Reference article: Erker, T. and P.A. Townshend. 2019. Trees in cool climate cities may increase atmospheric carbon by altering building energy use. Environmental Research Communications 1: 081003. DOI: 081003. doi.org/10.1088/2515-7620/ab37fd
Planners and activists have increasingly looked to urban trees and greenspaces as unambiguously helpful for reducing emissions and buffering people from negative climate impacts, despite occasional local pushback. But a new study out of the University of Wisconsin-Madison highlights the potential energy and emissions penalties associated with city trees, and calls for more nuance in urban planning as municipal authorities try to recruit trees into the climate preparedness fight.
Allies in the fight
Cities are the source of the bulk of human greenhouse gas emissions, and many of the approaching hazards due to climate change can be compounded in the urban environment. From excessive urban summertime heat to the heightened risk of coastal flooding with sea level rise and more intense storms, many of the battles to meet the challenges of climate change will be fought in our cities over the next few decades.
Several cities have begun looking at better management of trees, parks, and other “green infrastructure” as part of their formal climate planning process. Tree planting programs have been touted for their potential to help lock away CO2 in growing tree tissues via photosynthesis, helping cut down on emissions driving climate change. Expanding green spaces to reduce pavement cover and open up space for thirsty tree roots has been tapped as a means to help absorb excess runoff and reduce future flooding. And urban planners and policymakers have eyed the shade and evaporative cooling provided by urban park- and street trees as tools to reduce summertime heat extremes, with the added benefit of lowering energy use and the associated greenhouse gas emissions from power plants. The new study, however, casts shade on some of the rosier hopes about the role of trees to help cities cope with climate change.
Enemies in our ranks?
In their study, Erker and Townshend examined the evidence that greater amounts of tree cover, and the lower summertime temperatures they can create (Ziter et al., 2019), universally results in lower overall energy consumption and greenhouse gas emissions. Focusing on the city of Madison, Wisconsin, the researchers used high-resolution aerial photographs (and a lot of patience!) to map the extent of tree canopy within 60 m of buildings, as well as their relative positions (North, South, East, and West) and distances. The authors then related their canopy coverage and position data to a large database on energy consumption for about 25 thousand residential buildings across the city – split between natural gas used for heating, and electricity used for cooling.
What they found was surprising: While trees near buildings were associated with somewhat lower fossil C emissions due to reduced electricity use, the same trees also resulted in relatively large increases in natural gas C emissions. And tree cover specifically to the immediate East of buildings did not appear to even have the effect of reducing electricity demand. In net, every additional 100 m2 of tree canopy in Madison was associated with a net increase of about 0.17% more carbon emissions, equaling about 62 kg of C per year for the average house — or about the equivalent of additional 500 miles of car travel annually per household.
While many trees reduce electricity consumption in the summer due to the lower temperatures they bring and the reduced need for air conditioner use, it seems the amount of shading they create in the winter months also kept houses in Madison colder. This lack of morning sun caused heating (natural gas) demand to ramp up. East-positioned trees in particular make shade that falls during the cooler morning hours, and so are less helpful in reducing electricity demand. But similar to trees in other positions, east-positioned trees also brought clear penalties in greater gas use for winter heating. Since tree cover to the north, south, and west of residences offset the bulk of their wintertime carbon penalty with summertime reductions, these results imply that much of the net city-wide emissions rise with greater tree cover might be due to the double-hit of carbon penalties from tree shading of the east side of buildings.
Context is everything
Extrapolating their work to cities across the U.S., the authors suggests that many cool-climate cities, where energy demand and carbon emissions for heating outpaces demand for cooling, might show a net emissions penalty for increased tree cover. This effect, however, is also sensitive to the relative “dirtiness” of the local electric generation plants: In places with higher CO2 emissions per kWhr of electricity (e.g. Chicago), the summertime reductions in electricity demand may offset more of the winter penalty in heightened natural gas emissions.
The study underlines an issue that has been raised before: Trees and urban green spaces have a variety of effects that generate both services and dis-services (Pataki et al., 2011), and simple one-size-fits-all solutions will likely see serious shortcomings and inefficiencies. And while cool-climate cities may see somewhat higher energy-related greenhouse gas emissions with increased tree cover, the temperature reductions, air pollution abatement, or flood buffering that might come with more canopy may well prove worth the cost — particularly where immediate human health is under threat. As this surprising study demonstrates, human values and clear-eyed evaluation of trade-offs will need to inform how cities manage the stresses of ongoing climate change. To be effective soldiers in the fight for climate change preparation, we may need to get smarter about how trees are deployed, and how to prioritize the tasks we assign them.
Pataki, D.E., M.C. Carreiro, J.C. Cherrier, N.E. Grulke, V. Jennings, S. Pincetl, R.V. Pouyat, T.H. Whitlow and W.C. Zipperer. 2011. Coupling biogeochemical cycles in urban environments: ecosystem services, green solutions, and misconceptions. Frontiers in Ecology and the Environment 9(1): 27–36.
Ziter, C.D., E.J. Pedersen, C.J. Kucharik and M.G. Turner. 2019. Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer. PNAS 116(15): 7575–7580.