By David Keith
Today Lee Miller and I published a pair of papers on the interaction between wind turbines and the atmosphere. “Observation-based solar and wind power capacity factors and power densities” in Environmental Research Letters, and “Climatic impacts of wind power” in Joule. (Many thanks to the journals for arranging simultaneous publication.) Don’t miss Lee’s video abstracts for Joule and ERL.
From my perspective, there are two big takeaways. First, there are now two independent lines of high-quality data suggesting that models with atmosphere-turbine interactions are getting something important correct. Second, that wind power has a somewhat larger environmental footprint than many had assumed, that, specifically, the land footprint of wind is at least 10 times higher than that of solar.
What does this mean for public policy? In my opinion, it means more empirical research to answer specific questions about wind’s impacts. A wise reporter chided me that scientists always want more research while pushing me towards a policy relevant conclusion. For me, the strongest high-level conclusion is that, as policymakers push towards decarbonization, it’s worth pushing a bit harder on solar and a bit less hard on wind.
Context matters: the big problem is that policymakers should be doing much more to cut carbon emissions, most importantly by technology-neutral policies that penalize the use of the atmosphere as a cost-free disposal site for carbon pollution. Some thoughtful environmental activists who are fighting day-to-day against fossil fuel interests to accelerate the deployment of low-carbon power will ask, Why publish the stuff that hands ammunition to the other side? My answer is simply that no large-scale energy technology is without social and environmental impacts. And, as renewable energy grows out of its cradle into the energy mainstream, those whose goal is environmental protection must welcome careful analysis of its full environmental impacts, particularly when that analysis can inform energy choices to reduce future impacts.
Why the timescale comparison?
Much reporting will focus narrowly on the timescale comparison in the Joule paper. Reporters seem drawn to want a simple, over-the-top claim/sound bite along the lines of, Wind is worse than fossil fuels this century. Such a claim is total nonsense.
Why then, did we make the wind versus fossil comparison in the Joule paper? Simply reporting that we get a specific climate change for a specific large deployment scenario isn’t very helpful because it doesn’t provide a relevant comparison. We need to find a way to compare the relative environmental footprints of low-carbon energy sources like solar and wind. Policymakers need a rough metric of how much these climate impacts matter on a per-unit-energy basis. A single wind farm has negligible impact on global climate over the next century. Yet, if that single wind turbine provides an infinitesimal global benefit in the form of reduced emissions and climate change, and an infinitesimal climate impact in the form of non-local hemispheric-scale climate change caused by atmosphere-turbine interactions, it’s relevant to compare these two infinitesimal effects in order produce a crude estimate of the ratio of benefits to harms. Because the carbon benefits grow cumulatively with time while the turbine-atmosphere interactions are instantaneous, this ratio is not dimensionless, but instead has units of time.
Because both the benefits and harms are very roughly linear, the timescale metric is relevant to wind or solar power at any scale. It doesn’t depend on the specific half-terawatt scenario studied here.
What do these timescales mean to me? They’re very rough order-of-magnitude guides to the relevance of the climate changes caused by low-carbon power sources. If the timescale is on the order of decades or less, then I think it’s fair to completely ignore the climate impacts in practical policymaking. This is the case for solar power. If the timescale was thousands of years, then I think the climate impact poses a serious problem. For wind, our analysis suggests the timescale is, to a very rough order, a hundred years—scientist speak for more-than-decades and less-than-millennia. Given that, I think it’s fair to conclude that wind power’s climate impacts are non-negligible.
Statements like wind is worse than fossil fuels this century are nonsense both because they’re overly precise about the timescale, which is in fact contingent on a bunch of open-ended assumptions as we describe in the paper, and because the climate change from atmosphere-turbine interactions and CO2 are quite different. There may be significant benefits to the climate change from wind turbine drag. For example, all of the global models that have examined large-scale wind deployment scenarios (including Mark Jacobson’s, though he did not show climate results in this paper) show cooling over the Arctic. Years ago, Danny Kirk-Davidoff and I wrote a nerdy paper in Journal of the Atmospheric Sciences to try and understand the reasons for this cooling. If correct, this is an added climate benefit of wind power.
Q: What’s new? A: Observational support for the models.
For me, the importance of these papers is not the timescale, nor the turbine-induced climate change, both of which have been shown before. But, in the main, the environmental science community ignored them. In part, I suspect many people concluded that the results were simply not robust, were not backed up by observational evidence.
The ERL paper is the first observational estimate of the average power density of large-scale wind power. Power density matters because it determines how much land is required to supply a given amount of energy. Our results use newly released data on the location of all US wind turbines. We find an average density of 0.5 Wm-2consistent with physically based models and inconsistent with wind resource estimates that ignore interactions between wind turbines and the atmosphere. See this graphical summary. (Power density matters because it means that supplying a given amount of wind power takes more land than previously assumed. Roughly 3 times more than an important estimate by the US DOE and more than 10 times the amount from an important study used by the IPCC.)
As we catalogue in the Joule paper, warming has now been observed at least 28 operational wind farms in at least 10 separate studies. Most of the studies are based on changes in satellite-observed skin temperature before and after wind farm installation. For me, the major result of the Joule paper was that our model roughly matches the diurnal and seasonal cycle of warming, providing strong confirmation that we are capturing an important mechanism that causes wind power-induced climate change.
As I see it, the novelty of these two papers is the link between models and observations. My naïve hope is that readers will not over-interpret the specific results in the Joule paper, which are highly configuration-dependent, but rather hear the importance of observational confirmation of previously theoretical results and conclude that one cannot simply ignore these effects, concluding that wind power’s land footprint and climate impacts need to receive more serious consideration in strategic decisions about decarbonizing our energy system.