Scientists first speculated about a link between carbon dioxide and atmospheric temperatures in the 19th century. But highly accurate measurements of atmospheric carbon dioxide levels didn’t begin until 1958, when oceanographer Charles David Keeling of the Scripps Institution of Oceanography installed instruments atop Mauna Loa on the island of Hawaii. His findings of yearto- year increases are universally accepted. Beginning in the 1980s, analysis of deep ice-core samples extracted from ancient glaciers in East Antarctica and Greenland have yielded historical data on temperature and the atmosphere’s chemical makeup.
The ice-core research shows a striking correlation between carbon dioxide levels and temperature. Moreover, carbon dioxide levels are now higher than at any time in the past 440,000 years. Before the Industrial Revolution, atmospheric carbon dioxide concentrations remained relatively flat. Since then, they have climbed by nearly a third and are now increasing at the unprecedented rate of 0.4 percent a year. And since the last ice age, about half the world’s forest cover has been lost, and along with it an important mechanism for soaking up atmospheric carbon dioxide.
In the past century, average global temperatures at the earth’s surface climbed by about 1.08 degrees Fahrenheit (.6 degrees Celsius) to their highest in at least a millennium. During the past 25 years, the rate of temperature increase has been even greater, about 3.6 degrees F, if extrapolated over a century. The ten warmest years since 1860 have all occurred since 1990, with 1998 being the warmest of all, according to the federal NationalClimaticDataCenter. The years 2002 and 2003 are virtually tied for second place.
At the same time, researchers have documented changes consistent with an enhanced greenhouse effect. The world’s landmasses aren’t cooling off nearly as much as they used to when night falls. Less snow covers the Northern Hemispherein winter. Less sea ice appears in the Arctic in spring and summer. Glaciers are retreating and shrinking, sometimes drastically; Mount Kenya’s largest glacier has shrunk by more than 90 percent; Kilimanjaro’s glaciers, by more than 70 percent; and 14 of Spain’s 27 glaciers have disappeared altogether since 1980. Habitats for a number of plants and animals are moving to higher (cooler) latitudes. Warming spells threaten tropical reefs.
Scientists say they have ruled out natural causes, such as a periodic surge in the sun’s output and volcanic activity, as an explanation for the climate change. “There is new and stronger evidence that most of the warming observed over the past 50 years is attributable to human activities,” concludes Climate Change 2001, the third assessment report of the Intergovernmental Panel on Climate Change (IPCC), established by the World Meteorological Organization and the United Nations Environmental Programme. The report represents some 1,300 authors and 1,200 expert reviewers and editors and 100 national governments. Similar conclusions were reached by a 2001 U.S. National Research Council panel and U.S. Climate Action Report 2002, a multiagency project that Secretary of State Colin Powell submitted to the U.N.
Despite such an emerging scientific consensus, the Bush administration says that additional scientific data are needed to warrant the economic disruptions that mandatory carbon dioxide reductions would cause. U.S. Senior Climate Negotiator Harlan Watson has said the administration’s approach to reducing greenhouse gas emissions was based principally on voluntary efforts and developing new, more fuel-efficient technologies. Yet the White House stirred controversy last year when it deleted sections of the Environmental Protection Agency’s draft report on the state of the environment that referred to human contributions to climate change. Meanwhile, the federal Energy Information Administration is forecasting that U.S. carbon dioxide emissions from energy use will grow by nearly 40 percent in the next 20 years.
To the extent that disagreement exists within the scientific community, it relates to the future—how severe global warming is likely to be and what its effects will look like. Scientists use global climate models to develop scenarios. All the models project a warmer future: between 2.5 to 10.4 degrees F warmer by 2100. Even at the lowest end of the projected range, temperatures will climb more than twice as much this century as they did during the 20th. The projected increase, the IPCC report says, “is very likely to be without precedent during at least the last 10,000 years.” The models also project that global sea levels will rise between 3.5 and 34.6 inches this century—and continue to rise for centuries. Even a lower-end sea-level rise of 11.8 inches would cause a typical shoreline to retreat 98 feet. Most of the projected rise will occur because water expands as it warms, but some of it will come from the melting of glaciers and ice caps.
Still, accurately gauging sea level is complicated and can quickly become a matter not of science but of politics, as Tuvaluans have recently learned.
On Funafuti’s deep-sea wharf is a structure that resembles a portable toilet enclosed in a wire-mesh fence. It is one of 12 monitoring stations around the Pacific that the Australian government has established since 1992 “to measure sea level and associated meteorological parameters.”
The controversy began in 2000, when then director of Australia’s National Tidal Centre (NTC), Wolfgang Scherer, announced that after seven years of measurements around the Pacific “there is no acceleration in sea level rise—none that we can discern at all.” Tuvalu, in particular, got a pie in the face: the NTC announced that sea level at Funafuti had actually fallen by 3.42 inches since 1993. “Falling Sea Level Upsets Theory of Global Warming” read a headline from the LondonTelegraph at the time.