Earth's Climate Approaches Dangerous Tipping Point

NEW YORK, New York, June 1, 2007 (ENS) - A stern warning that global warming is nearing an irreversible tipping point was issued today by the climate scientist who the Bush administration has tried to muzzle.

James Hansen, director of NASA's Goddard Institute for Space Studies in New York, today published a study showing that greenhouse gases emitted by human activities have brought the Earth’s climate close to critical tipping points, with potentially dangerous consequences for the planet.

"If global emissions of carbon dioxide continue to rise at the rate of the past decade," said Dr. Hansen, "this research shows that there will be disastrous effects, including increasingly rapid sea level rise, increased frequency of droughts and floods, and increased stress on wildlife and plants due to rapidly shifting climate zones."

Tipping points can occur during climate change when the climate reaches a state such that strong amplifying feedbacks are activated by only moderate additional warming.

Physicist James Hansen heads the NASA's Institute for Space Studies, a division of Goddard Space Flight Center in Greenbelt, Maryland. (Photo courtesy NASA)
Dr. Hansen has said in the past that a global tipping point will be reached by 2016 if levels of greenhouse gases like methane and carbon dioxide are not reduced.

This study finds that global warming of 0.6ºC in the past 30 years has been driven mainly by increasing greenhouse gases and only moderate additional climate forcing is likely to set in motion disintegration of the West Antarctic ice sheet and Arctic sea ice.

Amplifying feedbacks include increased absorption of sunlight as melting exposes darker surfaces and speedup of iceberg discharge as the warming ocean melts ice shelves that otherwise inhibit ice flow.

The research appears in the current issue of "Atmospheric Chemistry and Physics."

In January 2006, Dr. Hansen said that officials at NASA headquarters had ordered the public affairs staff to review his coming lectures, papers, postings on the Goddard website and requests for interviews from journalists.

Dr. Hansen said he would decline to adhere to the restrictions. NASA officials said its scientists were free to discuss science but not policy issues.

In March 2007, Hansen told the House of Representatives Committee on Oversight and Government Reform, "The effect of the filtering of climate change science during the current administration has been to make the reality of climate change less certain than the facts indicate, and to reduce concern about the relation of climate change to human-made greenhouse gas emissions."

From a combination of climate models, satellite data, and paleoclimate records, Hansen and co-author Makiko Sato of Columbia’s Earth Institute, conclude that the West Antarctic ice sheet, Arctic ice cover, and regions providing fresh water sources and species habitat are threatened from continued global warming.

NASA collects data with one of three Global Positioning System stations installed in Marie Byrd Land, West Antarctica. (Photo courtesy NASA)
The researchers used data on earlier warm periods in Earth’s history to estimate climate impacts as a function of global temperature,

They used climate models to simulate global warming, and satellite data to verify ongoing changes.

The researchers also investigated what would be needed to avert large climate change, thus helping define practical implications of the United Nations Framework Convention on Climate Change.

That treaty, signed in 1992 by the United States and almost all nations of the world, aims to stabilize atmospheric greenhouse gases "at a level that prevents dangerous human-made interference with the climate system."

Based on climate model studies and the history of the Earth, the Hansen and Sato conclude that additional global warming of about 1ºC (1.8ºF) or more, above global temperature in 2000, is likely to be dangerous.

In turn, the temperature limit has implications for atmospheric carbon dioxide, CO2, which has already increased from the pre-industrial level of 280 parts per million, ppm, to 383 ppm today and is rising by about two ppm per year.

Sato said, "The temperature limit implies that CO2 exceeding 450 ppm is almost surely dangerous, and the ceiling may be even lower."

The study also shows that the reduction of non-carbon dioxide greenhouse gases such as methane and black soot can offset some of the increase in carbon dioxide, but only to a limited extent.

Hansen today urged immediate action to limit climate change. "We probably need a full court press on both CO2 emission rates and non-CO2 forcings, to avoid tipping points and save Arctic sea ice and the West Antarctic ice sheet," he said.

In May another team of NASA and university scientists found the first evidence that extensive areas of snow melted in west Antarctica in January 2005 in response to warm temperatures. (Photo courtesy NASA)
In climate science, radiative forcing is loosely defined as the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system.

A positive forcing, with more incoming energy, tends to warm the system, while a negative forcing, with more outgoing energy, tends to cool it.

A computer model developed by the Goddard Institute was used to simulate climate from 1880 through today. The model included a more comprehensive set of natural and human-made climate forcings than previous studies, including changes in solar radiation, volcanic particles, human-made greenhouse gases, fine particles such as soot, the effect of the particles on clouds and land use.

Extensive evaluation of the model’s ability to simulate climate change is contained in a companion paper to be published in the journal "Climate Dynamics."

The authors use the model for climate simulations of the 21st century using both ‘business-as-usual’ growth of greenhouse gas emissions and an alternative scenario in which emissions decrease slowly in the next few decades and then rapidly to achieve stabilization of atmospheric CO2 amount by the end of the century.


Antarctic ice is melting (Photo courtesy British Antarctic Survey)
Climate changes are so large with the business-as-usual scenario, that Hansen concludes "business-as-usual would be a guarantee of global and regional disasters."

The business-as-usual scenario specifies additional global warming of 2 to 3ºC (3.6 to 5.4ºF).

But the study finds one-quarter to one-third less severe climate change when greenhouse gas emissions follow the alternative scenario.

"Climate effects may still be substantial in the alternative scenario, but there is a better chance to adapt to the changes and find other ways to further reduce the climate change,” said Sato.

While the scientists say it is still possible to achieve the alternative scenario, they warn that significant actions will be required to do so.

Emissions must begin to slow soon. "With another decade of business-as-usual," says Hansen, "it becomes impractical to achieve the alternative scenario because of the energy infrastructure that would be in place."

Dr. Hansen, 65, is a physicist who joined NASA in 1967. Since the 1970s he has worked on computer simulations of the Earth’s climate in a effort to understand humanity’s impact upon it.

For more about Dr. Hansen's work see:
House Panel Investigates Bush's Climate Science Manipulations

Outspoken American Climate Scientist Honored By WWF

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