At risk of dis­si­pa­tion?

Clouds are fast chang­ing char­ac­ter in a chang­ing cli­mate. Sci­en­tists are yet to keep pace

Down to Earth - - COVER STORY - Shree­shan Venkatesh

POW­ERED by winds, clouds have been cir­cu­lat­ing around the globe like magic car­pets car­ry­ing weather on their back. Un­der­stand­ing them not only holds the key to de­cod­ing chang­ing weather pat­terns, it is also cru­cial for pre­dict­ing a change in cli­mate.

What mat­ters to cli­mate is how the be­hav­iour and ex­tent of clouds have changed over 250 years or will change in the fu­ture, says Neil Don­ahue, Lord Pro­fes­sor of Chem­istry at Carnegie Mellon Univer­sity, Pitts­burgh. To un­der­stand these changes cli­mate sci­en­tists, armed with con­stantly im­prov­ing tech­nolo­gies and weather satel­lite data, are busy un­tan­gling the jour­ney of clouds—right from the level of mi­cro­scopic aerosols, which act as the seed of cloud droplets, till they be­come the most cru­cial player in the cli­matic sys­tem.

But clouds keep chang­ing and re­or­gan­is­ing them­selves. Their char­ac­ter has be­come even more un­cer­tain in a chang­ing cli­mate. And this has made the job of sci­en­tists more chal­leng­ing.

Aerosols: tiny par­ti­cles, big im­pact

One of the many puz­zles that cli­mate sci­en­tists are try­ing to un­ravel is whether aerosols re­leased by hu­man ac­tiv­i­ties, such as ve­hic­u­lar and in­dus­trial emis­sions, af­fect the for­ma­tion and be­hav­iour of clouds.

Go­ing by the con­ven­tional the­ory, high aerosol load­ing in the at­mos­phere re­sults in more num­ber of cloud droplets, and that make the sky ap­pear cloudy. Since the con­cen­tra­tion of aerosols would have been low be­fore the in­dus­tri- al era be­gan in the 1800s, it is be­lieved that the pre-in­dus­trial at­mos­phere must have been less cloudy than what is ob­served now.

Cloud droplets also re­flect part of the so­lar ra­di­a­tion back into space, and thus have a cool­ing ef­fect on earth. By this logic, clouds in the in­dus­trial era would have higher cool­ing ef­fect on earth. Some spec­u­late that this cool­ing ef­fect masks the warm­ing ef­fect of green­house gases ( ghgs) re­leased into the at­mos­phere in the in­dus­trial era. “Our best es­ti­mate is that about one-third of the ex­tra warm­ing that would have been caused by car­bon diox­ide has been masked by that pol­lu­tion, and we do not re­ally know ex­actly how much hot­ter it will get if and when we get rid of that,” says Don­ahue. The un­cer­tainty is due to the lack of ref­er­ence data on at­mo­spheric ob­ser­va­tions from the pre-in­dus­trial era.

Sci­en­tists at Switzer­land’s Euro­pean Or­ga­ni­za­tion for Nu­clear Re­search, known as cern, tried to ad­dress this un­cer­tainty early this year through a high-tech ex­per­i­ment, aptly named cloud ( Cos­mics Leav­ing Out­door Droplets). They sim­u­lated the pre-in­dus­trial era at­mos­phere un­der highly con­trolled con­di­tions, and found that in the ab­sence of ghg emis­sions, nat­u­ral aerosols or bio­genic vapours re­leased by trees were ca­pa­ble of sup­port­ing the cloud cover, sim­i­lar to the present-day sit­u­a­tion.

cern re­searchers are now study­ing the ef­fects of cos­mic par­ti­cles in so­lar ra­di­a­tion on cloud for­ma­tion to un­der­stand how present­day at­mos­phere is dif­fer­ent from that of the past.

Then there are stud­ies that are try­ing to bring

What mat­ters to cli­mate is how the re­flec­tiv­ity and ex­tent of clouds have changed over the past 250 years, or will change in the fu­ture

new in­sights into the for­ma­tion of cloud droplets. Sci­en­tists from the Univer­sity of Bris­tol in the UK and the eth Zurich Univer­sity in Switzer­land in 2012 de­bunked the con­ven­tional wis­dom about cloud for­ma­tion. They said droplet for­ma­tion on or­ganic aerosols, which are usu­ally byprod­ucts of com­bus­tion pro­cesses, is not a uni­form process, and that it could take any­where from less than a sec­ond to sev­eral hours de­pend­ing on the vis­cos­ity of the aerosol par­ti­cle. The less sol­u­ble an aerosol, the longer it would take to form droplets, said the re­searchers.

In March this year, re­searchers at the US De­part­ment of En­ergy’s Lawrence Berke­ley Na­tional Lab­o­ra­tory also tried to un­der­stand the mi­cro­pro­cesses in­volved in cloud for­ma­tion by us­ing a proxy mix of or­ganic and in­or­ganic aerosols to em­u­late the at­mos­phere. They ob­served that more than the sol­u­bil­ity of aerosols, what mat­tered most is the in­ter­ac­tion at the in­ter­face of the aerosol and con­densed wa­ter vapour. Or­ganic aerosol par­ti­cles ef­fec­tively push down the sur­face ten­sion of wa­ter to fa­cil­i­tate ef­fi­cient for­ma­tion of big­ger cloud droplets, they con­cluded.

While spe­cific con­nec­tions be­tween the

chem­istry of aerosol par­ti­cles and the for­ma­tion of cloud droplets is yet to be as­cer­tained, the find­ings hold tremen­dous value for un­der­stand­ing the mi­cro­pro­cesses that de­cide the bright­ness or pre­cip­i­ta­tion po­ten­tial of clouds.

Clouds of dual na­ture

Clouds are a key com­po­nent of the cli­mate sys­tem be­cause they help reg­u­late the planet’s tem­per­a­ture. Clouds are re­spon­si­ble for both heat­ing up and cool­ing down the planet, de­pend­ing on their type and where they are lo­cated (see ‘Pole­ward drift’). For in­stance, when lo­cated at lower al­ti­tude, clouds typ­i­cally con­trib­ute to the cool­ing ef­fect by shield­ing the planet and re­flect­ing around half of the sun­light that strikes them. It gives them the white glow. It is es­ti­mated that if cloud cover were ab­sent and all the wa­ter in the clouds ex­isted as wa­ter on the sur­face of earth, the planet would have ab­sorbed about 20 per cent more heat than what it does cur­rently. This would have made the earth warmer by about 12oC. But when si­t­u­ated higher up in the at­mos­phere, they trap the in­fra-red ra­di­a­tions bounc­ing off the earth’s at­mos­phere, thus warm­ing the planet to the tune of about 7oC.

Un­der­stand­ing this dual na­ture of cloud is im­por­tant be­cause it shows how changes in clouds will af­fect the en­ergy bal­ance and ra­di­a­tion bud­get of the planet.

“Clouds have a net cool­ing ef­fect in the cur­rent cli­mate,” says Joel Nor­ris, Pro­fes­sor of Cli­mate and At­mo­spheric Sciences at the Scripps In­sti­tu­tion of Oceanog­ra­phy in San Diego, Cal­i­for­nia. This is be­cause the cool­ing ef­fect of clouds out­weighs their warm­ing ef­fect. How­ever, there is a con­fu­sion. No one is sure if the net cool­ing ef­fect will be­come stronger or weaker as global warm­ing pro­gresses, he adds.

Mi­gra­tion cri­sis

How­ever, ad­vanced re­mote sens­ing tech­nol­ogy and satel­lite im­agery over the past three decades have helped sci­en­tists make some head­way on un­der­stand­ing how clouds have been chang­ing and re­or­gan­is­ing them­selves.

Re­searchers from the Univer­sity of Wash­ing­ton and Univer­sity of Ari­zona in the US have com­piled me­te­o­ro­log­i­cal data for the pe­riod of 1954-2008 from weather ships to map the changes ob­served in clouds over oceans. They have also cre­ated a map of cloud cover changes over land by us­ing weather sta­tion data be­tween 1971 and 2009. The maps in­di­cate sig­nif­i­cant changes in their ex­tent. While cloud cover over oceans shows sub­tle re­duc­tion, it is fast re­ced­ing over land. Be­sides, mid-lat­i­tude cloud cover is re­duc­ing over the trop­ics, while all kinds of clouds are ex­pand­ing over the sub-trop­ics and high-al­ti­tude clouds are mi­grat­ing to­wards the poles. nasa con­firmed the ob­ser­va­tion in May this year.

The mi­gra­tion of high-level clouds to­wards the poles is wor­ri­some. Con­sid­er­ing the mas­sive amounts of wa­ter stored at poles, the in­crease in warmth could ul­ti­mately con­trib­ute to sea level rise around the world. The cat­a­strophic re­sults are al­ready vis­i­ble. The world’s sec­ond largest ice sheet, si­t­u­ated in Green­land, is melt­ing rapidly due to the blan­ket­ing ef­fect of clouds which are rais­ing tem­per­a­tures in the re­gion by 2-3oC. It is es­ti­mated that a third of the to­tal global sea level rise cur­rently is be­ing forced by this melt­ing.

Re­searchers at the Scripps In­sti­tu­tion of Oceanog­ra­phy say clouds are not only mov­ing pole­wards, they are mov­ing higher up in the at­mos­phere. Nor­ris, lead au­thor of the pa­per, says ex­pan­sion of the sub­trop­i­cal dry zone will lower the re­flec­tion of so­lar ra­di­a­tion back to space by

clouds and al­low earth to ab­sorb more so­lar ra­di­a­tion. This would re­sult in a warmer cli­mate. “I have also found a rise in the height of the high­est cloud tops. This is ex­pected to oc­cur with global warm­ing,” Nor­ris adds. As the lower at­mos­phere warms up and up­per at­mos­phere cools down, it will al­low buoy­ant clouds to rise higher in the sky. These clouds would trap heat and this would re­sult in a con­sid­er­able warm­ing of the cli­mate.

Still a puz­zle

Re­search on clouds has no doubt picked up con­sid­er­able speed in re­cent years. Sev­eral data-gath­er­ing projects are si­mul­ta­ne­ously us­ing ground ob­ser­va­tions, re­mote sens­ing, weather bal­loons, spe­cially equipped planes and satel­lite im­agery to try and pro­duce a co­her­ent pic­ture of how clouds are re­spond­ing to cli­mate change.

De­spite this, the prob­lem of as­sim­i­lat­ing clouds in weather and cli­mate mod­els re­mains. “Clouds have not been in­cor­po­rated in cli­mate mod­els very well be­cause the res­o­lu­tion in mod­els is crude. They have to be de­duced, and this is im­per­fect science,” says Kevin Tren­berth, se­nior sci­en­tist at the Cli­mate Anal­y­sis Sec­tion, the US Na­tional Cen­ter for At­mo­spheric Re­search.

“Cli­mate mod­els can­not han­dle the physics of clouds di­rectly and pa­ram­e­terise (in­spired guess­work) their prop­er­ties,” says Roger Davies, pro­fes­sor of cli­mate physics at the Univer­sity of Auck­land in New Zealand. Nei­ther can they han­dle the het­ero­gene­ity of real clouds that af­fect their ra­dia­tive prop­er­ties, and work with av­er­age cloud prop­er­ties. Since the ra­dia­tive and mois­ture prop­er­ties of clouds av­er­age dif­fer­ently, model clouds tend to be ei­ther too bright or too dry, says Davies, adding that mod­ellers choose to make clouds too dry.

Davies says most con­fu­sion about cloud change is due to paucity of qual­ity satel­lite ob­ser­va­tions. Global cov­er­age is es­sen­tial to get enough sam­ples, and satel­lites are the only op­tion. The early satel­lites fo­cused more on weather im­agery than cli­mate mea­sure­ments. “We have only had about two decades of qual­ity mea­sure­ments, and it will take sev­eral more decades to draw strong ob­ser­va­tional con­clu­sions about cloud changes. In the mean­time, the con­clu­sions are sub­ject to change as more data be­comes avail­able,” Davies ex­plains.

While we wait for more in­for­ma­tion on how cloud be­havioural changes man­i­fest on the ground and how ad­verse ef­fects can be mit­i­gated, it seems safe to say that un­usual and ex­treme weather pat­terns are here to stay.

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