Ancient Climate Studies Suggest Earth On Fast Track To Global Warming

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."

Cool? If he think rapid climate change was due to periodical orbital anomalies 20 M years ago, why does he need a different mechanism for 55 M years ago? Are those orbital changes periodic or are they not periodic?

And why no mention of the much higher methane levels of the Paleocene and the 21 times stronger greenhouse potential of methane over CO2?

What I smell is a funding deficit. Orbital changes won't draw Ford, Pew or even NASA money. Methane isn't a by-product of industry and not a big field of study unless you want to be laughed at by the general public for studying cow farts.

But if you can have those billions of tons CH4 meet free O2 on their bubble to the surface and convert into that dastardly CO2, then you have something that you can beat industry over the head with to make it worthy of funding.

BTW. I wonder what happens to the 4 leftover hydrogen atoms in his model? Maybe he's come up with the key to a hydrogen economy --- farts in the bathtub, so to speak.

If he think rapid climate change was due to periodical orbital anomalies 20 M years ago, why does he need a different mechanism for 55 M years ago?

The PETM mechanism is (apparently) not related to orbital cycle forcing.

And why no mention of the much higher methane levels of the Paleocene and the 21 times stronger greenhouse potential of methane over CO2?

Why does that need to be mentioned? I'm not following you here.

The methane release scenario for the PETM has been around awhile. This isn't new. The rate-of-release comparison and the number of ocean cycles that it occurred over, comparing the PETM to the modern era, is what I haven't seen before. The conversion of methane to CO2 goes back to the "Ocean Burps" link I posted higher in the thread -- dated 2003, I think.

If CH4 is oxidized by O2, then the H gets converted to H20. I.e., CH4 + 2 O2 --> C02 + 2 H20.

First of all, there is far less scientific consensus than the press, the environmental "movement" (Luddites in my vernacular), and the political left would have you believe. In fact quite a few scientists (climatologists included) disagree with the whole conclusion that human activities are the cause of "global warming".

Dr. Frederick Seitz Past President, National Academy of Sciences and President Emeritus, Rockefeller University for one has set up a petition project to permit the voices of dissenting scientists to be heard. For an overview of the project visit the website: http://www.oism.org/pproject/s33p41.htm

One of the things about science is that, like the weather and climate, it is always in a state of flux. New theories challenge the old, become accepted, and are in turn replaced with others. Two major reasons to be skeptical of the "global warming" movement in particular are the smugness and intolerance of dissent displayed by its proponents, and the vast amounts of money involved.

What would be the mean global temperature during the interglacials only?

Taking the current interglacial period as "today", the mean temperature of the four previous ones is about 1 - 1.5 degrees C warmer.

The current interglacial period is about the same length (so far) as the most previous one (starting about 140 000 years ago). The earlier ones were a lot shorter.

Again, these are based on the Voshtok Ice Core data. Once I get ahold of the Dome C data (goes back about 600 000 years) I will do a comparison.

Two major reasons to be skeptical of the "global warming" movement in particular are the smugness and intolerance of dissent displayed by its proponents, and the vast amounts of money involved.

I'd add a third. What can be generously considered amateurish errors in interpreting data (more likely intentional fraud) i.e. Mann's famous "Hockey Stick" chart which the entire IPCC and Kyoto is predicated on. If their data were as strong as their propaganda machine, there would be not need to falsify results, but they have been caught doing it over and over again.

Thanks for the link. I did take a look. However such a conclusion is a real, real stretch. Where is the plain English explanation of the methodology, the assumptions, and the data used for such a prediction? I suspect that there will be really large holes in at least two of those areas.

Both methane and CO2 are greenhouse gases that change the Earth's radiative balance if their atmospheric concentration changes.

As other posters pointed out, methane is a much stronger GH gas. They change the radiative balance in different ways, increases in CO2 have less effect as the overall concentration increases while methane has more as well as being 20 times more heat absorbent at current concentrations. In short, rapid rises in methane could cause warming but rapid rises in CO2 would not without the forcing mechanism causing rises in water vapor. We've talked about forcing before and I will say it again, forcing is a figment of a model and a theory, yet to be proven or demonstrated outside of a model.

You agree then that we may be soon to freeze our butts off?

Based on all the data I've seen (and I'm talking about data, not computer models, which can be made to say anything you want), I agree, with the proviso that "soon" might mean a hundred years from now.

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