BANDELIER NATIONAL MONUMENT, N.M.—The dead pinon trees stretching across northern New Mexico's Pajarito Plateau stand out, stubborn clumps of gray still standing six years after they died.
They were not alone. Craig Allen, the scientist who chronicled their demise, ticked off a list of the things he watched die with them during the drought of 2002-03: juniper, blue gramma grass, ponderosa pine, Douglas fir, even cottonwoods.
Warming weather played a role here, and new evidence suggests the same thing may be happening around the world.
An international scientific team led by Allen reports that similar forest dieback might be rising globally.
While droughts have always come and gone, the evidence collected by Allen and his colleagues suggests that the added stress of higher temperatures may be taking its toll on Earth's forests.
The data are too sparse and inconsistent to be able to say for sure that global warming is killing forests, Allen and his colleagues report in a paper to be published in the peer-reviewed journal Forest Ecology and Management.
"We don't say there's a smoking gun," said Allen, a U.S. Geological Survey ecologist based at Bandelier National Monument.
The scientists found 88 cases in which increased temperatures and drought had caused increased mortality in forest trees, including examples in all types of forest and on most continents.
"This really looks pretty darn serious," said ecologist David Breshears, a co-author and leading expert on the effect of climate on pinon-juniper forests.
University of Arizona forest researcher Tom Swetnam, who was not involved in the project, praised the work, calling it "the most comprehensive review of tree and forest mortality patterns."
Swetnam agreed that there is not yet a smoking gun linking human-caused climate change to global forest death. But he noted that in forested areas of North America, there are clear signs that higher temperatures are causing more fires and increased tree death.
The difficulty in painting a clearer picture of global forest health and the effects of global warming comes from a lack of comprehensive data, Allen said.
To overcome that problem, Allen and his colleagues scoured the scientific literature and sought out scientists on every continent, looking for research on forests. The result is "the first snapshot" of forest health worldwide in the context of climate change, Breshears said.
The scientists call for a globally coordinated effort to better monitor forests to help better understand how they are responding to climate change.
The evidence Allen and his colleagues found on the Pajarito Plateau shows the difficulty of the problem.
In research published four years ago, Breshears, Allen and others tied the massive pinon forest death of 2002-03 to a drought combined with warmer weather. In terms of the amount of rain and snow that fell, the drought was no worse than conditions in the region in the 1950s. But far more trees died in 2002-03, the scientists found, because temperatures were higher.
Allen and his colleagues found studies showing similar heat-related forest death elsewhere in North America, along with Africa, Asia, Europe and Central and South America.
Allen said the difficulty is that scientists don't have a clear picture of the detailed biological mechanisms that kill the trees during hot, dry weather.
Without that understanding, he said, it's hard to predict how forests will respond to rising temperatures predicted as a result of greenhouse pollutants.
That is important, the scientists wrote, because forests contain huge amounts of carbon. It is released into the atmosphere as the trees decay. If global warming kills forests, it could create a feedback effect, releasing more carbon into the atmosphere and therefore warming the planet even more quickly.
Information from: Albuquerque Journal, http://www.abqjournal.com