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Methane 'belch' theory gets boost By Julianna Kettlewell

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    http://news.bbc.co.uk/2/hi/science/nature/3775181.stm Methane belch theory gets boost By Julianna Kettlewell BBC News Online science staff Scientists have
    Message 1 of 1 , Jun 7, 2004
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      http://news.bbc.co.uk/2/hi/science/nature/3775181.stm
      Methane 'belch' theory gets boost
      By Julianna Kettlewell
      BBC News Online science staff

      Scientists have found a series of vents in the Nordic Seas that may have
      burped enough methane to cause massive global warming 55 million years
      ago.

      The early Eocene Period witnessed a dramatic increase in temperature,
      which was triggered by a sudden surge of greenhouse gases in the
      atmosphere.

      But just where these gases came from has been something of a mystery.
      Nature magazine reports the discovery of gas vents dating from the right
      time and which could represent the source.

      Mammal evolution

      At the beginning of the Eocene, the Earth's temperature suddenly shot up
      by about
      five Celsius. It was one the most dramatic global warming events that has
      ever
      been recorded in geologic history.

      For human evolution, it was a vital period. The warm climate allowed
      primitive
      mammals to disperse across the Earth, diverging as they did so into many
      different
      groups. Among these newly emerging groups were mankind's primate
      ancestors.

      Scientists usually agree about the cause of this 100,000-year heatwave: a
      great
      pulse of carbon dioxide (CO2) hit the atmosphere to launch an extreme
      greenhouse event.

      However, what does cause a large amount of discussion within the
      scientific community is just how this gas was produced.

      One popular theory relates the existence of hydrates - a frozen mixture
      of water and
      methane produced by microbes - which are locked in ocean sediments. If
      the sea
      warmed up enough, the argument goes, then these hydrates would have
      melted,
      disgorging their load of methane.

      The methane (a potent greenhouse gas itself) would have then broken down
      to produce a CO2 surge. This process needs to be kick-started by minor
      global warming, and some scientists suggest that the collision of a comet
      or asteroid might have done the trick. But these accounts have suffered
      from a lack of direct evidence.

      Interesting find

      Now a team of Norwegian scientists have hit upon something that could
      explain the
      creation of these greenhouse gases.

      While studying charts of sedimentary rock produced by companies
      prospecting for
      petroleum, they noticed a series of hydrothermal vents.

      They believe that trapped organic matter was heated so intensely by angry
      seams of lava that it broke down into methane and forced its way to the
      surface forming these
      structures.

      "We believe these hydrothermal vents are a likely explanation for the
      gases that caused the climatic change," Bjorn Jamtveit, of the University
      of Oslo, told BBC News Online.

      However, the theory has been met with some scepticism. This is largely
      because the carbon these vents belched would have been rich in a form -
      or isotope - that does
      not tally with geological records.

      Rock deposits indicate the massive CO2 surge of 55 million years ago
      contained a
      lot of the isotope carbon-12. But the carbon produced when organic matter
      is heated
      intensely by lava contains a greater proportion of carbon-13.

      "The problem is that the carbon released is the wrong sort of carbon,"
      explained Mark Maslin, of University College London, UK. "I just don't
      find it a plausible theory."

      The only way this theory could explain the high levels of carbon-12 at
      the beginning
      of the Eocene is if truly enormous amounts of gas were produced. That
      way, even
      though it contains proportionally less carbon-12, the carbon cycle is
      still flooded
      with it.

      Ocean warning

      "It is a bit like having a swimming pool filled with white paint,"
      explained Gerald
      Dickens, of Rice University, Houston, US. "If you want to turn that
      swimming pool pink, you can either add a little red paint or a lot of
      pink paint."

      In other words, the gas produced by hydrothermals represents pink paint.
      To get the
      effect recorded, you need an awful lot of it.

      According to Professor Jamtveit, there was indeed an awful lot of it.
      "When we
      calculate the amount of gas that can be released by this process it is
      enormous," he
      said. "It corresponds to the amount of gas in the petroleum resources in
      the whole
      arth today."

      But Dr Maslin remains unconvinced: "I can't imagine that amount of carbon
      could be
      released just through the Nordic Seas," he said.

      If the hydrothermal vents did not cause the Eocene thermal maximum that
      could be
      worrying news for us.

      The vents only spewed methane because of a rare spate of laval activity.
      It could
      happen again, but probably not in the near future.

      On the other hand, if the hydrate-release theory is true; if minor global
      warming can
      cause gas hydrates in ocean sediments to melt and shed their methane
      load, we
      could be heading for trouble.

      Dr Maslin put it bluntly: "We've got global warming occurring - and if we
      keep it up
      will all the gas hydrate be released to cause super global warming?
      Unfortunately I
      think the answer to that is 'yes'."

      LINKS TO MORE SCIENCE/NATURE STORIES

      http://www.giss.nasa.gov/research/intro/schmidt_02/

      Ocean Burps and Climate Change?

      By Gavin Schmidt

      About 55 million years ago an event known as the Paleocene-Eocene Thermal
      Maximum (PETM) occurred. This was an episode of rapid and intense warming
      (up to 7�C at high latitudes) which lasted less than 100,000 years (see
      Figure 1). Curiously, the PETM was accompanied by an
      exceptionally large
      change to the global carbon
      cycle as indicated by a large drop in
      the isotopic ratio of 13C to
      12C in the ocean and on land.

      The ratio of these carbon isotopes generally changes as a
      function
      of biological activity, since
      carbon in living matter tends to be
      preferentially made up of 12C
      as opposed to 13C. Thus an
      increase in biological activity
      "uses up" more 12C, and therefore
      the ratio of 13C to 12C in the
      remaining carbon increases.
      Conversely, a decrease of
      biological uptake, leads to a decrease
      of the isotopic ratio (i.e., it
      gets "lighter").

      The change at the PETM was so
      large that it would have required
      a decrease in biological
      activity equivalent to roughly three times
      the total present-day
      terrestrial biosphere. In other words, if all of
      the terrestrial carbon today
      (in forests, animals, soils, etc.) were
      converted to carbon dioxide and
      returned to the global inorganic
      carbon pool, the change in the
      global carbon isotopic ratio would
      only be a third as big as that observed during the PETM! However,
      no such event is seen at the
      PETM, and thus another source for
      very "light" carbon must be
      found.

      Volcanos are another source of light carbon as carbon dioxide gas
      within eruptions. But this
      source would also imply an enormous,
      and highly unlikely, amount of
      volcanism to match the
      observations. In fact, only one source of carbon that is isotopically
      light and available in large enough quantities has been pinpointed
      so far, this is the reservoir of methane hydrate deposits (Figure 2)
      buried on the continental
      shelves of the oceans (Figure 3).

      Bacteria produce methane as
      they decompose organic matter in
      the ocean sediments, and in
      cold, high-pressure environments,
      methane hydrates will form.
      This is an ice-like solid that consists
      of methane surrounded by water
      molecules in a lattice structure.
      However, if the temperature
      warms, or the pressure is reduced
      (for instance if local sea level decreases), the hydrate will break
      up
      and release the methane as gas which can bubble up through the
      ocean and enter the atmosphere.

      What would be the consequences
      of such a large emission of
      methane into the atmosphere? At
      present, methane has a
      residence time of about 10
      years before it is oxidized to carbon
      dioxide. However, the chemistry
      of this process is highly
      non-linear, and as emissions
      increase, the capacity of the
      atmosphere to deal with the
      excess methane decreases and the
      residence time lengthens. This can lead to quite large increases in
      the methane concentration. This
      matters because molecule for
      molecule, methane is a more
      powerful greenhouse gas than
      carbon dioxide. The climate
      consequences depend very strongly
      on exactly how long the extra
      methane hangs around.

      Our research (described in
      Schmidt and Shindell 2003) has
      examined these chemical and radiative effects. Based on
      plausible
      scenarios for what may have
      occurred at the PETM, we tried to
      estimate the history of the
      methane and carbon dioxide
      concentrations. Using radiation modeling we estimated how strong
      the climate forcing would be
      for each scenario, and then ran
      general circulation models to
      see how that forcing would change
      the climate.

      We found that for some
      scenarios, the methane levels can stay
      high enough and remain long enough to play the dominant role in
      the subsequent climate warming.
      The temperature changes are
      close enough to those observed through the PETM to support both
      the hypothesized scenario and
      our modelling efforts. While there
      are huge uncertainties in
      almost every aspect of this study, this
      research shows that we can
      "connect the dots" from a methane
      hydrate forcing to the observed
      global warming.

      Methane plays a large role in
      present day climate forcing (see
      "Trends of Measured Climate
      Forcing Agents" for more details)
      and has more than doubled in
      concentration since the
      pre-industrial period. This study goes a long way in
      quantifying its
      role in paleoclimate
      variability as well.

      January 2003

      Reference

      Schmidt, G.A., and D.T.
      Shindell 2003. Atmospheric
      composition, radiative
      forcing, and climate change as a
      consequence of a massive
      methane release from gas
      hydrates.
      Paleoceanography 18, 1004,
      doi:10.1029/2002PA000757


      Contact

      Please address all inquiries about this research to Dr. Gavin
      Shmidt.


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