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Global Temperature Records Start with 10,200 Volunteers

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Long-term measurements are critical to understanding climate change

For 120 years, a global network of land-based weather stations has monitored Earth’s surface temperature. In the United States, steadfast volunteers have tended these stations since 1890, measuring daily temperature, rain and snow depth to support weather forecasts and, over time, monitor a changing climate.

The National Weather Service’s Cooperative Observer Program (Coop) and its 10,200 volunteers in 50 U.S. states and territories produce weather information that is used nationwide by farmers, government officials, insurance companies, utility companies, manufacturers and business planners.

The global average temperature records are vital to understanding how climate is changing, and data from the Coop program ― the nation’s largest and oldest weather network ― are the source of much of the U.S. information. The network initially was organized to monitor weather for agriculture, but its consistent, long-term historical climate data have established an irreplaceable record of climate in the United States.

“The way we measure things,” Joel Cline, who manages the program for the National Weather Service in Silver Spring, Maryland, told America.gov, “we are the ground truth” ― a term used in meteorology, map making and satellite imagery to mean information collected directly from a site.

The Coop program is one of many that contribute data to the National Climatic Data Center (NCDC), the world’s largest archive of weather and climate data, to produce global records for average temperature. NCDC gets data from more than 30,000 weather stations in 200 countries.

NCDC in Asheville, North Carolina, is one of three centers around the world ― one in the United Kingdom and two in the United States ― that independently combine data about conditions on Earth’s land and sea surfaces to calculate the planet’s average temperature. The calculations contribute to findings like those in the periodic assessment reports of the Nobel Prize-winning Intergovernmental Panel on Climate Change, whose next report will be published in 2013.

ADDING VALUE TO THE NUMBERS

Every morning at 7 a.m., no matter what kind of weather they’re having in Lyles, Tennessee, 49-year-old Mina Dressler walks outside to the weather station near her house and records the maximum and minimum temperatures from the previous day.

If the weather was wet, she records total rainfall or snowfall and snow depth, and writes down anything she thinks the Weather Service might like to know, like if there’s local damage from a storm. Once a month, she walks across the road and downloads data from a recording rain gauge she keeps on her property and sends the data to Nashville. The rain gauge is part of the NCDC’s U.S. Hourly Precipitation Data Network.

Dressler runs the East Hickman Public Library on Highway 100 in Lyles, a small town 47 miles (76 kilometers) southwest of Nashville, and she’s been collecting daily observations for the National Weather Service Forecast Office in Nashville for 10 years.

“Somebody has to step in and do something when other people won’t,” she told America.gov. “That’s where your volunteers come in. They don’t do it for personal glory, they just do it because it needs to be done.”

Other meteorological networks across the nation are automated, but people do some things better than machines.

“There are several ways to measure snowfall, but you just can’t beat a human,” Ralph Troutman, observing program leader at the National Weather Service Forecast Office in Nashville, told America.gov, “and so many of the folks include comments in the record. I have one lady who every day has a comment about sunrise, sunset, how many clouds there are, the amount of lightning ― anything she sees during the 24-hour period that she thinks might be significant. A lot of them do that. High and low temperature only tells so much of the story. Their comments add value to the record.”

ASSESSMENT AND CORRECTION

Individual or institutional volunteers take observations near oceans, on top of mountains, in cities, on the plains and at schools, water treatment plants, churches and airports. For decades, some have been sending in data to local forecast offices online, over the phone and even on paper forms.

Over the years, weather stations change hands, move into new surroundings or have cities, parking garages and other heat sources rise up around them. Volunteers take observations at different times of day, make mistakes, take vacations or fall ill.

At the NCDC, data come in from 1,200 of the best Coop stations around the country. Experts put the data through a rigorous quality-control process before the data becomes part of the Global Historical Climatology Network dataset.

“If you look over time, particularly in the last few decades,” NCDC Director Tom Karl said at a recent briefing in Washington, “these stations have undergone changes in instrument types, they’ve moved, some have been affected by urbanization, some have been maintained better than others, and for some stations the site characteristics have changed substantially and they’re no longer what we would call ideal stations to measure climate change.”

“What that means is,” Karl said, “if you’re going to use these data they have to undergo an intensive amount of assessment and correction to see if they can be useful to measure climate change.”

The first thing they correct for is the time observations were taken ― what NCDC calls time-dependent inconsistencies in the record ― because temperatures taken in the morning are cooler than those taken in the afternoon. Most volunteers, but not all, take observations at 7 a.m. local time.

NCDC also corrects for changes in a weather station’s location, surroundings or instrumentation, and it compares the data with other climate change indicators, including the dates of lake and river thaws, glacial ice volume, Arctic sea ice, ocean heat content and the response to climate of plants and animals.

“The important component here is that no single station is relied upon,” Karl said. “It’s the multitude of stations that gives the power to this technique.”