New paper finds no evidence that climate has become more variable or extreme, says many models predict decreased variability

A paper published today in Environmental Research Letters finds that there is "no firm evidence" that the global climate has or will become more variable or extreme. According to the authors, 

"debate rages in the literature about whether the climate has or will become more variable. There is currently no firm evidence that temperature variability has or will increase because questions have been raised about the methods used to reach this conclusion."  

"The IPCC has so far been unable to draw firm conclusions about whether temperature variability is increasing. Indeed SREX even contained the question 'is the climate becoming more extreme?'. To paraphrase the answer, 'we don't know'." 

"Once issues such as these [data analysis problems] have been accounted for, there appears to be little overall change in global temperature variability. In addition, Huntingford et al indicate that many climate models predict decreases in variability in the future, while [another model] essentially points to no change."

Debate heating up over changes in climate variability

Lisa Alexander and Sarah Perkins

2013 Environ. Res. Lett. 8 041001 doi:10.1088/1748-9326/8/4/041001 Published 28 October 2013

Abstract

Heatwaves have profound socio-economic impacts. Increases in temperature variability would exacerbate these impacts but debate rages in the literature about whether the climate has or will become more variable. There is currently no firm evidence that temperature variability has or will increase because questions have been raised about the methods used to reach this conclusion. However, irrespective of changing temperature variability, the impact from increases in the frequency and intensity of heatwaves will be a major problem for the future.[non-sequitur]

Heatwaves kill people and cause enormous economic and environmental damage [1, 2]. In 2012, the Intergovernmental Panel on Climate Change (IPCC) Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) concluded that there was medium confidence that the length and/or number of heatwaves had increased since the middle of the 20th century and that it was very likely that the length, frequency, and/or intensity of these events would increase over most land areas by the end of the 21st century [3]. These conclusions were reiterated and strengthened by the IPCC Fifth Assessment Report (AR5) [4], released last month. However the IPCC has so far been unable to draw firm conclusions about whether temperature variability is increasing. Indeed SREX even contained the question 'is the climate becoming more extreme?'. To paraphrase the answer, 'we don't know'. If it is, then the impacts from changes in extreme heat could be far greater than if only mean temperatures were increasing.

In recent years several studies have tried to address this question directly. Some have analysed daily observational data [5, 6] but these data are not available across the whole globe. Methodological differences between these studies mean that the results are hard to compare. Most studies, including Coumou and Robinson [7], who investigated both past and future temperature variability, have used monthly or seasonal temperature data as a proxy for severe heatwaves [8–10]. This allows a more global assessment. However, although a relationship exists between extreme monthly mean temperatures and the presence of heatwaves [8, 9], monthly temperatures do not reflect the intensity and duration of heatwaves—the aspects that are likely to have the most severe impact on people and the environment. In contrast to the studies based on daily observations, the studies using monthly or seasonal data [8–11] can be compared but disagree on temperature variability.

There are two camps: one that says that temperature variability is increasing globally [9] and one that says that it isn't [10, 11] (or at least that the jury is still out [6]). The validity of the method used by the first camp [9] and also incorporated into the Coumou and Robinson study, has been questioned by studies in the second camp [10–12]. The main issue relates to the normalization of temperature anomalies, creating the impression that overall variability has increased [10]. Other problems relate to the existence of trends in time series and the changing number of observing stations over time [11]. Once issues such as these have been accounted for, there appears to be little overall change in global temperature variability. In addition, Huntingford et al [10] indicate that many climate models predict decreases in variability in the future, while [7] essentially points to no change. However there does seem to be some consistency between the studies in both camps that variability has increased in some regions e.g. western Europe.