By Pete Wyckoff
Is the planet cooling? "I've just completed Mike's Nature trick...to hide the decline," writes climate scientist Phil Jones in a stolen 1999 e-mail which has caused a frenzy. FoxNews.com tells us that we finally have a 'smoking gun'--proof that scientists are manufacturing a global warming crisis so that they can... they can...(I've never really understood the goals of the evil scientific conspirators).
The planet is warming. The data are unequivocal and based on measured temperatures (corrected for things like the "heat island" effect, so please don't write an angry response claiming that the thermometers are wrong). What Phil Jones was referring to is something else: past temperatures estimated via tree rings. Since 1960, the rings in trees seem to have lost some of their power to record temperature.
Why should tree rings indicate temperature at all? As most of us learned in childhood, the trunks of trees at our latitude tend to put on a distinct growth ring every year. All other things being equal, when the trees are happy, they put on a large ring. When the going gets tough, the rings get thin. What makes a tree happy? Light, nutrients, lack of disease, and warmth (to a point). What do trees despise? Drought. By careful interpretation of past tree growth patterns, we can learn a lot about past climates.
Scientists have spent many years developing the techniques needed to reconstruct climate via tree rings. The problem is that in the past few decades, the tree ring-climate relationships seem to have become "decoupled" in many areas. Why? The main cause seems to be increasing atmospheric carbon dioxide. While carbon dioxide is famously a gas that heats the planet (the greenhouse effect is real and uncontroversial), carbon dioxide also directly impacts plants. Carbon dioxide fuels photosynthesis, and increased carbon dioxide in the air can both speed-up plant growth and make plants less sensitive to drought.
Decreased drought sensitivity is an expected response for plants exposed to high levels of carbon dioxide. All along the underside of a plant's leaves are little holes called "stomata." These holes can open and close. A tree must open its stomata to take in carbon dioxide for photosynthesis. Unfortunately, plants lose water out of their open stomata. Plants growing in air that has lots of carbon dioxide can reduce the amount of time their stomata are open, thus making them lose less water and become less susceptible to drought.
Biologists call the concept here "water-use efficiency," and it is of crucial interest to farmers and foresters alike. Carbon dioxide causes warming that will likely make west central Minnesota a drier place in the future. At the same time, increased carbon dioxide in the air makes plants growing in our region less susceptible to drought. The balance between these two forces will be crucial.
The changing relationship between climate and tree growth is a hot topic of research at your local university. Last Friday, Dr. Chris Cole and Dr. Jon Anderson, of the University of Minnesota, Morris, published a paper in the journal "Global Change Biology" showing that aspen trees in Wisconsin are growing faster than they used to, and much of the increase is attributed to increased atmospheric carbon dioxide. Two weeks ago, a former student and I published a paper in the "Journal of Ecology" showing that oak trees in west central Minnesota became less sensitive to drought during the 20th century. If "dust bowl"-severity droughts come again soon, we project that the local oaks will suffer 50 percent less mortality than they likely did in the 1930s.
So what does this all mean? The relationship between tree rings and climate is becoming muddied by the rapid recent increase in atmospheric carbon dioxide. For most of the past 10,000 years, carbon dioxide levels in the atmosphere remained reasonably stable. Now they are skyrocketing. Modern tree rings are no longer the reliable recorders of temperature they once were. It is a good thing that we now have thermometers.
What does Phil Jones' stolen e-mail not mean? It does not mean that global warming is a hoax. It does not mean that there are really any cracks in the scientific consensus that humans are causing dangerous alterations to the global climate.
We humans are changing the climate, largely by emitting vast quantities of carbon dioxide via the way we heat our houses, fuel our cars, and generate our electricity. This is unwise. Yes, the future climate, along with the increased carbon dioxide, may be good for some. For most people, however, the downsides of climate change are likely to far outweigh the benefits. Don't let Fox News mislead you. As a prudent, conservative people, we should take serious steps to mitigate our impact.
Dr. Pete Wyckoff is Associate Professor of Biology at the University of Minnesota, Morris.