October 12, 2009
Scientists Look At Ancient CO2 Levels
Scientists say that a new historical record of carbon dioxide levels is showing that current political targets on climate may be "playing with fire."
The researchers used ocean sediments to map CO2 levels dating back to 20 million years ago.
The researchers noted in the journal Science that this extends knowledge of the link between CO2 and climate back in time.
Antarctica's temperatures and atmospheric content have left a series of chemical clues in the layers of its ice cores, helping scientists map the last 800,000 years accurately by drilling.
However, a further look into history has been more problematic. The new record shows more precise estimates of historical records than what were previously available for the 20 million year timeframe.
The new research could look back as far as the Miocene period, which began over 20 million years ago.
Carbon dioxide concentrations in the atmosphere were at roughly 400 parts per million at the start of the period, before beginning to decline about 14 million years ago.
The high concentrations were probably sustained by volcanic activity that took place in what is now the Columbia River basin of North America.
Carbon dioxide concentrations have been much lower in the intervening millennia. In the last few million years, they have cycled between 180ppm and 280ppm, which is in sequence of ice ages and warmer interglacial periods.
The scientists believe with humanity's emissions today that the CO2 will be more in the 400ppm range within a decade.
"What we have shown is that in the last period when CO2 levels were sustained at levels close to where they are today, there was no icecap on Antarctica and sea levels were 25-40m higher," said research leader Aradhna Tripati from the University of California at Los Angeles (UCLA).
"At CO2 levels that are sustained at or near modern day values, you don't need to have a major change in CO2 levels to get major changes in ice sheets," she told BBC News.
The elevated CO2 and sea levels were associated with temperatures about 5-11F higher than today.
The researchers collected the data from ratios of boron and calcium in the shells of tiny marine organisms called foraminifera.
The ratio indicates the pH of seawater at the time the organisms grew. This let scientists calculate the carbon dioxide content of the atmosphere.
The shell fragments were collected from cores drilled from the Pacific Ocean's floor.
Jonathan Overpeck said this provides a more accurate look at how past CO2 values relate to climate than previous methods.
"This is yet another paper that makes the future look more scary than previously thought by many," said the University of Arizona scientist.
"If anyone still doubts the link between CO2 and climate, they should read this paper."
Tripati said the new research does not imply that reaching CO2 levels this high would definitely result in huge sea level changes, but that sustaining such levels on a long timescale might produce such changes.
"There aren't any perfect analogies in the past for climate change today or in the future," she said.
"We can say that we've identified past tipping points for ice sheet stability; the basic physics governing ice sheets that we've known from ice cores are extended further back, and... I think we should use our knowledge of the physics of climate change in the past to prepare for the future."
Governments pledged to stabilize greenhouse gas concentrations at the Rio Earth Summit of 1992 "at a level that would prevent dangerous anthropogenic interference with the climate system."
That level has been debated through the years, but currently people are more supportive of the 450ppm figure.
The International Energy Agency (IEA) released its prescription on Tuesday for taking on climate change. It expects concentrations of greenhouse gases to peak at the equivalent of 510ppm of CO2 before finally stabilizing at 450ppm.
The Boxer-Kerry Bill, which just entered the U.S. Senate, also focuses on the number being 450ppm.
"Trouble is, we don't know where the critical CO2 or temperature threshold is beyond which ice sheet collapse is inevitable," said Dr Overpeck.
"It could be below 450ppm, but it is more likely higher - not necessarily a lot higher - than 450ppm.
"But what this new work suggests is that... efforts to stabilize at 450ppm should avoid going up above that level prior to stabilization - that is, some sort of 'overshoot' above 450ppm on the way to stabilization could be playing with fire."
The Association of Small Island States (Aosis) is pushing for adoption of the much lower figure of 350ppm because of concurs about short-term sea level rise.
However, with concentrations already substantially higher, political support for that is scanty outside Aosis members, which includes low-lying countries like The Maldives, Palau and Grenada.
Image Caption: Core samples were gather by the drilling ship, Joides Resolution. Image Courtesy NASA/JOI Alliance/IODP
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