Trees absorb and store more carbon than grasslands, but they also capture more sunlight (warmth) than grass, which is more reflective. As with everything related to climate, it’s complex. Illustration: Hilch. Photo: Gregory Dean.
Forests are complex systems. The Earth’s climate is a complex system. When you put them together, you get something even more complex. Complexity upon complexity, as it were.
It’s been known for quite a while that the planet’s forests have a profound effect on its climate. In some areas of the globe they even seem to be a determining factor in terms of local conditions. Yet too often the interrelationship of forests and climate gets boiled down to the equivalent of a self-storage locker, with trees plucking carbon dioxide from the air and then “storing” it in their trunks, limbs, roots, and leaves.
They do that, yes, but they do a lot more, says R. Alec Giffen, a former chief of the Maine Forest Service who is now affiliated with the New England Forestry Foundation and the Clean Air Task Force, which is partnering with the Woods Hole Oceanographic Institute. For the past few years, Giffen has been combing the scientific literature for studies on forests and climate to get a better handle on how each influences the other.
“Based on what we know, forests have a very large potential to help us mitigate climate change,” said Giffen. “However, it’s not just about carbon sequestration. That’s not to say that’s not important, but it involves a wide variety of other factors as well.” Forests influence the climate by producing and oxidizing methane, a powerful greenhouse gas; transpiring water into the air, thus creating clouds; absorbing gases like nitrogen oxides, carbon monoxide, ozone, and sulfur dioxide; providing shade that cools buildings and streets; affecting airflow across the planet; and influencing greenhouse gas emissions through everything from the burning of wood waste, to natural decay on the forest floor, to use of wood products.
Sometimes the forest’s influence works in favor of mitigating climate change, sometimes against it. There are lots of contradictions inherent in what we think we know right now about forests and climate. Sometimes one forest in one part of the globe can have a mitigating effect on climate change, while a forest in another part has the exact opposite effect. It’s all very...complex.
One example Giffen likes to point to: When forests invade areas that have long been grasslands – in the western US, say – they absorb and store more carbon than the grass or sage they replace, but they also capture more sunlight and thus the additional warmth that the grasslands (or sagelands) would have reflected.
A similar example; as forests advance on the Arctic tundra they – again – boost carbon storage, but also absorb more light, which would otherwise have been reflected during much of the year by the bright, snow-covered tundra. (This reflection is known as albedo.) This makes the micro-climate warmer, in the same way that when higher temperatures reduce arctic ice cover, the dark waters that are revealed absorb more sunlight, which further reduces the ice cover. It’s a feedback loop of global proportions.
Microclimates also come into play in urban areas: Researchers have shown that deploying more trees in cities increases evaporative cooling, reduces heat absorbed by buildings and pavement, cuts air conditioning use, and even curbs “fugitive emissions” of volatile organic compounds from auto gas tanks by shading the vehicles. Think about it: don’t you prefer to park in the shade on a hot summer day?
Cows like to park in the shade, too, and more trees in agrarian settings could have a similarly positive cooling effect. Silvopasturing can make for happier cows and increase milk production, in addition to increasing the carbon storage potential of the land. And some tree species fix nitrogen from the air, thereby improving the soil. Trees interspersed in row crops – otherwise known as agroforestry – can increase crop yields while reducing chemical fertilizer use.
Farmers also have a vested interest in the biogenic volatile organic compounds that trees produce. These chemicals, released into the air as part of a tree’s metabolic activity, vary from species to species and forest to forest. Their effects can be direct: they block and scatter sunlight, for instance. Or indirect: they help increase the reflectivity of clouds by boosting the number of droplets in them, potentially making them longer lived.
Give and Take
One of the things that tends to be overlooked when considering how trees affect the climate is that forests actually emit CO2. While standing forests are excellent carbon sinks, the flow of CO2 isn’t all one way. “There’s a very big emission [of CO2] from forests,” said Giffen. “It doesn’t nearly equal the sequestration factor, but it does offset it.”
That offsetting occurs in the natural course of things. When trees die in the forest, they start to decay and the carbon dioxide they’ve stored goes back into the atmosphere. This happens a lot because only a tiny fraction of trees will ever make it to maturity. Most will die young as they lose the race for resources. That might be called the slow-release CO2 pill. The fast one occurs when forests catch fire and all that carbon pours into the atmosphere at once, something that’s been happening more frequently around the globe, from Indonesia to California to Europe to Australia.
Right now, US forests sequester more carbon than they emit, Giffen notes in a climate mitigation report that he published online. But he and many in the forest products industry suggest that a lot more carbon would be locked up if CO2 emissions from dead and dying trees in managed forestland were cut substantially.
“We have millions of acres in New England that have far more trees on them than are going to be there in 10, 20, or 30 years,” he said. “Suppressed trees, weaker trees are going to die and decompose. One of the things we can do is target those stands for thinning, which is going to increase the growth and sequestration potential of the trees that are left. Since the emissions [from the suppressed trees] would end up in the atmosphere anyway, we can use the wood more productively for something else, for waferboard, or MDF, or at the very least as a substitute for fossil fuels.”
There’s unquestionably sound, albeit broad-brushed, logic to this idea, but the details of where and how it would work vary. Scientists are still working to quantify the carbon footprint associated with various kinds of timber harvests, as the treatment technique, the site specifics, and how the harvested wood is used all swing the needle.
Robert Perschel, the executive director of the New England Forestry Foundation, said that if humans want to truly get a grip on how forests and forest management affect climate change, we need to get a better handle on all the variables: carbon storage, using wood instead of carbon-intensive materials like concrete, steel, and plastic, and the whole array of bio-effects that occur when we tinker with or alter forests.
Perschel is convinced that “when each of these three variables are accounted for, it will reflect an astounding ability of sustainable forest management to mitigate climate change.” But a “more thorough analysis is required to tell us which forests to concentrate our management on, which techniques to use, and how much we can optimally grow and harvest,” he added.
Giffen urges powering up the development of analytical or computer modeling tools to see how forests’ various effects on climate play off each other. But developing algorithms to calculate all of the many ways that trees affect the climate is not going to be an overnight game. According to the Intergovernmental Panel on Climate Change, keeping local warming to 2 degrees centigrade over pre-industrial levels is necessary to avoid the worst of what rising temperature will do. “We don’t know everything, but we know enough to start,” Giffen said. “Forests are green geoengineering that we know how to do.” In fact, we’ve been doing it for centuries. Sometimes badly and sometimes well.
After analyzing the research that’s out there, Giffen concluded that the first thing we should concentrate on is keeping forests as forests. The second is to better manage forests and improve production of solid wood products to maximize carbon storage and improve forest health. Other actions that he feels would help mitigate climate change over the next, say, 20 to 30 years? Create a system of “strategically located ecological reserves” big enough to protect biodiversity and allow for climate change adaptation. Also, refrain from harvesting old-growth forests, focus sustainable forest management on forests that have been managed in the past. And “embrace exemplary forest management” – harvest to foster forest health and resilience while developing guidelines for so-called “climate beneficial management” in bioregions throughout the country.
“There’s no need for us to be paralyzed,” he said. “We need to be careful, but there are things we can do now to capitalize on forests’ ability to help us keep cool.”
Joe Rankin is a forestry writer. He lives in central Maine.