Counting Carbon: Understanding Carbon Footprints of Buildings

Historically the amount of CO2 in the atmosphere hovered just under 300 parts per million (ppm), but it’s now approaching 400 ppm. CO2 is not the most powerful of the greenhouse gases on a per-molecule basis—not by a long shot—but it is by far the most common and most significant of those generated by humans. Various targets have been proposed as acceptable levels of CO2, most famously 450 ppm, above which the resultant temperature rise would likely cause extreme disruption to Earth’s ecological and social systems. Many policy initiatives give lip service to this goal, but current actions are inadequate to reach it. Based on more recent scientific findings, author Bill McKibben has launched a campaign to reset that target at 350 ppm, a point we passed in 1988. That’s a much more ambitious goal, but one that, if achieved, would more likely lead to a future climate that resembles our own.

Regardless of the target, there is general agreement that we have to slow the growth in carbon emissions and then shrink those emissions. As researchers seek ways to reduce human-generated carbon emissions at a cost that society will accept, buildings consistently emerge as the best opportunity. “Buildings are the biggest and lowest-hanging fruit in dealing with greenhouse gases in the atmosphere,” says architect and researcher Hal Levin, who chairs the Project Committee on Carbon Emissions Tool Development of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). Since carbon emissions from buildings generally follow energy use, we’ll go a long way simply by making buildings more energy efficient.

Historically the amount of CO₂ in the atmosphere hovered just under 300 parts per million (ppm), but it’s now approaching 400 ppm. CO₂ is not the most powerful of the greenhouse gases on a per-molecule basis—not by a long shot—but it is by far the most common and most significant of those generated by humans. Various targets have been proposed as acceptable levels of CO₂, most famously 450 ppm, above which the resultant temperature rise would likely cause extreme disruption to Earth’s ecological and social systems. Many policy initiatives give lip service to this goal, but current actions are inadequate to reach it. Based on more recent scientific findings, author Bill McKibben has launched a campaign to reset that target at 350 ppm, a point we passed in 1988. That’s a much more ambitious goal, but one that, if achieved, would more likely lead to a future climate that resembles our own.

Regardless of the target, there is general agreement that we have to slow the growth in carbon emissions and then shrink those emissions. As researchers seek ways to reduce human-generated carbon emissions at a cost that society will accept, buildings consistently emerge as the best opportunity. “Buildings are the biggest and lowest-hanging fruit in dealing with greenhouse gases in the atmosphere,” says architect and researcher Hal Levin, who chairs the Project Committee on Carbon Emissions Tool Development of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). Since carbon emissions from buildings generally follow energy use, we’ll go a long way simply by making buildings more energy efficient.

But energy use, whether it’s measured in dollars or kilowatt-hours, in absolute terms or as a percent reduction against code, is not exactly a measure of carbon emissions. How energy is generated and distributed changes how much carbon is released in the process. And energy used in the building is not the whole picture when it comes to greenhouse gas emissions. What you count and how you count it can change both the answers you get and what you do about them.