(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 <a href = “https://www.arborday.org/media/zones.cfm”>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.
I spent last week at a meeting on the “Societal Impacts of Decadal Climate Variability” and it is clear that some scientists view decadal climate variability as an underappreciated subject. There is good reason for such concerns as many important societal decisions — like flood insurance, engineering design, and risk models — depend up understanding variability in the context of human-caused changes. The NYT article includes a passage that suggests that some people view the trend to be most important, while others think variability matters more:
The Agriculture Department is in the process of redoing the [planting guideline] map itself. But critics have taken issue with the department’s decision to use 30 years of temperature data, saying it will result in cooler averages and fail to reflect the warming climate. The 1990 U.S.D.A. map used 13 years of data; the Arbor Day map used 15 years ending in 2004.
Cameron P. Wake, a climatologist at the Climate Change Research Center of the University of New Hampshire, said “a 30-year period would include several cycles of multiyear effects like El Nino, with an underlying assumption that climate is stable and varies around a mean. Warming, on the other hand, is not variability, its a long term trend,” Dr. Wake said. “I would say the U.S.D.A. doesn’t want to acknowledge there’s been change.”
Kim Kaplan, a spokeswoman for the departments Agricultural Research Service, defended the decision to use 30 years, saying the longer time period would strike a balance between weather, which can vary greatly from year to year, and climate.
For planting your garden such debates may not be so relevant, but for decisions related to risk management and adaptation to climate more generally, this debate is central. Effective planning depends on knowing what range of possibilities to expect in the immediate and longer-term future. Use too long a record from the past and you may underestimate trends. Use too short a record and you miss out on longer time-scale variability. Throw in the politicization of the issue where trend partisans are associated with advocacy for action on greenhouse gas reductions and variability partisans are associated with those opposing action, and you have a very challenging decision making context indeed.
Out of this, one thing seems clear — improved decision making is unlikely to occur if basic issues of climate science are simplified to such a degree that they are misrepresented, especially by an authoritative source like the New York Times.
Roger Pielke, Jr.
Center for Science and Technology Policy Research
University of Colorado