It’s now accepted in certain quarters that climate change AKA anthropogenic global warming (AGW) is causing more frequent extreme weather events, and about once a week along comes another extreme event that gets blamed on AGW, with the most recent being the Chilean floods. Before that we had the California drought, Cyclone Pam, the frigid US winter, the Boston blizzard and the Australian bushfires. We can in fact trace a chain of extreme events allegedly caused in whole or in part by AGW that goes all the way back to hurricane Katrina in 2005, the “superstorm” that got the ball rolling.
Now, however, a report that calls these claims into question has come to Energy Matters’ attention. We are not presently at liberty to disclose which report it is, but it’s written by a group of climate scientists whose academic credentials are impeccable and who have numerous peer-reviewed publications to their credit. The report concludes that with the arguable exception of heat waves there is no compelling observational evidence for global increases in the frequency of any extreme weather events, meaning that the oft-heard claims that AGW is causing more of them are totally without foundation.
Here is a brief summary of what the report says.
We begin with Heat Waves. A global increase in heat waves would be a logical expectation because mean global surface air temperatures have risen. But because of conflicting trends in different regions and other uncertainties (such as how to define a heat wave) the report has only “medium confidence” that there has been such an increase.
(T)here has been a likely increasing trend in the frequency of heatwaves since the middle of the 20th century in Europe and Australia and across much of Asia where there are sufficient data. However confidence on a global scale is medium due to lack of studies over Africa and South America but also in part due to differences in trends depending on how heatwaves are defined. There is also evidence in some regions that periods prior to the 1950s had more heatwaves (e.g., over the USA, the decade of the 1930s stands out) while conversely in other regions heatwave trends may have been underestimated due to poor quality and/or consistency of data.
This combined with issues with defining events, leads to the assessment that there is medium confidence that globally the length and frequency of warm spells, including heat waves, has increased since the middle of the 20th century.
Next Extreme Precipitation. Because of conflicting results, lack of data and more definitional uncertainties (in this case how to define “extreme”) the report is unable to reach a firm conclusion as to whether extreme precipitation events are on the rise globally or not:
Given the diverse climates across the globe it has been difficult to provide a universally valid definition of ‘extreme precipitation’. In general, statistical tests indicate (that) changes in precipitation extremes are consistent with a wetter climate, although with a less spatially coherent pattern of change than temperature, in that there are large areas that show increasing trends and large areas that show decreasing trends and a lower level of statistical significance than for temperature change.
In summary …. it is likely that since 1951 there have been statistically significant increases in the number of heavy precipitation events (e.g., above the 95th percentile) in more regions than there have been statistically significant decreases, but there are strong regional and subregional variations in the trends.
On to Floods. The report finds no “clear and widespread” evidence for a global trend. In fact it can’t even define the sign of the global trend, if there is one, with any confidence:
While the most evident flood trends appear to be in northern high latitudes, where observed warming trends have been largest, in some regions no evidence of a trend in extreme flooding has been found, e.g., over Russia. Other studies for Europe and Asia show evidence for upward, downward or no trend in the magnitude and frequency of floods, so that there is currently no clear and widespread evidence for observed changes in flooding except for the earlier spring flow in snow dominated regions.
In summary, there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale.
Likewise Droughts. For much the same reasons the report also can’t identify a global trend in droughts with better than low confidence, and again it doesn’t say whether the trend is up or down.
Because drought is a complex variable and can at best be incompletely represented by commonly used drought indices, discrepancies in the interpretation of changes can result. For example, Sheffield and Wood found decreasing trends in the duration, intensity and severity of drought globally. Conversely, Dai found a general global increase in drought, although with substantial regional variation and individual events dominating trend signatures in some regions. Studies subsequent to these continue to provide somewhat different conclusions on trends in global droughts and/or dryness since the middle of the 20th century.
In summary, the current assessment concludes that there is not enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought or dryness (lack of rainfall) since the middle of the 20th century, due to lack of direct observations, geographical inconsistencies in the trends, and dependencies of inferred trends on the index choice.
Next Severe Local Weather Events. The report reaches effectively the same conclusion as it reaches for floods and droughts:
(S)evere local weather phenomena (include) hail or thunderstorms. These are not well observed in many parts of the world, since the density of surface meteorological observing stations is too coarse to measure all such events. Moreover, homogeneity of existing reporting is questionable. Alternatively, measures of severe thunderstorms or hailstorms can be derived by assessing the environmental conditions that are favourable for their formation but this method is very uncertain.
In summary, there is low confidence in observed trends in small-scale severe weather phenomena such as hail and thunderstorms because of historical data inhomogeneities and inadequacies in monitoring systems.
Next Tropical Storms: For once the report is unequivocal. There are no global trends:
Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century and it remains uncertain whether any reported long-term increases in tropical cyclone frequency are robust, after accounting for past changes in observing capabilities. No robust trends in annual numbers of tropical storms, hurricanes and major hurricanes counts have been identified over the past 100 years in the North Atlantic basin.
Finally Extratropical Storms. The assessment concludes with more accounts of conflicting and/or ambiguous results and votes of low confidence:
Studies that have examined trends in wind extremes from observations or regional reanalysis products tend to point to declining trends in extremes in mid-latitudes and increasing trends in high latitudes. Other studies have compared the trends from observations with reanalysis data and reported differing or even opposite trends in the reanalysis products. On the other hand, declining trends reported over China between 1969 and 2000 were generally consistent with trends in NCEP reanalysis. Trends extracted from reanalysis products must be treated with caution however.
In summary, confidence in large scale changes in the intensity of extreme extratropical cyclones since 1900 is low. There is also low confidence for a clear trend in storminess proxies over the last century due to inconsistencies between studies or lack of long-term data in some parts of the world (particularly in the SH). Likewise, confidence in trends in extreme winds is low, due to quality and consistency issues with analysed data.
So there it is. Except maybe for heat waves the available data provide no good evidence for global increases (or decreases) in any extreme weather events over the period of observational record. Barack Obama, John Kerry, Ban Ki-moon, the US National Climatic Data Center, the UK Met Office, Al Gore, Nature magazine and many others, please take note.