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Western Forests Could Become Carbon Source, Not Sink

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Human-forced climate change has already many effects that are visible today.  An article that appears in today’s Science introduces another candidate: trees in the Western U.S. are living only half as long as they did 50 years ago.  In the climate most of us grew up in, western forests acted as carbon sinks.  Their growth “scrubbed” carbon from the atmosphere.  Climate change has introduced conditions that are drier than normal; severe drought has ensued across the region.  As a result, trees are growing less and dying earlier than they used to.  That could result in less carbon being removed from the atmosphere, creating yet another positive feedback loop in the climate system.

Researchers focused on what’s called “background” mortality – trees dying from events that do not include infestations of insects like the mountain pine beetle currently afflicting the West, which is identified as “abrupt” mortality.  They studied 76 plots where trees were at least 200 years old.  They were undisturbed by logging (harder to do every year), bark beetle epidemic or wildfire.  Trees being studied in Colorado were largely wiped out by the mountain pine beetle epidemic currently moving across the state’s forests, itself a trend linked to climate change.  Temperature data in the research plots (across the entire West) showed an increase in every season of the year.  The warming and drought conditions we’ve experienced in Colorado has also made its presence felt across a much larger region.

Our forests are suffering from multiple coincident effects of a warming planet and regionalized drying.  Direct human pressures such as increased population in the inter-mountain West and hundreds of years of logging aren’t helping matters.  Efforts need to be made today to decrease our forcing on the climate system.  Carbon emissions need to be drastically reduced so that concentrations in the atmosphere can be reduced later this century.  If forests are unable to play their historic role of a carbon sink, those efforts become all the more critical.  Unfortunately, it will likely increase their cost, something environmentalists cited regularly during the past eight years’ of climate inaction.

The Denver Post has, surpringly to me, a pretty good article on this.  Joseph Romm (Center for American Progress Senior Fellow & Climate Progress blogger) has a more scientifically-rigorous discussion of the article and its implications.  I recommend reading Romm’s analysis if you don’t want to read the article itself.

Cross-posted at SquareState.

[Update]: While reading the article again, something important popped out at me. The authors note the following:

From the 1970s to 2006 (the period including the bulk of our data; table S1), the mean annual temperature of the western United States increased at a rate of 0.3° to 0.4°C decade -1, even approaching 0.5°C decade -1 at the higher elevations typically occupied by forests.

So between 1.2°C and 2°C warming has already occurred since the 1970s.  That means the forests of the future are in for bad times.  If we could somehow magically stop emitting greenhouse gases today, the climate system would still get warmer for the next 100+ years due to the forcing “in the pipeline”, as climatologists refer to it.  The climate system hasn’t fully responded to the gases emitted in the last 5, 10, or 50 years.

Of course, no such magic is going to occur.  Emissions will have to stop increasing (stabilize) then start decreasing.  Which means there is plenty of additional forcing (warming) that will occur.  The 2007 IPCC assessment relied on models that didn’t demonstrate the warming that has already occurred very well.  Policy decisions based on that report would therefore be poorly suited for the task we face.

[2nd Update]: NPR’s Science Friday discussed this paper with one of the researchers today.  The segment can be found here.


One thought on “Western Forests Could Become Carbon Source, Not Sink

  1. Pingback: Four-Corners Tree Die-Off Tied To Global Climate Change Drought « Weatherdem’s Weblog

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