The Best Trees To Plant For Global Warming Have Three Blades & Generate Electricity

Recently, I was asked whether it was better to plant trees or build a wind farm to fight global warming. It’s an interesting question, given that reforestation has been in the headlines so much recently. The answer, calculated further down, is that if there’s an option to build wind energy, that’s better than planting trees.

Wind turbines among trees image courtesy US Department of the Interior (usgs.gov)

A wind farm is about 8 times more effective at reducing CO2e annually than a forest, and the reduction is permanent, not temporary. Additionally, it eliminates a bunch of other air and water pollution, and reduces habitat destruction.

This isn’t to say “Don’t plant trees!” of course. That would be silly. But where there’s a good wind resource, if you can swing building a wind farm instead of spending the money on a forest, the world will be better off in the long run.

First though, the question raised an interesting false dichotomy. A false dichotomy is a dichotomy that is not jointly exhaustive (there are other alternatives), or that is not mutually exclusive (the alternatives overlap), or that is possibly neither.

Wind generation and trees (shorter ones, not California’s red giants) can co-exist easily and often do. One might think an actual dichotomy would be wind vs solar energy. But no, hybrid wind solar farms work just fine.

A hybrid wind-solar farm. Image courtesy: Washington State Department of Commerce

And sometimes, there isn’t even an option to planting trees.

As Mark Z. Jacobson, the Stanford professor behind the 100% renewable plans for 139 countries by 2050, said when I asked him about this:

Wind turbines occupy the least footprint on the ground of any energy technology. The spacing area between them is always dual-purpose, so can be used for forestry, agricultural land, or solar panels.

That said, wind farms have one set of environmental advantages and trees have an overlapping set of advantages.

At the level of CO2 and pollution, assuming you took a large area without trees, perhaps 25 square kilometers that is roughly 5 kilometers on a side, and either planted trees or built a wind farm, there’s a set of numbers that could be worked up.

The average wind turbine built in the USA in 2018 was 2.6 megawatt (MW) in nameplate capacity. Let’s take a Vestas V100-2.6 wind turbine as the base for the numbers. Its rotor diameter is an easy to use 100 meters. That’s important as you typically want wind turbines set about 10 rotor diameters apart downwind and 4-6 diameters across the wind. That suggests that all else being equal (it never is), you could put up a wind farm with about 10 turbines across by 5 turbines deep, or 50 turbines in total. That gives a nameplate capacity of 130 MW. At the average 40% capacity factor, they’ll generate about 450 gigawatt-hours (GWh) of electricity per year.

Every GWh of electricity generated by wind displaces 0ne GWh of fossil fuel generation. That’s been shown by study after study globally, and will persist until fossil fuels are a small minority on the grid. Coal generation averages about 1,000 metric tons of CO2e per GWh. Gas averages about 500 metric tons of CO2e emissions per GWh generated. Most grids still have more coal than gas, but let’s split the difference and say about 750 metric tons of CO2e per GWh, suggesting that the hypothetical wind farm would prevent the emission of about 340,000 metric tons of CO2e per year.

Trees, on the other hand, capture CO2, at least temporarily. Each will take up, on average, about 22 kg of CO2e per year, and about 0.9 metric tons after 40 years. Of course, unless you do something with the wood, the majority of that CO2e ends up in the air again, but at least for 40 years there’s abatement. And you can plant a lot more trees. Normally dense forests might have about 75,000 trees per square kilometer. That suggests about 1.9 million trees for our square of land. That in turn suggests about 41,000 tons of CO2e sequestered per year in our hypothetical, unwooded, 25 square kilometer area.

Yeah, the wind farm would prevent the emission of about 8 times as much CO2e per year as the trees. And it would also prevent the sulphur dioxide, nitrous oxide, particulate, and hydrocarbon pollution emissions. And it would prevent the habitat destruction and water pollution related to the extraction, refinement and shipping of the fossil fuels.

I have to say that if there’s a location that’s good for wind energy, the priority should be on building a wind farm, not planting a forest. But there are a lot of places in the world that are good for trees but not good for wind farms. And as pointed out, sometimes you can do both.

There are some other factors, of course.

Wind farms and trees both make good habitats for a lot of ground animals. Trees make better habitats for flying animals. One question is related to endangered bird and bat species. At first blush, you would assume that the wind farm would be the poorer choice if there are endangered bird or bat species in or around the 25 square kilometers. You might be wrong though. If the area is unwooded, that’s the habitat the birds and bats are adapted to. Planting a massive forest would disrupt their entire ecosystem. In many cases, it would put the species at risk, as grassland species don’t do well in forests.

And wind farms are spread widely apart and, depending on the species and characteristics, can often be set far enough away from bird nesting grounds, for example, to be non-disruptive. Similarly, increasing cut-in speed, the wind velocity at which the blades start turning and generating electricity, is proven by studies to eliminate bat deaths. And along whooping crane migration corridors, wind farms have wildlife biologists who monitor the migration and have the authority to shut down the turbines when cranes get within a couple of kilometers of them. There are mitigations, in other words.

Fossil fuel generation kills a lot more birds per GWh of generation than wind energy does between air pollution, water pollution, habitat destruction and birds directly impacting with fossil fuel infrastructure.

The specific 25 square kilometers would have to have an environmental assessment, in other words, but all else being equal, I’d still likely go with wind farms being a better choice than forests when there is a choice. Then, plant a lot of trees in places where wind farms don’t make sense.