Stories Told by Trees and Corals
From the Diaries of Giants
Imagine a peaceful giant several thousand years old, living high in the mountains and keeping a seasonal diary on climate change, a diary he/she is willing to share with anyone who wants to know. There are in fact many such giants, some more than 4000 years old. The oldest are in California in the White Mountains, known as "bristlecone pines" (Pinus longaeva) and in the Sierra Nevada, known as "giant sequoias" (Sequoiadendron giganteum).
Trees record the conditions of growth in their rings. A wide ring means plenty of warm days and sufficient water, a narrow ring means nasty conditions, either a short growing seasons because summer was late in coming (up on the mountain), or a severe water shortage (in the foothills, in areas where water is limiting). The mixture of conditions recorded (time of snow-melt, intensity of winter rain, temperature in June, etc.) depends on what a given tree cares about in terms of growth. Hence, a tree is a "biased reporter," and the same is true for all other organisms recording climate change. What a scientist can extract from tree rings depends on how many properties of a ring can be measured (width, density of early wood, density and width of wood grown late in the season), how clever the statistical methods are, and how well the items of interest (say, spring temperature or annual rainfall) are correlated with the properties measured. For instance, special measurements can be made on the isotope chemistry of the wood. This kind of information can yield insights on the composition of the rainfall (from oxygen isotopes) and on the rate of photosynthesis (from carbon isotopes).
photo of a white spruce, showing mid century tree ring growths. Note the thin rings, indicating dry years, and the thicker rings, indicating wetter years.
The Field of Dendrochronology
The science of studying tree-rings is called "dendrochronology," which may be loosely translated as "tree history." In fact, in regions where no written documents are available the first (and commonly only) detailed record of climate change is from trees. The pioneer of dendrochronology was Andrew Douglass (1867-1962), a professor of astronomy and physics at the University of Arizona, who was interested in the history of the 11-year sunspot cycle. He searched for this cycle in tree rings, hoping that trees would respond to climate change provoked by the solar cycle. In the process, he founded tree-ring science. The characteristic sequence of ring widths in any one region, which tends to agree between different trees, contains the clues to climate change and is also useful in dating ancient wood in archeology. In the Southwest, where Douglass did his work, the sequence contained the record of great droughts, which influenced the history of settlements in the region. The tree-ring record now goes back many thousands of years, both in North America and in western Europe, and records are being established on all continents (excepting Antarctica).
An important result of providing a precise tree-ring chronology was the opportunity to calibrate the radiocarbon (14C) time scale by comparing the measured 14C age an individual tree-ring with its “real” age, determined from just counting the annual rings. It turns out that the14C ages and “real” ages (referred to as calendar ages) are not the same because the abundance of radiocarbon in the atmosphere varies through time, both as a result of changes in production (from cosmic rays converting nitrogen-14 to carbon-14) and as a result of changes in the intensity of mixing of the ocean (which affects the exchange of carbon between the ocean and the atmosphere). Whenever cosmic rays are less intense, radiocarbon production drops. Whenever mixing is less intense, radiocarbon does not enter the ocean as readily and becomes more abundant in the atmosphere.
Paleotemperatures derived from examinations of coral reefs from the Galápagos Islands off the coast of Ecuador. The upper graph shows the Sea Surface Temperature (SST) Anomaly. Red indicates warmer than average temperatures, while blue indicates colder than average. The lower graph shows the oxygen isotope anomaly, a proxy
for paleotemperature. The yellow bars indicate El Niño years – note how they correlate very well to the Pacific coral record of warming. (Data from Shen et al. and graph modified from NOAA at National Geophysical Data Center
From the Diaries of Corals
Coral growth rings are analogous to tree rings in that they are a biased report on conditions that are of interest to the organism. Coral skeleton is composed of calcium carbonate. The skeleton formed in the winter has a different density than that formed in the summer because of variations in growth rates related to temperature and cloud cover conditions. Thus corals exhibit seasonal growth bands very much like those observed in trees. Corals respond to and record water temperature and light and nutrient availability. Thus, their growth rate and chemical composition reports on these items. We may assume that the availability of light does not change much from one year to the next, so we should expect growth to reflect temperature and nutrient availability. Nutrients are much more abundant at depth, than in the nutrient-poor surface waters where corals grow, such that variations in the intensity of mixing by winds (and tides) are likely to be important in determining changes in the rate of coral growth.
As with tree wood, special measurements can be made on the isotope chemistry of the coral skeleton. Oxygen isotope composition of the coral’s calcium carbonate can yield information on sea-surface temperature (SSTs). Since there is a correlation between the number of 18O present in the coral skeleton and the temperature of the water, less 18O present than 16O indicates that the air and ocean temperatures were warmer at the time the coral formed. Information on past upwelling history can be inferred from ratios of Cadmium [Cd] to Calcium [Ca] and Barium [Ba] to Calcium. Because upwelled waters have more barium and cadmium such that a high ratio of Ba/Ca and Cd/Ca preserved in the coral skeleton indicates higher upwelling rates. Scientists use as many clues as possible to put the pieces of past climate together.
Coral records are now being used to reconstruct the amplitudes and frequencies of El Niño events in the Pacific (see Figure 8.6.4). A search for coral growth cycles yielded the following periods: 37.5, 30.8, 27.0, 21.7, 7.5 and 6.0 years. Note that here is no peak near 11 years, the cycle of sunspots. Four of these peaks resemble a series of multiples of 7.5 years (7.5, 15, 22.5, 30, 37.5) but with the 15 and 30-year cycles missing. This suggests a dominant influence of something called the "North Atlantic Oscillation" with a period near 7.5 years. The period near 22 years is twice the major solar cycle. Thus, if solar cycles are important in stimulating the system, their energy is subsumed into internal oscillations, possibly locked in near 22 years. The coral record also may reflect volcanic eruptions. The elevated growth rate after the 1601 A.D. Peruvian eruption suggests favorable effects occurred from increased ocean mixing during harsh winter storms, leading to higher nutrient content in this nutrient-starved region.