That Sinking Feeling

By Nick Atkinson

Illustration ©Dave Cutler/

Aren’t trees great? There’s something so utterly natural about meandering through a leafy woodland glade, staring up at that immense, almost overwhelming beech tree, and giving the big old thing a hug. And not only are trees our friends, but they can also save the planet. Right? According to the website of the CarbonNeutral Company (formerly known as Future Forests), “the invisible nature of greenhouse gases contrasts with how easy it is to understand how trees absorb CO2 and put out oxygen.” So there you have it. We dig fossil fuel out of the ground, burn it, and fill the atmosphere with carbon dioxide—and the trees soak it right back up.

If only it were that simple. After decades of intensive research, scientists are still arguing over the role forests play in the global carbon cycle. Aside from the difficulties of measuring carbon uptake, other equally huge question marks hang in the rapidly warming air. How will trees respond to already-unstoppable climate change? Where should we concentrate efforts to conserve existing forests or replant “converted forests” (surely a euphemism that ranks alongside “friendly fire”)? What species should we plant, fast growing exotics or slower growing natives?

The tantalizing hope of finding answers to such questions is what makes scientists get up in the morning. But if we haven’t agreed on the science behind tree planting and carbon offsets, how can we agree on a policy for dealing with climate change? Ken Caldeira, a professor at the Carnegie Institution’s Department of Global Ecology at Stanford University, sums up the problem of scoring the effectiveness of forests in keeping an increasingly hothouse planet cool. “The problem with offsets is that they always involve comparing a real situation with a counterfactual situation.” And if we’re comparing the real world with a hypothetical one, we need to be careful.

Doing the Math

We know that trees take CO2 out of the atmosphere, but, in order to use forests as a tool for negating our emissions, we need to know exactly—or at least roughly—how effective they are. “It’s actually very difficult,” says Caldeira, “to measure how much carbon is stored in a forest.” At any moment in time, a typical stand will include saplings, young trees, mature trees, senescent trees, dead trees, and decaying trees. Some are absorbing carbon; others are releasing it. Measure the amount of carbon even in the same stand at a different time, and a different figure will emerge. Carbon storage fluctuates in a sawtooth fashion, appropriately enough, making it necessary to come up with an average figure over time for a given area.

Measuring the carbon in trees themselves isn’t too tough a problem. “A crude estimate is that a quarter of the live biomass is carbon,” says Nick Brown, a forestry scientist at Oxford University. But things get more complicated when it comes to quantifying the carbon in the dead and decaying stuff in the soil. This is especially so in temperate regions, where measuring below-ground organic matter at a landscape level is a formidable task. However, it’s not a chore to be swept under the carpet, so to speak, because soil can contain the majority of organic carbon locked within the forest.

In principle, little more is required than a sample scooped from the forest floor. The sample is dried, weighed, burned, and weighed again; the difference between the two weights gives a measure of how much organic matter was present. But try scaling up those findings, and the problems start. Not only does soil organic carbon vary from place to place, it is also vulnerable to climate change.

Most of a forest’s carbon is in partially decomposed plant material, Brown explains, but it decomposes slowly in temperate zones because it is cold, wet, and acidic. But as things warm up, the rate of decomposition increases and when that happens, carbon is released back into the atmosphere. “We can make fairly good predictions about how much carbon will be held in live trees,” he says, “but when we don’t have a handle on what’s going to happen to the soil organic matter, the predictions for the forest break down.”

Measuring the effectiveness of a carbon offset program requires comparing this hard-to-measure amount of total forest carbon with the nigh-on impossible estimate of how much carbon would have been locked up by the forest (or whatever land it stands upon) had the offset project never taken place. Baseline data are collected before a project begins, but from there the data must be extrapolated. In time, those projections become increasingly uncertain. There is no doubt that forests in general exert a positive role on the natural world. But they don’t do it in an easily quantifiable way. And to complicate the equation further, trees aren’t just affected by the climate—they can change it, too.

Why Not in Our Backyard?

Caldeira’s recent work has focused on another property of forests: their color. Leaf canopies tend to be darker than, say, open grassland. As a consequence, they absorb more of the sun’s heat and can, according to Caldeira’s computer models, actually contribute to global warming. Convinced by his findings, Caldeira kicked up a storm recently with a provocative suggestion: “The preservation and restoration of forests outside the tropics will do little or nothing to slow climate change and could even accelerate warming,” he wrote last January in The New York Times. (1)

Caldeira refers to a phenomenon known as the “albedo effect.” Albedo is the degree to which the Earth’s surface reflects sunlight, and it differs markedly from place to place. He argues that, by adding trees in northern forests, we are effectively dampening local reflectivity. In winter, for example, smooth, highly reflective snowfields are swapped for a more broken, darker surface. The net result is extra heat. “The absorption of sunlight by boreal forests means they exert a net warming influence on global temperatures,” he says. In other words, temperate forests don’t cool the planet; they warm it.

But why should that be any different in the tropics? In short, the answer is clouds. Trees affect the water cycle, a phenomenon that Rob Jackson, a biology professor at Duke University, has studied in detail. Tropical forests such as those in the Amazon recycle much of their water. Transpired vapor rises and forms clouds, which eventually release the water back down to the forest. “When you cut the trees down,” he says, “you break that cycle.”

Not only is water lost, but an important cooling mechanism is also undermined. Caldeira points out that their cloud-forming ability could mean that tropical forests provide a double climate advantage. Clouds reflect heat in the same way that snow and ice do. So the blanket of mist that forms above rainforests potentially could help keep the tropics cool. On the other hand, temperate forests (at least in the U.S.) have a different effect. “The water doesn’t go high enough into the atmosphere to get a lot of conductive rainstorms back from it. A lot of that water ultimately ends up over the Atlantic Ocean.”

Hares or Tortoises?

Different tree species grow at different rates and so lock up carbon at different rates as well. From a CO2 mitigation viewpoint, wouldn’t it make sense to focus on fast-growing species in the tropics to maximize the amount of carbon that gets dragged out of the air? And would shorter rotation times—the number of years between successive crops—also help us deal with the more changeable climatic conditions we are likely to see over the next hundred years?

Unfortunately, the answer to both questions is negative. Fast-growing trees such as eucalyptus have a less-desirable characteristic, says Jackson: “they’re water hogs.” Understanding the water cycle is every bit as important as understanding the carbon cycle. Planting inappropriate species will likely cause ecological devastation. Ultimately, if the water goes, the forest goes. Instead, the focus should be on a mix of native, albeit slower-growing, species that bring other benefits, especially in terms of conserving biodiversity. A hundred million hectares of eucalyptus in South America might help scurry away a few parts per million of atmospheric CO2, but at what cost to the natural order of things?

Planting slower-growing, native species also makes sense from a sequestration standpoint. Somewhat counterintuitively, shorter rotation times are arguably worse than longer ones. “The more frequently a forest is harvested, the more carbon is emitted,” Brown says. Land preparation, management during the growth period, and, finally, felling and logging operations all put significant amounts of CO2 into the atmosphere. “As soon as there’s a carbon cost attached to managing a forest, it really doesn’t take very much at all for the forest to become a net emitter rather than a net sink. It happens somewhere between the second and third rotation.”

At last, biodiversity gets a look in. It’s one of the few aspects of the debate that everyone seems to agree on. “We should try to protect and restore natural ecosystems regardless, rather than focusing narrowly on climate,” Caldeira says. Whatever the benefits of forests as a means to clean up the atmosphere, they have a proven role as centers of speciation. Ian Swingland, chief scientist at Sustainable Forestry Management, a for-profit organization based in the U.K., agrees. Forests directly either harbor or, through their effect on the global climate, support most of the world’s biodiversity. Initiatives to preserve pristine forests for carbon sequestration should, and indeed do, lean heavily on that as a significant fringe benefit. As Swingland says, “There’s so much to play for.”

Something in the Air

Trees and rising temperatures are not a good combination. Large-scale die-offs from drought, fire, and disease outbreaks loom large in current ecological models of climate change and have the capacity to turn a carbon sink into a carbon source almost instantaneously. Direct carbon emissions from forest fires in Korea in 2000, for example, negated one to three percent of the global forest carbon uptake.(2) A greater forest area coupled with an increased risk of disaster is surely tempting fate.

Even the relatively damp forests of temperate regions like the U.K. are vulnerable. They have not had to withstand fire since at least the last glaciation, and their species composition reflects that. Even a relatively small change in summer temperature and rainfall pattern could turn the forests tinderbox dry, a disaster waiting to happen. A major fire would leave behind a “moonscape,” says Brown—our hopes of carbon sequestration up in smoke.

Recent computer simulations carried out by Jing Ming Chen’s research group at the University of Toronto underscore the risk. Al-
though China’s forests sequestered an estimated 13 percent of the total CO2 absorbed by the world’s forests during the 1990s, their model suggests a rather different picture a hundred years from now.(3) Under some conditions brought about by plausible changes in the climate and atmospheric makeup, the forests could even become net carbon emitters. So if we are counting on those forests to take carbon out of the system, we could be in trouble.

Chainsaw Reactions

The Stern Report, commissioned by the British government to assess likely economic and social impacts of climate change, states that deforestation currently accounts for around 18 percent of global CO2 emissions, more than the entire transport sector. It clearly makes sense to stop cutting trees down. Swingland makes a passionate call for avoided deforestation to qualify for carbon credits under the Kyoto Protocol’s Clean Development Mechanism. “It is immoral for organizations or governments to stand by and allow the existing primary ecosystems to be destroyed,” he says.

However, deforestation prevention is hardly an active step toward reduction of our carbon emissions. Paying someone to do something, such as grow a new plantation, provides much more tangible results than paying someone not to do something, Jackson explains. “They might not have done anything anyway.” Leakage, where damage is done outside the geographical bounds of the project, is another problem. “We could easily set up a system that provides a financial incentive for people to grow trees when all they’re doing is cutting them down somewhere else to provide the wood that would otherwise have been harvested.”

Nonetheless, of all the arguments for and against the use of forests as carbon sinks, the most persuasive is simply to leave our existing forests alone. Avoided deforestation conserves biodiversity and natural ecological processes while minimizing the likelihood of undesirable side effects. But even if corruption can be controlled, the answer to our looming climate roller coaster ride is not in the forests of South America or Indonesia, it’s in our own behavior. Perhaps the greatest risk lies in the conscience-salving effect of funding forestry projects as carbon offsets. We want to believe that we can mitigate global warming—and carry on making short-haul flights or driving inefficient cars. The simple truth is that we can’t.

Literature cited:

1. Caldeira, K. “When being green raises the heat.” The New York Times, January 16, 2007.

2. Choi, S.D., Y.S. Chang, and B.K Park. 2006. Increase in carbon emissions from forest fires after intensive reforestation and forest management programs. Science of the Total Environment, 372(1):225-235.

3. Ju, W. et al. Future carbon balance of China’s forests under climate change and increasing CO2. Journal of Environ mental Management doi:10.1016/j.jenvman.2006.04.028