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blatherskiteA little knowledge is a dangerous thing, and a Whole Lotta Knowledge isn't necessarily much safer. I frequently fall into the trap of thinking I know more about a subject than I really do, and I'm constantly vigilant about what I actually know and what I just think I know. For those who have advanced degrees and extreme expertise in areas that I only vaguely grasp, the temptation to opine upon matters they have not really studied must be even greater. This leads to frequent cases of commenting authoritatively on matters the author is at best naïve about.
An excellent example is provided by a recent New York Times article To Save the Planet, Don't Plant Trees by Nadine Unger, an "assistant professor of atmospheric chemistry at Yale". Unger, who by the evidence needs refresher courses in ecology and plant physiology, proposes the dubious conclusion that planting forests may actually be counterproductive when it comes to fighting global warming. Her logic is at best weak, and ranges from the seemingly disingenuous to the downright puzzling. She starts out well, correctly reminding us of the complexity of ecosystems and suggesting that there are no simple answers, then proceeds to ignore that complexity. That approach is both bad rhetoric and worse science. A few choice examples:
Unger: "The dark color of trees means that they absorb more of the sun’s energy and raise the planet’s surface temperature... Their conclusion is that planting trees in the tropics would lead to cooling, but in colder regions, it would cause warming."
It's old news that trees capture large amounts of solar energy, as do all plants. They then use this to transform carbon dioxide into oxygen, sugar, and eventually, more plants. This is called photosynthesis, for those who aren't familiar with the term. Whether they increase Earth's surface temperature and whether that increase is meaningful is a complex issue, but can be reduced to a simple question: If global warming was not accelerating dangerously before the industrial revolution or when the missing original forests were present (the ones that are being replaced by afforestation), why would restoring the lost vegetation change that?
It also seems odd to me that trees cool the Earth's surface in the tropics if their net effect is to trap radiation; there's a lot more solar radiation in the tropics, and if you increase the amount trapped, temperatures will rise. Simple thermodynamics. And what about other vegetation types? Grasslands also trap large amounts of energy, but aren't singled out for criticism. The unexplained reason behind Unger's proposition likely relates to the evapotranspiration that all plants perform (that is, the passage of water from the soil to the atmosphere via the plant). The result can lead to warming, because water vapor is a potent trap for longwave radiation, but can also lead to cooling, because transforming liquid water into water vapor absorbs energy and produces a cooling effect. That cooling effect is likely to be more important for tropical vegetation, but the relationship between cooling and warming is not trivial and cannot be summarized in a single glib statement.
It's worth noting that the original forests that will nominally be restored by many afforestation programs weren't considered hazardous to the Earth's health; they were not causing net heating before they were harvested, so it's hard to imagine how they would cause net heating when they are restored to their original condition. I've seen no credible suggestion that widespread eradication of forests would stop global warming, which would be an obvious strategy if trees really do cause warming as Unger suggests. Ruductio ad absurdum.
The absorption of solar radiation by trees (and any other vegetation) is complex: it depends on the vegetation mixture in a community (particularly the mixture of C3 and C4 plants), the development stage of the plants ("phenology"), the availability of water to the plants, soil nutrient levels, and a great many other factors, all of which interact dynamically. One hopes that Unger's own computer models are more sophisticated in dealing with these interactions than would be suggested by her argument in this op ed piece.
Unger: "Worse, trees emit reactive volatile gases that contribute to air pollution and are hazardous to human health."
So the solution is to cut down all trees and replace them with flowers or perhaps low-VOC asphalt? The emission of toxic compounds by trees is well known, as anyone who has visited the Blue Ridge Mountains of Virginia, can attest. Yet even this extreme case of emissions by trees is not considered problematic by anyone who's actually studied forests -- or human biology for that matter. Certainly, I've never seen a travel advisory warning people to avoid visiting this scenic area, nor is the local government planning to evacuate residents to safer areas.
Unger: "As these compounds mix with fossil-fuel pollution from cars and industry, an even more harmful cocktail of airborne toxic chemicals is created."
So the solution is to ban trees, not force car manufacturers and other industries to reduce their pollutant emissions? Citing "reactive volatile gases" in this context is scaremongering. Maybe we should be worried about all that dangerously reactive oxygen in the atmosphere? After all, everyone knows that it causes fires, and moreover, that it rusts our metals. Speaking of oxygen:
Unger: "Moreover, it is a myth that photosynthesis controls the amount of oxygen in the atmosphere."
This statement is so misguided and wrongminded that I hardly know where to begin. I have to take a deep breath (of oxygen produced by plants) and assume that she's pointing out that oxygen levels also depend on a large number of uptake processes that remove oxygen from the air and prevent oxygen levels from rising continuously. For forests specifically, there's no question that they are net producers of oxygen, at least before the trees becomes senescent, at which point their respiration rate (= rate of oxygen consumption) may exceed its photosynthetic oxygen production. The disingenuous nature of her statement is revealed by a simple question: What other source contributes as much oxygen to the atmosphere as photosynthesis by trees and other vegetation? There is none. Nothing even comes close. In the absence of a net oxygen source (i.e., plants), the atmospheric oxygen content would decline continuously until "life as we know it" became impossible.
Unger: "In fact, almost all the oxygen the Amazon produces during the day remains there and is reabsorbed by the forest at night. In other words, the Amazon rain forest is a closed system that uses all its own oxygen and carbon dioxide."
This statement seems to rely on the belief that there is no such thing as an atmospheric circulation pattern. So if the atmosphere does not move, then "wind" is clearly also a myth, and all gases stay local. Like most medieval Europeans, the gases never travel beyond sight of their birthplace. It's hard to imagine how an atmospheric scientist could mis-state the case so badly. More specifically, I've seen no evidence whatsoever that any pre-senescence forest plays a zero-sum game with oxygen consumption and production. That statement is based on 25 years of editing journal manuscripts on this subject. I don't have a PhD, but I've probably read more about this subject than some PhDs.
On the topic of carbon sequestration by forests, Unger is closer to not being outright misleading. The magnitude of carbon sequestration by ecosystems is an open and ongoing debate; the actual results vary widely depending on the ecosystem and on the climate and vegetation details, they change over time even for the same ecosystem, and they change depending on the estimation method that is used. (It's a complex measurement problem with no easy answers.) But stating that trees are not necessarily a good carbon storage source because they inevitably die or burn badly misrepresents the situation. When they die or burn, they are replaced by new vegetation that eventually recaptures all that carbon. It's more correct to state that carbon sequestration changes during the succession process that occurs as pioneer species are replaced by climax species, and that once you reach the climax ecosystem, sequestration generally tends to stabilize. This is simple math: when there is no net growth and no increase in soil storage of carbon, then by definition, there is no net sequestration. So at some point, all the tree planting in the world will no longer affect carbon sequestration: sequestration will have reached its saturation point as uptake of carbon is balanced by release of carbon. That is, the system will reach an equilibrium state.
All this being said, there are a few good reasons to be wary of afforestation as a panacea, and some of my Chinese colleagues are discovering and reporting them. (The work of Cao Shixiong is particularly notable.) The "smoking gun" is the highly significant hydrological effect of trees, which goes unmentioned by Unger: Where forests are the natural climax vegetation, they develop in equilibrium with the available water supply and maintain a relatively stable hydrological regime. In such areas, forests are a brilliant solution: they maintain the flows of water through the biosphere at stable levels. Unfortunately, many afforestation programs extrapolate from this "trees are good" message to assume that this is true in all environments, thereby forgetting the underlying assumption: that there must be enough water to sustain the trees. Many such projects rely on fast-growing non-native trees because of the perceived urgency of stabilizing degraded sites quickly. These trees often use more water than the environment provides, and in so doing, they deplete the available near-surface water. As the water table drops, shallow-rooted surface vegetation that can no longer reach the soil water dies; if the deficit between available water and tree needs is sufficiently large, the water table continues to decline until even the trees can't reach the water, and the trees themselves die.
The net result is a massive loss of vegetation cover, leading to desertification rather than the intended ecosystem restoration. In many environments, such as northern China, trees are being planted over millions of acres, and it's slowly being revealed to be a disastrous choice in many areas. A far better solution would be to restore native vegetation, which is usually steppe or grassland plants capable of using the available water thriftily. That vegetation coevolved with the local climate, and will be a much more sustainable solution in the long run.
Lesson one of this post is that ecology is wonderfully, hideously complex, and whether it's more wonderful or more hideous depends on whether you're admiring it or trying to model it, respectively. Lesson two of this post is that people who are experts in one area should not assume that they are, by default, experts in another area. Expertise takes years of specialized, focused study to achieve, and you can't obtain it by reading a few journal articles, usually taken out of the larger context of an entire field of study's body of knowledge.
An excellent example is provided by a recent New York Times article To Save the Planet, Don't Plant Trees by Nadine Unger, an "assistant professor of atmospheric chemistry at Yale". Unger, who by the evidence needs refresher courses in ecology and plant physiology, proposes the dubious conclusion that planting forests may actually be counterproductive when it comes to fighting global warming. Her logic is at best weak, and ranges from the seemingly disingenuous to the downright puzzling. She starts out well, correctly reminding us of the complexity of ecosystems and suggesting that there are no simple answers, then proceeds to ignore that complexity. That approach is both bad rhetoric and worse science. A few choice examples:
Unger: "The dark color of trees means that they absorb more of the sun’s energy and raise the planet’s surface temperature... Their conclusion is that planting trees in the tropics would lead to cooling, but in colder regions, it would cause warming."
It's old news that trees capture large amounts of solar energy, as do all plants. They then use this to transform carbon dioxide into oxygen, sugar, and eventually, more plants. This is called photosynthesis, for those who aren't familiar with the term. Whether they increase Earth's surface temperature and whether that increase is meaningful is a complex issue, but can be reduced to a simple question: If global warming was not accelerating dangerously before the industrial revolution or when the missing original forests were present (the ones that are being replaced by afforestation), why would restoring the lost vegetation change that?
It also seems odd to me that trees cool the Earth's surface in the tropics if their net effect is to trap radiation; there's a lot more solar radiation in the tropics, and if you increase the amount trapped, temperatures will rise. Simple thermodynamics. And what about other vegetation types? Grasslands also trap large amounts of energy, but aren't singled out for criticism. The unexplained reason behind Unger's proposition likely relates to the evapotranspiration that all plants perform (that is, the passage of water from the soil to the atmosphere via the plant). The result can lead to warming, because water vapor is a potent trap for longwave radiation, but can also lead to cooling, because transforming liquid water into water vapor absorbs energy and produces a cooling effect. That cooling effect is likely to be more important for tropical vegetation, but the relationship between cooling and warming is not trivial and cannot be summarized in a single glib statement.
It's worth noting that the original forests that will nominally be restored by many afforestation programs weren't considered hazardous to the Earth's health; they were not causing net heating before they were harvested, so it's hard to imagine how they would cause net heating when they are restored to their original condition. I've seen no credible suggestion that widespread eradication of forests would stop global warming, which would be an obvious strategy if trees really do cause warming as Unger suggests. Ruductio ad absurdum.
The absorption of solar radiation by trees (and any other vegetation) is complex: it depends on the vegetation mixture in a community (particularly the mixture of C3 and C4 plants), the development stage of the plants ("phenology"), the availability of water to the plants, soil nutrient levels, and a great many other factors, all of which interact dynamically. One hopes that Unger's own computer models are more sophisticated in dealing with these interactions than would be suggested by her argument in this op ed piece.
Unger: "Worse, trees emit reactive volatile gases that contribute to air pollution and are hazardous to human health."
So the solution is to cut down all trees and replace them with flowers or perhaps low-VOC asphalt? The emission of toxic compounds by trees is well known, as anyone who has visited the Blue Ridge Mountains of Virginia, can attest. Yet even this extreme case of emissions by trees is not considered problematic by anyone who's actually studied forests -- or human biology for that matter. Certainly, I've never seen a travel advisory warning people to avoid visiting this scenic area, nor is the local government planning to evacuate residents to safer areas.
Unger: "As these compounds mix with fossil-fuel pollution from cars and industry, an even more harmful cocktail of airborne toxic chemicals is created."
So the solution is to ban trees, not force car manufacturers and other industries to reduce their pollutant emissions? Citing "reactive volatile gases" in this context is scaremongering. Maybe we should be worried about all that dangerously reactive oxygen in the atmosphere? After all, everyone knows that it causes fires, and moreover, that it rusts our metals. Speaking of oxygen:
Unger: "Moreover, it is a myth that photosynthesis controls the amount of oxygen in the atmosphere."
This statement is so misguided and wrongminded that I hardly know where to begin. I have to take a deep breath (of oxygen produced by plants) and assume that she's pointing out that oxygen levels also depend on a large number of uptake processes that remove oxygen from the air and prevent oxygen levels from rising continuously. For forests specifically, there's no question that they are net producers of oxygen, at least before the trees becomes senescent, at which point their respiration rate (= rate of oxygen consumption) may exceed its photosynthetic oxygen production. The disingenuous nature of her statement is revealed by a simple question: What other source contributes as much oxygen to the atmosphere as photosynthesis by trees and other vegetation? There is none. Nothing even comes close. In the absence of a net oxygen source (i.e., plants), the atmospheric oxygen content would decline continuously until "life as we know it" became impossible.
Unger: "In fact, almost all the oxygen the Amazon produces during the day remains there and is reabsorbed by the forest at night. In other words, the Amazon rain forest is a closed system that uses all its own oxygen and carbon dioxide."
This statement seems to rely on the belief that there is no such thing as an atmospheric circulation pattern. So if the atmosphere does not move, then "wind" is clearly also a myth, and all gases stay local. Like most medieval Europeans, the gases never travel beyond sight of their birthplace. It's hard to imagine how an atmospheric scientist could mis-state the case so badly. More specifically, I've seen no evidence whatsoever that any pre-senescence forest plays a zero-sum game with oxygen consumption and production. That statement is based on 25 years of editing journal manuscripts on this subject. I don't have a PhD, but I've probably read more about this subject than some PhDs.
On the topic of carbon sequestration by forests, Unger is closer to not being outright misleading. The magnitude of carbon sequestration by ecosystems is an open and ongoing debate; the actual results vary widely depending on the ecosystem and on the climate and vegetation details, they change over time even for the same ecosystem, and they change depending on the estimation method that is used. (It's a complex measurement problem with no easy answers.) But stating that trees are not necessarily a good carbon storage source because they inevitably die or burn badly misrepresents the situation. When they die or burn, they are replaced by new vegetation that eventually recaptures all that carbon. It's more correct to state that carbon sequestration changes during the succession process that occurs as pioneer species are replaced by climax species, and that once you reach the climax ecosystem, sequestration generally tends to stabilize. This is simple math: when there is no net growth and no increase in soil storage of carbon, then by definition, there is no net sequestration. So at some point, all the tree planting in the world will no longer affect carbon sequestration: sequestration will have reached its saturation point as uptake of carbon is balanced by release of carbon. That is, the system will reach an equilibrium state.
All this being said, there are a few good reasons to be wary of afforestation as a panacea, and some of my Chinese colleagues are discovering and reporting them. (The work of Cao Shixiong is particularly notable.) The "smoking gun" is the highly significant hydrological effect of trees, which goes unmentioned by Unger: Where forests are the natural climax vegetation, they develop in equilibrium with the available water supply and maintain a relatively stable hydrological regime. In such areas, forests are a brilliant solution: they maintain the flows of water through the biosphere at stable levels. Unfortunately, many afforestation programs extrapolate from this "trees are good" message to assume that this is true in all environments, thereby forgetting the underlying assumption: that there must be enough water to sustain the trees. Many such projects rely on fast-growing non-native trees because of the perceived urgency of stabilizing degraded sites quickly. These trees often use more water than the environment provides, and in so doing, they deplete the available near-surface water. As the water table drops, shallow-rooted surface vegetation that can no longer reach the soil water dies; if the deficit between available water and tree needs is sufficiently large, the water table continues to decline until even the trees can't reach the water, and the trees themselves die.
The net result is a massive loss of vegetation cover, leading to desertification rather than the intended ecosystem restoration. In many environments, such as northern China, trees are being planted over millions of acres, and it's slowly being revealed to be a disastrous choice in many areas. A far better solution would be to restore native vegetation, which is usually steppe or grassland plants capable of using the available water thriftily. That vegetation coevolved with the local climate, and will be a much more sustainable solution in the long run.
Lesson one of this post is that ecology is wonderfully, hideously complex, and whether it's more wonderful or more hideous depends on whether you're admiring it or trying to model it, respectively. Lesson two of this post is that people who are experts in one area should not assume that they are, by default, experts in another area. Expertise takes years of specialized, focused study to achieve, and you can't obtain it by reading a few journal articles, usually taken out of the larger context of an entire field of study's body of knowledge.