Around 717 million years ago, Earth’s humid landscapes and roiling blue waters transformed into a frigid, barren world. Scientists nicknamed this stage of geological history, and others like it, Snowball Earth.
What exactly froze the planet nearly solid has been a mystery, as has how it remained that way for 56 million years. On Wednesday, a team of researchers at the University of Sydney said they have it figured out. Earth’s glaciation, they say, may have come from a global drop in carbon dioxide emissions, a result of fewer volcanoes expelling the gas into the atmosphere.
Less carbon dioxide makes it more difficult for Earth’s atmosphere to trap heat. If the depletion were extreme enough, they argued, it could have thrust the planet into its longest ice age yet.
The theory, published in the journal Geology, adds insight to the way geological processes influenced Earth’s past climate. It may also help scientists better understand trends in our current climate.
“These days, of course, humans are having a large impact on CO2 in the atmosphere,” said Adriana Dutkiewicz, a sedimentologist at the University of Sydney who led the study. “But back in time, there were no humans, and so everything was basically modulated by geological processes.”
There are many ideas about what turned Earth into a snowball. One popular theory suggests that minerals released by the weathering of igneous rock sucked enough carbon dioxide from the atmosphere to set off a deep freeze.
Perhaps that helped kick off a global glaciation, Dr. Dutkiewicz said, but it couldn’t have kept Earth frozen for 56 million years on its own.
“So there has to be another mystery mechanism that would have sustained the glaciation for that long,” she said.
Dr. Dutkiewicz and her colleagues turned their eyes to volcanoes because of a newly available model of Earth’s shifting tectonic plates. As the continents spread apart, they studied the changing length of the mid-ocean ridge — a chain of underwater volcanoes — predicted by the model.
The team then calculated the amount of volcanic gas emissions at the beginning of, and throughout, the ice age. Their results showed a drop in atmospheric carbon dioxide sufficient to initiate and sustain a 56-million-year glaciation.
A reduction in volcanic gas emissions has been proposed as an explanation for Snowball Earth before. But according to Dr. Dutkiewicz, this is the first time researchers have proved that the mechanism was viable through modeled computations.
Dietmar Müller, a geophysicist at the University of Sydney and an author of the study, said the work was one way “to distinguish between alternative models for this very ancient part of Earth’s evolution.” If scientists know there was an ice age, Dr. Müller explained, “then we can say this one reconstruction model is perhaps more likely than the other one.”
Of course, a model is still just that: a model. Without real-world data to back it up, the researchers can’t rule out other possibilities.
“One thing about geology, there are no definite answers,” Dr. Dutkiewicz said. “But based on a combination of different lines of evidence, we can suggest that this is a very likely process.”
Francis Macdonald, a geologist at the University of California, Santa Barbara, who was not involved in the work, said studies like this were important for learning about why climates fail. But he is hesitant to readily accept outcomes from models of the ancient seafloor, because there is little data revealing what Earth’s oceanic crust was like at that time.
“How do we actually test that?” Dr. Macdonald asked about the team’s model. “I think it’s a really big challenge.”
Still, Dr. Müller thinks it is important to try to put bounds on the amount of volcanic gas emitted in the past, particularly when it comes to running climate models for the future. “Usually, that’s the most uncertain parameter,” he said.
Research like this can help scientists distinguish the influence of geological activity from human-induced climate change. But could a natural drop in volcanic emissions ever save us from the amount of carbon we have pumped into our atmosphere today?
“Unfortunately not,” Dr. Dutkiewicz said. “We can study these ancient perturbations,” she added, “but the human-induced change is a different kind of beast.”
Karibu miaka milioni 717 iliyopita, mazingira yenye unyevunyevu ya Dunia na maji yake ya bluu yaligeuka kuwa dunia baridi na tupu. Wanasayansi waliita hatua hii ya historia ya kijiolojia, na nyingine kama hiyo, Dunia ya Snowball. Kwa nini dunia ilibaki kama barafu kwa miaka milioni 56? Kwa mujibu wa ripoti ya utafiti wa Chuo Kikuu cha Sydney, utafiti huo umefanyika kwa njia ya "Wanasema kwamba huenda barafu za dunia zilitokana na kupungua kwa uzalishaji wa kaboni dioksidi duniani kote, kwa sababu ya idadi ndogo ya volkano zilizotupa gesi hiyo angani.""" Kupungua kwa kaboni dioksidi hufanya iwe vigumu zaidi kwa angahewa la Dunia kukamata joto. Kama ukosefu huo wa maji ungekuwa mkubwa vya kutosha, wangedokeza, ungeweza kuingiza sayari katika kipindi chake kirefu zaidi cha barafu. Nadharia hiyo, iliyochapishwa katika jarida la Geology, inaongeza ufahamu wa jinsi michakato ya kijiolojia ilivyoathiri hali ya hewa ya zamani ya Dunia. Pia yaweza kuwasaidia wanasayansi kuelewa vizuri mwelekeo wa hali yetu ya hewa ya sasa. "Kwa kweli, siku hizi wanadamu wana athari kubwa kwa CO2 katika anga, ""alisema Adriana Dutkiewicz, mtaalamu wa sedimentolojia katika Chuo Kikuu cha Sydney ambaye aliongoza utafiti huo." "Lakini nyuma katika wakati, hakukuwa na wanadamu, na hivyo kila kitu kilikuwa kimsingi modulated na michakato ya kijiolojia. " Kuna mawazo mengi kuhusu nini akageuka Dunia katika mpira wa theluji. Nadharia moja inayopendwa sana inadokeza kwamba madini yaliyotokezwa na hali ya hewa ya miamba ya volkano yalinyosha kaboni dioksidi ya kutosha kutoka angahewa ili kusababisha baridi kali. "Huenda hiyo ilisaidia kuanzisha barafu ya dunia, ""Dkt Dutkiewicz alisema, ""lakini haingeweza kuifanya dunia iwe imefungwa kwa miaka milioni 56 peke yake.""" "Kwa hiyo lazima kuwe na utaratibu mwingine wa fumbo ambao ungedumisha barafu kwa muda mrefu", alisema. Dutkiewicz na wenzake waliangalia volkano kwa sababu ya mfano mpya wa mabadiliko ya mabamba ya dunia. Mabara hayo yalipokuwa yakitawanyika, walichunguza mabadiliko ya urefu wa mteremko wa katikati ya bahari - mlolongo wa volkano za chini ya maji - uliotabiriwa na mfano huo. Kisha, kikundi hicho kilihesabu kiasi cha gesi zilizotokezwa na volkano mwanzoni mwa kipindi hicho cha barafu na wakati wote huo. Utafiti huo ulionyesha kuwa kiwango cha kaboni dioksidi katika angahewa kilipungua kiasi cha kutosha kuanzisha na kudumisha kipindi cha barafu cha miaka milioni 56. Kupunguzwa kwa uzalishaji wa gesi za volkano kumependekezwa kama ufafanuzi wa Dunia ya Snowball kabla. Hata hivyo, Dutkiewicz anasema ni mara ya kwanza kwa watafiti kuthibitisha kwamba mbinu hiyo inawezekana kwa kutumia mifano. "Dietmar Müller, mwanasayansi wa sayansi ya ardhi katika Chuo Kikuu cha Sydney na mwandishi wa utafiti huo, alisema kazi hiyo ni njia moja ya ""kufanya tofauti kati ya mifano mbadala kwa sehemu hii ya kale sana ya mageuzi ya Dunia."" ""Ikiwa wanasayansi wanajua kulikuwa na enzi ya barafu, basi tunaweza kusema kwamba mfano huu wa ujenzi ni uwezekano zaidi kuliko mwingine.""" Bila data ya ulimwengu halisi kuunga mkono, watafiti hawawezi kuondoa uwezekano mwingine. "Kitu kimoja kuhusu jiolojia, hakuna majibu ya uhakika", Dakt. Dutkiewicz alisema. "Lakini kulingana na mchanganyiko wa mstari tofauti wa ushahidi, tunaweza kupendekeza kwamba hii ni mchakato unaowezekana sana. "" Francis Macdonald, mtaalamu wa jiolojia katika Chuo Kikuu cha California, Santa Barbara, ambaye hakuhusika katika kazi hiyo, alisema masomo kama haya yalikuwa muhimu kwa kujifunza juu ya kwa nini hali ya hewa inashindwa." Lakini anashindwa kukubali matokeo ya mifano ya sakafu ya bahari ya kale, kwa sababu kuna data chache zinazoonyesha jinsi ganda la bahari la Dunia lilivyokuwa wakati huo. "Tunawezaje kupima hilo?" Dakt. Macdonald aliuliza kuhusu mfano wa timu hiyo. "Mimi nafikiri ni changamoto kubwa sana, ""Muhammad Müller anasema, akisema ""Ni muhimu kujaribu kuweka mipaka juu ya kiasi cha gesi ya volkano iliyotolewa katika siku za nyuma, hasa linapokuja suala la kuendesha mifano ya hali ya hewa kwa ajili ya siku zijazo.""" "Kwa kawaida, hiyo ni parameter isiyo na uhakika zaidi", alisema. Utafiti kama huu unaweza kuwasaidia wanasayansi kutofautisha ushawishi wa shughuli za kijiolojia na mabadiliko ya hali ya hewa yanayosababishwa na wanadamu. Lakini je, kupungua kwa gesi inayotokana na volkano kunaweza kutukomboa kutokana na kiasi cha kaboni ambacho tumepeleka angani leo? "Kwa kusikitisha sivyo", Dakt. Dutkiewicz akasema. "Tunaweza kujifunza machafuko haya ya kale", aliongeza, "lakini mabadiliko yanayosababishwa na binadamu ni aina tofauti ya mnyama".