US?s hotter than ever

And it?s definitely because of greenhouse gases

Last summer, the US recorded one of the steepest declines in concern about global warming among global markets. That was on the back of one of the snowiest winters in Americans? recent memory. This summer, expect a reverse trend. The US National Oceanic and Atmospheric Administration (NOAA) now reports that the past 12 months have been the warmest ever recorded in the country since record-keeping began in 1895.

Record high temperatures have caused drought conditions and scorched crops across two-thirds of the US, putting more than 110m people under extreme heat advisories and burning up 1.3m acres last month alone. Just last week saw close to a million people suffer power outages in some areas, even as others have been taken aback by baseball-sized hail. The NOAA underlines that the odds of every single month during the June 2011-June 2012 period ranking among the warmest third of their historical distribution are 1 in 1,594,323!

There has been growing talk that the planet has entered a new geological age called the Anthropocene, aka the age of man, where the hitherto dominant natural cycle has been dramatically impacted by our species. But while it is increasingly accepted that this could be behind the growing frequency, intensity and erraticness of extreme weather events, what makes the NOAA report ground-breaking is that it identified human influence on weather extremes within months rather than years. Climate change sceptics have already begun using this to bludgeon the report?s findings, saying its fingerprinting of individual extreme weather events as evidence of global warming will not stand the test of time. As it appears in the latest bulletin of the American Meteorological Society, the Explaining Extreme Events of 2011 from a Climate Perspective report counters: ?Scientific thinking on this issue has moved on and now it is widely accepted that attribution statements about individual weather or climate events are possible, provided proper account is taken of the probabilistic nature of attribution.?

What lends NOAA?s argument credibility is (a) that report authors admit that in delivering to the ?hot and bothered? public?s clamour for an explanation of unusual environmental phenomena, they are only proffering contingent conclusions, and (b) the report does not ascribe every 2011 extreme weather event to man-made global warming. With respect to the Thailand floods last year, for example, it emphasises that non-meteorological factors like the changing hydrography of the Chao Phraya river, conversion of agricultural land to much more vulnerable industrial usage, and reservoir operation policies were much more important in setting the scale of the disaster.

What corroborates NOAA?s findings is that the US Climate Extremes Index, which tracks the highest and lowest 10% of extremes in temperature, precipitation, drought and tropical cyclones across the contiguous US, was a record-large 44% during the January-June period, over twice the average value. Extremes in warm daytime temperatures (83%) and warm night-time temperatures (70%) covering large areas of the country contributed to the record high value.

The NOAA report is also of note in the context of the well-publicised findings of the Intergovernmental Panel on Climate Change that developing regions are more vulnerable to climate changes than are developed countries. Nobody is disputing this. But if public concern about anthropogenic extreme weather events rises in the developed world, this could pull the global climate change treaty conversation out of stasis. There is unfortunately no evidence of this yet if one goes by the preoccupations of the US presidential campaign trail.

Still, NOAA has made a strong statement. As its director Thomas Karl told PBS in an interview, a series of scientific teams across the world have come together to look at selected extreme climate events of last year, to determine whether these would have occurred all by themselves, and to conclude that these wouldn?t have been as intense if it were not for the increase of greenhouse gases in the atmosphere.

An analogy from the report is worth reproducing here because it combines archetypical American colour with broader global communicability. Take a baseball player who starts taking steroids and then hits 20% more home runs in a season. This would legitimise an attribution statement that, all other things being equal, steroid use increased his probability of home runs by 20%. ?The job of the attribution assessment is to distinguish the effects of anthropogenic climate change or some other external factor from natural variability.? Replace baseball with cricket, home runs with sixes, steroids with greenhouse gases, and you will understand how NOAA concluded that last year?s Texas drought was 20 times more likely than it would have been in the 1960s with similar weather patterns.

renuka.bisht@expressindia.com