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3Qs: Hot, hot heat

6/12/12

A record-​​breaking heat wave hit the East Coast last week, fol­lowed by a spate of rain and thun­der­storms. We asked Auroop Gan­guly, an asso­ciate pro­fessor of civil and envi­ron­mental engi­neering whose exper­tise lies in under­standing cli­mate change and extreme weather, to explain the rela­tion­ship between heat waves and global warming.

How do you define and track a heat wave?

Like many other extreme weather phe­nomena, there is no single way to define or track heat waves, but common def­i­n­i­tions take into account their poten­tial impacts. The heat index, for example, is an aggre­gate mea­sure that com­bines tem­per­a­ture and humidity — two pri­mary weather fac­tors that deter­mine human per­cep­tion of heat. On the other hand, human mor­tality, in the dev­as­tating heat waves of Chicago or Paris, for example, is often caused not just by high day­time tem­per­a­tures but by con­sec­u­tive night­time tem­peratures exceeding cer­tain thresh­olds. One other mea­sure for heat waves is cooling degree days, which attempts to relate tem­per­a­tures to energy demand for air conditioning.

For agri­cul­ture and food secu­rity, heat waves and tem­per­a­ture extremes may be defined through spe­cific thresh­olds beyond which cer­tain crops cannot sur­vive. Occa­sion­ally while studying cli­mate vari­ability or change, we track tem­per­a­tures that set new records, which would in turn vary depending on the region and the season. One sta­tis­tical method to define and track extremes is through the rig­orous “extreme value theory,” which lead to met­rics such as 100-​​year return periods. This is the prob­a­bility that a given tem­per­ature will be exceeded once every 100 years on the average.

On the whole, the def­i­n­i­tion and tracking of heat waves is very much depen­dent on the end use. Mete­o­rol­ogists often prefer relying on the heat index or record tem­per­a­tures for given regions and sea­sons. Energy com­pa­nies and plan­ners tend to look at cooling degree days. Agri­cul­tural plan­ners may be inter­ested in the pos­si­bility of tem­per­a­tures exceeding cer­tain thresh­olds. Cli­mate sci­en­tists inter­ested in adap­ta­tion to climate change or in the phys­ical sci­ences have devel­oped an array of met­rics based on the con­se­quences of heat waves or on their link­ages to atmos­pheric physics or a combination.

Do global warming and higher temperatures go hand in hand?

In gen­eral, it is very unlikely if not impos­sible that any one heat wave, cold snap, dry spell or inci­dence of heavy rain­fall could be related to longer-​​term nat­ural cli­mate vari­ability or anthro­pogenic, or man-​​made, global warming. Cer­tainly, a heat wave at this time of the year in the North­east is not uncommon and should be viewed as weather phe­nomena rather than cli­mate phe­nomena. Weather is what’s hap­pening at any given moment and cli­mate is average weather over a period of time.

On the other hand, global warming is pro­jected to increase the occur­rence of heat waves, implying that longer lasting, more fre­quent and more intense heat waves are con­sis­tent with what one would expect as the Earth con­tinues to get warmer. How­ever, large uncer­tain­ties remain when assigning causality to spe­cific instances of regional heat waves or other weather events. In one of my 2009 papers pub­lished in the journal Pro­ceed­ings of the National Academy of Sci­ences, for example, we found that while heat waves are likely to grow even worse than pre­vi­ously thought, the remaining uncer­tain­ties and geo­graph­ical vari­ability are also more than pre­vi­ously believed.

What causes climate variability, such as changes in precipitation patterns, and what impact may it have on the environment?

Cli­mate vari­ability may be caused by nat­ural cycles ranging across dif­ferent time scales, such as the inter-​​annual El Nino to the 60– to 70-​​year Atlantic Mul­ti­decadal Oscil­la­tion. The growing tem­per­a­ture trends owing to anthro­pogenic global warming, which is super­posed over and above nat­ural vari­ability, has been noticed or is pro­jected to occur at multi-​​decadal to cen­tury scales.

At the other end of the time scale are very long-​​term changes, including changes in mon­soon rain­fall patterns. These pat­terns have been sug­gested as a likely cause for the decay of a 4000-​​year-​​old long-​​lost urban civ­i­liza­tion in the Indus valley. These dif­ferent time scales — from day-​​to-​​day weather and nat­ural cli­mate oscil­la­tions of sev­eral decades to century-​​scale warming trends and longer-​​term nat­ural vari­ability in the Earth’s cli­mate system — are impor­tant to under­stand and acknowl­edge when attempting to understand cli­mate change.

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