Press "Enter" to skip to content

Who Is Driving Whom? Climate and Carbon Cycle in Perpetual Interaction and Changing Feedback Mechanisms

The research vessel JOIDES Resolution in Fremantle (Australia) the morning before the ship sailed on Expedition 356. The results are based on samples taken from this drilling vessel as part of the International Ocean Discovery Program IODP. Credit: William Crawford, IODP JRSO

Bremen climate scientists disclose changing feedback mechanisms between climate and global carbon cycle over the last 35 million years.

The current climate crisis underlines that carbon cycle perturbations can cause significant climate change. New research reveals how carbon cycle and global climate have been interacting throughout the last 35 million years of geologic history, under natural circumstances. The study, led by David De Vleeschouwer from MARUM – Center for Marine Environmental Sciences at the University of Bremen, was now published in Nature Communications.

Man-made global heat­ing has long been presen­ted as a re­l­at­ively simple chain of cause and ef­fect: hu­mans dis­rupt the car­bon cycle by burn­ing fossil fuels, thereby in­crease the con­cen­tra­tion of CO2 in the at­mo­sphere, which in turn leads to higher tem­per­at­ures around the globe. “However, it be­comes in­creas­ingly clear that this is not the end of the story. Forest fires be­come more fre­quent all over the world, re­lease ad­di­tional CO2 into the at­mo­sphere, and fur­ther re­in­force the global warm­ing that en­hanced forest fire risk in the first place. This is a text­book ex­ample of what cli­mate sci­ent­ists call a pos­it­ive feed­back mech­an­ism,” stresses David De Vleeschouwer, a postdoc­toral re­searcher at MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences at the Uni­versity of Bre­men.  

To re­veal these kind of cli­mate-car­bon cycle feed­back mech­an­isms un­der nat­ural cir­cum­stances, David De Vleeschouwer and col­leagues ex­ploited iso­topic data from deep-ocean sed­i­ment cores. “Some of these cores con­tain sed­i­ments of up to 35 mil­lion years old. Des­pite their re­spect­able age, these sed­i­ments carry a clear im­print of so-called Mil­anković cycles. Mil­anković cycles re­late to rhythmic changes in the shape of the Earth’s or­bit (ec­cent­ri­city), as well as to the tilt (ob­liquity) and ori­ent­a­tion (pre­ces­sion) of the Earth’s ro­ta­tional axis. Like an as­tro­nom­ical clock­work, Mil­anković cycles gen­er­ate changes in the dis­tri­bu­tion of solar in­sol­a­tion over the planet, and thus pro­voke ca­denced cli­mate change,” ex­plains David De Vleeschouwer. “We looked at the car­bon and oxy­gen iso­tope com­pos­i­tion of mi­cro­fossils within the sed­i­ment and first used the ec­cent­ri­city, ob­liquity and pre­ces­sion ca­dences as geo­lo­gical chro­no­met­ers. Then, we ap­plied a stat­ist­ical method to de­term­ine whether changes in one iso­tope sys­tem lead or lag vari­ab­il­ity in the other iso­tope.”

His col­league Max­imilian Vah­len­kamp adds: “When a com­mon pat­tern in both iso­tope sys­tems oc­curs just a little earlier in the car­bon sys­tem com­pared to the oxy­gen iso­tope sys­tem, we call this a car­bon-iso­tope lead. We then in­fer that the car­bon cycle ex­er­ted con­trol over the cli­mate sys­tem at the time of sed­i­ment de­pos­ition.” Pa­leo­cli­mato­lo­gists and pa­leocean­o­graph­ers of­ten use car­bon iso­topes as an in­dic­ator of car­bon-cycle per­turb­a­tions, and oxy­gen iso­topes as a proxy for changes in global cli­mate state. Changes in the iso­topic com­pos­i­tion of these deep-sea mi­cro­fossils may in­dic­ate, for ex­ample, an in­crease in the con­tin­ental car­bon stor­age by land plants and soils, or global cool­ing with a growth of ice caps. 

“The sys­tem­atic and time-con­tinu­ous ana­lysis of leads and lags between car­bon cycle and cli­mate con­sti­tutes the in­nov­at­ive char­ac­ter of this study. Our ap­proach al­lows to se­quence Earth’s his­tory at high res­ol­u­tion over the past 35 mil­lion years,” says Prof Heiko Pä­like. “We show that the past 35 mil­lion years can be sub­divided in three in­ter­vals, each with its spe­cific cli­mate-car­bon cycle modus op­erandi.” On av­er­age, the au­thors found oxy­gen iso­topes to lead car­bon iso­tope vari­ations. This means that, un­der nat­ural con­di­tions, cli­mate vari­ations are largely reg­u­lat­ing global car­bon cycle dy­nam­ics. However, the re­search team fo­cused on times when the op­pos­ite was the case. In­deed, De Vleeschouwer and col­leagues found a few ex­amples of an­cient peri­ods dur­ing which the car­bon cycle drove cli­mate change on ap­prox­im­ately 100,000-year times­cales, just as it is the case now on much shorter times­cales – “but then of course without hu­man in­ter­ven­tion,” states Pä­like.

Dur­ing the old­est in­ter­val, between 35 and 26 mil­lion years ago, the car­bon cycle took the lead over cli­mate change mostly dur­ing peri­ods of cli­mate sta­bil­ity. “Peri­ods of cli­mate sta­bil­ity in the geo­lo­gic re­cord of­ten have an as­tro­nom­ical cause. When the Earth’s or­bit around the sun is close to a per­fect circle, sea­sonal in­sol­a­tion ex­tremes are trun­cated and more equable cli­mates are en­forced,” ex­plains David De Vleeschouwer. “Between 35 and 26 mil­lion years ago, such as­tro­nom­ical con­fig­ur­a­tion would have been fa­vour­able for a tem­poral ex­pan­sion of the Ant­arc­tic ice sheet. We pro­pose that un­der such a scen­ario, the in­tens­ity of gla­cial erosion and sub­sequent rock weath­er­ing in­creased. This is im­port­ant, be­cause the weath­er­ing of silic­ate rocks re­moves CO2 from the at­mo­sphere, and thus ul­ti­mately con­trols the green­house ef­fect.”  

But around 26 mil­lion years ago, the modus op­erandi rad­ic­ally changed. The car­bon cycle took con­trol over cli­mate at times of cli­mate volat­il­ity, not sta­bil­ity. “We be­lieve this change traces back to the up­lift of the Hi­m­alayan moun­tains and a mon­soon-dom­in­ated cli­mate state. When sea­sonal in­sol­a­tion ex­tremes are amp­li­fied through an ec­cent­ric Earth or­bit, mon­soons can be­come truly in­tense. Stronger mon­soons per­mit more chem­ical weath­er­ing, the re­moval of CO2 from the at­mo­sphere and thus a car­bon-cycle con­trol over cli­mate.”  

The mech­an­isms pro­posed by the au­thors not only ex­plain the ob­served pat­terns in car­bon and oxy­gen iso­topes, they also provide new ideas as to how the cli­mate sys­tem and the car­bon cycle in­ter­ac­ted through time. “Some hy­po­theses need fur­ther test­ing with nu­mer­ical cli­mate and car­bon cycle mod­els, but the pro­cess-level un­der­stand­ing presen­ted in this work is im­port­ant be­cause it provides a glimpse at the ma­chinery of our planet un­der bound­ary con­di­tions that are fun­da­ment­ally dif­fer­ent from today’s,” says De Vleeschouwer. Moreover, this work also provides scen­arios that can be used to eval­u­ate the abil­ity of cli­mate-car­bon cycle mod­els when they are pushed to the ex­treme scen­arios of the geo­lo­gic past.

Reference: “High-latitude biomes and rock weathering mediate climate–carbon cycle feedbacks on eccentricity timescales” by David De Vleeschouwer, Anna Joy Drury, Maximilian Vahlenkamp, Fiona Rochholz, Diederik Liebrand and Heiko Pälike, 6 October 2020, Nature Communications.
DOI: 10.1038/s41467-020-18733-w

Source: SciTechDaily