Global temperatures will resume their long term growth trend within five to 10 years ending the so called pause in global warming, a leading climate scientist has predicted.
The pause – which on some measures has gone on since the mid-1990s - continued into 2014 on the basis of global temperature data released last week by US space agency NASA and the National Oceanic and Atmospheric Administration (NOAA) of the US.
However, the warming effect of greenhouse gases such as carbon dioxide will grow sufficiently to overcome the combined impact of various natural climate cooling factors, journalists on a telephone news conference were told last week by Gavin Schmidt, director of NASA's Goddard Institute of Space Studies.
There is evidence that volcanoes and a slightly dimmer Sun have acted to cool the Earth recently and so offset the warming impact of greenhouse gases, according to Schmidt, widely seen as a strong advocate for the case that humans are causing climate change. But Schmidt said that he did not expect the global warming pause – which he referred to as the hiatus - to persist.
This is because the warming impact, or forcing, due to greenhouse gases in the atmosphere would keep growing with continuing emissions of greenhouse gases, Schmidt said, and "in five to ten years time it is changes in greenhouse gases that will dominate".
NASA and NOAA announced last week that 2014 was the warmest year recorded since measurements began but the fact is that the margin is so small as to be statistically meaningless. NASA itself ranks the probability that 2014 was the warmest year at 38 per cent while NOAA is slightly more confident putting the probability at 48 per cent.
The difference between global mean surface temperature in 2014 and the previous warmest years on record, 2010 and 2005, is measured in just hundredths of a degree on both the NASA and NOAA analyses. This is within the margin of error of the data which means that there is no statistical difference between global temperatures in 2005, 2010 and 2014.
Independent climate research institute Berkeley Earth, sometimes seen as sympathetic to climate sceptics, put it this way: “The global surface temperature average (land and sea) for 2014 was nominally the warmest since the global instrumental record began in 1850; however,within the margin of error, it is tied with 2005 and 2010 and so we can’t be certain it set a new record.”
Furthermore, satellite measurements of the temperature of the atmosphere do not rank rank 2014 as the warmest year. The analysis of satellite data performed by the University of Alabama in Huntsville (UAH) puts 2014 in third place while an analysis of data provided by US firm Remote Sensing Systems places 2014 in sixth place. Both datasets report that 1998 was the warmest year since satellite measurements began in 1979.
The existence of the pause in global warming was acknowledged by the Intergovernmental Panel on Climate Change in its most recent report published in 2013 but there has been significant debate about the actual duration of this hiatus with some commentators alleging that the length is exaggerated by cherry-picking the start date as 1998 – a particularly warm year due to a particularly strong El Nino Pacific Ocean warming event.
Schmidt himself alluded to this last week when he told the press conference that the “hiatus question is a complicated one”. He pointed out that the El Nino year of 1998 was a “stand out” year and that if a line is drawn from 1998 to 2014 then global surface temperature “doesn't look like it has changed very much”.
Schmidt went on to point out that “2014 is exactly where we would have expected to be before 1998”. There is no statistical evidence for a change in trends and no evidence of a break point between 1997 and 1998, he said. “There is no evidence that the long term trend is really much different to what it has been,” he added.
However, an analysis of global temperature data published last year uses statistics to avoid the charge of cherry picking and indicates that the pause dates back to 1995. Ross McKitrick from the Department of Economics at the University of Guelph in Canada, who analysed the data, is seen as sceptical about global warming but his research was published in the peer-reviewed Open Journal of Statistics and its findings have yet to be challenged.
McKitrick's analysis dating the start of the the pause in surface temperatures to 1995 agrees with the view of acknowledged climate sceptic physicist Professor Richard Lindzen of MIT who has written there has been “no statistically significant warming since 1995”.
A number of possible explanations have been put forward to explain the pause These include a build up of heat in the deep oceans, a weakening of solar activity and aerosols of volcanic ash in the atmosphere that reflect the sun’s rays back into space. However, the impact of solar activity and volcanoes does not appear sufficient to explain the problem and the accumulation of deep ocean heat appears to be somewhat elusive – the measured increase in ocean heat content being less than that required to explain the pause.
Recent research has implicated long term cycles in the oceans but there is no agreed mechanism with some papers attributing the pause to Pacific Ocean cycles, other research pointing to changes in the Atlantic and one recent paper saying that all the oceans are involved.
There is also a suggestion that the pause is an artefact of the way the data is analysed and that it only appears to exist because faster warming in the Arctic has been excluded from the various global temperature analyses. Another possibility is that the pause is an entirely natural variation in the climate cycle around an underlying upward trend in global temperatures.
None of these explanations has gained widespread acceptance.
See our report on 2014 temperature data from NOAA and NASA here.
See our story on Ross Mitrick's statistical analysis of the pause here.
See here for a report on research that links the Sun with the pause.
Berkeley Earth here.
Site by Accentika