Embassy of the United States Bogotá, Colombia

(Results rely on land and sea observations, historical and satellite data) (918)

By Cheryl Pellerin
Science Writer

Washington ? Each month, scientists at three centers ? one in the United Kingdom and two in the United States ? independently combine data about conditions on Earth's surface to calculate the planet's average temperature.

The centers collect and process the data in different ways but, for decades, they have shown similar results: Global average temperature has increased over the past century and warming has been especially rapid since the 1970s.

"We're reporting what we see, and we see warming," said David Easterling, chief of the Scientific Services Division at the National Oceanic and Atmospheric Administration's National Climatic Data Center (NCDC). "There's no sleight of hand. People can have all sorts of consternation about it but we're just trying to be an unbiased source of information. The bottom line is that the science says it's warming. The line is going up."

The centers are the NCDC; NASA's Goddard Institute for Space Studies (GISS) in New York City; and the Met (meteorological) Office, the United Kingdom's Exeter-based national weather and climate information service, which works with the University of East Anglia to produce its monthly global average temperature reports (abbreviated as HadCRUT3).

One Planet, Many Temperatures

Global average temperature records are vital to understanding how the climate is changing. The calculations contribute to findings like those in the periodic assessment reports of the Nobel Prize-winning Intergovernmental Panel on Climate Change (IPCC).

But none of the centers maps actual temperatures, which are taken all over the world on land and sea at different heights from the ground, in different conditions, at different times of day, and with different kinds of equipment. Instead, the centers calculate temperature anomalies ? deviations from normal temperature.

"Rather than say how warm or cold it is, you say how much warmer or colder it is compared to normal," GISS scientist Reto Ruedy said. "Ten degrees higher than normal means the same thing in summer and winter. It means the same thing on top of a mountain or near the equator ? deviation from the norm."

Another reason scientists use temperature anomalies rather than absolute temperature, Peter Stott, head of climate monitoring at the Met Office's Hadley Centre for Climate Change, told America.gov, is that not all parts of Earth are equally covered by weather stations or other observing systems.

"If you're missing data from a place that's abnormally cold, like the Arctic," Stott said, "the global temperature record for that month would warm up a bit. So you would have a change that wasn't due to climate change; it was due to the fact that you'd lost one of your stations. If you use anomalies you don't have that problem ? missing data does not bias the temperature record."

Calculating Global Temperature

Every day, thousands of temperature observations are taken around the planet. Land stations use the daily readings to create a monthly average that is sent off for use by climate researchers. Ocean observations from ships and buoys are sent to national meteorological services through a World Meteorological Organization network called the Global Telecommunication System (GTS).

The HadCRUT3 record, produced by the Met Office with the University of East Anglia's Climatic Research Unit, takes in observations from about 2,000 land stations each month. Sea-surface temperature observations come from 1,200 ocean buoys and 4,000 ships in a voluntary observing program. Other buoys are moored in the tropics and coastal regions, mainly around the United States. Together they collect 1.5 million observations a month.

NCDC in North Carolina is the world's largest archive of weather and climate data. Its data come from NOAA and sometimes NASA; temperature, precipitation and other instrument readings from up to 30,000 weather stations worldwide; international data over the GTS; and data from 10,000 volunteers who monitor weather stations for the U.S. Cooperative Observer Network program. NCDC also has long-term data-exchange efforts with other countries, including a 20-year program with China.

The data go through rigorous quality-control procedures before they enter the Global Historical Climatology Network, which NCDC uses to monitor long-term trends in temperature and precipitation. This data set was used in the IPCC Fourth Assessment Report and other international assessments. NCDC data are freely available through its Web site.

At GISS, a small part of the work is to calculate once a month its global average temperature anomaly maps and graphs. They download the collection of mean temperatures that NCDC creates from the monthly or daily reports of individual weather stations.

"GISS then combines these data to provide concise information about the current state of our planet compared to previous times," Ruedy said.

Each global temperature center uses different observation sets, but there are large overlaps in the data. Each has its own ways to check and process data and to make the final calculation of global average temperature.

"The way you average up numbers from a whole bunch of climate stations around the world into one number for the globe can be done in an infinite number of ways," Easterling said. "We all do it a little bit differently and end up with slight differences in the numbers. But they all say the same thing ? it's warming."

More information on climate change ( http://www.america.gov/global/environ.html ) is available from America.gov. Want to do something about climate change? Join the global conversation on Facebook ( http://www.facebook.com/ConversationsClimate ) and share your thoughts below.

(This is a product of the Bureau of International Information Programs, U.S. Department of State.  Web site: http://www.america.gov)