|Trees, climate friend or foe?|
It’s fun to be a contrarian, to point out cases in which commonly held conceptions falter, or when the opposite is true.
But contrarian points often require quite a bit of nuance, and seldom do they completely invalidate the common ideas they critique. Also, there is a real danger that in the popularization of contrarian scientific ideas by those without expertise in the field in question, much of the nuance and qualifications gets lost, and readers can be mislead into believing things that do not reflect the actual results or opinions of the researchers whose work is cited.
This fall’s release of SuperFreakonomics by economist Steven Levitt and journalist Stephen Dubner follows on their best-selling Freakonomics in exposing how many things – from driving drunk to prostitution – may be contrary to what we expect. But it’s their chapter on climate science that has drawn considerable criticism:
Trees and Solar Panels … Bad for the Climate?
As so many critics have addressed other substantive criticisms of the book’s climate change chapter, let’s focus on two less-noted but interesting contrarian positions that Levitt and Dubner bring up in their book: that both trees and solar panels may be bad for the environment because they absorb more light than the surfaces they rest on.
Both are examples of an argument having a grain of truth but misleading when generalized to all cases.
Both arguments relate to the concept of albedo, the amount of light reflected by a material. Reflected light goes back up into space as shortwave radiation without substantively warming the Earth, while absorbed light is reradiated as heat (e.g., longwave radiation). Unlike shortwave radiation, a considerable portion of longwave radiation is trapped by greenhouse gases in the atmosphere and raises the surface temperature as long as the heat source remains.
Dark surfaces have a lower albedo than light surfaces, so they absorb more light as heat. That explains, for example, why black asphalt can burn your feet on a hot summer day, or why black cars get hotter in the sun than white cars. Placing a darker object on a lighter surface, whether a tree over snow-covered ground or a solar panel on desert sand, will produce more heat and increase Earth’s temperature by a small amount.
Heat from Albedo (Flow) ≠ Warming (Stock) from GHGs
However, and this is important, the heating produced via albedo changes differs structurally from the heating produced by greenhouse gases. You can think of the albedo heating like an electric space heater. As long as the space heater is on, the room remains warm (with the warmth trapped by the insulation in your walls, or the greenhouse gases in the atmosphere). Turn the heater off, and the room will cool down until it reaches the temperature there before you turned the heater on.
The point is that albedo represents a “flow” effect (e.g., the strength of the forcing depends on the current heat flow from the albedo change), but greenhouse gases are a “stock” effect where the heating is due to the stock of GHGs in the atmosphere rather than the emissions of any particular year.
Instead of a space heater, you can think of greenhouse gases in the atmosphere like a bathtub. A certain amount flows in through the tap and out from the drain; but if you increase the flow to let in more than the drain lets out, the bathtub will fill up. Even if you turn off the tap, the bathtub will take some time to drain.
Source: NASA (View larger image)
Similarly, carbon dioxide and other greenhouse gases accumulate in the atmosphere because we’ve turned up the tap by burning fossil fuels, and this extra concentration takes hundreds if not thousands of years to be removed.
The instantaneous forcing of albedo means that if you put a solar panel in a desert today and remove it 50 years from now, the extra heating produced by the panel for 50 years will disappear rapidly when it’s removed. Given that the worst effects of climate change are not expected to occur until later this century, focusing on albedo in the near-term makes sense only if we expect those albedo changes to remain indefinitely.
Solar Panel Albedo Concerns? Think of Alternative
In the case of solar panels, the albedo argument raised by Levitt and Dubner again suffers from their failure to examine the alternative; namely, the amount of heat produced by conventional electricity production.
In the worst-case scenario for solar panels, when they are located in high-albedo desert environments, the waste heat produced is only 11 percent greater than that produced by burning coal. In other conditions, such as rooftop photovoltaic installations, there will actually be less heat generated by the panels than by the conventional electric generation it is replacing.
Even if we ignore the waste heat from conventional fossil fuels, the albedo forcing from solar panels in the worst-case scenario is still only a fraction of that of the CO2 reduced over the lifetime of the panels. Indeed, the hypothetical “albedo debt” from our desert panels is made up in less than a year; over a 20-year lifespan of the panels, they will avoid 25 times more GHG-induced warming as they would contribute from albedo.
Finally, Levitt and Dubner cite Nathan Myhrvold, formerly of Microsoft and now with Intellectual Ventures, who claims that, in addition to albedo, solar panels are disadvantaged as a result of the high carbon emissions associated with producing the materials that constitute the panels.
While lifecycle carbon emissions from solar panel manufacturing are not trivial, they are not nearly large enough to negate the large carbon savings over the lifetime of the panels. Using the fairly high indirect emission estimates that Myhrvold himself cites, a solar installation producing 100 MWh annually over a 20-year lifespan would require around 150 tons of indirect CO2-equivalent emissions to construct. However, with annual CO2 savings of 50 tons per year compared to U.S. average grid mix electricity (e.g., around 0.5 kg CO2 per kWh), the solar installation would make up for its initial lifecycle debt in three years, and go on to reduce more than 800 tons of CO2 over the next 17 years. The figure below shows the expected lifetime CO2 savings taking both albedo (in the worst case, completely ignoring the avoided waste heat from conventional generation) and lifecycle emissions into account. If we assume the lifetime of the solar installation to be 30 years rather than 20 years, the net CO2 reduction of course becomes even greater.
|Nuanced View of Tree Albedo Needed|
Levitt and Dubner make similarly misleading statements about tree albedo. Loosely quoting Ken Caldeira of Stanford University, they say that he “mentions a most surprising environmental scourge: trees. Yes trees.”
They go on to explain that “… his research has found that planting trees in certain locations actually exacerbates warming because comparatively dark leaves absorb more incoming sunlight than, say, grassy plains, sandy deserts, or snow-covered expanses.”
Again, there is a core grain of counterintuitive truth here that nuances, rather than overturns, conventional knowledge. Recent research indeed suggests that in certain areas albedo effects from aforestration would outweigh benefits from carbon sequestration to provide positive net forcing. However, these locations are limited mainly to areas like the boreal forests having snow cover for a significant portion of the year. For the world as a whole, the carbon sequestration benefits from trees far outweigh the albedo forcing, and land use changes (particularly in the tropics) are responsible for about 18 percent of annual greenhouse gas emissions.
One of the seminal papers on the subject on which Caldeira was a co-author, Bala et al (2007), points out that:
“… [in high latitude areas] the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that aforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude aforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.”
The nuanced view of tree albedo perhaps can be better understood as: trees in the arctic and sub-arctic regions probably warm more than they cool; trees in temperate regions are somewhat offset by albedo but still for the most part good; and trees in the tropics are unambiguously good.
That’s a long way from simply labeling trees an “environmental scourge.”
While Levitt and Dubner’s contrarianism is often quite interesting, witty, and entertaining, when they tackle climate science in their latest book their lack of nuance can leave many readers with a false impression of what the science actually says.
That’s too bad. These nuances are important, because they explain why the conventional knowledge is still correct in many cases.