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Confusion on Climate Variability and Trends
(Posted by Olive on behalf of Roger)
Even the venerable New York Times is prone to completely botching a discussion of the science of climate change. In a front page article today, the NYT reports on how the National Arbor Day Foundation has updated plant hardiness maps to reflect recent changes in climate. (A plant hardiness map presents the lowest annual temperature as a guideline to what plants will thrive in what climate zones.) The NYT misrepresents understandings of variability and trend and in the process confuse more than clarify.
The new map updates a 1990 USDA map based on 1974-1986 data, and replaces it with data from 1990-2006. In most places the range of increased average minimum temperature has moved north as can be seen from a difference map between the two time periods. The difference map, shown here, has the horizontal lines because the zones used are so broad -- 10 degrees -- that the differences are only noticeable at the margins of the zones.
The New York Times reports that these differences can all be attributed to human-caused climate change, using the case of Atlanta as an illustration:
Using data from the National Oceanic and Atmospheric Administration, the Arbor Day map indicates that many bands of the country are a full zone warmer, and a few spots are two zones warmer, than they were in 1990, when the map was last updated.Atlanta, which was in Zone 7 in 1990, is now in Zone 8, along with the rest of northern Georgia. That means that areas in the northern half of the state where the average low temperature was zero to 10 degrees Fahrenheit are now in a zone where the average low is 10 to 20 degrees. A scientific consensus has concluded that this warming trend has largely been caused by the human production of heat-trapping gases.
Because the zones span 10 degrees (or 5 degrees in the case of the 1990 USDA map) and the largest change shown on the difference map is 2 zones (i.e., >10 degrees!, now corrected), then clearly no location has jumped 2
zones! This is just an error.
More important than this simple mistake is the claim in the NYT that the changes in temperature observed in Atlanta can be attributed to human-caused greenhouse gases. In fact, the IPCC argues that it needs 30 years of records to detect trends, much less make attribution. In fact, the IPCC report just out has reported that the U.S. southeast has actually cooled over the period of record as shown below.
The underlying issue has to do with understanding the role of human-caused climate change in the context of climate variability on long time scales.
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