Commentary: Plant zones and climate change
The Free For All section of the February 4 Washington Post carried the following letter I wrote under the caption “So, why are the plant zones changing?.”
The interesting front-page story about the shifts in the Agriculture Department’s plant hardiness zone maps since 1990 [“New plant map shifts area to warmer zone,” Jan. 26] included this headline on the continuing page:“Plant map doesn’t measure climate change.” However, nothing in the article discusses the relationship of the zone changes to climate change, whether global, national or regional.
The nearly uniformly northward shifting zones reflect increases in “average winter low temperatures between 1976 and 2005 at 8,000 weather stations.” While this doesn’t fully measure all the changes in climate, if this nationwide pattern is not attributable to global climate change—specifically to the global warming that scientists have concluded is unequivocal—what, pray tell, is responsible? The failure of The Post, not only to make the connection with global climate change but also to seemingly disavow it, is most puzzling.
I thought I should provide more information than allowed by the brief letter both on the U.S. Department of Agriculture’s (USDA) map and the evidence that these shifting zones reflect a clear winter warming trend across most of the United States since the mid-1970s that is consistent with global warming.
The online 2012 USDA Plant Hardiness Zone Map is intended to be the standard by which gardeners and growers can determine which plants are the most likely to thrive at a location.
Each 10-degree F zone “represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976–2005.” The previous edition of the USDA PHZM, revised and published in 1990, was drawn from weather data for 1974–1986. So, the new map is not intended to reflect the minimum temperature conditions that have prevailed from 1987 through 2005 in contrast to those that prevailed during the previous 13 year period but to reflect the full 30-year period.
The USDA states: “Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.” It is probably this caveat (based on 30-year averages, just single minimum temperature measurements for a year) that the Agriculture officials were trying to convey when they “stressed that the new map is not a tool to measure climate change.” But, the Post headline writer stated this in a way that is likely to mislead the reader that the shifting zones are unrelated to climate change. A closer look indicates that they are related to climate change and, in fact, under-represent it.
“The new PHZM is generally one half-zone warmer than the previous PHZM throughout much of the United States, as a result of a more recent averaging period (1974–1986 vs. 1976–2005).“ The warming is consistent with the 0.4 to more than 1.2°F per decade warming trend in mean temperature over most of the continental U.S. during the months of January, February and March from 1976 through 2005, based on Weather Service analysis. This 1976-2005 period corresponds exactly with the period considered for the new PHZM. However, if the annual minimum temperature data for just the 1987-2005 period had been used to construct the new map, the plant zones would certainly have shifted more.
Moreover, this warming trend is consistent with the increase in global mean temperature and specifically with the warming during this time period in the northern latitude zone in which the U.S. lies as shown below based on NASA analysis. This global warming has been termed unequivocal by the Intergovernmental Panel on Climate Change and has been attributed by the National Academy of Sciences largely to human activities, particularly to the build-up of greenhouse gases in the atmosphere.
I was motivated to write the letter not only to point out the Post’s poor construction that leaves readers with an important misconception, but to ask why this national-class newspaper based in our nation’s capital, did not do a more thorough job in examining the rather obvious connection. The story by Seth Borenstein of the Associate Press clearly did this and ironically was carried by the Post online. Also, the Capital Weather Gang’s blog on the Post’s website observed that the shift in the planting zones “is a rock solid indicator of climate change.” That blog included a NOAA graph showing the preponderance of much-above-normal annual minimum temperatures in the U.S. since 1990.
There is a back story to the new map and the reticence of the USDA spokesperson to relate its shifts to climate change, captured in this USA Today story. An earlier attempt by the American Horticultural Society to update the 1990 plant hardiness zone map resulted in a draft map in 2003 was rejected by the USDA, which stated that a longer, 30-year period was required in order to develop a new climate map (despite the fact that only 13 years of data were included in the 1990 map). Some suggested that the map was squelched by officials in the George W. Bush Administration to limit the public’s awareness of climate change.
So, in 2006 the National Arbor Day Foundation released a new, unsanctioned map, based on minimum temperature data from 1991 to 2005 that showed marked warming and northward movement of zones. Coming nearly five years later, the USDA’s 2012 map, while based on more data and providing much more geographic detail, is generally very similar to the Arbor Day Foundation map. However, the USDA map is slightly cooler and arguably more out-of-date because it averages over the full 1976-2005 time period, rather than just post-1990 as did the Arbor Day Foundation map.
The USDA should have made it clear that, while the map itself is not a particularly good instrument for determining climate change, the changes in minimum temperatures on which they are based are very consistent with more thorough documentations of climate change by NOAA, NASA and such Federal reports as Global Climate Change Impacts in the United States. USDA apparently recognizes climate change as a reality as it has a program to coordinate assessment, mitigation and adaptation. The Washington Post should have, through interviews or citing these sources, indicated that the new plant map does, in fact, reflect climate change. I know that almost everything is being politicized these days, but gardening?
President, University of Maryland Center for Environmental Science