ScienceDaily, October 21, 2011
Global warming
is real, according to a major study released Oct. 20. Despite issues raised by
climate change skeptics, the Berkeley Earth Surface Temperature study finds
reliable evidence of a rise in the average world land temperature of
approximately 1°C since the mid-1950s.
Analyzing
temperature data from 15 sources, in some cases going as far back as 1800, the
Berkeley Earth study directly addressed scientific concerns raised by skeptics,
including the urban heat island effect, poor station quality, and the risk of
data selection bias.
On the basis of its
analysis, according to Berkeley Earth's founder and scientific director,
Professor Richard A. Muller, the group concluded that earlier studies based on
more limited data by teams in the United States and Britain had accurately
estimated the extent of land surface warming.
"Our biggest
surprise was that the new results agreed so closely with the warming values
published previously by other teams in the U.S. and the U.K.," Muller said.
"This confirms that these studies were done carefully and that potential
biases identified by climate change skeptics did not seriously affect their
conclusions."
Previous studies,
carried out by NOAA, NASA, and the Hadley Center, also found that land warming
was approximately 1°C since the mid-1950s, and that the urban heat island
effect and poor station quality did not bias the results. But their findings
were criticized by skeptics who worried that they relied on ad-hoc techniques
that meant that the findings could not be duplicated. Robert Rohde, lead
scientist for Berkeley Earth, noted that "the Berkeley Earth analysis is
the first study to address the issue of data selection bias, by using nearly
all of the available data, which includes about 5 times as many station
locations as were reviewed by prior groups."
Elizabeth Muller,
co-founder and Executive Director of Berkeley Earth, said she hopes the
Berkeley Earth findings will help "cool the debate over global warming by
addressing many of the valid concerns of the skeptics in a clear and rigorous
way." This will be especially important in the run-up to the COP 17
meeting in Durban, South Africa, later this year, where participants will
discuss targets for reducing Greenhouse Gas (GHG) emissions for the next
commitment period as well as issues such as financing, technology transfer and
cooperative action.
The Berkeley Earth
team includes physicists, climatologists, and statisticians from California,
Oregon, and Georgia. Rohde led the development of a new statistical approach
and what Richard Muller called "the Herculean labor" of merging the
data sets. One member of the group, Saul Perlmutter, was recently announced as
a winner of the 2011 Nobel Prize in Physics (for his work in cosmology).
The Berkeley Earth
study did not assess temperature changes in the oceans, which according to the
Intergovernmental Panel on Climate Change (IPCC) have not warmed as much as
land. When averaged in, they reduce the global surface temperature rise over
the past 50 years -- the period during which the human effect on temperatures
is discernable -- to about two thirds of one degree Centigrade.
Specifically, the
Berkeley Earth study concludes that:
•
The urban heat island
effect is locally large and real, but does not contribute significantly to the
average land temperature rise. That's because the urban regions of Earth amount
to less than 1% of the land area.
•
About 1/3 of
temperature sites around the world reported global cooling over the past 70
years (including much of the United States and northern Europe). But 2/3 of the
sites show warming. Individual temperature histories reported from a single
location are frequently noisy and/or unreliable, and it is always necessary to
compare and combine many records to understand the true pattern of global
warming.
•
The large number of
sites reporting cooling might help explain some of the skepticism of global
warming," Rohde commented. "Global warming is too slow for humans to
feel directly, and if your local weather man tells you that temperatures are
the same or cooler than they were a hundred years ago it is easy to believe
him." In fact, it is very hard to measure weather consistently over decades
and centuries, and the presence of sites reporting cooling is a symptom of the
noise and local variations that can creep in. A good determination of the rise
in global land temperatures can't be done with just a few stations: it takes
hundreds -- or better, thousands -- of stations to detect and measure the
average warming. Only when many nearby thermometers reproduce the same patterns
can we know that the measurements were reliably made.
•
Stations ranked as
"poor" in a survey by Anthony Watts and his team of the most
important temperature recording stations in the U.S., (known as the USHCN --
the US Historical Climatology Network), showed the same pattern of global
warming as stations ranked "OK." Absolute temperatures of poor
stations may be higher and less accurate, but the overall global warming trend
is the same, and the Berkeley Earth analysis concludes that there is not any
undue bias from including poor stations in the survey.
Four scientific
papers setting out these conclusions have been submitted for peer review and
will form part of the literature for the next IPCC report on Climate Change.
They can be accessed on: www.BerkeleyEarth.org. A video animation graphically shows global
warming around the world since 1800.
Berkeley Earth is
making its preliminary results public, together with its programs and dataset,
in order to invite additional scrutiny. Elizabeth Muller said that "one of
our goals is to make the science behind global warming readily accessible to
the public." Most of the data were previously available on public
websites, but in so many different locations and different formats that most
people could access only a small subset of the data. The merged database, which
combines 1.6 billion records, is now accessible from the Berkeley Earth
website: www.BerkeleyEarth.org.
What Berkeley Earth
has not done is make an independent assessment of how much of the observed
warming is due to human actions, Richard Muller acknowledged. As a next step,
Berkeley Earth plans to address the total warming of the oceans, with a view to
obtaining a more accurate figure for the total amount of global warming
observable.
More
information about Berkeley Earth is available at www.BerkeleyEarth.org.
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