An iceberg floats off Greenland. Climate scientists say that among the… (Joe Raedle, Getty Images )
Scientists sounded alarms Tuesday with a pair of studies challenging the idea that climate change is occurring gradually over the century and that its worst effects can be avoided by keeping emissions below a critical threshold.
A National Research Council report says the planet is warming so quickly that the world should expect abrupt and unpredictable consequences in a matter of years or a few decades. Among the changes already underway are the sudden decline in Arctic sea ice and climbing extinction rates, the report found.
Scientists based their findings, in part, on the study of climate history as recorded in tree rings, ocean sediment and ice cores. They found the timeline punctuated by big, sudden changes, including ocean circulation shifts and mass extinctions.
As a result of the burning of fossil fuels, industrial activity and deforestation, the amount of carbon dioxide and other heat-trapping greenhouse gases in the atmosphere has soared to levels not seen in millions of years, with global temperatures rising by about 1.5 degrees. The scientists say the accelerating gas levels increase the risk of reaching various "tipping points," leaving nature and society little time to react.
"To willfully ignore the threat of abrupt change could lead to more costs, loss of life, suffering and environmental degradation," said the council, an arm of the National Academies that produces reports for the U.S. government.
Even slow and steady changes in climate can have sudden repercussions, the report says. A slight increase in sea level could be enough to overtop a levee during a storm, and a tiny rise in ocean acidity from carbon dioxide could make seawater inhospitable to coral.
The scientists also call for an early warning system to anticipate sudden climate shifts. They envision a network of satellites and land-based monitors tracking ocean temperatures near polar ice sheets and methane levels in the Arctic. Those monitors would work alongside existing networks for drought, famine and groundwater depletion.
"We watch our streets, we watch our banks ... but we do not watch our environment with the same amount of care and zeal," said James White, a climatologist at the University of Colorado, Boulder and chairman of the committee that wrote the report.
A separate study published Tuesday challenges a widely held view that the world can avoid the most dangerous consequences of climate change by limiting emissions to 1 trillion metric tons of carbon since preindustrial times. Staying below that ceiling would keep global temperatures from rising more than 3.6 degrees, a threshold the United Nations-backed Intergovernmental Panel on Climate Change has endorsed.
An international group of scientists, health experts, legal scholars and economists under the auspices of Columbia University says the cap is too high. Fossil fuel emissions must be kept to half that and global warming held to about 1.8 degrees to avert disastrous consequences, they say, including many feet of sea level rise and irreversible ecological harm.
"If we don't get on a downward emissions pathway this decade, young people are likely to inherit a climate system with dramatic consequences out of their control," said James Hansen, the climate scientist who led the study.
Stabilizing Earth's climate would also require restoring 100 billion metric tons of carbon to forests and soils through better management, the authors say. Going beyond typical research papers, they call for a global tax on carbon to facilitate a transition to nuclear and renewable energy.
Hansen, who retired this year as director of the NASA Goddard Institute for Space Studies to join a policy-oriented initiative at the Earth Institute at Columbia University, is an outspoken advocate of aggressive climate-change-fighting measures who has sometimes faced criticism from fellow scientists.
Martin Hoerling, a meteorologist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory, said he does not agree with some of the conclusions of Hansen's group, including predictions that the 3.6-degree warming target would eventually raise sea level about 20 feet. But he shares their sense of urgency.
"Our planet can't sustain the comfort zone to which we've become adapted unless we change our pace of emissions," he said.
Other experts said a cap on carbon is pointless unless technological advances make renewable energy an affordable alternative.
The National Research Council report was not entirely grim.
Scientists now believe several threats once of imminent concern, including the potential shutdown of an important circulation pattern in the Atlantic Ocean and a massive release of methane from Arctic permafrost, are no longer likely to happen this century.
But the near-term likelihood of other big changes, such as a collapse in the West Antarctic Ice Sheet that would unleash a sudden rise in sea level, are unknown, the report said.