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Science: Global warming is killing U.S. trees, a dangerous carbon-cycle feedback

Contrary to the popular notion that increases in carbon dioxide emissions increase vegetation, a “stunningly important paper,” in Science finds the reverse has been true.

Led by scientists from the US Geological Survey and USDA, “Widespread Increase of Tree Mortality Rates in the Western United States,” finds:

Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions.

After examining a variety of potential causes, the study concludes:

Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

And this is only the tip of the (disintegrating) iceberg — the planet is on an emissions path to warm 10 times as much in the coming century as we warmed during the period examined in this study.

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As a co-author warned the BBC, this is another potential major amplifying feedback that could itself accelerate global warming in the coming years and decades:

“We may only be talking about an annual tree mortality rate changing from 1% a year to 2%, but over time a lot of small numbers add up,” said co-author Professor Mark Harmon from Oregon State University.

He feared that the die-back was the first sign of a “feedback loop” developing.

As regional warming caused an increased number of trees to die, there would be less living trees to absorb carbon dioxide from the atmosphere.

Yet there would be an increased proportion of decaying trees, releasing the carbon that had been locked away inside the trees’ wood.

And indeed, as I’ve previously reported (see “Something else for the deniers to deny: The ocean is absorbing less carbon dioxide”), the Global Carbon Project analysis of the “natural land and ocean CO2 sinks” finds:

… the efficiency of these sinks in removing CO2 has decreased by 5% over the last 50 years, and will continue to do so in the future. That is, 50 years ago, for every ton of CO2 emitted to the atmosphere, natural sinks removed 600 kg. Currently, the sinks are removing only 550 kg for every ton of CO2 emitted, and this amount is falling.

Of course, no discussion of the impact of global warming on Western tree mortality would be complete without a mention of climate-driven pest infestations:

Warmer temperatures might also increase the number and prevalence of insects and diseases that attack trees, the team added.

They used the example of recent outbreaks of tree-killing bark beetles in the US, which have been linked to a rise in temperatures.

Glad to see the major British media outlet get this important part of the story right (see “Even conservative San Diego Union knows climate change is killing Western forests” but “The New York Times blows the bark beetle story”).

Science’s news story on the piece, “Western U.S. Forests Suffer Death by Degrees” (subs. req’d) notes:

“This is a stunningly important paper,” says David Breshears, an ecologist at the University of Arizona, Tucson. For years, he and others have lamented massive diebacks that occur when fungal and insect pests ravage stands of trees. “What’s harder to detect,” he explains, is any subtle but significant shift in the trees’ background death rate. “They have done a very thorough job” of documenting it….

Ultimately, “the finger seems to be pointed to warming.” says Breshears. Temperatures in the [Western] United States have risen about 0.4°C per decade in the past 40 years. Snowpack of the regions examined diminished over the time period they studied and is melting earlier, effectively lengthening the summer drought. Warmer air also leads to more evaporative loss, exacerbating the effect.

Julio Betancourt of USGS adds “Models suggest that most of this change was due to the buildup of greenhouse gases.”

The ultimate danger is that what we are seeing so far is just the beginning or ‘linear’ stages of what will ultimately be a nonlinear catastrophe. After all, the planet has warmed only about 0.6°C in the past 40 years. We are on track to warm close to 10 times as much this century (see “Hadley Center: Catastrophic 5–7°C warming by 2100 on current emissions path”). What would that mean?

Another member of the team, Dr Nate Stephenson, said increasing tree deaths could indicate a forest that was vulnerable to sudden, widespread die-back.

“That may be our biggest concern,” he warned.

“Is the trend we’re seeing a prelude to bigger, more abrupt changes to our forests.

So add this study to the list of key amplifying feedbacks that the major climate models are missing:

The time to act is now.