Environment

First independent confirmation of global land warming

Print edition : May 03, 2013

A UNIQUE new observational study that did not use temperature recordings from land stations has confirmed global land warming, according to a scientist at the National Oceanic and Atmospheric Administration’s (NOAA) Cooperative Institute for Research in Environmental Sciences (CIRES) in the United States. The finding refutes concerns that artefacts in land-based observing systems have led to an artificial global land-warming trend.

Since 1952, using a network of weather stations dotted around the globe to take daily readings, scientists have recorded an increase of 1.2 degrees Celsius in the earth’s air temperature over land. Several scientists have, however, questioned the accuracy and representativeness of the land station observations that were used to determine this warming trend and, therefore, do not have confidence in it. “Imagine that a house is built next to the thermometer that was taking the measurements,” said CIRES scientist and lead author Gilbert Compo of the NOAA’s Earth System Research Laboratory. “How much does that affect the long-term trend at that measuring site?”

Changes in a weather station’s surroundings—such as trees being replaced by concrete—can affect the temperatures recorded at that station. Urban development uses materials that effectively retain more heat than natural cover, leading to a local warming. Similarly, even minor relocation of a weather station can introduce an inconsistency in the recorded temperatures over time, especially if it involves a change in altitude. “So you need to try and get rid of that unrepresentative warming.” Scientists have made corrections to the recorded temperatures to compensate for urban warming and have also corrected several other factors that would cause the observed data to inaccurately represent the true situation. The question is: Did those corrections work?

To determine whether the observed warming trend is accurate, Compo and his colleagues used a different approach to investigate land surface temperature trends. The scientists used 20th Century Reanalysis (20CR), a physically based, state-of-the-art data assimilation system that circumvents the problems faced in using weather station temperature data. “20CR doesn’t have those problems because we never used a thermometer to determine air temperatures over land,” Compo said. Given the variables of barometric pressure, sea surface temperature, sea-ice concentration, and carbon dioxide, volcanic and solar variations, the scientists were able to use the 20CR method to infer the air temperatures over land across the globe. The derived temperatures agreed both annually and centennially with those found by weather stations. The scientists published their findings in the recent issue of Geophysical Research Letters.

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