Truthout - September 24, 2020
Behind the apocalyptic wildfires in California and Oregon, another ominous trend is creeping across the globe: Everywhere in the world, trees are dying with the biggest trees going first. Entire forests are threatened worldwide.
The 83 large wildfires burning in mid-September in Arizona, California, Colorado, Oregon, Montana, Nevada, South Dakota and Wyoming are only one symptom of what’s happening to forests everywhere. Forests are dying.
Forests can adapt to change, but recent atmospheric warming is driving changes faster than many trees can adapt to.
Between 2010-2016, a research group at Los Alamos National Laboratory in New Mexico, led by Nate McDowell, studied southwestern forests and developed a “forest drought-stress index” based on more than a thousand years of tree ring data, which reveal past drought conditions. They concluded that by the 2050s, average annual drought conditions in the southwestern U.S. will be worse than any drought in the past 1,000 years. From this, they concluded that sometime after 2050, conifer forests in the southwestern U.S. will disappear. Since then, the McDowell team has concluded that during the remainder of this century, half (or more) of all conifer forests in the northern hemisphere will disappear, and, furthermore, that forests worldwide are endangered by what they call “hotter drought.” Forests are hanging by a thread.
The causes of expanding tree death are complicated, but they all start with warmer air. According to NASA, since about 1890, the average temperature across the entire planet has risen 1.6 degrees Fahrenheit (0.9°Celsius.) This may not seem like much, but the temperature rise over land is 50 percent greater than the global average rise, so temperature over land has risen an average of 2.4°F (1.3°C). This means less snow, more rain, earlier springtime, and most importantly, warmer air.
Warmer air can hold more moisture, so, as the air warms, it draws moisture out of soils and plants, including trees. This is called “atmospheric moisture demand” or “vapor pressure deficit.” This important principle explains why a wet towel dries faster in a dry-heat sauna than it does in a steam room. “The air literally sucks the moisture out of the soil and plants,” explains forest researcher A. Park Williams, author of a major study of the effects of drought on forests.
As air temperature rises, atmospheric moisture demand rises even faster. For trees, this creates a double-whammy. In different ways, trees are harmed by both the rising atmospheric moisture demand and the rising heat. ...
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