April 8, 2013
First independent confirmation of global land warming
A unique and innovative new observational study that did not use temperature recordings from land stations has confirmed global land warming, according to a scientist at NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES). The finding refutes concerns that artifacts in land-based observing systems have led to an artificial global land warming trend.
“This shows that global warming over land is real,” said CIRES scientist lead author Gilbert Compo who works at NOAA’s Earth System Research Laboratory. “It is not an artifact of the observing system. It is happening.”
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 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,” Compo said. “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.
“Urban warming is real, but local,” Compo said. “So you need to remove the contribution of it 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 out?” Compo said.
To answer this question and determine whether the observed warming trend is real and accurate, Compo, CIRES Fellow Prashant Sardeshmukh, NOAA scientist Jeff Whitaker and their colleagues used an entirely different approach to investigate land surface temperature trends. The scientists used an approach termed the 20th Century Reanalysis (20CR), a physically based, state-of-the-art data assimilation system (see sidebar) that circumvents the problems faced in using weather station temperature data.
“20CR doesn’t have those problems because we never used a thermometer over land 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 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 journal Geophysical Research Letters on April 8, 2013.
“One thing we found was that the barometer is even more valuable than we thought,” Compo said. “We were able to reproduce the hour-by-hour, day-by-day variations in temperature using only barometric pressure as a starting point.”
The agreement confirms that deficiencies in land-based station temperatures have been corrected adequately, Compo said. It also affirms that the conclusions based on large-area averages of land temperatures are robust, i.e., the climate is warming, he said.
“Do we have global warming if you only specify the sea surface temperatures and update with the pressure?” Compo said. “The answer is: yes.”
Other co-authors of the project are CIRES scientists Prashant D. Sardeshmukh and Chesley McColl, Philip Brohan at the Met Office Hadley Center in the United Kingdom, Philip D. Jones from the University of East Anglia, U.K. and the Center of Excellence for Climate Change Research in Saudi Arabia.