tree rings


The Science of Reconstructing Past Climate

To find out what tree rings are telling us about droughts in the Colorado Basin, and to get some current perspective on the current eleven-year drought in the region, listen to my radio story for The California Report and view the slide show of my journey to the region. — Gretchen Weber

With cores from trees like this one, scientists have been able to reconstruct more than 1,000 years of climate history in this region. (Photo: Gretchen Weber)

Abbie Tingstad is a paleoclimatologist whose doctoral work at UCLA involved reconstructing climate in the Upper Colorado River Basin, using tree rings and lake sediments.

By Abbie Tingstad

Unlike biology, chemistry, or most mainstream sciences, it’s hard to envision what someone who studies paleoclimatology actually does. I run into a lot of blank stares at dinner parties. So I’ve started describing the field as “climate forensics.”

Paleoclimatology and forensics of the Law & Order or Bones variety share the basic goal of reconstructing something that has happened in the past. In the latter, of course, the sequence of events that led to a crime is put together. In the former, researchers identify past variations in climate.  These sciences also have quite a lot in common when it comes to the basic methodology: Continue reading

Thousand-Year-Old Trees Get a Growth Spurt

Bristlecone pine. Photo: US Forest Service

Bristlecone pine in the Inyo National Forest. Photo: US Forest Service

There’s a lot of history packed into a tree with more than 4,000 annual growth rings. Scientists who count them (dendrochronologists) have been able to learn a lot about the drought history of California and the West.

The Great Basin bristlecone pines that grow along the spine of the Sierra are the oldest living things on Earth–older, even, than the giant sequoias. Studying the uppermost trees, around 12,000 ft., researchers stumbled on a strange trend. The trees, legendary for their slow rate of growth, have been growing faster over the last 50 years or so, than at any time in the last three millennia.

If you missed it this week, Malcolm Hughes, one of the study’s lead researchers and a professor of dendrochronology at the University of Arizona’s Laboratory for Tree-Ring Research, spoke to NPR’s All Things Considered about the possible cause.

There’s more on the study in a recent post on the RealClimate blog.

You can see these astonishing trees for yourself in the Ancient Bristlecone Pine Forest of Inyo National Forest–but you might want to wait until spring. The visitor center is not staffed between November and May and winter access is iffy at 10,000 feet. Worse yet, the original vistor center burned down in the fall of last year. The Forest Service is using a temporary (trailer) facility until a permanent one is rebuilt. According to the Forest Service website:

“…the visitor center is being designed to be a model of energy efficiency, utilizing the latest in “green” building practices.   According to Bristlecone Pine Forest Manager John Louth, some of the improvements that visitors will see will be a state-of-the art solar power system, updated exhibits addressing the impacts of global warming on the ancient trees, a small research library, a slightly larger theatre room and a fire/intrusion detection & suppression system.”

Decoding California’s Drought History

Abbie Tingstad is a paleoclimatologist at UCLA. She specializes in reconstructing drought records in the western United States, and takes us along on some of her field research in this guest post:

Part of a Piñon pine beam under the collapsed rock shelter. This beam was one of several sampled for tree ring analysis. Photo by Abbie Tingstad.

Part of a Piñon pine beam under a collapsed rock shelter. This beam was one of several sampled for tree ring analysis. Photo by Abbie Tingstad.

By Abbie Tingstad

The site was so remote we needed a team of archaeologists and a couple of heavy-duty 4x4s to get us there.

Deep within the rocky piñon-juniper cliffs of northwestern Colorado was a secret so well hidden I didn’t see it until I was physically inside, face-to-face with a series of hand prints made over a thousand years ago. This rock shelter was occupied during Medieval times by Fremont Indians, contemporaries of the Anasazi whose cultural center was further south in the Four Corners Region. The site had already been excavated, but our interest as dendroclimatologists was not in artifacts. We had come to take samples from the ancient piñon and juniper beams that once supported this structure for the valuable paleoclimate information contained within their annual growth rings.

Tingstad sampling a live Piñon pine tree in northeastern Utah. This tree is about 550 years old. Photo by Glen MacDonald.

Tingstad sampling a live Piñon pine tree in northeastern Utah. This tree is about 550 years old. Photo by Glen MacDonald.

Gathering Medieval climate information from tree rings, lake sediments, and other natural climate archives in the Western US is critically important for understanding the implications of increasing temperatures in this region, particularly when it comes to future water supply and demand.

Research has confirmed that temperatures rose in the Western U.S. from about A.D. 800-1300, which translated into a series of droughts. The most devastating of these occurred in the mid-11th and 12th Centuries, when dry conditions persisted for several decades and may have contributed to the collapse of the Anasazi and Fremont cultures.

Paleoclimate data from tree rings and other sources also suggest that the mechanism driving drought during the “Medieval Warm Period” was eastern Pacific Ocean cooling. Like a widespread, extended La Niña event, cool sea surface temperatures may have strengthened the persistent moisture-blocking system of high-pressure off the West coast, nudging storm tracks north.

While the Medieval period is an instructive analogue for the warming we are beginning to experience, it is an imperfect one. Two major factors separate the episode the Fremont and Anasazi experienced a thousand years ago from what we are just beginning to undergo today. First, Medieval warming appears to have been fostered by a combination of increased solar irradiance and decreased volcanic activity, rather than anthropogenic greenhouse gas emissions. Secondly, Medieval times were characterized mainly by summer warming, while winter and spring temperatures are expected to increase most dramatically in the future. These differences manifest themselves in many ways, but notable for the water-starved West are the implications for decreased winter snowpack and earlier spring river discharge.

The Medieval Warm Period may not offer a precise preview of our future, but it serves as a valuable warning about the tenuous balance of water supply and demand in California and the Western US, something the occupants of the Fremont rock shelter we visited were likely aware of.

Since the turn of the new Millennium, drought has been the norm rather than the exception in this region and the end is not in sight: As of May 1, 2009 surveys suggest that the Sierra Nevada snowpack is two-thirds of normal. What we can learn from Medieval times is not to expect “normally” moist conditions to return any time soon, and to plan accordingly.