A team of US and Canadian scientists claims to have found the strongest evidence yet that human activities are contributing to global warming.
Building a history of Arctic climate from tree rings, ice cores, silt layers on lake bottoms, and tiny marine fossils, the researchers say that from 1840 to about 1950, the region warmed to its highest temperatures in 400 years.
Natural factors account for warming during the early part of the 110-year period, say scientists. But during the second half, especially since 1920, natural factors fail to account for all the heating.
"It's the most definitive sign yet that [human-induced] greenhouse-gas warming is under way," says Jonathan Overpeck, head of the National Oceanic and Atmospheric Administration's Paleoclimatology Program and leader of the research group.
The study's publication in yesterday's issue of Science comes at time of ardent debate within the United States and among industrial countries over proposals to reduce carbon dioxide emissions. Next month, these nations meet in Kyoto, Japan to seek a common response to global warming.
This study supports other research showing rising temperatures in the Arctic region. At a conference in Anchorage, Alaska, earlier this year, scientists noted that Nome, now enjoys at least 10 more days each year of 70-degree temperature, compared with two or fewer days a year between 1915 and 1970. As a result, hardy willow-type plants are appearing for the first time in valleys in the Brooks Range, which spans the northern part of the state.
The NOAA study is also the latest in a growing body of evidence that European nations are likely to use to support their push for a prompt cut in C02 emissions - to 15 percent below 1990 levels - by 2010. The Clinton administration, concerned about stalling economic growth, wants a more modest cut over a longer period.
CO2, a "trace" component of Earth's atmosphere, traps heat. The gas is introduced to the atmosphere by a range of natural processes, from volcanic eruptions and lightning-induced forest fires to leaves and fallen trees decomposing on a forest floor. But with the advent of industrialization and the wide use of fossil fuels, concentrations of atmospheric CO2 have been building at a more rapid pace.
In this latest climate study, Dr. Overpeck and his colleagues have attempted to fill a gap in what is known about global warming. Before the 1950s, there were only sporadic Arctic temperature readings. This study turned to indirect, or proxy, indicators of the Arctic's climactic past to put temperature readings taken before the 1950s into a longer context. Those readings pointed to an average increase of 0.6 degrees C since the turn of the century, greater than the average increase for the hemisphere as a whole.
The use of proxies such as tree rings and ice cores injects some uncertainty into the five-year project, because "one needs to make assumptions about what the proxies represent," says Douglas Hardy, a research associate at the University of Massachusetts at Amherst. Still, proxies represent the only viable approach to reconstructing the climate record in this region. The NOAA data sets came from 29 locations throughout North America and Eurasia above the Arctic Circle.
Over the past 400 years, the Arctic's climate has varied widely, the researchers note. For the first several hundred years, the various proxies didn't always agree on what was happening. Moreover, the climate swings the team gleaned from their data often correlated with natural factors such as slight increases in solar irradiance, which would boost warming, or increased concentrations of tiny particles called aerosols from volcanic eruptions, which have a cooling effect. Natural cycles in ocean currents also affected the trends seen over the past four centuries.
Indeed, the team attributes the first part of the latest warming period, which began in 1840, to the end of the "Little Ice Age," which occurred from about 1600 to the mid-1800s. From 1850 to 1950, "all of the different [proxy] records we used are very coherent. All show a dramatic change upward in temperature, Dr. Hardy says.
Other published research shows C02 and methane increased only slightly from 1840 to 1920. The team attributes the warming during that time frame to a decrease in volcanic activity and an increase in solar irradiance. After 1920, the amount of trace gases built in the atmosphere at an exponential rate, the team writes, "probably" giving them "an increasingly dominant role" when compared with the heating expected through natural processes.
"That is most suggestive of anthropogenic effects," Hardy concludes.