Published by Cambridge University Press, the book builds upon scientific findings presented at the "Avoiding Dangerous Climate Change" conference held in Exeter, England, in February last year. The conference was sponsored by the United Kingdom Department of Environment, Food and Rural Affairs.
The conference brought together more than 200 scientists and political leaders from 30 nations. Major themes included key vulnerabilities of the climate system and critical thresholds, socio-economic effects, and technologies to limit greenhouse-gas emissions.
Based on his talk at the conference, Schlesinger contributed a book chapter titled "Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation."
Higher temperatures caused by global warming could add fresh water to the northern North Atlantic Ocean by increasing the precipitation and by melting nearby sea ice, mountain glaciers and the Greenland ice sheet, Schlesinger said. This influx of fresh water could reduce the surface salinity and density, leading to a shutdown of the thermohaline circulation.
"We have evidence dating back to 1965 that shows a drop in salinity around the North Atlantic," Schlesinger said. "So far, the salinity change is small, but we could be standing at the brink of an abrupt and irreversible climate change."
Among the talking points Schlesinger will cover at the news conferences:
- The observed warming during 1856-1990 was predominantly human-induced. "Using a simple climate/ocean model, we calculated the contributions to the observed changes in global-mean, near-surface temperature caused by human and volcano forcing, and putative variations in the irradiance of the sun for the years 1856-1990," Schlesinger said. "We found the human effect has steadily increased and is now the dominant external factor. Variations in solar output played only a minor role in the observed temperature change, and we found no significant contribution from volcanoes."
- The observed melting of alpine glaciers, the Greenland and West Antarctic ice sheets, the freshening of the North Atlantic Ocean, and the slowdown of the Atlantic thermohaline circulation are the "smoking gun" of global warming. "We are seeing dangerous, human-induced climate change," Schlesinger said. "The melting of the Greenland ice sheet would raise sea level by 18 feet. Melting of the Antarctic ice sheet would raise sea level an additional 22 feet. Most coastal cities would be inundated."
- These observed changes in climate and ongoing research have shown that human-induced warming is proceeding more quickly than anticipated. "Not only are the Greenland and Antarctica ice sheets melting much faster than models predicted, measurements show a significant freshening (influx of fresh water) of the North Atlantic Ocean and a 30 percent reduction of North Atlantic circulation within the past 50 years," Schlesinger said. "What we are seeing is very worrisome. It is now clear that we have no time to spare -- we must act immediately."
- If the present course of increasing emissions continues, there is a high likelihood that the Atlantic thermohaline circulation will shut down during the next 200 years. The thermohaline circulation is driven by differences in seawater density, caused by temperature and salinity. Like a great conveyor belt, the circulation pattern moves warm surface water from the southern hemisphere toward the North Pole. Between Greenland and Norway, the water cools, sinks into the deep ocean, and begins flowing back to the south.
"This movement carries a tremendous amount of heat northward, and plays a vital role in maintaining the current climate," Schlesinger said. "If the thermohaline circulation shut down, the southern hemisphere would become warmer and the northern hemisphere would become colder. The heavily populated regions of eastern North America and western Europe would experience a significant shift in climate."
Two major factors affect the range of possible future temperature increases: Scientists don't know precisely how sensitive the climate system will be to future emissions; and they don't know exactly how much humankind will emit. People can only control one of the factors. By reducing emissions, the amount of future warming and associated impacts can be reduced.
"Recent work by five independent research teams has shown that climate sensitivity could be larger than the 4.5 degrees Celsius upper bound published by the Intergovernmental Panel on Climate Change," Schlesinger said. "In fact, climate sensitivities as high as 9 degrees Celsius are not implausible. Paralysis in near-term action to significantly reduce emissions could make mitigation nearly impossible to attain."
Two other authors and one of the book's editors will also participate in the news conferences. The May 9 news conference will begin at noon EDT at JP Morgan-Chase corporate headquarters in Manhattan. The May 10 news conference will begin at 4 p.m. in Room 485 of the Russell Senate Office Building.
Editor's note: To reach Michael Schlesinger, call 217-778-9891; e-mail: firstname.lastname@example.org.