Emis­sions off­set ice age

Hu­mans’ ef­fect on Earth is coun­ter­act­ing ten­dency of glacial cy­cles

The Denver Post - - NEWS - By Chris Mooney

At a time of in­tense plan­e­tary warm­ing, it’s odd to con­tem­plate a coun­ter­fac­tual world in which we might be in or head­ing into a glacial pe­riod, pop­u­larly called an “ice age.”

But new re­search pub­lished Wed­nes­day in the in­flu­en­tial jour­nal Na­ture sug­gests that we may have had a close scrape with such a pe­riod ear­lier in the cur­rent ge­o­log­i­cal epoch known as the Holocene — and that pre-in­dus­trial hu­man mod­i­fi­ca­tions of the cli­mate through agri­cul­ture, fires and de­for­esta­tion might have just barely staved it off.

“Hu­man­ity nar­rowly es­caped a glacial in­cep­tion in the middle of the Holocene, which was al­most sup­press­ing the for­ma­tion of civ­i­liza­tion,” says Hans Joachim Schellnhu­ber, one of the pa­per’s three au­thors and found­ing di­rec­tor of the Pots­dam In­sti­tute for Cli­mate Im­pact Re­search.

More­over, the study says, mas­sive hu­man green­house gas emis­sions since that time have prob­a­bly “post­poned” what might oth­er­wise be an­other ice age “by at least 100,000 years.”

The new re­search is based on the idea that there are two key fac­tors that shape whether Earth goes into an ice age. There’s one that hu­mans can in­flu­ence, as well as one they re­ally can’t.

The fac­tor out of our con­trol is Earth’s Mi­lankovitch cy­cles, which de­scribe the er­ratic way in which the planet or­bits the sun and spins on its axis over vast time pe­ri­ods. Earth’s or­bit grows slowly more and less el­lip­ti­cal, even as the an­gle of the planet’s ax­ial tilt and the wob­ble of the poles as the planet spins (much like what you see with a spin­ning top) also change slightly over thou­sands of years.

All of this can af­fect the de­liv­ery of sun­light over dif­fer­ent parts of Earth and the na­ture of the sea­sons and thus, whether it’s pos­si­ble to build up huge ice masses on land. But there’s also a se­cond fac­tor that’s in our con­trol — how much car­bon diox­ide is in the at­mos­phere. We are able to turn this knob by how many forests we cut down and how many fos­sil fu­els we burn, both pro­cesses that trans­fer car­bon into the at­mos­phere.

At­mo­spheric car­bon diox­ide traps heat, caus­ing an over­all warm­ing ef­fect, and this will hap­pen no mat­ter where the planet is in its var­i­ous or­bital cy­cles. And if there’s enough of it, it can coun­ter­act the ten­dency of th­ese cy­cles to make and then un­make ice ages.

Us­ing anal­y­sis of past plan­e­tary glacia­tions and a com­puter model of Earth that is able to pre­dict their oc­cur­rence, the re­searchers found that car­bon diox­ide con­cen­tra­tions were only slightly too high to push us into glacia­tion a few thou­sand years ago.

“The Earth sys­tem would al­ready be well on the way to­wards a new glacial state if the pre-in­dus­trial CO2 level had been merely 40 (parts per mil­lion) lower than it was dur­ing the late Holocene,” the au­thors write. In­deed, they note that about 800,000 years ago, or­bital align­ments were sim­i­lar, but car­bon diox­ide con­cen­tra­tions were around 240 parts per mil­lion, and glacia­tion did in­deed oc­cur.

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