Zachos said. "It will take tens of thousands of years before atmospheric carbon dioxide comes down to preindustrial levels. Even after humans stop burning fossil fuels, the effects will be long lasting."

Santa Cruz CA (SPX) Feb 16, 2006

Human activities are releasing greenhouse gases more than 30 times

faster than the rate of emissions that triggered a period of extreme

global warming in the Earth's past, according to an expert on

ancient climates.

"The emissions that caused this past episode of global warming

probably lasted 10,000 years. By burning fossil fuels, we are likely

to emit the same amount over the next three centuries," said James

Zachos, professor of Earth sciences at the University of California,

Santa Cruz.

Zachos will present his findings this week at the annual meeting of

the American Association for the Advancement of Science (AAAS) in

St. Louis. He is a leading expert on the episode of global warming

known as the Paleocene-Eocene Thermal Maximum (PETM), when global

temperatures shot up by 5 degrees Celsius (9 degrees Fahrenheit).

This abrupt shift in the Earth's climate took place 55 million years

ago at the end of the Paleocene epoch as the result of a massive

release of carbon into the atmosphere in the form of two greenhouse

gases: methane and carbon dioxide.

Previous estimates put the amount of released carbon at 2 trillion

tons, but Zachos showed that more than twice that amount--about 4.5

trillion tons--entered the atmosphere over a period of 10,000 years

(Science, June 10, 2005). If present trends continue, this is the

same amount of carbon that industries and automobiles will emit

during the next 300 years, Zachos said.

Once the carbon is released into the atmosphere, it takes a long

time for natural mechanisms, such as ocean absorption and rock

weathering, to remove excess carbon from the air and store it in the

soil and marine sediments. Weathering of land rocks removes carbon

dioxide permanently from the air, but is a slow process requiring

tens of thousands of years. The ocean absorbs carbon dioxide much

more rapidly, but only to a point. The gas first dissolves in the

thin surface layer of the ocean, but this surface layer quickly

becomes saturated and its ability to absorb more carbon dioxide

declines.

Only mixing with the deeper layers can help restore the ability of

the surface water to absorb additional carbon dioxide from the

atmosphere. But the natural processes that mix and circulate water

between the ocean surface and deeper ocean layers work very slowly.

A complete "mixing cycle" takes about 500 to 1,000 years, Zachos

said.

The greenhouse emissions that triggered the PETM initially exceeded

the ocean's absorption capacity, allowing carbon to accumulate in

the atmosphere. Unfortunately, humans appear to be adding carbon

dioxide to the air at a much faster rate: about the same amount of

carbon (4.5 trillion tons), but within a few centuries instead of

10,000 years. What was emitted 55 million years ago over a period of

about 20 ocean mixing cycles is now being emitted over a fraction of

a cycle.

"The rate at which the ocean is absorbing carbon will soon

decrease," Zachos said.

Compounding this concern is the possibility that higher temperatures

could retard ocean mixing, further reducing the ocean's capacity to

absorb carbon dioxide. This could have the kind of "positive

feedback" effect that climate researchers worry about: reduced

absorption, leaving more carbon dioxide in the air, causing more

warming.

Higher ocean temperatures could also slowly release massive

quantities of methane that now lie frozen in marine deposits. A

greenhouse gas 20 times more potent than carbon dioxide, methane in

the atmosphere would accelerate global warming even further.

Such positive feedback or "threshold" effects probably drove global

warming during the PETM and a few other ancient climate extremes,

Zachos said, and they could happen again. It is possible that we

already are in the early stages of a similar climate shift, he said.

"Records of past climate change show that change starts slowly and

then accelerates," he said. "The system crosses some kind of

threshold."

Clues to what happened during the PETM lie buried deep inside the

sediment at the bottom of the sea, which Zachos and his colleagues

have probed during several cruises of the Ocean Drilling Program

(ODP). Composed mainly of clay and the carbonate shells of

microplankton, this sediment accumulates slowly, but steadily--up to

2 centimeters every millennium--and faithfully records changes in

ocean chemistry. The layer of sediment deposited during the PETM,

now buried hundreds of meters below the seafloor, tells a clear and

compelling story of sudden change and slow recovery, he said.

During the PETM, unknown factors released vast quantities of methane

that had been lying frozen in sediment deposits on the ocean floor.

After release, most of the methane reacted with dissolved oxygen to

form carbon dioxide, which made the seawater more acidic. Acidic

seawater corrodes the carbonate shells of microplankton, dissolving

them before they can reach the ocean floor and reducing the

carbonate content of marine sediment.

Zachos led an international team of scientists that analyzed

sediment cores recovered from several locations during an ODP cruise

in the southeastern Atlantic. Collected at depths ranging from 2.5

to 4.8 kilometers (1.6 to 3.0 miles), each sediment core bore a

telltale PETM imprint: a 10- to 30-centimeter layer of dark red

carbonate-free clay sandwiched between bright white carbonate-rich

layers.

by relating the thickness of the clay layer to the rate of

accumulation of marine sediment, Zachos estimated that it took

100,000 years after the PETM for carbon dioxide levels in the air

and water to return to normal. This finding is consistent with what

geochemists have predicted using models of how the global carbon

cycle will respond to carbon dioxide emissions from the burning of

fossil fuels.

"We set out to test the hypotheses put forward by a small group of

geochemists who model the global carbon cycle, and our findings

support their predictions," Zachos said. "It will take tens of

thousands of years before atmospheric carbon dioxide comes down to

preindustrial levels. Even after humans stop burning fossil fuels,

the effects will be long lasting."

http://www.terradaily.com/reports/Ancient_Climate_Studies_Suggest_Ear

th_On_Fast_Track_To_Global_Warming.html

posted by Pat N on the Yahoo Group ClimateConcern