2009 Was Second Warmest Year on Record
2009 Second Warmest Year of Warmest Decade on Record
Environmental News Service, Jan. 25, 2010
The year
2009 was tied for the second warmest year in the modern record,
according to a new analysis of global surface temperature from the
National Aeronautics and Space Administration. Conducted by NASA's
Goddard Institute for Space Studies in New York City, the analysis also
shows that in the Southern Hemisphere, 2009 was the warmest year since
modern records began in 1880.
January
2000 to December 2009 was the warmest decade on record. Throughout the
last three decades, the GISS surface temperature record shows an upward
trend of about 0.2°C (0.36°F) per decade.
"There's
a contradiction between the results shown here and popular perceptions
about climate trends," said climatologist Dr. James Hansen, director of
the Goddard Institute for Space Studies. "In the last decade, global
warming has not stopped."
Although
2008 was the coolest year of this past decade, due to strong cooling of
the tropical Pacific Ocean, 2009 saw a return to near-record global
temperatures.
The past
year was only a fraction of a degree cooler than 2005, the warmest year
on record, and tied with a cluster of other years - 1998, 2002, 2003,
2006 and 2007 - as the second warmest year since recordkeeping began.
"There's
always an interest in the annual temperature numbers and on a given
year's ranking, but usually that misses the point," said Dr. Hansen.
"There's
substantial year-to-year variability of global temperature caused by
the tropical El Niño-La Niña cycle. But when we average temperature
over five or 10 years to minimize that variability, we find that global
warming is continuing unabated," he said.
The GISS
analysis shows that the largest temperature increases over the past
decade have been in the Arctic and the Antarctic Peninsula.
Since
1880, the year that modern scientific instrumentation became available
to monitor temperatures precisely, a clear warming trend is present,
though there was a leveling off between the 1940s and 1970s.
The
near-record temperatures of 2009 occurred despite an unseasonably cool
December in much of North America. High air pressures in the Arctic
decreased the east-west flow of the jet stream, while also increasing
its tendency to blow from north to south and draw cold air southward
from the Arctic. This resulted in an unusual effect that caused frigid
air from the Arctic to rush into North America and warmer mid-latitude
air to shift toward the north.
"Of
course, the contiguous 48 states cover only 1.5 percent of the world
area, so the U.S. temperature does not affect the global temperature
much," said Hansen.
In total, average global temperatures have increased by about 0.8°C (1.4°F) since 1880.
"That's
the important number to keep in mind," said Gavin Schmidt, another GISS
climatologist. "In contrast, the difference between, say, the second
and sixth warmest years is trivial since the known uncertainty, or
noise, in the temperature measurement is larger than some of the
differences between the warmest years."
Climate
scientists agree that rising levels of carbon dioxide and other
greenhouse gases trap incoming heat near the surface of the Earth and
are the key factors causing the rise in temperatures since 1880, but
these gases are not the only factors that can impact global
temperatures.
Three
other key factors - changes in the Sun's irradiance, oscillations of
sea surface temperature in the tropics, and changes in aerosol levels -
can also cause slight increases or decreases in the planet's
temperature.
Overall,
the GISS scientists say, the evidence suggests that these effects are
not enough to account for the global warming observed since 1880.
El Niño
and La Niña are prime examples of how the oceans can affect global
temperatures. They describe abnormally warm or cool sea surface
temperatures in the South Pacific that are caused by changing ocean
currents.
Global
temperatures tend to decrease in the wake of La Niña, which occurs when
upwelling cold water off the coast of Peru spreads westward in the
equatorial Pacific Ocean.
La Niña,
which moderates the impact of greenhouse-gas driven warming, lingered
during the early months of 2009 and gave way to the beginning of an El
Niño phase in October that's expected to continue in 2010.
An
especially powerful El Niño cycle in 1998 is thought to have
contributed to the unusually high temperatures that year, and Hansen's
group estimates that there's a good chance 2010 will be the warmest
year on record if the current El Nino persists.
At most, scientists estimate that El Niño and La Niña can cause global temperatures to deviate by about 0.2°C (0.36°F).
Warmer
surface temperatures also tend to occur during particularly active
parts of the solar cycle, known as solar maximums, while slightly
cooler temperatures occur during lulls in activity, called minimums.
A deep
solar minimum has made sunspots a rarity in the last few years. Such
lulls in solar activity, which can cause the total amount of energy
given off by the sun to decrease by about a tenth of a percent,
typically spur surface temperature to dip slightly. Overall, solar
minimums and maximums are thought to produce no more than 0.1°C
(0.18°F) of cooling or warming.
"In 2009,
it was clear that even the deepest solar minimum in the period of
satellite data hasn't stopped global warming from continuing," said
Hansen.
Small
particles in the atmosphere called aerosols can also affect the
climate. Volcanoes are powerful sources of sulfate aerosols that
counteract global warming by reflecting incoming solar radiation back
into space.
In the
past, large eruptions at Mount Pinatubo in the Philippines and El
Chichon in Mexico have caused global dips in surface temperature of as
much as 0.3°C (0.54°F). But volcanic eruptions in 2009 have not had a
significant impact.
Meanwhile,
other types of aerosols, often produced by burning fossil fuels, can
change surface temperatures by either reflecting or absorbing incoming
sunlight. Hansen's group estimates that aerosols probably counteract
about half of the warming produced by man-made greenhouse gases, but he
cautions that better measurements of these elusive particles are needed.
Critics of GISS' analyses have accused the institute of manipulating data, but Schmidt says they are just plain wrong.
"Indeed,
there are people who believe that GISS uses its own private data or
somehow massages the data to get the answer we want. That's completely
inaccurate," he said. "We do an analysis of the publicly available data
that is collected by other groups. All of the data is available to the
public for download, as are the computer programs used to analyze it.
One of the reasons the GISS numbers are used and quoted so widely by
scientists is that the process is completely open to outside scrutiny.
To
conduct its analysis, GISS uses publicly available data from three
sources: weather data from more than a thousand meteorological stations
around the world; satellite observations of sea surface temperature;
and Antarctic research station measurements.
These
three data sets are loaded into a computer program, which is available
for public download from the GISS website. The program calculates
trends in temperature anomalies - not absolute temperatures - but
changes relative to the average temperature for the same month during
the period of 1951-1980.
Other
research groups also track global temperature trends but use different
analysis techniques. The Met Office Hadley Centre, based in the United
Kingdom, uses similar input measurements as GISS, for example, but it
omits large areas of the Arctic and Antarctic, where monitoring
stations are sparse.
In
contrast, the GISS analysis extrapolates data in those regions using
information from the nearest available monitoring stations, and thus
has more complete coverage of the polar areas.
If GISS
didn't extrapolate in this manner, the software that performs the
analysis would assume that areas without monitoring stations warm at
the same rate as the global mean, an assumption that doesn't line up
with changes that satellites have observed in Arctic sea ice, Schmidt
explained. Although the two methods produce slightly different results
in the annual rankings, the decade-long trends in the two records are
essentially identical.
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