Constructed Climates

In this chapter I consider carbon and energy balances within cities and between cities and nature, and the question of the long-term sustainability of these quantities. Americans use a lot of energy, dominated a century ago by coal, then overtaken by petroleum and natural gas, and now, once again, dominated by coal. Even longer ago, trees supplied most of our energy, and recent hopes have pinned the future on biomass fuels. Certainly, modern-carbon energy has advantages over fossil-carbon energy, but our total energy use far exceeds any hopes for a substantial biomass solution.

Energy use varies across the states of the United States, with the highest density states having the lowest per capita energy use for both gasoline and electricity. An energy shortage ought not worry us: We have several centuries worth of coal reserves that could power a new era of coal-fired, plug-in cars.

Photosynthesis makes the link between our energy use, our carbon emissions, and growing vegetation, and, of course, it provided the fossil fuels people use today. I quickly overview how photosynthesis strongly connects atmospheric carbon dioxide (CO₂), water, and light, providing the important context for sustainability calculations. Following on from the examination of sunlight and impervious surfaces producing urban heat islands, I demonstrate how light interacts with trees as part of fixing carbon and shading the ground. Recent emissions from burning fossil fuels over the last 100 years added back carbon sequestered from the biosphere over millions of years. These emissions drove the last century’s increase in atmospheric carbon dioxide, and I show both the dramatically strong correlation between CO₂ and temperature over the last 400 millennia, as well as recent warming trends. Several examples show how global warming affects other species directly in the way their lives take place. In addition to global warming, local warming from heat island effects also show direct changes in organism-level properties. These changes take place due to both increased temperatures and increased CO₂ concentrations. Finally, urbanization changes the behavior of soils, or at least the microbial life they contain, increasing respiration over periods of many decades.

A simple carbon footprint calculation for Durham County, North Carolina, shows that even with complete countywide coverage by natural vegetation, citizens’ carbon footprints greatly exceed their county boundaries. As for urban vegetation, even though large trees hold large amounts of carbon and grow by many kilograms each year, cities must check that growth to preserve services that citizens demand and limit damages to publicly and privately held assets. Urban trees present maintenance challenges, like a bull in a china shop, demanding careful cost-benefit considerations. This maintenance requires the use of fossil-fuel-powered implements and vehicles, and these carbon costs need accounting in any carbon sequestration calculation.

Leading up to an “energy footprint” calculation, I show that urban trees could play an important role in reducing energy use, but mainly in cooling our houses through shade. Trees also reduce winds, and during the summer, wind reduction increases energy demands. In the case of winter heating, which uses more energy than summer cooling, shade increases energy demands while wind reduction decreases it. These competing trade-offs, balanced differently in different areas of the globe, means that using urban trees for energy reduction needs careful consideration.