The tropical regions of the world have been expanding since 1979, according to a study by US climatologists who are unsure whether the phenomenon is caused by global warming or natural climate change.(AFP/HO-NASA)
The Earth is the hottest it has been in at least 400 years, probably
even longer. The National Academy of Sciences, reaching that conclusion in a
broad review of scientific work requested by Congress, reported Thursday that
the "recent warmth is unprecedented for at least the last 400 years and
potentially the last several millennia."
A panel of top climate scientists told lawmakers that the Earth is running
a fever and that "human activities are responsible for much of the recent
warming." Their 155-page report said average global surface temperatures
in the Northern Hemisphere rose about 1 degree during the 20th century.
The report was requested in November by the chairman of the House Science Committee,
Rep. Sherwood Boehlert , R-N.Y., to address naysayers who question whether global
warming is a major threat.
Last year, when the House Energy and Commerce Committee chairman, Rep. Joe
Barton , R-Texas, launched an investigation of three climate scientists, Boehlert
said Barton should try to learn from scientists, not intimidate them.
The Bush administration also has maintained that the threat is not severe enough
to warrant new pollution controls that the White House says would have cost
5 million Americans their jobs.
Climate scientists Michael Mann, Raymond Bradley and Malcolm Hughes had concluded
the Northern Hemisphere was the warmest it has been in 2,000 years. Their research
was known as the "hockey-stick" graphic because it compared the sharp
curve of the hockey blade to the recent uptick in temperatures and the stick's
long shaft to centuries of previous climate stability.
The National Academy scientists concluded that the Mann-Bradley-Hughes research
from the late 1990s was "likely" to be true, said John "Mike"
Wallace, an atmospheric sciences professor at the University of Washington and
a panel member. The conclusions from the '90s research "are very close
to being right" and are supported by even more recent data, Wallace said.
The panel looked at how other scientists reconstructed the Earth's temperatures
going back thousands of years, before there was data from modern scientific
For all but the most recent 150 years, the academy scientists relied on "proxy"
evidence from tree rings, corals, glaciers and ice cores, cave deposits, ocean
and lake sediments, boreholes and other sources. They also examined indirect
records such as paintings of glaciers in the Alps.
Combining that information gave the panel "a high level of confidence
that the last few decades of the 20th century were warmer than any comparable
period in the last 400 years," the academy said.
Overall, the panel agreed that the warming in the last few decades of the 20th
century was unprecedented over the last 1,000 years, though relatively warm
conditions persisted around the year 1000, followed by a "Little Ice Age"
from about 1500 to 1850.
The scientists said they had less confidence in the evidence of temperatures
before 1600. But they considered it reliable enough to conclude there were sharp
spikes in carbon dioxide and methane, the two major "greenhouse" gases
blamed for trapping heat in the atmosphere, beginning in the 20th century, after
remaining fairly level for 12,000 years.
Between 1 A.D. and 1850, volcanic eruptions and solar fluctuations were the
main causes of changes in greenhouse gas levels. But those temperature changes
"were much less pronounced than the warming due to greenhouse gas"
levels by pollution since the mid-19th century, it said.
The National Academy of Sciences is a private organization chartered by Congress
to advise the government of scientific matters.
HEATING UP: The Earth is the hottest it has been in at least
400, maybe more.
SCIENTISTS AGREE: The National Academy of Sciences studied
tree rings, corals and other natural formations, in part, to conclude that the
heat is unprecedented for potentially the last several millennia.
HUMAN FAULT: Human activities are responsible for much of
the recent warming, the Academy says.
On the Net: National Academy of Sciences: http://nationalacademies.org
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