Tuesday, May 20

The other drought monitor and what it portends

First, a note on my use of the term "megadrought" in the previous post: While a megadrought can be considered a drought of longer than a decade's duration, the definition established by science is "a prolonged drought lasting two decades or longer," according to Wikipedia.  See Wikipedia for more discussion of the issue, and also note that the Climate Central article in this post quotes two scientists who speak of the current drought of 13 years' duration in the USA as a megadrought.

One thing the scientists agree on:  "The term megadrought is generally used to describe the length of a drought and not its acute intensity," according to Wikipedia. The Dust Bowl drought, while not a megadrought, was 10 years of hell for the people who lived through it. 

And yet it was human actions in the Dust Bowl region that helped create some of the worst conditions for families who hacked it out.  So there was the cyclic, drought-inducing weather pattern, and there was that other factor: people. 
May 15, 2014; Wired Magazine: Betsy Mason writes about a U.S. Drought Monitor/ NASA Earth Observatory map showing drought conditions in the USA as of May 6:  Map Shows Half of the US Suffering Drought Conditions. "[...] As scary as the map is, it doesn’t convey the true severity of the situation because the impacts are cumulative from several years of drought, particularly in the southwest centered on Northern Texas."

The United States Drought Monitor, which relies greatly on satellite technology, is only 14 years old, but it's been a great boon. However, there's another drought monitor, one far older. It's trees and tree rings.

It wasn't until this era that the reading of tree rings for their information on periods of drought weather became a science, known as dendrology, and also made it possible to 'read' trees that were preserved underwater, sometimes for centuries, for the data they could reveal on drought periods.

The thing about trees is that they're very accurate record-keepers. They don't make mistakes.  Let's see what the trees are telling dendrologists about megadroughts in the USA. From Caroline Fraser's June 20, 2013, analysis for Yale's e-360, Megadrought in U.S. Southwest: A Bad Omen for Forests Globally:

[...] With a highly variable climate, the Southwest boasts perhaps the best-studied megadrought history in the world. It’s the home of dendrology, the science of studying tree-rings, first developed at the University of Arizona.

The pronounced seasonality of hot summers followed by cold winters produces well-defined rings, while archaeological fascination with Southwestern cultures — Chaco Canyon, Mesa Verde, and other sites where ancient peoples flourished and disappeared — has supported the collection and study of centuries of tree-ring data.

Temperate-zone trees lay down wider rings in wet years, which narrow or vanish during drought. What’s more, rings can be precisely dated, with sets matched against each other, revealing burn scars and patterns of climate, precipitation, drought stress, and tree mortality.

Park Williams, a young bioclimatologist and postdoctoral fellow at Los Alamos National Laboratory, has teamed up with other specialists at the U.S. Geological Survey (USGS) and the University of Arizona to wring new insight from the data set spanning the years 1000 to 2007.

Driving recently into the Jemez Mountains near his office, we pass rust-red pines, dead or dying from drought. Later, kneeling next to a freshly cut stump, he points to a ring near the bark.

“That thick ring right there is probably 1998,” he says, a wetter El Niño year.

Armed with 13,147 such site-specific cross-sectioned specimens, gathered from more than 300 sites, Williams and his co-authors devised a new “forest drought-stress index,” integrating tree-ring measurements with climatalogical and historical records for a paper published earlier this year in Nature Climate Change.

Winter precipitation has long been thought important to tree growth, but another key variable leapt from this fresh examination of the data, related to a warmer, dryer climate: the average vapor pressure deficit during summer and fall, which is driven by temperature.

As air grows warmer, its capacity to hold water vapor increases exponentially, which speeds evaporation and sucks more moisture out of trees’ leaves or needles, as well as the soil itself.

If the vapor pressure deficit sucks out enough moisture, it kills trees, and there’s been a lot of that going on.

Looking back in time through the tree rings, Williams determined that the current Southwest drought, beginning in 2000, is the fifth most severe since AD 1000, set against similarly devastating megadroughts that have occurred regularly in the region.

One struck during the latter 1200s (probably driving people from the region) and another in 1572-1587, a drought that stretched across the continent to Virginia and the Carolinas. Few conifers abundant in the Southwest — including piñon, ponderosa pine, and Douglas fir — survived that latter event, despite lifespans approaching 800 years; those species have since regrown.

The forest drought stress index correlates strongly with these periods, while 20th-century temperature records show a connection between drought and tree mortality associated with huge wildfires and bark-beetle outbreaks, such as the devastating ones of the past two decades.

Williams’ study is also supported by satellite fire data from the past few decades, revealing an exponential relationship between drought stress and areas killed by wildfire.

His projections, based on climate forecasts, sparked grim headlines throughout the region: If the climate warms as expected, forests in the Southwest will be suffering regularly from drought stress by 2050 at levels exceeding previous megadroughts.

After 2050, he calculates, 80 percent of years will exceed those levels. “The majority of forests in the Southwest probably cannot survive in the temperatures that are projected,” he says.[...]

This still doesn't explain whether the current drought in the U.S. is a true megadrought.  For insight on the answer, which is still a matter of debate among climatologists, I turn to the best plain-English examination of the question I've found on the Internet. (See the website for source links.):

Is the West’s Dry Spell Really a Megadrought?
By Bobby Magill
December 12, 2013
Climate Central

SAN FRANCISCO — The drought that has been afflicting most of the Western states for the past 13 years may be a “megadrought,” and the likelihood is high that this century could see a multi-decade dry spell like nothing else seen over the past 1,000 years, according to research presented at the American Geophysical Union Fall Meeting on Wednesday and Thursday.

Today, drought or abnormally dry conditions are affecting every state west of the Mississippi River and many on the East Coast, with much of the Southwest under long-term severe, extreme or exceptional drought conditions. While drought conditions nationwide are down this year, they remain entrenched in the West.

Since 2000, the West has seen landscape-level changes to its forests as giant wildfires have swept through the Rockies and the Sierra Nevada, bark beetles have altered the ecology of forests by killing countless trees and western cities have begun to come to terms with water shortages made worse by these changes as future snowpack and rainfall becomes less and less certain in a changing climate.

“The current drought could be classified as a megadrought — 13 years running,” paleoclimatologist Edward Cook, director of the Tree Ring Laboratory at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y., said at an AGU presentation Wednesday night. “There’s no indication it’ll be getting any better in the near term.

But the long period of drought the West is currently experiencing may not be a product of human-caused climate change, and could be natural, he said.

“It’s tempting to blame radiative forcing of climate as the cause of megadrought,” Cook said. “That would be premature. Why? There’s a lot of variability in the system that still can’t be separated cleanly from CO2 forcing on climate. Natural variability still has a tremendous impact on the climate system.”

Tree ring data show that decades-long droughts have occurred before humans started emitting greenhouse gases that fuel climate change. Long-lasting drought events have been tied to fluctuations in ocean conditions, which can alter large-scale weather patterns. For example, when the tropical Pacific Ocean is cooler than average, but the Atlantic Ocean is unusually mild — as has been the case during the past several years — there is a higher risk of drought in parts of the West and Central U.S.

The area of the West that was affected by severe drought in the Medieval period was much higher and much longer than the current drought, tree ring data show.

It is “indeed pretty scary,” Cook said. “One lasted 29 years. One lasted 28 years. They span the entire continental United States.”

Two megadroughts in the Sierra Nevada of California lasted between 100 and 200 years.

Cook is among the first to suggest that the current drought in the West is a megadrought, which is typically defined as a widespread drought lasting for two decades or longer, Cornell University assistant professor of earth and atmospheric sciences Toby Ault said during an AGU presentation Thursday.

But the idea that the current 13-year dry spell will be of similar magnitude of the megadroughts found in tree ring records is subject of debate.

“Are we in a megadrought? I guess we are,” Ault said. “They are a threat to civilization in the future.”

Ault is studying the probability that the U.S. will experience a megadrought this century on the order of no other dry period seen here at any time in the last millennium.

Data gleaned from tree rings and other sources show that the chance of a decade-long drought in the U.S. this century would be about 45 percent, and a multi-decade-long drought less than 10 percent, he said. 

“That’s not the whole picture because we’re going to see climate change in this century,” he said.

He said that the chances of a widespread multi-decade megadrought are high in the worst-case scenario, but he quoted University of Arizona geosciences professor Jonathan Overpeck to characterize the chances of megadrought in less severe scenarios: “It’s extremely non-negligible, the risk of prolonged multi-decadal megadrought.”

The bottom line: “The picture looks like we’re going to have to take this seriously,” Ault said.

Such dry spells would have severe implications for the nation’s water supply, and the U.S. is going to have to adapt and find smarter ways to cope, he said.

The current drought is occurring at a time of sweeping and abrupt changes in the nation’s forests as a result of both the extended dry period and human-caused climate change, said Lisa Graumlich, dean of the College of the Environment at the University of Washington.

Speaking at AGU on Wednesday, Graumlich said vast ecosystem changes are happening at an unprecedented scale across the country as tree mortality in Western forests is increasing dramatically, partly because bark beetles are spreading widely as summer warm seasons are longer than before.

“The time in which forests are burning in the West is much longer than it was in previous decades,” she said. “Forest insects are erupting across the West.”

Those changes and others including loss of sea ice, longer growing seasons in the Arctic, tundra being replaced by forests and shrubs, are occurring across an area scientists haven’t seen before, Graumlich added.

“We’re seeing right now ecosystem tipping points. They’re at an unprecedented spatial scale. They’re related to timing of biological events that ecologists are finding surprising.”


No comments: