Forests Struggle to Survive in U.S. Heat Wave
By Matt Owens Oct. 1, 2012
A new USGS-backed study in the October 2012 issue of Nature Climate Change fills in another piece of the climate puzzle. The findings are based on comparing tree ring data to reconstructed past climate conditions in the region; and where this study really stands out is that it hones in on the relationship between heat and water availability in causing tree-death.
The team, led by A. Park Williams of the Department of Energy's Los Alamos National Laboratory, have come up with a forest drought-stress index (FDSI) for the region. When the FDSI reaches a certain threshold, the trees start dying. The FDSI is based on relative humidity, which is an indirect product of both rainfall and temperature. As the temperature rises, the air can hold more absolute quantities of moisture – so, as anyone living in Texas knows, really hot air feels extremely dry and can kill you from dehydration quickly.
With the increasingly high air temperatures in the southwest (and everywhere else really), total moisture capacity rises steeply. The amount of rainfall needs to rise by a large amount in order to preserve the same relative humidity under such a steep rise in temperature. This hasn't happened in the southwest, and this is what is really stressing (and killing) the trees.
2011 and 2012 both reached what the researchers call megadrought levels according to their FDSI numbers. But it will take several more years of prolonged conditions like this year to start the complete region-wide die-off.
This new research also uses existing climate model projections for the region as a baseline to determine when the FDSI might reach levels causing complete die-off. The answer: 2050.
However, the climate models have some key flaws, and using them as a basis for planning is unwise according to many who've been following climate change theory and reality. Looking at their results based on the climate models, shows the FDSI continuing to plunge towards a sustained megadrought – but then pulling out of the dive, and from there gently trending downwards and finally settling into permanent megadrought conditions by 2050.
If the models are wrong, and the next few years are like this one and last year, then the forests of the southwest will undergo that massive die-off.
Considering how the models have been underestimating observed climate conditions, it may be prudent to expect such an event. Furthermore, it seems that the study used the climate model results that were already in existence before this year's Arctic sea ice meltdown, which may have sustained rippling effects throughout the global climate. Also, we are now entering into a new solar maximum period, which should increase the heat just a little bit more. Coincidently, the record crushing heat waves, melt events, and droughts of the past several years have been happening in one of the longest recorded solar minimums.
Stepping back from the theories and science, one thing is very certain: dead trees burn well. And they burn best when there's a lot of them. The new study also showed that recent increases in tree mortality is leading to more wildfires. Keep that in mind if you live in the southwest.
Once the forests are gone, according to a USGS news release about the study, "the current forests will give way to ecosystems more tolerant of prolonged, severe drought." That could mean shrublands and grasslands they say.
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