I first became involved in global change research a little over 30 years ago at Oak Ridge National Laboratory. Then, the fundamental question to be solved was whether the Earth would become a source or a sink for carbon as climate and atmospheric chemistry continued to change.

The answer to this question is critical for defining the nature of the risks we face in the future. If global warming and increases in atmospheric CO₂ result in the Earth sequestering and storing more carbon (say, because trees cover more area and CO₂, a plant nutrient, makes them grow at greater densities than today), the impacts from climate changes forced by excess atmospheric CO₂ will be considerably less than our calculations suggest. On the other hand, if warming forces the stored carbon from forests and rangelands (say, because today’s trees become climatically “obsolete” and undergo widespread dieback), then the problem becomes even more challenging than we thought, with more CO₂ begetting more warming, more warming begetting still more CO₂, and so on.

In the intervening 30 years since 1977, when we began obtaining research grants to study this issue, much has been learned about the global carbon cycle. There is even a “current” answer to the question: The vegetation of the Earth is now a net carbon sink and should continue sequestering carbon at least until about mid-21st century, when it is expected to become “carbon saturated.”

Yet, this outcome rests on global vegetation models that assume, rather than know, the answer to the question we were asking in 1977: They assume that warming will permit trees to cover more area, and that CO₂ will enhance vegetation density. At the same time, the models do not simulate such things as the consequences if trees undergo significant dieback when the climate they require (their “climate envelope”) moves away to higher latitudes and altitudes. Yet how we manage the land to reduce climate impacts depends entirely on that unanswered question.

One reason the question has not been answered is that we still do not yet know if, or how, increasing atmospheric CO₂ will change the climate envelope to which each species is thought to be limited. For example, Forest Service research has shown that tree seedlings grown under higher concentrations of CO₂ more efficiently use water to photosynthesize; that is, they can sequester more carbon for every gallon of water they use. If this process works the same way in wildland vegetation as in controlled experiments, the moisture limits of the climate envelope for say, loblolly pine, could be expanded, permitting loblolly to grow in more arid climates, or to withstand greater droughts where it grows today.

Conversely, CO₂ has increased about 30 percent in the last century, with little or no tree growth effect detected, while forest diebacks have become more common, especially from climate change-induced increases in insects and wildfires. Should we manage our forests on the expectation that the carbon “carrying capacity” will increase on the land, or that it will decrease? That today’s forests will emit less water to the atmosphere in the future, or will lose more water during hotter summers?

These are critical questions from the past 30 years that are still not answered satisfactorily today. Yet, they must be answered to meet the climate change and water challenges Chief Kimbell recently enunciated, and that Forest Service scientists at the Southern Research Station have focused on answering.

In this issue of Compass, you can read about expectations for future change in southeastern climate, forests, and life. Consider the article on research into the ability of forests to sequester more carbon under higher CO₂ concentrations in the future entitled “That Carbon Dance.” Another article describes some surprising results of long-term CO₂ fumigations of trees in the Duke Forest. You can examine new research on models that calculate future water supplies in the region. Other SRS research reviewed delves into implications for Coastal Plain ground water of rising sea levels, also induced by climate change. These studies aim at the most important uncertainties in global climate change and their implications for our local and regional environmental issues.