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:

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.

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.

The world stopped getting warmer almost 16 years ago, according to new data released last week.

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.