Global temperatures between 1998 and 2013 indicate a slowdown in global warming, but this was due to natural cooling fluctuation and not due to any decrease in greenhouse gas emissions. This has been proved by statistical analysis of the temperatures, conducted by McGill University physics professor Shaun Lovejoy.
The deceleration in rising temperatures during this 15-year period is sometimes referred to as a "pause" or "hiatus" in global warming, and has raised questions about why the rate of surface warming on Earth has been markedly slower than in previous decades.
The findings have been published in a paper published this month in Geophysical Research Letters.
The statistical methodology was developed by the McGill researcher who used it in an earlier paper to show that global warming in the industrial era was not a natural fluctuation in the earth's climate. He had used proxy figures for the pre-industrial temperatures.
In his new paper, Lovejoy applies the same approach to the 15-year period after 1998, during which globally averaged temperatures remained high by historical standards, but were somewhat below most predictions generated by the complex computer models used by scientists to estimate the effects of greenhouse-gas emissions.
Despite continued emissions, the temperature has not risen as predicted by most climate models. Critics have used the present pause to argue that global warming in the industrial era was also a natural phenomenon and not connected to emissions.
The new study concludes that there has been a natural cooling fluctuation of about 0.28 to 0.37 degrees Celsius since 1998, in line with variations that occur historically every 20 to 50 years, as evidenced from tree rings, ice cores and lake sediment.
The cooling effect observed between 1998 and 2013 exactly follows a slightly larger pre-pause warming event, from 1992 to 1998, so that the natural cooling during the "pause" is no more than a return to the longer term natural variability, Lovejoy concludes.