Extreme Weather: More heat waves or colder and snowier winters?

These three recent papers published in high-profile Journals seem to send mixed and confusing signals among the readers namely "Will there be more heat waves in future or colder and snowier winters like the past one (2010/11) over most of North America and elsewhere?"

The first two papers use regional/global climate models to simulate projection of 'extreme hot events' in the conterminous US and also in various other regions of the world. Diffenbaugh & Ashfaq use a nested regional climate model (RegCM3) and obtain increasing frequency of heat waves over conterminous US during the decades 2010-2020, 2021-2029 and 2030-2039. The intensification of hot extremes, according to their study, is associated with a shift towards more anticyclonic atmospheric circulation (leading to surface drying) during the warm season. The study further concludes that intensification of hot extremes could result from relatively small increase in greenhouse gas concentration and as such, constraining global warming to 2°C (a threshold value agreed at the 2009 Copenhagen Conference of the UNFCCC) may not be sufficient to avoid dangerous climate change. The paper by Clark et al. uses an ensemble of 224 simulations of the equilibrium response to doubled CO₂ using a variant of the HadCM3 global climate model. Their study obtains significant changes in single-day hot events ranging between 2°C and 6°C for large parts of Europe, North America and Asia. Changes in soil moisture appear to be an important contributor to future extreme heat waves.

The latest paper (by Petukhov & Semenov) attempts to "link the reduced Barents-Kara sea ice concentration to the cold winters of northern hemisphere, in particular the severe cold winters of 2005-06 over Europe."

It is instructive to take a close look at these three studies:

1. The studies on heat waves DO NOT use observed data on heat waves, especially for the recent decade (2000-2009) for which data are now available. Further the study by Diffenbaugh & Ashfaq does NOT include the decade of the 1930s which has been adjudged as the hottest decade in the conterminous US and possibly over northern hemisphere and elsewhere. Several heat waves, some very severe, were reported during this hottest decade of the twentieth century and four of the years of that decade are now identified as among the hottest years in the conterminous US. 2. Both the studies on heat waves project drying of the land surface (soil moisture deficit) as an important contributor to future extreme heat waves. How the global and regional hydrologic cycle respond to future anthropogenic warming is not known at this time. A small increase in soil moisture together with changes in boundary-layer cloud cover could reduce the projected number of 'extreme heat waves' in future decades. 3. The study by Petukhov & Semenov uses the ECHAM5 general circulation model to simulate 'large scale easterlies accompanying an anti-cyclonic vortex developed over the heating source (due to open waters of Berents-Kara Sea) dominating the flow patterns in the sub-polar latitudes'. These easterly flow patterns, according to the authors, have brought colder winters over Europe in particular. The study however does NOT explain why most of North America has witnessed significantly colder and snowier winters in the first decade of this millennium. (ex., 2002/03, 2007/08, 2009/10 and the most recent 2010/11). The study also ignores the fact that most of South America has witnessed much colder winter season in recent years, e.g., the winter seasons of 2007 and also past winter (June-July 2010) were significantly colder over most of South America. Several additional examples of recent colder winters in various parts of the world are provided in Khandekar (2010), which the authors of this study appear to have ignored.

In summary, none of the above studies provide any assessment against observed weather extremes (cold or warm) of the past. Many other model simulations studies reported in recent literature lack a reality check against observed extreme weather events of recent years. There is an urgent need to develop a comprehensive database of all weather extremes (cold or warm) and analyze their mechanics and dynamics in the context of present debate on global warming and climate change.

Will there be "more heat waves OR colder and snowier winters in the next decade and beyond?" The studies reported above do not provide a definitive answer at this point in time.

Additional Reference
Khandekar, M. L. 2010. Extreme weather events of summer 2010: global warming or natural variability. Energy & Environment 21: 1005-1010 .