Ce méthane va accélérer le réchauffement qui va donc accélérer cette libération de méthane, pour
A cette époque les dinosaures avaient disparus depuis 10 millions d'années, il n'y avait pas de glaces importantes aux pôles, même avant, et donc bien moins de gaz méthane stockés aux fonds des océans, que
Le passage du Paléocène à l'Eocène, il y a 55,8 millions années, a été marqué par la plus rapide et importante perturbation climatique du Cénozoïque. Un événement soudain a provoqué le réchauffement de la planète, conduisant au Paleocene-Eocene Thermal Maximum (PETM) (maximum thermique du Paléocène-Eocène), associé à des changements dans les circulations océanique et atmosphérique, à l'extinction de nombreux foraminifères benthiques, et à l'important renouvellement de la faune de mammifères terrestres qui coïncida avec l'émergence de bon nombre des principaux ordres de mammifères actuels.
L'événement a vu les températures mondiales augmenter d'environ 6°C sur seulement 20 000 ans, avec une hausse correspondante du niveau des mers en même temps que l'ensemble des océans se réchauffaient1. Les concentrations atmosphériques de dioxyde de carbone (CO2) ont augmenté, entraînant une élévation de la lysocline. L'anoxie de certaines eaux profondes peut avoir joué un rôle dans les extinctions marines. L'événement est lié à une diminution de l'isotope δ13C, qui se déroula sur deux périodes courtes (environ 1 000 ans). Celle-ci est sans doute la conséquence du dégazage des clathrates (dépôts de « glace de méthane »), qui a accentué une tendance préexistante au réchauffement. La libération de ces clathrates, et, finalement, le PETM lui-même, peuvent avoir été déclenchés par une série de causes.
Une quantité de carbone à peu près aussi grande que les gisements actuels de charbon, de pétrole et de gaz naturel pénétra dans l'atmosphère terrestre lors du PETM. Déjà chaude, la terre se réchauffa en moyenne d'encore 5°C, puis mit plus de 150 000 ans pour absorber l'excès de carbone et se refroidir.
A tremendous release of methane gas frozen beneath the sea floor heated the Earth by up to 13 degrees Fahrenheit (7 degrees Celsius) 55 million years ago, a new NASA study confirms. NASA scientists used data from a computer simulation of the paleo-climate to better understand the role of methane in climate change. While most greenhouse gas studies focus on carbon dioxide, methane is 20 times more potent as a heat-trapping gas in the atmosphere.
In the last 200 years, atmospheric methane has more than doubled due to decomposing organic materials in wetlands and swamps and human aided emissions from gas pipelines, coal mining, increases in irrigation and livestock flatulence.
However, there is another source of methane, formed from decomposing organic matter in ocean sediments, frozen in deposits under the seabed.
"We understand that other greenhouse gases apart from carbon dioxide are important for climate change today," said Gavin Schmidt, the lead author of the study and a researcher at NASA's Goddard Institute for Space Studies in New York, NY and Columbia University's Center for Climate Systems Research. "This work should help quantify how important they have been in the past, and help estimate their effects in the future."
The study will be presented on December 12, 2001, at the American Geophysical Union (AGU) Fall Meeting in San Francisco, Calif.
Generally, cold temperatures and high pressure keep methane stable beneath the ocean floor, however, that might not always have been the case. A period of global warming, called the Late Paleocene Thermal Maximum (LPTM), occurred around 55 million years ago and lasted about 100,000 years. Current theory has linked this to a vast release of frozen methane from beneath the sea floor, which led to the earth warming as a result of increased greenhouse gases in the atmosphere.
A movement of continental plates, like the Indian subcontinent, may have initiated a release that led to the LPTM, Schmidt said. We know today that when the Indian subcontinent moved into the Eurasian continent, the Himalayas began forming. This uplift of tectonic plates would have decreased pressure in the sea floor, and may have caused the large methane release. Once the atmosphere and oceans began to warm, Schmidt added, it is possible that more methane thawed and bubbled out. Some scientists speculate current global heating could eventually lead to a similar scenario in the future if the oceans warm substantially.
When methane (CH4) enters the atmosphere, it reacts with molecules of oxygen (O) and hydrogen (H), called OH radicals. The OH radicals combine with methane and break it up, creating carbon dioxide (CO2) and water vapor (H2O), both of which are greenhouse gases. Scientists previously assumed that all of the released methane would be converted to CO2 and water after about a decade. If that happened, the rise in CO2 would have been the biggest player in warming the planet. But when scientists tried to find evidence of increased CO2 levels to explain the rapid warming during the LPTM, none could be found.
The models used in the new study show that when you greatly increase methane amounts, the OH quickly gets used up, and the extra methane lingers for hundreds of years, producing enough global warming to explain the LTPM climate.
"Ten years of methane is a blip, but hundreds of years of atmospheric methane is enough to warm up the atmosphere, melt the ice in the oceans, and change the whole climate system," Schmidt said. "So we may have solved a conundrum."
Schmidt said the study should help in understanding the role methane plays in current greenhouse warming.
"If you want to think about reducing future climate change, you also have to be aware of greenhouse gases other than carbon dioxide, like methane and chlorofluorocarbons," said Schmidt. "It gives a more rounded view, and in the short-term, it may end up being more cost-efficient to reduce methane in the atmosphere than it is to reduce carbon dioxide."
ScienceDaily (Nov. 9, 2011) — The release of massive amounts of carbon from methane hydrate frozen under the seafloor 56 million years ago has been linked to the greatest change in global climate since a dinosaur-killing asteroid presumably hit Earth 9 million years earlier. New calculations by researchers at Rice University show that this long-controversial scenario is quite possible.
Nobody knows for sure what started the incident, but there's no doubt Earth's temperature rose by as much as 6 degrees Celsius. That affected the planet for up to 150,000 years, until excess carbon in the oceans and atmosphere was reabsorbed into sediment.
Earth's ecosystem changed and many species went extinct during the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago, when at least 2,500 gigatonnes of carbon, eventually in the form of carbon dioxide, were released into the ocean and atmosphere. (The era is described in great detail in a recent National Geographic feature.)
A new report by Rice scientists in Nature Geoscience suggests that at the time, even though methane-containing gas hydrates -- the "ice that burns" -- occupied only a small zone of sediment under the seabed before the PETM, there could have been as much stored then as there is now.
This is a concern to those who believe the continued burning of fossil fuels by humans could someday trigger another feedback loop that disturbs the stability of methane hydrate under the ocean and in permafrost; this change could warm the atmosphere and prompt the release of large amounts of methane, a more powerful greenhouse gas than carbon dioxide.
Some who study the PETM blame the worldwide burning of peat, volcanic activity or a massive asteroid strike as the source of the carbon, "but there's no crater, or any soot or evidence of the burning of peat," said Gerald Dickens, a Rice professor of Earth science and an author of the study, who thinks the new paper bolsters the argument for hydrates.
The lead author is graduate student Guangsheng Gu; co-authors are Walter Chapman, the William W. Akers Professor in Chemical Engineering; George Hirasaki, the A.J. Hartsook Professor in Chemical Engineering; and alumnus Gaurav Bhatnagar, all of Rice; and Frederick Colwell, a professor of ocean ecology and biogeochemistry at Oregon State University.
In the ocean, organisms die, sink into the sediment and decompose into methane. Under high pressure and low temperatures, methane molecules are trapped by water, which freezes into a slushy substance known as gas hydrate that stabilizes in a narrow band under the seafloor.
Warmer oceans before the PETM would have made the stability zone for gas hydrate thinner than today, and some scientists have argued this would allow for much less hydrate than exists under the seafloor now. "If the volume -- the size of the box -- was less than today, how could it have released so much carbon?" Dickens asked. "Gu's solution is that the box contains a greater fraction of hydrate."
"The critics said, 'No, this can't be. It's warmer; there couldn't have been more methane hydrate,'" Hirasaki said. "But we applied the numerical model and found that if the oceans were warmer, they would contain less dissolved oxygen and the kinetics for methane formation would have been faster."
With less oxygen to consume organic matter on the way down, more sank to the ocean floor, Gu said, and there, with seafloor temperatures higher than they are today, microbes that turn organic matter into methane work faster. "Heat speeds things up," Dickens said. "It's true for almost all microbial reactions. That's why we have refrigerators."
The result is that a stability zone smaller than what exists now may have held a similar amount of methane hydrate. "You're increasing the feedstock, processing it faster and packing it in over what could have been millions of years," Dickens said.
While the event that began the carbon-discharge cycle remains a mystery, the implications are clear, Dickens said. "I've always thought of (the hydrate layer) as being like a capacitor in a circuit. It charges slowly and can release fast -- and warming is the trigger. It's possible that's happening right now."
That makes it important to understand what occurred in the PETM, he said. "The amount of carbon released then is on the magnitude of what humans will add to the cycle by the end of, say, 2500. Compared to the geological timescale, that's almost instant."
"We run the risk of reproducing that big carbon-discharge event, but faster, by burning fossil fuel, and it may be severe if hydrate dissociation is triggered again," Gu said, adding that methane hydrate also offers the potential to become a valuable source of clean energy, as burning methane emits much less carbon dioxide than other fossil fuels.
The calculations should encourage geologists who discounted hydrates' impact during the PETM to keep an open mind, Dickens said. "Instead of saying, 'No, this cannot be,' we're saying, 'Yes, it's certainly possible.'"
Je n'avais jamais entendu parler de ce lieu!
