Constructed Climates

Figure 3.13: A calculation of Durham County citizens’ carbon footprint. On a per capita basis, carbon emissions represent 1.32 acres of net primary production by plants, but the county only has 0.83 acre on a per citizen measure. These numbers underestimate total U.S. per capita carbon emissions; using numbers from Figure 3.1 yields about 10,000 kg per person, not 2,300 kg.

Carbon footprint calculations aren’t rocket science. The more you fill your gas tank, the more you fly, the bigger your house, the colder your air conditioning setting, the higher your heating setting, the more goods you buy, the bigger your carbon footprint.

Here’s an approximate carbon calculation for people living in Durham County, assuming average gasoline and electricity use (see Figure 3.2). In my calculation, the question I’ll ask is how many acres of vegetation do I need to pull the carbon I’ve used, in terms of my fossil-fuel use, back out of the atmosphere? I’ll call that a carbon offset calculation, telling us how our fossil-carbon use compares to carbon use by the biosphere.[56]

Calculating carbon emissions from gasoline requires simple multiplication of gallons used and carbon content per gallon, but emissions for electricity in the Durham area depends on the mix of energy sources, either coal, natural gas, or nuclear. Typically, carbon emissions for electricity generation range from about 0.04–0.13 kgC/kWh, and I’ll assume 0.1 kgC/kWh.[57] As the table in Figure 3.13 shows, the average Durham resident each year emits 2,300 kg of carbon, about 2.5 tons.

Just to make the calculation simpler, suppose that every acre of Durham County has trees fixing carbon.[58] Using growth values of about 20 kg/tree/yr for 30-45 cm DBH trees (see Figure 3.10) and crown width and area (see Figure 2.13) of 10 m and 80 m², respectively, gives about 0.25 kgC/m². This number agrees well with the net primary productivity values of Figure 1.2, with a United States average of 0.43 kgC/m², which I use here. Other estimates for eastern and northeastern forests range upwards of 1 kgC/m² per year.[59] At the low end, the arid shrublands of the southwestern United States fix less than 0.1 kgC/m² per year.[60] Using the United States average, each acre of forested land absorbs about 1,740 kg of carbon, meaning that the average Durham resident needs about 1.3 acres of forest to offset their emissions, an area nearly twice the per capita area of the county.[61]

Whatever the detailed assumptions,[62] it is clear that, on average, citizens of Durham County emit more carbon from their fossil fuel use than the trees in their county can absorb. In this sense, Durham residents export their carbon sequestration needs somewhere else, relying on some other place’s trees to balance atmospheric carbon levels.[63] Of course, most Americans are just like Durham’s residents, and that’s why atmospheric carbon dioxide levels are rising. We’re using fossil carbon beyond the ability of the biosphere to counteract it, and whose job is it to make up the difference? Perhaps areas that have positive sequestration could be compensated for that service to urban residents?

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[56]Carbon offset calculations have an element of nonreality. These calculations outrageously assume that the carbon dioxide pulled out of the atmosphere and fixed by vegetation gets locked away, never to be seen again by the biosphere. That idea is not true. Vegetation dies and decomposes, and decomposition releases the carbon dioxide back into the atmosphere. Fossil carbon release represents new carbon added to the biosphere — actually, mainly the atmosphere. I don’tworry too much about making sure these calculations are perfectly precise: People’s direct and indirect energy use differs greatly, vegetation patterns vary greatly, and even the sequestration point I’ve made several times makes me cautious about the underlying offset calculation assumptions.

[57]Price et al. (2002) discuss emissions for electricity generation, and provide an emissions range for electricity generation of about 0.04–0.13 kgC/kWh.

[58]Not every acre of Durham County has trees: See urbanized Durham in Figure 2.1.

[59]Pan et al. (2006) estimated carbon sequestration in northeastern forests.

[60]Cleveland et al. (1999) estimated carbon sequestration in eastern and northeastern forests, and arid shrublands like the southwestern United States.

[61]Another way to think about sequestration is that the 2,300 kg of personally used carbon emissions represents the carbon fixed by about 115 30-45 cm DBH trees, meaning our energy use is equivalent to cutting down about two of these trees each week.

[62]The numbers I report in Figure 3.13 greatly underestimate total U.S. per capita carbon emissions. Dividing total 2008 energy use from Figure 3.1, about 100 quadrillion Btus, by the U.S. population of 300 million leads to 3.3 × 10⁸ Btu/American. Dividing this amount by the 14,000 Btus in a kilogram of wood yields 24,000 kg wood equivalents. That’s about half carbon, meaning about 12,000 kg carbon emissions per American, much more than the 2,300 kg in my carbon budget table. Commercial, industrial, and transportation energy use makes up the difference.

[63]I once saw a Web posting announcing that the United Nations had planted more than a billion trees in 2007. That’s one tree for each of 7.7 people. It helps, of course, but the problem of increasing carbon dioxide greatly exceeds that solution. Worse yet, even ecological society meetings herald being carbon neutral because organizers paid someone to plant thousands of seedlings, then claimed they offset carbon emissions for a meeting today with carbon sequestration tomorrow. That sounds very much like deficit spending arguments, not true carbon neutrality, from people who should know better, yet chose to fly to attend an international meeting. These tree plantings are a start, of course, but the only real solution involves finding and using fossil-carbon-free energy sources. Planting trees cannot offset our massive fossil-fuel energy consumption.