So-called global warming 'pause' is due to natural climate fluctuations

Thursday, July 24, 2014, 2:39 PM - Working through the climate record, McGill University researcher Shaun Lovejoy has taken a close look at the so-called global warming 'pause' or 'hiatus' and discovered that, rather than it showing that global warming has stopped, it is simply the result of a natural cooling trend that has shown up from time to time throughout history.

There is ample evidence that the Earth continues to accumulate heat due to global warming and global temperatures are running at levels higher than we've seen throughout human history (click here to see the evidence). However, it's difficult to have any discussion about global warming and climate change without hearing about the 'pause' that has been seen over the past 15 years or so. Many climate change 'skeptics' claim that since global surface temperatures have shown no increase since after 1998, the rises in temperatures before that and this period show that it's all just natural fluctuations in Earth's climate. Thus, the claim continues, global warming is not real, and there is no threat from carbon dioxide emissions or climate change.

Climate scientists studying this slow-down in the rise of global surface temperatures have found several contributing factors to the pattern - the intensity of the El Niños and La Niñas since 1998, sulphur dioxide from volcanic eruptions, changes in wind patterns over the Pacific Ocean, etc - however a new statistical analysis from Prof. Shaun Lovejoy of McGill University, who studies the physics of weather and climate, is showing that this 15-year period is due to a natural cooling trend that has been seen repeated in the climate record, which is temporarily masking the current warming trend.

Looking at weather and climate throughout history, Lovejoy has already examined the current phenomenon of global warming, since the beginning of the Industrial Revolution in the 1880s. His statistical analysis of the entire time period, published in April in the journal Climate Dynamics, ruled out - with at least 99 per cent certainty - the hypothesis that the current global warming trend is just a natural fluctuation in Earth's climate.

Turning that same method of analysis to look at the smaller-scale changes within the longer trend, Lovejoy has found two different recurring fluctuations. One is a warming seen every 30-40 years, while a there is cooling that shows up every 20-50 years, both of which influence how the trend in global warming due to greenhouse gas emissions is showing up in the temperature record. Three periods in particular stand out in the study. The post-war cooling period, between the years 1944 and 1976, had a contribution from the cooling fluctuation that dropped temperatures by about a half a degree C. The warming fluctuation showed up during the period from 1992-1998 ('pre-pause'), to enhance the warming trend by roughly a third of a degree. After that, the cooling fluctuation took over again from 1998-2013, dropping temperatures by anywhere from 0.28 to 0.37 degrees C, thus masking the overall warming trend and producing an apparent 'pause' in global surface temperature increases.

The strength of Lovejoy's analysis is that it doesn't rely on the use of global climate models (GCMs). Although these models have increased in sensitivity and accuracy over the years, they are still a large source of criticism for 'skeptics' and thus a method that simply uses the existing climate record and avoids basing conclusions on these models would avoid that 'easy' criticism.

"Being based on climate records, this approach avoids any biases that might affect the sophisticated computer models that are commonly used for understanding global warming," Lovejoy said in a McGill press release.

It isn't known how long this current cooling fluctuation will last, but this study shows that its effects - and thus this 'pause' - are definitely a temporary thing, and eventually we're going to see temperatures start to rise again.