forcings that should have led to cooling, we still had the warmest decade in the observational record.

So, the question is, given it did cool several times in the historical period under broadly parallel circumstances in terms of the forcing, why has it not cooled since 1998 by 0.3C or 0.4C, and how come we broke the records for the warmest decade?

There has been time (its 15 years while previous cooling occurred in 10 years) for cooling of 0.3C or 0.4C to have occurred. There really is a case to argue that we should have cooled to close to the values measured in around 1990 and definitely not broken the record for the warmest decade on record.

A plausible answer is that we have underestimated the climate sensitivity.

We know, for certain, that aerosols, natural variability and solar variability have cooled the climate in the past. This time, they have not.

One way that this makes sense is if climate scientists have underestimated how dominant CO2 and other greenhouse gases are in warming the climate. In other words, CO2 and other greenhouse gases are countering the cooling effects of natural variability by much more than we anticipated.

If correct, this means that the capacity of CO2 and other greenhouse gases to accelerate warming – once natural variability, solar variability and aerosols decline in influence – has been underestimated.

A second possible explanation is that the warming by CO2 has led to a sufficiently different climate system that natural variability now functions differently. This seems extremely unlikely but is certainly anything but comforting.

If you see the slowing of warming over the last 15 years as a hint that climate scientists might have been wrong and that global warming is less of a problem than predicted, you are very likely being lulled into a false sense of security.

The lack of cooling of 0.3C or 0.4C since 1998 is most easily explained by the effect of increasing CO2 and other greenhouse gases masking the cooling that would otherwise have occurred.

It follows that when we next see an El Niño, and the solar cycle is more average, or if developing countries clean up their aerosol emissions, we will see an acceleration of warming rates observed prior to 1998.

In short, the slowing of warming rates since 1998 is not a good news story. It is very likely a hint that climate scientists have underestimated the sensitivity of climate to increasing CO2 and the slowing of warming is lulling us into a very false sense of security.


  1. August Global Sea surface temperatures were at a record high, according to NOAA. So, yes, the calm before the storm. Next strong El Nino should push global surface temperatures to new records.

  2. From the Guardian, following climategate. James Lovelock's excoriating view of the lying, dumb, little scumbags who do modern climate science.

    on CRU scientists

    I was utterly disgusted. My second thought was that it was inevitable. It was bound to happen. Science, not so very long ago, pre-1960s, was largely vocational. Back when I was young, I didn't want to do anything else other than be a scientist.

    They're not like that nowadays. They don't give a damn. They go to these massive, mass-produced universities and churn them out. They say: "Science is a good career. You can get a job for life doing government work." That's no way to do science.

    I have seen this happen before, of course. We should have been warned by the CFC/ozone affair because the corruption of science in that was so bad that something like 80% of the measurements being made during that time were either faked, or incompetently done

    on computer models

    I remember when the Americans sent up a satellite to measure ozone and it started saying that a hole was developing over the South Pole. But the damn fool scientists were so mad on the models that they said the satellite must have a fault. We tend to now get carried away by our giant computer models. But they're not complete models.

    They're based more or less entirely on geophysics. They don't take into account the climate of the oceans to any great extent, or the responses of the living stuff on the planet. So I don't see how they can accurately predict the climate.

    on predicting temperatures

    If you look back on climate history it sometimes took anything up to 1,000 years before a change in one of the variables kicked in and had an effect. And during those 1,000 years the temperature could have gone in the other direction to what you thought it should have done. What right have the scientists with their models to say that in 2100 the temperature will have risen by 5C?

    The great climate science centres around the world are more than well aware how weak their science is. If you talk to them privately they're scared stiff of the fact that they don't really know what the clouds and the aerosols are doing. They could be absolutely running the show.

    We haven't got the physics worked out yet. One of the chiefs once said to me that he agreed that they should include the biology in their models, but he said they hadn't got the physics right yet and it would be five years before they do. So why on earth are the politicians spending a fortune of our money when we can least afford it on doing things to prevent events 50 years from now? They've employed scientists to tell them what they want to hear.

    on scientists

    Sometimes their view might be quite right, but it might also be pure propaganda. This is wrong. They should ask the scientists, but the problem is scientists won't speak. If we had some really good scientists it wouldn't be a problem, but we've got so many dumbos who just can't say anything, or who are afraid to say anything. They're not free agents.