At Sonnblick, where measurements are made every minute, the air gets so cold that Elke Ludewig and her crew must remove ice from frozen instruments. The thermometers transmit averages to Austria’s official Central Institution for Meteorology and Geodynamics, which routes them into key global temperature databases.
With that information, climatologists at independent and government agencies, such as NASA and the National Oceanic and Atmospheric Administration, map temperature patterns from the past to the present.
These maps rely on the fact that temperatures from nearby stations tend to correlate with one another over time — if the air around one station heats up, it probably also gets hotter at nearby stations. This correlation lets scientists pool the readings from multiple stations to calculate the temperature across the planet’s entire surface. In this way, scientists combine readings over space, and because the temperature record extends to the 19th century, they also are able to compare readings over time.
There are complications. Older measurements were “taken in any number of weird or wonderful ways,” said Peter Thorne, a climate researcher at the National University of Ireland at Maynooth.
Some stations recorded temperatures in a variety of units now antiquated, such as Réaumurs, rather than Celsius; others fixed their prime meridian at Paris rather than Greenwich, England, the location of the now globally accepted prime meridian. Such inconsistencies force today’s climate scientists to double as historians, digging into the methodologies of old temperature records to determine the appropriate adjustments.
Past ocean temperatures can be particularly treacherous to calculate consistently. When sailors on merchant ships or explorer expeditions hoisted buckets from the ocean to record temperature, some of the saltwater would evaporate before reaching the deck. Because ships vary in size, the resulting difference in evaporation could produce changes in the temperature readings. Climatologists must adjust for these variations, too.
Some observations can’t be adjusted because they do not exist. During World War II, for instance, stations in Europe were forced to stop recording. Vast volumes of historical records have yet to be digitized; others have been lost or destroyed.
Despite these inconsistencies, and the fact that different research groups apply different adjustments, their conclusions show broad agreement on the degree of warming. There is such an abundance of reliable data now that any incremental adjustment or new method of computation can’t alter the overall number.
“The bottom line is that any adjustments are relatively small in the global mean,” said Gavin Schmidt, who directs NASA’s Goddard Institute for Space Studies. “Neither are the calculated uncertainties anywhere close enough to affect the overall trends.”
[Extreme climate change has arrived in America]
The Washington Post analyzed four global temperature data sets to find the earth’s fastest-warming locales for our year-long series, 2ºC: Beyond the Limit. We found that roughly one-tenth of the globe has already warmed by more than 2 degrees Celsius (3.6 degrees Fahrenheit), when average temperatures over the past five years are compared with those of the mid- to late 1800s. Catastrophic changes could be unleashed if that threshold is breached globally, a United Nations panel has warned.
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