The biggest and most horrifying story I stumbled across at the AGU involves forest mortality, as mentioned in this 2012 story in the NYTimes: Los Alamos National Laboratory studies tree deaths

It's good on the technical aspects, and really helped me understand the mechanism of "hydraulic failure" — how heat can not just challenge, but kill trees. The story doesn't want to be the last word on the subject, perhaps to its credit. It helps us understand the details: 

To monitor how trees might succumb to thirst, researchers are measuring water flow inside each trunk. Normally ropes of water molecules are pulled up from the soil and roots by the atmosphere, moving through very small channels called xylem. When the air is warm, it exerts a greater pull on the water, increasing tension. If the tension gets high, the rope breaks and air is introduced. Like an embolism that can kill a person, air bubbles can block the flow of water. A tree can dry out and die.

It's helpful but I must say,it's not what the researcher in question, Nate McDowell, said at the at the AGU a couple of weeks ago. He framed it differently: as forest mortality. 

In which case, the Times' approach almost literally misses the forest (mass mortality) for the trees (how individual trees succumb to climactic conditions). 

In any case, talking to McDowell at the AGU, I mentioned that I was walking the Pacific Crest trail and had seen one burned out forest after another walking north through Southern California. Many huge fires have hit the trail before and after I have been walking just these past two years. Just weeks after myself and Chris Nottoli passed through the San Jacinto Mountains they were hit with a major fire, the Mountain fire, that consumed over 30k acres of pines near Idyllwild,and forced a long difficult roadwalk detour for those coming on the trail post-June 2013.  

In Section C, I encountered another large area –more than 16k acres – of burned forest to the east and north of Big Bear Lake. Huge pines. Big Bear Fire. 2007. Took a full day or more to walk through the dead and twisted trees and scorched earth.

In Section D, coming down from the San Gabriel Mountains and turning north towards Agua Dulce, I had to walk through the vast scar left by the Station Fire of 2009, which burned over 160k acres and filled the sky with the life of thousands upon thousands of trees. 

Then in Section E about thirty miles of trail north of Green Valley were completely destroyed by the Powerhouse Fire. A ranger told me that the soil itself had been changed by the extreme heat of the blaze. The trail had simply vanished. 

Joe Anderson, who with his wife Terry takes care of hundreds of hikers passing through the trail near his town of Green Valley, told me that one hiker who did go through the burn emerged entirely blackened below his shoulders after walking through miles and miles of chaparral and pinyon pine turned to charcoal. 

"It's like that the whole length of the trail, all the way up to Canada," said Nate McDowell, a couple of weeks ago, in the press room at the AGU. 

I don't want to be alarmist, but McDowell and his friend and fellow scientist Craig Allen believe that the forests of the Southwest are doomed. They have a date in mind, for when they will have died off.  

2045-2050. 

[for those curious about the mechanism of this catastrophe, the hypotheses and the studies, I've put some resources below the fold.]

Here's the study from 2010 referenced above, and a paragraph that adds a new detail to the explanation of what happens when a pinyon pine or a juniper — both species long accustomed to the droughts of the Southwest — are challenged by heat and drought. 

In addition to hydraulic failure and carbon starvation, a third
physiological mechanism predisposing plants to mortality may
exist—cellular metabolism limitation. This hypothesis suggests that
low tissue water potentials during drought may constrain cell
metabolism (Wu¨ rth et al., 2005; Ryan et al., 2006; Sala and Hoch,
2009), thereby preventing the production and translocation of
carbohydrates, resins, and other secondarymetabolites necessary for
plant defense against biotic attack. The common observation that
trees which succumb to insect attacks have weak resin flow and are
unable to pitch out attacking insects is consistent with constraints on
photosynthetic carbon uptake, cellular carbon metabolism, and/or
treewater relations.A likely sequence formany isohydric species that
is consistent with Manion’s cascade (Manion, 1991) is that climatestressed
trees starve for carbon, perhaps due to poor edaphic position
combinedwithdrought,which causespoor resinflow and an inability
to defend against insect attack, which subsequently allows fungi that
are symbiotic with the beetles to colonize and occlude the sapwood,
causing transpiration to cease, drying of the canopy, and eventual
mortality (McDowell et al., 2008, 2009).

In a 2009 paper, Allen and McDowell identified the same three factors in tree/forest mortality, but did not put a date on the forests' demise: https://www.fort.usgs.gov/sites/default/files/products/publications/22659/22659.pdf

Here's a paper by Park Williams of Lamont-Doherty, another researcher into this question, who offers a quantitaive stab at measuring the stress of drought on forests, and also graphs Vapor Pressure Deficit for these forests over time. Temperature as a potent driver of region forest drought stress and mortality 

Expect to add to these resources over time: hope to write about this crisis and these forests. I've only just discovered pinyon pine — I can't bear to see it go away from my backyard so soon!