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:

1. Collection of evidence
I’ll use the example of tree rings, something I have direct experience working with from my research in northeastern Utah. Based on river gauge data, this area of the Colorado River Basin originates 10-15% of Upper Basin water, which is why my colleagues and I were interested in understanding the incidence of past droughts here.

In this case, my climate “fingerprints” were moisture-sensitive Piñon pines. We searched for sites that appeared to have very old living trees — identifiable by trunk size, the open appearance of the branches, and the presence of dead limbs — and lots of dead trees. We used a manual coring device to take pencil-sized samples from both living and dead trees.

2. Lab work
Back at UCLA, I mounted the delicate samples onto wooden holders with an adhesive. Then my assistants and I sanded each sample using progressively finer sandpaper, until individual cells within annual rings were visible under a light microscope.

I counted the annual rings on each of the live samples back in time, starting with the ring closest to the bark. I noted recurring patterns of narrow rings, which are generally associated with climate events unfavorable for tree growth (for example, during the Dust Bowl drought). These patterns allowed me to identify when the dead trees lived. As I had hoped, some of the dead trees were older than any of the living trees, allowing a longer reconstruction.

After dating the samples, I measured the widths of the annual rings using a microscope attached to a computer. I then used statistical software to check the accuracy of my dating and to generate one time sequence per site, based on the ring widths of all the individual trees.

3. Reconstruction of events
In a crime lab, analysts will compare fingerprints, residue samples, or other forms of evidence with a database to place the evidence into context. Paleoclimatologists do the same, except we use instrumental climate information. In the Uinta Mountains research, I found that the tree rings correlated well with measures of moisture such as streamflow and snowpack over the 20^th century time period for which instrumental data are available. Using a statistical technique called linear regression, I developed a mathematical expression to estimate these climate variables based on tree ring widths. Applying these models to the entire data sets, I developed reconstructions for Uinta Mountains streamflow and snowpack going back several centuries.

My colleagues and I used these estimates of past hydrology to analyze drought frequency, length, and severity. This information extends knowledge of baseline conditions for water management beyond the 20th century.

4. Comparison with other lines of evidence
Even if a paleoclimate record may appear to be a “smoking gun,” researchers validate their work by developing climate reconstructions for a particular area based on different types of data and/or by comparing their work with that of others. As in criminal investigations, the strongest case is made when independent forms of evidence all point to the same conclusion.

At the end of a paleoclimate investigation, researchers are able to present strong evidence for things like long droughts in the Western U.S. during Medieval times, or shifts in the way El Niño behaves, or that late-20^th century temperatures are high in the context of relatively recent Earth history. However, paleoclimatology is an imperfect science. Tree rings, lake and ocean sediments, corals, glaciers, and other archives imprinted by past climates cannot compete with modern instruments. But like many good crime scene reconstructions, a few distorted or missing bits don’t make the big picture wrong.

Paleoclimatologist Abbie Tingstad holds samples of tree cores at the base of an ancient pinon pine at her research site in northeastern Utah. (Photo: Gretchen Weber)

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.”

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.

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.