Cli­mate sci­ence

Global warm­ing may not cause storms, but it in­creases their im­pact. The cost of de­nial is grow­ing. g. By Bob Ward rd

The Observer - - FRONT PAGE - Bob Ward is pol­icy and com­mu­ni­ca­tions di­rec­tor at the Gran­tham Re­search In­sti­tute on Cli­mate Change and the En­vi­ron­ment at the London School of Eco­nom­ics and Po­lit­i­cal Sci­ence

As the US comes to terms with its sec­ond ma­jor weather disaster within a month, an im­por­tant ques­tion is whether the dev­as­ta­tion caused by hur­ri­canes Har­vey and Irma will con­vince Don­ald Trump and his ad­min­is­tra­tion of the re­al­ity of cli­mate change.

The pres­i­dent’s lux­u­ri­ous Mara-Lago es­tate in Florida may es­cape Irma’s wrath, but with the deaths of so many Amer­i­cans, and bil­lions of dol­lars in dam­age to homes and busi­nesses, the costs of cli­mate change de­nial are be­gin­ning to pile up at the door of the White House.

Just days be­fore Har­vey formed in the At­lantic last month, Trump signed an ex­ec­u­tive or­der to over­turn a pol­icy, in­tro­duced by his pre­de­ces­sor Barack Obama, to help Amer­i­can com­mu­ni­ties and busi­nesses be­come more re­silient against the risks of flood­ing, which are rising be­cause of cli­mate change.

But the mer­ci­less as­sault on the US main­land by Har­vey and Irma should be forc­ing the pres­i­dent to recog­nise the con­se­quences of his ar­ro­gance and com­pla­cency in dis­miss­ing the re­search and anal­y­sis car­ried out by sci­en­tists.

The flooded streets of Hous­ton and the wind-rav­aged homes of south Florida bear the un­mis­tak­able fin­ger­print of ex­treme weather made worse by man­made green­house gas emis­sions.

A hurricane is a huge, ro­tat­ing clus­ter of thun­der­storms that forms above a sea sur­face that has a tem­per­a­ture of at least 26.5C. It is like a gi­ant en­gine, trans­fer­ring heat from the sea sur­face up into the at­mos­phere and gen­er­at­ing strong winds and heavy rain in the process.

The At­lantic hurricane sea­son of­fi­cially runs from 1 June un­til 30 November, and typ­i­cally pro­duces 12 trop­i­cal storms, of which six reach hurricane strength – with sus­tained wind speeds of more than 73mph. There have been 11 storms so far this year, in­clud­ing six hur­ri­canes. In early Au­gust, the Na­tional Oceanic and At­mo­spheric Ad­min­is­tra­tion raised its pre­dic­tion for the sea­son to “above nor­mal”, sug­gest­ing that the fi­nal tally could be up to 19 storms, with nine of them hur­ri­canes.

But this sea­son has al­ready been re­mark­able in one re­spect be­cause two ma­jor hur­ri­canes, with sus­tained winds of more than 110mph, have hit the US main­land, af­ter a pe­riod of 12 years when none of the strong­est storms made Amer­i­can land­fall.

Hurricane Irma has also set records for be­ing the strong­est hurricane to have oc­curred in the open At­lantic, and for hav­ing sus­tained wind speeds of at least 185mph over the long­est pe­riod.

Cli­mate change can­not be blamed for the hurricane count in any sin- gle sea­son, nor for the oc­cur­rence of any sin­gle storm, but there are three ways in which it is mak­ing the con­se­quences worse.

First, al­though the in­ten­sity of a hurricane de­pends on many fac­tors, warmer seawa­ter tends to pro­mote stronger storms. Av­er­age sea sur­face tem­per­a­tures have been rising, and some parts of the North At­lantic and Gulf of Mexico are warmer than av­er­age at the moment, which is a key rea­son why both Har­vey and Irma be­came so strong so quickly.

Sec­ond, a warmer at­mos­phere can hold more water vapour, which can re­sult in heav­ier rain­fall. That is true not only for hur­ri­canes but also for weaker storms across the world. Even rel­a­tively mild trop­i­cal storms can cause great dam­age by drop­ping huge vol­umes of rain over one area.

Third, apart from strong winds and heavy rain­fall, hur­ri­canes cause dam­age through storm surges as their winds push seawa­ter ahead of them. Storm surges can inun­date ex­ten­sive low-ly­ing coastal ar­eas, sweep­ing away ev­ery­thing in their path. Sea lev­els have been grad­u­ally rising glob­ally, mak­ing storm surges big­ger and dead­lier.

Sci­en­tists are still not sure about the other ways in which cli­mate change may be im­pact­ing hur­ri­canes. The main rea­son Har­vey cre­ated such ex­treme flood­ing around Hous­ton was that it stalled over the city and dumped rain for sev­eral days with­out mov­ing on. We do not know if cli­mate change played a role in cre­at­ing the at­mo­spheric con­di­tions that made that hap­pen.

Nor can we yet pre­dict whether cli­mate change will af­fect the num­ber of hur­ri­canes that oc­cur ev­ery year. Some stud­ies have sug­gested that while num­bers will drop, strengths will in­crease.

Also un­cer­tain is how nat­u­ral cli­mate vari­abil­ity af­fects hur­ri­canes. Num­bers have in­creased markedly in the North At­lantic since the 1990s, but this seems to be due, at least partly, to large-scale changes in ocean cir­cu­la­tion that oc­cur over many years or decades.

How­ever, it is clear that the lives and liveli­hoods of mil­lions of Amer­i­cans will be at risk if Trump and his ad­min­is­tra­tion con­tinue to deny the ex­is­tence of cli­mate change and its im­pact on the threat posed by hur­ri­canes.

The storms’ as­sault should be forc­ing the US pres­i­dent to recog­nise the con­se­quences of dis­miss­ing sci­ence

Nasa/Reuters

Hurricane Irma (bot­tom right) ad­vances across the At­lantic past Puerto Rico and over the Do­mini­can Repub­lic to­wards the US main­land, 6 Septem­ber.

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