Global Warming Pause Goes Mainstream But No One Can Explain It

The Pause: temperatures flat-line while carbon dioxide concentrations continue to grow. Courtesy: RSS and NOAA

The pause: temperatures flat-line while carbon dioxide concentrations continue to grow. Courtesy: RSS and NOAA

Global temperature data for 2013 was released yesterday by two major US government agencies and the announcements confirm that the pause in global warming that began in the late 1990s has continued for another year.

America’s National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) simultaneously released their reports on global temperature trends in 2013. NASA data shows that 2013 tied with 2006 and 2009 as the seventh warmest year on record while NOAA data places 2013 joint fourth with 2003. The small differences between the two sets of data are as insignificant as the variations in global surface temperatures over the last decade or so – fractions of a degree.

Both tell the same story: the inconvenient truth, that is becoming harder for politicians to ignore, is that average global surface temperatures are statistically flat and have been throughout this century. This trend is also apparent in the other major global temperature records: the two satellite temperature records maintained by Remote Sensing Systems (RSS) and by the University of Alabama in Huntsville (UAH) and also the UK Met Office’s record which is the oldest continuous temperature record in existence. The Met Office will be announcing its full year 2013 analysis shortly and it will almost certainly confirm that the pause has continued for another year.

The differences between the two data sets are minor. They tell the same story: temperatures are high at the moment but they have stopped rising.

The differences between the two data sets are minor. They tell the same story: temperatures are high at the moment but they have stopped rising. Courtesy: NASA and NOAA

Warmest decade on record…

Now it is a fact that temperatures in the last decade are the warmest on record, but it is also a fact that the rise has slowed significantly. This has been going on since the late 1990s. Some suggest that this is an artifact caused by “cherry picking” a particularly warm start year in 1998 but the analysis is more subtle than that. If you track back from the most recent data you will find that the uncertainties around the temperatures in individual years – what scientists call the error bars – overlap. There has been no statistically significant warming. It just happens that this pause effect can be tracked back to 1998 – or even to 1997 in the case of the RSS data.

Make no mistake about it, the pause is real and it has moved out of the climate sceptic community and into the mainstream of climate science. A fact that was acknowledged last week by leading scientific journal Nature – which has never been a supporter of climate sceptic arguments – when it ran a welcome news report discussing the pause by its US correspondent Jeff Tollefson which stated:

“For several years, scientists wrote off the stall as noise in the climate system: the natural variations in the atmosphere, oceans and biosphere that drive warm or cool spells around the globe. But the pause has persisted, sparking a minor crisis of confidence in the field. Although there have been jumps and dips, average atmospheric temperatures have risen little since 1998, in seeming defiance of projections of climate models and the ever-increasing emissions of greenhouse gases.”

The importance of Nature’s article can not be underestimated. For it signals that it is okay for climate scientists to talk openly about the pause without being labelled as sceptics and deniers.

The fact of the pause has also been acknowledged by leading climate scientist Dr Kevin Trenberth, of the US National Center for Atmospheric Research, who recently published a paper that suggests the pause may have been triggered by a particularly strong El Nino Pacific Ocean warming event in 1998 and is linked to twenty-plus year cycles in the Pacific Ocean – known as the Pacific Decadal Oscillation (PDO). Trenberth’s contention is that the heat is building up in the oceans – and he may be right, but the evidence is sketchy and other climate scientists are not convinced.

No one knows why

Climate scientist Dr Judith Curry of the Georgia Institute of Technology, who is seen by many as a being somewhat sceptical, summed up her doubts over ocean heating this way in her blog this week: “All in all, I don’t see a very convincing case for deep ocean sequestration of heat.  And even if the heat from surface heating of the ocean did make it into the deep ocean, presumably the only way for this to happen involves mixing (rather than adiabatic processes), so it is very difficult to imagine how this heat could reappear at the surface in light of the 2nd law of thermodynamics.”

NASA held a telephone press conference to coincide with the release of its 2013 data and this brought together two other leading climate scientists: Dr Gavin Schmidt , deputy chief of NASA’s Goddard Institute for Space Studies, and Dr Thomas Karl, Director of NOAA’s National Climatic Data Center.

Schmidt, in particular, is seen as a forceful advocate of the case for human-driven climate change and if a simple explanation for the pause were available then he would have given it. Schmidt talked in terms of the the trend in increasing global average temperatures in the last 10 to 15 years being lower compared with the trend seen in the years before. “This appears to be a function of internal variability in the system”, he told reporters.  He pointed to the impact of natural variability linked to more frequent La Nina Pacific Ocean cooling events, a slight increase in volcanic activity and a dimming in the sun. He also highlighted a major uncertainty over the influence of particulate air pollution – known as aerosols.

Karl suggested that the long term changes in the Pacific Ocean – the PDO – may be playing a part and he pointed out that the PDO was now in a cooling phase.

There is also a possibility that computer climate models failed to predict the pause because they have assumed that the atmosphere is more sensitive to carbon dioxide build-up than is really the case. This climate sensitivity issue remains a delicate subject. The evidence of computer model projections remains the bedrock of the case put forward by the Intergovernmental Panel on Climate Change (IPCC) – the body that produces the the climate bible reports clutched by green-tinged politicians in the UK and elsewhere. But the longer the pause continues, the stronger the case becomes that the computer climate models may need to be tuned down.


So the pause could be due to the natural variations in the Pacific, volcanoes, solar activity or it could be due to man made particulate air pollution or to other factors. In short, climate scientists do not know.

In all the comments that are made about the climate, it is important to come back to some basic facts:

  • Carbon dioxide is a greenhouse warming gas.
  • The concentration of carbon dioxide in the atmosphere is growing.
  • Simple physics suggests that the Earth should warm as a result of this continuing increase in carbon dioxide.
  • Despite this, global average surface temperatures have been statistically flat since the late 1990s and the warming predicted by computer climate models has failed to materialise.
  • Climate scientists can not explain this pause and the apparent lack of warming.

The pause is a mystery. It as brutally simple as that. At least now, perhaps, climate scientists can begin to discuss this mystery openly and publicly and get to the bottom of what is really happening to our climate.

The pause has gone mainstream – and that is good news.

Sources and references:

NOAA here. NASA here. UAH here. RSS here. Met Office here.

NASA press conference slide pack here. Nature article here. Judith Curry’s blog here.

Kevin Trenberth’s paper here. My report on the NASA press conference here.

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One Response to “Global Warming Pause Goes Mainstream But No One Can Explain It”
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