Into the un­known realm

Cloud­seed­ing­bringsrain­fall,but­can­havead­ver­seim­pact

Down to Earth - - COVER STORY -

is an emerg­ing branch of cli­mate science to find out ways in which plan­e­tary pro­cesses can be mod­i­fied to our ad­van­tage. Clouds are one of the most ac­ces­si­ble av­enues where such changes can be made. The fact that aerosols (mi­cro­scopic solid or liq­uid par­ti­cles float­ing in air) have the abil­ity to in­flu­ence cloud prop­er­ties can be used to make them re­flect a greater amount of so­lar ra­di­a­tion or to bring rain.

Cloud seed­ing is the ma­nip­u­la­tion of clouds through the use of aerosols to in­crease pre­cip­i­ta­tion. The process in­volves in­fus­ing clouds with ad­di­tional aerosols (the most com­mon is sil­ver io­dide) from an air­craft. An in­crease in aerosols means an in­crease in the num­ber of par­ti­cles around which wa­ter vapour can con­dense. Un­der suit­able en­vi­ron­men­tal con­di­tions, this can in­crease rain­fall.

Clouds have an­other crit­i­cal role. They reg­u­late the amount of sun­light reach­ing the earth and the amount of heat earth ra­di­ates into space. This dual role can be ma­nip­u­lated through aerosols to mit­i­gate an­thro­pogenic warm­ing. The idea is to whiten clouds by in­fus­ing it with aerosols and to in­crease their re­flec­tiv­ity so that more sun­light is re­flected. This re­quires shoot­ing up jets of sea­wa­ter sprays into the at­mos­phere so that the salt aerosol par­ti­cles can in­duce whiter and more re­flec­tive cloud cover.

An­other am­bi­tious pro­posal in­volves seed­ing aerosol par­ti­cles in the up­per tro­po­sphere, where icy clouds trap the in­frared ra­di­a­tion re­flected by earth. The idea is to make this layer thin­ner so that it al­lows more in­fra-red ra­di­a­tion to es­cape into space.

In In­dia, cloud seed­ing ex­per­i­ments have yielded mixed re­sults over the past six years. But the tech­nol­ogy is be­ing ex­ten­sively stud­ied in coun­tries like China, which has spent over a bil­lion dol­lars on rain cre­ation to meet in­creas­ing de­mands for fresh­wa­ter since 2008.

Geoengineering, how­ever, faces se­vere crit­i­cism as it of­fers short-term mit­i­ga­tion. It could un­leash a cas­cade of short- and longterm im­pacts that are be­yond imag­i­na­tion be­cause the vari­ables in at­mo­spheric and oceanic sciences are still rel­a­tively un­known.

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