Climate Close-up

by Holli Riebeek· design by Robert Simmon· December 22, 2005

“This history in trees tells us the climatic story of the Southwest with amazing accuracy. When a real theory of climate has been developed and we can predict drought and flood over a period of years, this Arizona story in tree rings will have played a creditable part in developing that climatic foresight which is perhaps the most valuable economic advantage yet lying beyond our reach.” –Andrew Ellicott Douglass, 1929

While cave rocks and ice cores provide a long-term, annual record of past climate (see “Written in the Earth” and “The Ice Core Record” in this series), some other climate proxies can offer a detailed record of seasonal temperature or rainfall changes. As they grow from season to season, coral reefs in the oceans and trees on the land both record small variations in the climate. These records can tell scientists about growing conditions in the oceans or on the land, but the record only stretches across the collective lifetimes of the organisms that have been preserved through the centuries. Thus, even though their records are more detailed, reefs and trees cannot provide records that are as long or continuous as ice or sediment core records.

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  Photograph of bristlecone pines along the Methuselah Trail, Inyo National Forest

Tree Rings

Squat and gnarled, Methuselah clings to the rocky slopes of the White Mountains in Southern California as it has for the past 4,770 years. When the ancient bristlecone pine took root, the earliest Greek civilization was being established and the Egyptians were just beginning construction of the Pyramids of Giza. Thousands of years later, both those civilizations are long gone, but Methuselah lives on. It is one of the Earth’s oldest known living organisms.

The barren limestone soil around the tree is bare of grass or other plants, supporting only a widespread grove of scraggly Bristlecone Pine trees, at least one of which is even older than Methuselah. At about 11,000 feet above the arid Great Basin Desert, the trees receive precious little water—hardly a location hospitable to any life, let alone the oldest of living organisms. Ironically, it is the barrenness of the location that has allowed the trees to live so long. With no surrounding fuel, lightning-ignited forest fires can’t engulf the grove.


The Methuselah Walk, high in the White Mountains of California, winds among the oldest known trees in the world. The ancient and twisted bristlecone pines grow extremely slowly, preserving a history of climate in their annual growth rings. The bristlecone climate record goes back 9,000 years, contained in living and dead wood as old as the last ice age. (Photograph copyright Dave Westwood)

  Close-up photograph of bristlecone pin tree rings

The inhospitable environment has also made the trees excellent recorders of rainfall. Each year, the trees grow wider, adding another ring to their girth. Most people have counted the number of rings in a tree stump to find out how old the tree was, but the rings also tell about growing conditions the year it formed. High in the Great Basin Desert, where water is scarce, the growing conditions are most directly influenced by rainfall.


Variation in the closely spaced rings of a bristlecone pine correspond to annual changes in rainfall and temperature. (Photograph copyright Henri D. Grissino-Mayer)

  Photographs of Andrew Elicott Douglass and Edmund Schulman in the field

In the 1890s, a young astronomer at the Lowell Observatory in Flagstaff, Arizona, was trying to understand how sun spot cycles might affect plant growth. In his research, Andrew Ellicott Douglass noticed that the thickness of each ring in the pines and Douglas firs in the region depended on how much rain fell during the year. He wrote, “Through long-past ages and with unbroken regularity, trees have jotted down a record at the close of each fading year—a memorandum as to how they passed the time; whether enriched by added rainfall or injured by lightning and fire…. So, in the rings of the talkative pines we find lean years and fat years recorded. The same succession of drought and plenty appears throughout the forest.” Because all of the trees in the area exhibited the same pattern of thick and thin rings, Douglass was able to construct a tree calendar going back to AD 700 by piecing together the tree ring patterns of living trees and patterns found in wood preserved in Native American Pueblo villages.


Andrew Ellicott Douglass (left) and Edmund Schulman (right) pioneered dendrochronology—the science of dating past events using tree rings. Douglass is shown coring a tree on the slopes of Mount Lemmon, above Tucson, Arizona. Schulman is pictured studying a pine cone in the same area. (Photographs by Charles Herbert, copyright University of Arizona Laboratory of Tree-Ring Research.

  Photograph of logs amid the ruins of an ancient pueblo village.

In the 1950s one of Douglass’ former students and a respected tree researcher in his own right, Edmund Schulman, headed into the White Mountains to look at the trees rumored to be very old. He discovered Methuselah and the old bristlecone pines surrounding it. Around the trees, even older dead trees remained on the ground. Together, they gave a climate record of the Southwest United States that extends back 9,000 years, the longest record for a single tree species. In Europe scientists have combined the ring-records from various trees to piece together the past 11,000 years of Europe’s climate history.


The first long-term record from tree rings was assembled from logs used in ancient Native American pueblos in the American Southwest. (Photograph copyright Scott August)

8000-year graph of precipitation derived from bristelcone pine tree trings

Douglas’ rings tell about rainfall in the southwestern United States, but trees also respond to changes in sunlight, temperature, and wind, as well as non-climate factors like the amount of nutrients in the soil and disease. By observing how these factors combine to affect tree rings in a region today, scientists can guess how they worked in the past. For example, rainfall in the southwestern United States is the factor that affects tree growth most, but in places where water is plentiful, like the Pacific Northwest, the key factor affecting tree ring growth may be temperature. Once scientists know how these factors affect tree ring formation, scientists can drill a small core from several trees in an area (a process that does not harm the tree) and determine what the climate was in previous years. The trees may also record things like forest fires by bearing a scar in a ring.


Short- and long-term variability of rainfall along the eastern margin of the Sierra Nevada is recorded in bristlecone tree rings. Several long and intense droughts that appear in the tree-rings are also found in sediments in nearby Mono Lake. (Graph derived from Hughes 1996)

  Photograph of fire scars on a tree cross section  

Individual events such as forest fires are recorded in tree-rings. The dark arcs that interrupt the sequence of rings in this sample were caused by fires in the 19th century. (Photograph copyright Henri D. Grissino-Mayer)


Climate Close-up: Coral Reefs


The warm, shallow ocean waters of the tropics have talkative “forests” of their own. Brightly colored mounds of coral grow in the warm ocean waters, quickly when nutrients are plentiful and more slowly when they are not. Like their land-based counterparts, corals add seasonal layers, which appear as bands in their hard calcium-carbonate shells. Corals respond to small changes in temperature, rainfall, and water clarity in a matter of months, making them a uniquely sensitive climate record. From a small core from the coral, scientists can put together a very detailed picture of climate in the Tropics—significant because much of Earth’s weather is controlled by conditions in the Tropics.

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  Photograph of a colorful coral reef

The bands in the coral’s shell can change in thickness with changes in temperature, water clarity, or nutrient availability, so while each band can record the season’s climate, the interpretation of the record depends on how the three factors are related. Cool water rising from the ocean floor brings extra nutrients in many areas, so the shells are often thicker when the water is cool. In other areas, the cold may slow growth. Scientists have to couple their observations of patterns in the seasonal bands to other measurements, including modern observations of coral growth, to determine what the bands say about climate change.


Vibrant coral reefs harbor diverse communities of life in the tropical oceans. Like trees, corals produce annual rings that store a record of past conditions. Chemical analyses reveal details about past temperature, nutrient availability, salinity, and other information. (Photograph courtesy NOAA Photo Library)

x-ray of a coral core showing annual layers

One of the most significant clues to climate in coral comes from the chemistry of the bands. The chemicals in each layer reflect conditions in the ocean when the layer formed. Like the scaly coverings of foraminifera and other marine organisms, the ratio of heavy and light oxygen in coral growth bands provide a record of temperature and rainfall during the growing season. Both more rain and higher temperatures result in a higher concentration of light oxygen in the ocean. The concentrations of other chemicals can help scientists separate the temperature and rainfall records implied by the oxygen ratio. In coral, the balance between strontium and calcium is largely determined by temperature. By comparing this ratio to the heavy-to-light-oxygen ratio, scientists can more accurately determine whether changes in coral skeletons are because of climate change involving temperature, or ocean salinity, which changes with rainfall, or a combination of both.


Each of the light/dark bands in this x-ray of a cross-section of a coral core formed during a year of growth. The surface of the coral (grown most recently) is on the left, and older bands extend to the right. (X-ray image courtesy Thomas Felis, Research Center Ocean Margins, Bremen)

Graph showing El Nino patterns recorded in a coral core

Coral can also tell scientists when heavy rains or floods carried extra sediment into the ocean. Sediment in the water can change the color of the coral as it absorbs elements from the land. Further, reef coral has a symbiotic relationship with algae that use photosynthesis to produce energy. When the water is clouded with sediment, the algae, and therefore the coral, cannot grow as quickly because it doesn’t receive as much sunlight. This slow-down in growth appears in the growth layers pulled from core samples just as drought shows up in the growth rings of trees.

The climate record left in coral reefs is detailed, but limited. First, coral reefs don’t exist everywhere in the world. They can only tell scientists about climate in warm, tropical waters. Scientists have discovered some deep water coral that may yield a detailed climate record of other regions, but the work is still in its early stages. Second, coral are living things that die. The record they preserve only covers the lifetime of the individual—a few hundred years, then an older coral from the reef has to be found to stretch the record further back. Piecing together a continuous record can be very difficult and requires numerous samples from both living and fossil corals.


Scientists use coral cores to study cyclical events like El Niño. The upper graph shows the Southern Oscillation Index—a meteorological measurement of the intensity of El Niño. Low values correspond to El Niño events, high values to La Niña events. The lower graph shows change in oxygen-18 isotopes measured in coral cores on Tarawa Island. The Southern Oscillation Index and the coral oxygen isotope measurements rise and fall together, and they generally match historical records of weak (light gray bars) and strong (dark gray bars) El Niños. (Graphs adapted from Cole, 1993)

  Photograph of two researchers drilling a coral core in the Flower Garden Banks National Marine Sanctuary

Scientists can place the coral reef record in the timeframe recorded by other climate proxies once they know when the reef lived. They can date coral by measuring how much thorium and uranium it contains. Like speleothems, coral contains a large amount of uranium when it forms. Over time, the uranium decays into thorium until there are roughly equal amounts of uranium and thorium—a process that takes about three to four hundred thousand years. From the time the coral forms until the uranium decay evens out a few hundred thousand years later, scientists can tell exactly how old the coral is by measuring how much thorium it contains. This gives them a timeframe to relate to other climate records.

  • Abbott, C., 1946: ”Our newest and oldest almanac...„ trees, Arizona Highways, 22, 3, 4–9
  • Bradley, R., 1999: Paleoclimatology, Academic Press, Harcourt Brace and Company, San Diego, California.
  • Cole, J.E., R.G. Fairbanks and G.T. Shen, 1993: ”Recent variability in the Southern Oscillation: isotopic results from a Tarawa Atoll coral,„ Science, 260, 1790–1793.
  • Douglass, A. E., 1929: ”The Secret of the Southwest Solved by Talkative Tree Rings,„ National Geographic, 56, 6, 736–770.
  • Hughes, M. K. and L. J. Graumlich, 1996: ”Climatic variations and forcing mechanisms of the last 2000 years,„ Multi-millenial dendroclimatic studies from the western United States, NATO ASI Series, Volume 141, 109–124.
  • Imbrie, J. and K. P. Imbrie, 1979: Ice Ages, Enslow Publishers: Hillside, New Jersey.

These researches are drilling a living coral head in the Flower Garden Banks National Marine Sanctuary, located in the Gulf of Mexico. Records derived from living coral are extended by matching their growth rings with those of fossil corals. Scientists also use carbon-14 and other types of radioisotopic dating to build a chronology from corals. (Photograph courtesy Simone Francis, Texas A&M Ecosystem Modeling Group)