The Human Touch: A Decade of Weather Extremes

The Human Touch: A Decade of Weather Extremes
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The ostensibly large number of recent extreme weather events has triggered intensive discussions, both in- and outside the scientific community, on whether they are related to global warming. Here, we review the evidence and argue that for some types of extreme — notably heatwaves, but also precipitation extremes — there is now strong evidence linking specific events or an increase in their numbers to the human influence on climate. For other types of extreme, such as storms, the available evidence is less conclusive, but based on observed trends and basic physical concepts it is nevertheless plausible to expect an increase.

Figure 1: World map showing the record extremes

The numbers refer to the year in the twenty-first century. Blue symbols represent rainfall; red symbols represent heatwaves/droughts; yellow symbols represent hurricanes/cyclones; and the green symbol represents a tornado outbreak.

Le Monde: “In 2003, Europe experienced its hottest summer in 500 years (resulting in 70,000 deaths), an unprecedented heat wave engulfed the Australian bush in 2009 and 2010, Russia was facing a heat wave affecting its population (11,000 people have died in Moscow alone) and crops (30% drop in cereal production due to drought and fire, forcing Moscow to ban wheat exports).

These extreme heat are increasingly frequent. Summers, especially, as shown in this Gaussian curve of the average climate between 1500 and 2002 – with, in blue, the peak years of summer freshness, and in red, those of heat. The bottom graph shows the frequency of ten summers extremes (defined as 5% of the hottest period between 1500 and 2002).

But these extremes are not just happening during summers. Today, throughout the year, the number of records reached by monthly temperatures on the globe is three times higher than that should be observed in a stationary climate.

Nine of the ten warmest years on record have been during the last decade – 2010, 2005 and 1998 are the three records of mean temperatures since records began in 1880. For the two researchers, a link exists between these heat waves and global warming caused by man. Their study joined by many scientific studies, including those of the laboratory of NASA’s Goddard Institute for Space Studies led by James Hansen, for which the temperature rise is largely driven by increased concentrations of greenhouse gases in the atmosphere, particularly carbon dioxide produced by human activities (energy production, transport, industry, etc.). The concentration of CO2 in the atmosphere of 285 parts per million in 1880, has now exceeded 390 parts per million, higher than the maximum “acceptable” level of 350 ppm.

In a nutshell, extreme weather events have increased over the last decade, and if we can not link each one of them to climate change, it is responsible for this trend. “Extreme weather events may be related to regional climatic oscillations such as El Niño or La Niña, the authors concede. But today, these processes take place in a context of global warming. It can transform an extreme event in an record event. It is very likely that even more extreme events unprecedented in the last decade would not have occurred without the anthropogenic global warming.

By anthropogenic, understand Man made global warming.

Sources: Nature Climate Change (2012): A decade of weather extremes / Audrey Garric Blog / Le Monde

Photo Credit: Hurricane Irene Makes Landfall in North Carolina / FlickR