Mounting evidence suggests early agriculture staved off global cooling
A new analysis of ice-core climate data, archeological evidence and ancient pollen samples strongly suggests that agriculture by humans 7,000 years ago likely slowed a natural cooling process of the global climate, playing a role in the relatively warmer climate we experience today.
A study detailing the findings is published online in a recent edition of the journal Reviews of Geophysics, published by the American Geophysical Union.
"Early farming helped keep the planet warm," said William Ruddiman, a University of Virginia climate scientist and lead author of the study, who specializes in investigating ocean sediment and ice-core records for evidence of climate fluctuations.
A dozen years ago, Ruddiman hypothesized that early humans altered the climate by burning massive areas of forests to clear the way for crops and livestock grazing. The resulting carbon dioxide and methane released into the atmosphere had a warming effect that "cancelled most or all of a natural cooling that should have occurred," he said.
That idea, which came to be known as the "early anthropogenic hypothesis" was hotly debated for years by climate scientists, and is still considered debatable by some of these scientists. But in the new paper, Ruddiman and his 11 co-authors from institutions in the United States and Europe say that accumulating evidence in the past few years, particularly from ice-core records dating back to 800,000 years ago, show that an expected cooling period was halted after the advent of large-scale agriculture. Otherwise, they say, the Earth would have entered the early stages of a natural ice age, or glaciation period.
The Earth naturally cycles between cool glacial periods and warmer interglacial periods because of variations in its orbit around the sun. We currently are in an interglacial period, called the Holocene epoch, which began nearly 12,000 years ago.
In 2003, Ruddiman developed his early anthropogenic hypothesis after examining 350,000 years of climate data from ice cores and other sources. He found that during interglacial periods, carbon dioxide and methane levels decreased, cooling the climate and making way for a succeeding glacial period. But, only during the Holocene era, these gas levels rose, coinciding, he said, with the beginning of large-scale agriculture. He attributed the rise to this human activity, which began occurring millennia before the industrial era.
He attributed the rise in carbon dioxide emissions to the slash and burn techniques widely used by early farmers to make available large areas of land for crops. Ruddiman found that carbon dioxide levels rose beginning 7,000 years ago, and that methane began rising 5,000 years ago. He said this explains why a cooling trend didn't happen that likely otherwise would have led to a new glacial period.
In the new study, Ruddiman and his colleagues have delved more deeply into the climate record using Antarctic ice-core data, dating back to 800,000 years ago. This use of a deeper historical data set clearly shows, they say, that the Holocene is unlike other interglacial periods in its abundance of carbon dioxide and methane, further implicating the impact of humans.
In the development of his hypothesis, Ruddiman and colleagues have drawn from numerous studies across scientific disciplines: climatology, anthropology, archaeology, paleoecology, and population dynamics, all to better understand how humans may have affected climate beyond the relatively recent industrial revolution and the widespread burning of fossil fuels.
They cite a recent study that also summarized archaeological studies and found that early rice irrigation, which releases methane gas to the atmosphere, explains most of the anomalously high rise in atmospheric methane beginning about 5,000 years ago. A proliferation of livestock farming during that time period also may explain part of the methane increase.
"After 12 years of debate about whether the climate of the last several thousand years has been entirely natural or in considerable part the result of early agriculture, converging evidence from several scientific disciplines points to a major anthropogenic influence," Ruddiman said.
Provided by University of Virginia
Uchunguzi mpya wa data ya hali ya hewa ya barafu, ushahidi wa akiolojia na sampuli za chavua za kale zinaonyesha kuwa kilimo cha wanadamu miaka 7,000 iliyopita kinaweza kupunguza mchakato wa baridi ya asili ya hali ya hewa ya ulimwengu, na kucheza jukumu katika hali ya hewa ya joto tunayoiona leo. Utafiti huo umefunuliwa katika toleo la hivi karibuni la jarida la Reviews of Geophysics, lililochapishwa na Shirika la Geofizikia la Marekani. "Kulima mapema ilisaidia kuweka sayari joto, ""alisema William Rudiman, mwanasayansi wa hali ya hewa wa Chuo Kikuu cha Virginia na mwandishi mkuu wa utafiti huo, ambaye ni mtaalamu wa kuchunguza sediment ya bahari na rekodi za barafu kwa ushahidi wa mabadiliko ya hali ya hewa." Ruddiman alidai kwamba wanadamu wa mapema walifanya mabadiliko ya hali ya hewa kwa kuchoma misitu ili kuondoa ardhi kwa ajili ya malisho na malisho. "Kutokana na hilo, kaboni dioksidi na methani iliyotolewa angani ilikuwa na athari ya joto ambayo ""iliondoa sehemu kubwa au yote ya baridi ya asili ambayo ingekuwa imepaswa kutokea,"" alisema." "Hiyo ni kwa sababu nadharia ya ""anthropogenic hypothesis"" ilizungumziwa kwa muda mrefu na wanasayansi wa hali ya hewa na bado inaonekana kuwa ya kutatanisha." Katika makala yake ya hivi karibuni, Ruddiman na waandishi wenzake 11 kutoka taasisi za Marekani na Ulaya wanasema kwamba ushahidi wa kukusanya katika miaka michache iliyopita, hasa kutoka kwa rekodi za barafu ya miaka 800,000 iliyopita, zinaonyesha kwamba kipindi cha baridi kilichotarajiwa kilisimamishwa baada ya kuanzishwa kwa kilimo kikubwa. Kama sivyo, wanasema, Dunia ingekuwa imeingia katika hatua za mapema za enzi ya barafu ya asili, au kipindi cha barafu. Dunia huzunguka kwa kawaida kati ya vipindi vya baridi vya barafu na vipindi vya joto vya kati ya barafu kwa sababu ya tofauti katika mzunguko wake kuzunguka jua. Katika kipindi cha kati ya barafu, kinachoitwa Holocene, miaka 12,000 iliyopita, Mwaka 2003, Ruddiman alianzisha nadharia yake ya anthropogenic baada ya kuchunguza miaka 350,000 ya data ya hali ya hewa kutoka kwa nyuklia za barafu na vyanzo vingine. Katika kipindi cha kati ya barafu, viwango vya kaboni dioksidi na methani vilipungua, na hivyo hali ya hewa ikawa baridi na kufungua njia kwa ajili ya kipindi kingine cha barafu. Lakini, ni wakati wa enzi ya Holocene tu, viwango hivi vya gesi viliongezeka, ikilinganishwa, alisema, na mwanzo wa kilimo kikubwa. Alidai kwamba ongezeko hilo lilitokana na utendaji huo wa kibinadamu, ambao ulianza maelfu ya miaka kabla ya enzi ya viwanda. "Mwanasayansi huyo wa sayansi anasema kwamba ongezeko la uzalishaji wa kaboni dioksidi lilitokana na mbinu za ""kuchoma na kuchoma"" zilizotumiwa na wakulima wa mapema ili kutengeneza mashamba makubwa kwa ajili ya mazao." Ruddiman alibainisha kuwa kiwango cha kaboni dioksidi kiliongezeka kuanzia miaka 7,000 iliyopita na kwamba methani ilianza kuongezeka miaka 5,000 iliyopita. "Hii inaelezea kwa nini mwenendo wa baridi haukufanyika, ambayo ingesababisha kipindi kipya cha barafu.""" Katika utafiti huu mpya, Ruddiman na wenzake wamechunguza kwa undani zaidi rekodi ya hali ya hewa kwa kutumia data ya barafu ya Antarctic, ambayo ni ya miaka 800,000 iliyopita. "Kutumia data ya kina zaidi ya kihistoria inaonyesha wazi kwamba Holocene ni tofauti na vipindi vingine vya kati ya barafu katika wingi wake wa kaboni dioksidi na methane, ikionyesha zaidi athari za wanadamu. """ Katika kuendeleza nadharia yake, Ruddiman na wenzake wametumia masomo mengi katika taaluma za kisayansi: hali ya hewa, anthropolojia, akiolojia, paleoekolojia, na mienendo ya idadi ya watu, yote ili kuelewa vizuri jinsi wanadamu wanaweza kuwa wameathiri hali ya hewa zaidi ya mapinduzi ya hivi karibuni ya viwanda na kuchoma kwa wingi kwa mafuta ya mafuta. "Wanasema utafiti wa hivi karibuni ambao pia ulifupisha masomo ya akiolojia na kugundua kwamba umwagiliaji wa mchele wa mapema, ambao hutolewa gesi ya methane kwenye anga, unaelezea zaidi ya ongezeko kubwa la methane ya angahewa kuanzia miaka 5,000 iliyopita. """ Kuenea kwa ufugaji wa mifugo katika kipindi hicho cha wakati pia kunaweza kueleza sehemu ya ongezeko la methani. "Baada ya miaka 12 ya mjadala juu ya kama hali ya hewa ya maelfu kadhaa ya miaka iliyopita imekuwa ya asili kabisa au kwa sehemu kubwa matokeo ya kilimo cha mapema, ushahidi unaoungana kutoka kwa taaluma kadhaa za kisayansi unaonyesha ushawishi mkubwa wa binadamu. """ Inatolewa na Chuo Kikuu cha Virginia