SCIENCE

Research suggests man-caused emissions may have very long-term impacts

Most current climate models only consider global warming's effects for the next century or so, and the results are worrisome enough; the decline of many different species, sea-level rise and unpredictable weather patterns are just a few of the trends scientists now predict.

But two recent papers suggest that even under relatively modest emissions scenarios, the impacts of man-caused climate change could carry on for tens of thousands of years.

One study introduces a new way of looking at climate feedbacks, or the ways in which different earth systems enhance or dampen global warming. By integrating short-term feedbacks with feedbacks that take millenia to kick in, the author calculates that human-caused emissions could have ripple effects for up to 165,000 years.

The second paper, published in the journal Science, focuses specifically on how high greenhouse gas levels may affect oceans, projecting that the consequences of man-caused warming will affect some marine ecosystems for at least 100,000 years.

"It's truly a very long-term problem, and that only gets worse the longer we continue putting CO2 into the atmosphere," said Sandra Kirtland Turner, research fellow at the University of Bristol in the United Kingdom and co-author of the Science paper.

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Sediment from past oceans may be a mirror

Turner and her co-authors used a unique climate model to examine long-term changes on ocean chemistry, using an emissions scenario in which humans release about 2,000 gigatons of carbon into the atmosphere over the next century.

This model was able to show how man-caused emissions might affect the sedimentary record of the future -- in other words, it showed what future geologists might see in an ocean sediment core harvested a few hundred thousand years from now, if today's emissions levels continue to rise.

The authors then compared this future record with actual sediment records taken during a period known as the Paleocene-Eocene Thermal Maximum (PETM), which took place about 56 million years ago, around the time mammals began to dominate the globe. Although it's still unclear to scientists exactly how this happened, a massive release of greenhouse gases caused a swift rise in the world's temperatures for a period of about 200,000 years.

"It's the best approximation we think we have to the future in terms of the rate of change," Turner said.

It turned out that the hypothetical sedimentary record of the future "might look a lot like the PETM," Turner said, meaning the way the oceans reacted to a fast increase of greenhouse gases 56 million years ago could provide a rough preview of what's to come.

During the PETM, only a modest number of species became extinct in the surface oceans, the paper states. But the impact on coral reefs and the deep-sea species was far more severe with major die-offs occurring in each ecosystem.

"We can't predict the exact outcome for any individual species or groups of species, but we do see that the combination of things like a reduced oxygen content ... and increased acidity of the deep ocean are things that are really challenging for deep-sea organisms," Turner said. "Across the PETM, one group of organisms, called benthic foraminifera, had an extinction rate of about 50 percent."

There are important differences between conditions during the PETM period and our own. The initial concentration of atmospheric carbon dioxide was higher, so sea surface temperatures were probably warmer. Scientists also think it's unlikely that there were permanent ice caps on the poles.

Additionally, the researchers suspect the release of greenhouse gases into the atmosphere that took place happened at a far slower rate than what is happening today.

"If we believe the PETM was slower than our present emissions rate, then this means that the severity of some future impacts could be worse," Turner said. During this time period, extreme temperatures persisted for around 50,000 years. At minimum, the model predicts that "major environmental change" will take place for at least 20,000 years, Turner said, with other effects lasting for up to 100,000 years.

Second study suggests longer, stronger warming

Turner stressed that how different climate feedbacks might reduce or enhance the concentration of greenhouse gases in the future is uncertain, and in need of further research.

But a second paper, published recently in the journal Proceedings of the National Academy of Sciences, suggests that climate feedbacks may do more harm than good, exacerbating the impacts of man-made greenhouse gas emissions.

In the paper, Richard Zeebe, oceanography professor at the University of Hawaii, Manoa, proposes that past climate models don't take into account how fast climate feedbacks, like the reflection of sunlight from sea ice and cloud cover, will play into climate feedbacks that take thousands of years to play out, such as methane emissions from vegetation and reflection due to land ice cover.

"What is new here, which is actually a relatively simple idea, is just to combine these two and look how the fast feedback sensitivity evolves over time into earth system sensitivity, and then make predictions for the next few centuries to millennia," Zeebe said.

When both types of feedbacks are added into the equation, Zeebe projects that the climate will become even more sensitive, making the Earth even warmer for a longer period of time. This increases the possibility of major sea-level rise and the melting of large ice sheets, as well as an uptick in carbon release from permafrost and oceanic methane hydrates.

Peak warming under Zeebe's scenario could last for about 10,000 years, while the full range of man-caused warming's impacts could play out for between 23,000 and 165,000 years -- a wide margin of error, Zeebe allowed, but still longer than earlier studies have implied.

"Even if you were to bring emissions to zero tomorrow, there's a long tail of climate change that we're already committed to," Zeebe said. "The overall concentration of CO2 will remain elevated in the atmosphere for tens of thousands of years."

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