This is the first in a series of weekly UPI articles examining the arguments and controversies surrounding the issue of global climate change.
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BOULDER, Colo., Nov. 17 (UPI) -- The jury is still out and the verdict is pending, but the stack of evidence implicating human activity in rising global temperatures is growing higher, although things are taking an interesting twist.
Consider two papers published just last week in the journal Science.
In the first, a group of investigators led by University of Oklahoma meteorologist David Karoly compiled new data strongly suggesting human influence has contributed significantly to average warming in North America in the second half of the 20th century.
In the second, Australian researchers, using a clever technique for determining the historical extent of the Antarctic ice shelf, tracked a 20 percent decline in sea ice extent on Antarctica's Law Dome since 1950.
Karoly's paper examined five different indices of climate change in North America:
-- mean surface air temperature over land;
-- the difference between land and ocean temperatures;
-- temperature differences between high and middle latitudes;
-- seasonal temperature differences, and
-- the daytime differences between maximum and minimum temperatures.
"In the period from 1950 to 1999, there are either negligible or negative effects from changes in solar radiation and volcanic aerosols (dust)," Karoly told United Press International. "The changes in temperatures with the increases in greenhouse gases agree very well."
In other words, natural processes do not seem to be responsible for the temperature increases -- to a point.
"On the basis of these results, it is likely that there has been a significant human influence on the observed North American warming in the second half of the 20th century, associated with increasing atmospheric concentrations of greenhouse gases and sulfate aerosols," the paper concludes. "Over the 20th century, this influence is manifest not only in mean temperature changes, but also in changes of the north-south temperature gradient, the temperature contrast between land and ocean, and reduction of the diurnal temperature range."
This is not a "case closed" situation, however, according to Karoly. "In the earlier part of the century, the greenhouse gas contribution is much smaller and the warming is due to solar and volcanic activity," he explained. "We can't rule out that the warming in the first part of the century was due to natural causes."
Regarding the sea ice research, Mark Curran, ice core chemist with the Australian Antarctic Division in Hobart, Tasmania, said the findings seem to bring the observations on sea ice extent into general agreement with atmospheric models that show global warming.
"Prior to the 1970s, the previous data did not show a change in sea ice." he told UPI. "The record would suggest an increase in sea ice. This goes against the models. There was not a previously measured decline in sea ice extent. That was a question people were pondering."
Curran and his group used observations of methansulphonic acid, or MSA, concentrations to show there likely has been a 20-percent decline of sea ice in Antarctica's Law Dome since about 1950.
"We have this single cell phytoplankton which produces MSA and it is then transported to Antarctica and is locked away in ice cores," he told UPI. "We developed a method for measuring MSA content in ice cores. We've discovered that the amount is related to the northerly extent of sea ice."
A substantial decline in sea ice could have widespread consequences. Antarctic sea ice is major contributor to the engine that drives global ocean currents. Loss of too much sea ice could shut down or dramatically change these currents, resulting a stagnant ocean or, paradoxically, another ice age in the northern latitudes.
More immediately, Curran said, the loss of sea ice could have substantial effect on the krill and phytoplankton that feed the great whales, which migrate to the area to feed.
The two papers conflict with research in which scientists have argued other natural factors, such as solar forcing of Earth's climate -- rather than anthropogenic CO2, as human-caused emissions are called -- are responsible for the observed increases in surface temperatures.
"Generally, people have been saying for several years now that because the solar activity has increased almost double in the last hundred years, this correlates rather well with the first half of global warming, up to about 1970," Brian Tinsley, a professor of physics at the University of Texas at Dallas, who has studied the cosmic ray effects on the atmosphere, told UPI.
"The last 30 years don't seem to be explainable by any solar mechanisms," however, Tinsley added. "And besides, there is good reason to think that man-made carbon dioxide is responsible for the most recent half of the warming."
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Dan Whipple covers environmental policy for UPI Science News. E-mail [email protected]
