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Joshua Tree Bloom Due To Climate Change? Not So Fast

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This tree in Arizona bloomed pretty well in 2007 | Photo: cobalt123/Flickr/Creative Commons License

The California desert's signature tree has gotten a lot of attention lately: the story of this year's record bloom (which I'm proud to say broke right here on has reached a global audience. And the developing narrative is that the unusual bloom can be credited to climate change.

As someone who's warned of the danger of human-created climate change since the early 1970s, you might expect me to be completely on board with this analysis. I'm not.

Articles on the extraordinary bloom have seen "print" on websites ranging from National Geographic to the New Scientist. The actual scientists quoted in the coverage really do try to convey just how much we don't know about what's going on here, Cameron Barrows of UC Riverside being an excellent and circumspect example. Nonetheless most of the writers come down solidly on the conclusion that the bloom is probably a result of global warming.

It's a tempting conclusion. We have no record of the trees ever blooming in such a seemingly coordinated manner, so this bloom appears to be something new. And things that haven't happened before tempt those of us who are concerned about climate change to find a connection.

That connection may well exist, but despite the headlines it isn't a slam dunk.

How could climate change spur a massive bloom of Joshua trees? Naturalist Jim Cornett, who's been studying the trees for a couple of decades, suggested to the Riverside Press Enterprise's David Danelski -- who wrote an admirably cautious piece on the bloom -- that the last couple dry years combined with temperatures two or three degrees above normal have sparked a "stress response" in the trees, in which they set heavy bloom as a last-ditch effort to preserve their genetic lineages.

It's a plausible hypothesis. Plants do often respond to environmental stresses by flowering. The last couple years in the Mojave have indeed been dry, and warmer than usual to boot.

There are a couple of picky problems, though.

Screen shot 2013-04-23 at 10.28.28 AM-thumb-600x266-49546
Precipitation averages in the Mojave Desert | Image: USGS

As you can see from the graph above, which I've yoinked from this USGS study on climate in the Mojave, the Mojave Desert is no stranger to extended drought. There was a drought across the Mojave that lasted from 1941 to 1975 or so, for instance. During that period there were quite a few multi-year warm spells, and yet no records exist of similar desert-wide heavy blooms.

That's not necessarily because such blooms didn't happen: it's hard to know for sure when scientists haven't kept track. But people have been living in the desert for a very long time. When a rainy season hits, old-timers will readily trot out stories of the precise years in which more rain fell. During the very heavy rains of the winter of 2004-2005, I had an old man at a gas station tell me solemnly that he saw bigger rains as a kid in 1941, for instance. And yet none of the desert's resident octogenarians have piped up, to my knowledge, with stories of a bloom in '48 that was bigger and more beautiful than 2013's. Absence of evidence isn't evidence of absence, but the lack of such anecdotal evidence is interesting nonetheless.

Two other issues complicate the slam-dunk diagnosis of climate change as a cause of the Joshua tree bloom. The first is that though the desert has indeed been drier and wetter than average in the last couple years, that data is itself an average. There are places in the Mojave that have gotten a good dumping of water in the last 12 months, and there are places that haven't been all that much warmer than usual. Joshua trees growing in places like those are blooming anyway, it would seem. In fact, some observers are crediting heavy summer rains in 2012 for the bloom. That's a problematic hypothesis as well: it didn't rain everywhere in the desert. Joshua trees react to the specific conditions where they happen to be growing, not to conditions where the rain fell during a monsoon a hundred miles away -- nor to a theoretical average across the tree's whole range.

The other pesky issue with the climate stress hypothesis is that many of the trees that have bloomed abundantly show no visible signs of unusual stress. There are five individual examples I can see from my desk here. They don't have the slightly reflexed leaves common to highly stressed Joshua trees. They don't have entire limbs that have died back. They haven't been gnawed at by thirsty rodents. They all have abundant coats of deep green, healthy-looking leaves with active growing tips. They might be stressed more than is usual for a Joshua tree, but they sure aren't showing it. And they bloomed their fool heads off a month ago, and are now happily growing healthy-looking clusters of fruit.

The biggest overall problem with attributing this bloom to any cause is that we have precisely one data point: a massive bloom happened in Spring 2013. Any number of things might have caused this bloom: temperature, moisture, some heretofore-unknown biological clock in the trees similar to those found in some other plants that can cause them to bloom en masse despite being widely separated, an advent of a new environmental chemical over the last few years that mimics plant hormones, or some complex combination of several different factors.

We don't have another example of a mass flowering of Joshua trees to compare this one to. If we did, we might be able to find something that both events had in common. But we don't. And given that the majority of the science surrounding anything more than the utter basics of Joshua tree blooming is of the "we don't really know" variety, it's way too early to conclude that global warming did it, or that this bloom means the trees are in even more trouble than we already knew they were.

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