For all we know, planting trees in American cities could increase carbon emissions.
Energy use in buildings is central to the carbon footprint of urban trees. Unfortunately, key assumptions about season and climate haven’t been checked against real-world data.
Last month, the House of Representatives passed a bundle of clean energy initiatives. The legislation likely won’t become law unless Democrats win the Senate and abolish the filibuster. Nevertheless, it represents an opening foray by House Democrats in anticipation of what could be a productive 2021 Congress.
Included in that package was the TREE Act, a bipartisan bill introduced earlier in September by Rep. Cleaver (D-MO) and Rep. McKinley (R-WV). The act parallels a bipartisan Senate version introduced over the summer by Sen. Capito (R-WV) and former presidential hopeful Sen. Booker (D-NJ).
In keeping with the legislative vogue, the full title of the bill is the Tackling Residential Energy burdens Efficiently Act. Essentially, it aims to save Americans money on their energy bills by strategically planting over a million trees in urban areas.
Tree planting is having a moment. The Trillion Tree Campaign is gaining steam internationally and some U.S. cities are adopting their own million tree campaigns. At the same time, trees have become fig leaves for some GOP officials who lack serious ideas on climate. The climate benefits of tree planting were even touted by President Trump (mid-lie) during the first general election debate.
In any case, it’s easy to see how the TREE Act garnered bipartisan support. Trees are the most charismatic plants around, and if they help reduce costs and carbon emissions all the better.
The working theory is that trees provide shade and other cooling effects in the summer, and break up harsh winds in the winter, moderating the temperature year-round. By reducing energy usage, they reduce harmful carbon and air pollution emissions from power plants.
There’s one big problem: we don’t know whether planting trees in urban areas decreases or increases overall carbon emissions.
To be clear, trees do suck carbon directly out of the air. However, a tree releases its carbon when it dies — and many urban trees don’t live long. Once you account for the carbon costs of tree nurseries and maintenance the net impact can actually be detrimental. Carbon is only temporarily stored by trees, whereas the power plant emissions from, say, running a nursery are long-term additions to the carbon problem.
That’s a pretty uncontroversial notion. Rather than sequestration, most people who want to fight climate change with urban trees point to the effects on the energy used by our buildings. Naturally, this is the view taken by the authors of the Tackling Residential Burdens efficiently Act.
What evidence do we have for that claim? Let’s start with the research cited by Cleaver, Capito, and Booker’s press releases: a 2017 study by Nowak et al., a group comprised mostly of researchers for the U.S. Forest Service. The study found that existing urban trees in the U.S. save billions of dollars and thousands of tonnes of carbon through avoided heating and cooling costs.
Of course, this study modeled the benefits of existing urban trees, not adding X number of new trees. Still, it’s evidence that trees moderate urban climates, making both summer heat and winter cold less intense, saving everyone on their energy bill and reducing power plant emissions too. So far so good.
However, it’s important to take a critical look at the way those numbers were derived.
Nowak et al. utilized a simulation that allowed them to input data on trees near buildings and output estimated energy savings. The simulation takes three main effects of trees into account: shade from the canopy, ability to block wind, and transpiration (cooling caused by moisture evaporating from leaves).
But as a 2019 paper by Erker and Townsend points out, simulations like the one in Nowak et al. have never been tested rigorously against real-world energy data.
Instead, Nowak et al. attempts to estimate the different effects of trees in isolation. To estimate the wind breaking effect of trees, they rely on studies of small, model trees in wind tunnels — studies that were conducted in the 70’s. Shading effects, meanwhile, were calculated by inputting tree shape, size, and canopy density (based on factors like species and age) into a program developed in 1985 by Forest Service researchers.
Prior to 2019, we had no research that brought together tree data with actual energy use data (at least, not in a way that looked at both heating and cooling). This is important, because some of the things trees do have the potential to increase energy burdens. Blocking the sun’s rays in the summer is helpful, but blocking them in the winter could drive up your heating bill.
To address this gap in the research, Erker and Townsend conducted a study of Madison, WI and its bitter winters. Through remote sensing, they were able to catalog trees and their locations. Then, they combined that information with energy use data on 25,000 homes, controlling for other factors like building characteristics.
The results suggest that there are indeed important effects that previous stimulations hadn’t managed to capture.
In Madison, trees caused an overall energy penalty over the course of a year. Increased winter heating costs overwhelmed summer benefits — not too surprising, given Madison’s cold climate. As the authors note, the reason could be that previous simulations failed to correctly calibrate the relative impact of shading and wind breaking.
To get a more complete picture of the empirical evidence, we can look at a systematic review of the research. A 2018 analysis by Yekang Ko takes a comprehensive look at the research on trees and building energy use. Of the forty-one papers compiled, nearly half looked at real-world cases of trees and buildings — nineteen papers in total.
However, if we dig a little deeper, we see there are serious blind spots in that research. Three of the early studies were conducted on a single mobile home in a forest. Fourteen of them, the majority, looked exclusively at either cooling or summertime energy use — excluding cold winter days when the greatest heating penalty would occur.
Of the remaining five, three found no statistically significant energy savings (one of those found statistically significant costs), one found a “diminishingly small impact” from trees, and one found no statistically significant effect from shade, but statistically significant energy savings from wind breaking.
So what do we make of all these different findings?
Given the grave importance of the climate crisis, we can’t fall back on the conventional wisdom that trees are good for “the environment,” and therefore the climate. We have to accept that the relationship between emissions and urban trees are, at this point, not well-understood.
Of course, trees impact more than just the climate. Green space improves mental health, encourages exercise, and compliments the aesthetic of urban spaces.
It’s telling that when people have the choice and the means, they tend to live in leafy, tree-filled neighborhoods, rather than ones filled with asphalt and concrete. As many have noted, historically marginalized communities in America often suffer from a lack of green space. When combined with policies to ensure affordability and avoid green gentrification, greening our urban spaces is a worthwhile goal.
Nevertheless, it’s vital that our elected officials understand how their actions impact carbon emissions. Considering the mixed evidence, it should be concerning to us that some of our representatives are holding up the TREE Act as climate action.
It’s also worrying that urban tree planting campaigns often tout averted power plant emissions — both carbon and air pollution — as core arguments in their favor. While those campaigns are doing good by greening urban spaces, their emissions estimates may be giving local policy makers and the public an overly optimistic view of how their city is doing on both air quality and climate.
The silver lining is that we do actually know how to slash emissions from building energy use. The solution: replace all of the fossil fuel-powered machines in our buildings with electricity-powered ones. That means electric heat pumps, water heaters, and stoves. Smaller but significant gains can come from demand-side improvements like better insulation.
In a happy coincidence, the work of upgrading America’s homes would create millions of good, blue-collar jobs. Everyone would benefit from that investment, and we’d never have to worry about the energy effects of trees again.
