For­est study sug­gests global warm­ing could lead soils to fill air with added car­bon

Weekend Herald - - WORLD - Chris Mooney

One of the reg­u­lar complaints of cli­mate change doubters and scep­tics is that sci­en­tific pro­jec­tions of a dire fu­ture are too heav­ily based on com­puter sim­u­la­tions, or mod­els, which — they say — rest on a va­ri­ety of ques­tion­able as­sump­tions.

But a ma­jor cli­mate change study pub­lished yes­ter­day re­lied not on mod­els but ex­per­i­men­tal data — a 26- year record of ob­ser­va­tions, no less — to reach a con­clu­sion per­haps just as wor­ry­ing. The re­search, track­ing the emis­sions of car­bon from ar­ti­fi­cially heated plots of a for­est in Mas­sachusetts, re­in­forces fears about the pos­si­bil­ity of a cli­mate change “feed­back” in­volv­ing the planet’s soils, one that could pile on top of and sub­stan­tially worsen the on­go­ing warm­ing trend trig­gered by the burn­ing of fos­sil fuels.

“The study is one of the long­est if not the long­est cli­mate change ecosys­tem ex­per­i­ment, beyond the one we are run­ning in our own planet,” said Pep Canadell, an ex­pert on the Earth’s car­bon cy­cle at the Com­mon­wealth Sci­en­tific and In­dus­trial Re­search Or­gan­i­sa­tion in Aus­tralia. He was not part of the re­search.

Start­ing in 1991, a team of re­searchers have been study­ing the same 18 plots of for­est soil in the Har­vard For­est in Mas­sachusetts. Six of the plots are en­tirely undis­turbed, rep­re­sent­ing the nat­u­ral state of the for­est floor; six are ar­ti­fi­cially heated through un­der­ground ca­bles to 5C above the nor­mal tem­per­a­ture; and six are “dis­turbed”, mean­ing they con­tain heat­ing ca­bles, but the ca­bles are not ac­tu­ally pow­ered, so the tem­per­a­ture is not al­tered.

The re­searchers in­volved — af­fil­i­ated with the Ma­rine Bi­o­log­i­cal Lab­o­ra­tory at Woods Hole, Mas­sachusetts, the Univer­sity of New Hamp­shire, the Univer­sity of Mas­sachusetts Amherst, and New Hamp­shire- based Re­search De­signs — have con­tin­u­ally mea­sured the dif­fer­ence in car­bon diox­ide emis­sions em­a­nat­ing from the dif­fer­ent plots. The hy­poth­e­sis i s that warmer tem­per­a­tures would lead micro­organ­isms in the soil to be­come more ac­tive in break­ing down plant mat­ter and other ma­te­ri­als. Th­ese mi­crobes would then re­lease more soil car­bon into the at­mos­phere in the form of car­bon diox­ide gas, in a process known as res­pi­ra­tion.

Now, in the lat­est up­date of the find­ings pub­lished in the jour­nal Sci­ence, the team re­ports that 17 per cent of the car­bon has now been lost from the up­per layer of soil of the heated plots over the pe­riod of the study. Ex­trap­o­lat­ing to the scale of the planet, the re­searchers sug­gest, that could be a big deal.

“This mag­ni­tude of loss could amount to hun­dreds of peta­grams — bil­lions of met­ric tonnes — of car­bon flux­ing from the world’s for­est soils to the at­mos­phere, if those soils re­sponded to warm­ing like the Har­vard for­est soils have done over the ex­per­i­men­tal pe­riod,” said Jerry Melillo, a sci­en­tist at the Ma­rine Bi­o­log­i­cal Lab­o­ra­tory in Woods Hole who has over­seen the re­search for nearly three decades. ( A peta­gram is equiv­a­lent to a bil­lion tonnes.) The study sug­gests a pos­si­ble re­lease of about 190 peta­grams over the course of the cen­tury from the top me­tre of the Earth’s soil, which it cal­cu­lates is “equiv­a­lent to the past two decades of car­bon emis­sions from fos­sil fuel burn­ing”. And Melillo be­lieves that the pos­si­bil­ity of this oc­cur­ring isn’t ad­e­quately taken into ac­count in fu­ture cli­mate change pro­jec­tions.

“I think that everyone agrees that we don’t in­clude the mi­cro­bial feed­back mech­a­nisms very well in cli­mate mod­els,” said Melillo.

That said, the Har­vard For­est study con­tains some sub­tleties and sur­prises. No­tably, the soil car­bon did not stream out of the ground con­tin­u­ously over the 26- year pe­riod. Rather, the re­lease oc­curred in four phases.

First there was a large burst of car­bon that hap­pened over the ini­tial 10 years. Then, the car­bon re­leases ceased for the next roughly seven years, as the heated plots ceased to show any dif­fer­ence from the non­heated ones. Then the emis­sions re­sumed — and then, most re­cently, stopped again.

The pat­tern ap­pears to re­flect the kinds of or­gan­isms in the soil chang­ing so as to favour or­gan­isms that con­sume car­bon in dif­fer­ent ways. “One of the things that be­came very apparent is that we were chang­ing in sig­nif­i­cant ways the struc­ture of the mi­cro­bial com­mu­nity,” said Melillo.

The re­search ar­rives a lit­tle un­der a year af­ter another in­flu­en­tial study com­piled the re­sults of 49 sep­a­rate field stud­ies on soil car­bon emis­sions and con­cluded that this feed­back is in­deed real, based on the bal­ance of the re­search. That study con­cluded that the feed­back could lead to 50 peta­grams of emis­sions just by 2050.

Still, there are sev­eral im­por­tant cau­tions about the lat­est re­search, noted Canadell, who also heads up the Global Car­bon Project, which an­nu­ally re­leases fig­ures on how much car­bon fos­sil fuels and other parts of the Earth’s sys­tem have put in the at­mos­phere.

“By def­i­ni­tion, ma­nip­u­la­tive ex­per­i­ments like this one, are sys­tems treated with a shock per­tur­ba­tion overnight,” he noted by email. But a sud­den soil tem­per­a­ture in­crease of 5C “is un­re­al­is­tic to what is hap­pen­ing or will hap­pen the real world”, Canadell con­tin­ued, not­ing that global warm­ing ac­tu­ally pro­ceeds “at sev­eral dec­i­mal points of a de­gree warm­ing per decade”.

Canadell also ques­tioned the 190 peta­gram fig­ure for pos­si­ble global emis­sions by 2100, not­ing that the study “is in one type of ecosys­tem and in one single place in the world ( a few hun­dred square me­tres), and the one thing we know is that dif­fer­ent biomes and ecosys­tems are very dif­fer­ent, and within, there are no t wo soils patches the same”.

“This neg­a­tive as­sess­ment, how­ever, doesn’t de­tract one bit from the ex­tra­or­di­nary rel­e­vance of the data set and new un­der­stand­ing pro­duced by Melillo’s ex­per­i­ment,” Canadell con­tin­ued.

How­ever, the re­sults ul­ti­mately get in­cor­po­rated into our un­der­stand­ing of the planet, Melillo said. “I’m re­ally quite thank­ful that there is a place in sci­ence for some of th­ese long- term ex­per­i­ments at the sys­tems level, where mech­a­nisms un­fold slowly over time.”

Pic­ture / AP

The re­search tracks the emis­sions of car­bon from ar­ti­fi­cially heated plots of for­est.

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.