New global temperature trend data released by the Hadley Centre and the Climatic Research Unit (CRU) in Britain find essentially no upward trend in global average temperatures since 1997. The Mail on Sunday in Britain reported:
The world stopped getting warmer almost 16 years ago, according to new data released last week.
The figures, which have triggered debate among climate scientists, reveal that from the beginning of 1997 until August 2012, there was no discernible rise in aggregate global temperatures.
This means that the 'plateau' or 'pause' in global warming has now lasted for about the same time as the previous period when temperatures rose, 1980 to 1996. Before that, temperatures had been stable or declining for about 40 years.
So what does this mean? The Mail cited the views of two climatologists, Judith Curry from Georgia Tech and Phil Jones from the CRU (and a figure in the Climategate kefuffle):
'The new data confirms the existence of a pause in global warming,' Professor Judith Curry, chair of the School of Earth and Atmospheric Science at America's Georgia Tech university, told me yesterday.
'Climate models are very complex, but they are imperfect and incomplete. Natural variability [the impact of factors such as long-term temperature cycles in the oceans and the output of the sun] has been shown over the past two decades to have a magnitude that dominates the greenhouse warming effect.
'It is becoming increasingly apparent that our attribution of warming since 1980 and future projections of climate change needs to consider natural internal variability as a factor of fundamental importance.'
Professor Phil Jones, director of the Climate Research Unit at the University of East Anglia, who found himself at the centre of the 'Climategate' scandal over leaked emails three years ago, would not normally be expected to agree with her. Yet on two important points, he did.
The data does suggest a plateau, he admitted, and without a major El Nino event – the sudden, dramatic warming of the southern Pacific which takes place unpredictably and always has a huge effect on global weather – 'it could go on for a while'.
Like Prof Curry, Prof Jones also admitted that the climate models were imperfect: 'We don't fully understand how to input things like changes in the oceans, and because we don't fully understand it you could say that natural variability is now working to suppress the warming. We don't know what natural variability is doing.'
Yet he insisted that 15 or 16 years is not a significant period: pauses of such length had always been expected, he said.
Yet in 2009, when the plateau was already becoming apparent and being discussed by scientists, he told a colleague in one of the Climategate emails: 'Bottom line: the "no upward trend" has to continue for a total of 15 years before we get worried.'
But although that point has now been passed, he said that he hadn't changed his mind about the models' gloomy predictions: 'I still think that the current decade which began in 2010 will be warmer by about 0.17 degrees than the previous one, which was warmer than the Nineties.'
Only if that did not happen would he seriously begin to wonder whether something more profound might be happening. In other words, though five years ago he seemed to be saying that 15 years without warming would make him 'worried', that period has now become 20 years.
Meanwhile, his Met Office colleagues were sticking to their guns. A spokesman said: 'Choosing a starting or end point on short-term scales can be very misleading. Climate change can only be detected from multi-decadal timescales due to the inherent variability in the climate system.'
He said that for the plateau to last any more than 15 years was 'unlikely'. Asked about a prediction that the Met Office made in 2009 – that three of the ensuing five years would set a new world temperature record – he made no comment. With no sign of a strong El Nino next year, the prospects of this happening are remote.
I reported back in 2007, the folks at the Hadley Centre at the United Nations climate change conference in Bali made an especially bold prediction about global temperature trends through 2014:
Most interestingly, and to its credit, the Hadley Centre has now gone out on a risky prediction limb. The Centre has combined its weather prediction model with a climate change model to make definite forecasts about the world's climate for the next decade. To wit: "We are now using the system to predict changes out to 2014. By the end of this period, the global average temperature is expected to have risen by around 0.3 degrees Celsius compared to 2004, and half of the years after 2009 are predicted to be hotter than the current record hot year, 1998." Since various temperature records—surface, satellite and weather balloons—have shown a temperature trend that increases at about 0.2 degrees per decade or less, this is a truly bold prediction.
That prediction looks shakier and shakier.
Late last year, a study by Grant Foster (for those in the know, Grant is the climate blogger Tamino) and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research analyzed temperature data from 1979 to 2010 taking into account the effects of El Ninos, volcanoes, solar variation and found a consistent upward temperature trend of +0.014 per year.
The satellite data mavens over at the University of Alabama in Huntington analyzed their 33 year record of temperature data and reported at the end of last year that the overall average global temperature trend is about +0.09 degrees per decade.
To enjoy some of the measured and polite discussion that so characterizes the global climate change debate head on over to Judith Curry's website Climate Etc.