This post will cover a lot of ground. I’m writing it having just read a PNAS paper from 2005 about a massive vegetation die-off event being tied to global climate change-type drought in the four-corner states. I have been thinking that such a study should be done as I read and learned about the various pine beetle epidemics afflicting Western North America. The paper, “Regional vegetation die-off in response to global-change-type drought” contains the kind of information that is critical in piecing a number of different threads together to weave a coherent story. The results contained within the paper, which can also be seen at this PNAS website, provide a profound message about the impacts of climate change. Such impacts have already occurred. As I’ve written recently (here and here, for example), they are growing in number and intensity. We ignore them at our (and the Earth’s) peril.
A series of big messages I got from this paper can be summed up as follows. With human-forced climate change, warmer droughts are predicted to occur more often. One such drought has already occurred (and could be continuing to occur) in the southwestern U.S. That drought has had a profound impact on a large region’s worth of vegetation. That impact came in two waves: the drought weakened the vegetation which then fell to the beetle epidemic. The beetles were able to spread due to the warmth that characterized this drought. With this and other region-wide die-offs, the potential for large changes in carbon stores is real we will face with their consequences. As a result, carbon-related policies must be prepared to take such die-offs (and their after-effects) into account. The failure of region-wide ecosystems, a disaster on its own, would also present a real danger to our society.
The paper identifies regional-scale mortality of overstory trees. Such events alter ecosystems and land surface properties for decades. Greenhouse forcings are expected to amplify the periodic, cooler droughts found in previous climate regimes. The drought that has occurred across the southwest since 2000 offers evidence about how those forcings manifest. [On a short tangent, the widespread, severe drought in Australia provide additional evidence.] This paper focused on a Piñon pine die-off. Additional trees act as overstory species across the four states studied. At this time, I’m not aware of similar studies detailing the greenhouse-forcing-drought-beetle-die-off relationship as it relates to those species. It is something I will look for after writing this.
There is one figure in this paper that I want to draw particular attention to:
This figure shows annual average temperatures and precipitation values for all the stations included in the study. The yellow-shaded vertical bands point out two regional-scale droughts – the first in the 1950s and the second in the early 2000s. In particular, I want to draw attention to the rise in average annual temperatures from the mid-1990s to the mid-2000s (top panel): from ~10.7C to 12.3C. Combined with the corresponding drop in precipitation from 380mm to ~250mm, this is what the authors have characterized as a global-change-type drought. It contrasts with the 1950s drought by being statistically warmer to a significant level. The bottom two panels also deserve some attention. Two of the last four years in the study exhibited maximum average annual temperatures at the 90th percentile (panel c). Three of the last four years in the study exhibited minimum average annual temperatures well above the 10th percentile (panel d). That information isn’t available from the top panel
The reason I draw attention to the temperature rise in particular is the warning it provides about anticipated future warming across the region as the climate continues to respond to greenhouse forcing. Under scenarios now considered likely with the “warming in the pipeline”, temperatures across this region are expected to rise another 2-10C. As I wrote above, this paper demonstrates the impacts that warming has on dominant vegetation types: water stressing the plants and allowing bark beetle infestations to spread unabated. With even more warming, what effects will ecosystems in the region experience? I’ve written before about the bark beetle problem affecting the higher elevations of Colorado and other regions across the Rocky Mountains (see list below). Those trees were impacted in a similar fashion that the Piñon trees were in this study. How many additional species will be stressed to the point that they will also experience region-wide die-offs? Under those same climate change scenarios, annual precipitation is expected to continue to decrease. That decrease will be for all purposes permanent as far as humans are concerned. Desert-level precipiation amounts are quite possible for hundreds of years.
Now look at the graph more closely. We’ve seen the devastating effects just a small quantitative amount of warming has already had. That’s one of the real dangers of climate change: ecosystems are quite used to the climate of the 20th century (in a larger sense, that of the past few thousand years). There is no way of accurately foretelling how those ecosystems will respond to a significantly different climate, which we might already have entered into. The die-offs I’ve seen and read about; the shifting climate and ecosystems zones I’ve seen evidence of tell me that the climate at the end of the 21st century could be quite different than the one of the 20th century.
Expanding on this a bit: at what stage would prairie grass die-off? I can hear the denialist line about tree die-off and small animal die-off not being a big deal and not indicative of climate change. The level of tree die-off discussed in this paper was unprecedented in scope: all ages, all sizes were affected. Beyond that though, I wanted to come up with a scenario that would provide more visceral evidence of climate change impacts on human society. If grass or hay or the like experienced a regional die-off due to an expanding, long-term warm drought, what would we do? If cattle started dying by the millions due to water stress and epidemics, would more people take notice? I have to think so. I hope it doesn’t get to that level, but it might before we all aggressively look for greenhouse forcing solutions.
One additional question I have is what story does the post-2004 data tell? I will look for additional, related studies to this one to fill out the scene. It was somewhat surprising that this study was published in 2005 . Nobody I’ve spoken to about the ponderosa pine die-off was aware of this paper – which is part of the reason I’m writing about it. If anyone is aware of such a study, I’m all ears. Otherwise, I’ll write something up on whatever I find.
Cross-posted at SquareState.
Here is a list of some of the bark beetle epidemic posts I’ve written: