Page 109 - The Green Building Bottom Line The Real Cost of Sustainable Building

88 CHAPTER 3

54 degrees Fahrenheit warmer than it would be without an atmosphere. 12 GHGs are
necessary for life on Earth—otherwise our planet would be freezing. The problem
occurs when the levels of greenhouse gases sharply increase, as has been occurring at
an accelerated rate over the past two hundred years. This dramatic increase in GHGs
throws off the delicate atmospheric balance and contributes to a host of climatic
changes, the most significant being an ever-warming planet, which in turn contributes
to the melting of the polar ice caps, rising sea levels, and a dramatic shift in weather
patterns.
Carbon dioxide (CO ) is the primary greenhouse gas of interest, as human activities
2
are greatly increasing its concentration in the atmosphere, though there are six main
greenhouse gases that scientists measure. 13 In the past century, the concentration of
carbon dioxide in the atmosphere has risen almost 30 percent, methane levels have
more than doubled, and nitrous oxide concentrations have increased by 15 percent. 14
Furthermore, the growth in atmospheric greenhouse gas concentrations has been pro-
gressing at a more rapid rate over time. For example, concentrations of CO have
2
increased during the last forty years at a rate 50 percent higher than seen in the previ-
ous two hundred years (54 parts per million [ppm] increase from 1960 to 2000 vs. the

TABLE 3.1 COMMON GREENHOUSE GASES

ATMOSPHERIC GLOBAL WARMING POTENTIAL
GAS LIFETIME (YEARS) (100 YEAR)
Carbon Dioxide (CO ) 50–200 1
2
Methane (CH ) 9–15 21
4
Nitrous Oxide (N O) 120 310
2
Hydrofluorocarbons (HFCs)
HFC-23 260 11,700
HFC-125 29 2,800
HFC-134a 13.8 1,300
HFC-143a 52 3,800
HFC-152a 1.4 140
HFC-227ea 33 2,900
HFC-236fa 22 6,300
Perfluorocarbons
Perfluoromethane (CF ) 50,000 6,500
4
Perfluoroethane (C F ) 10,000 9,200
2 6
Sulfur Hexafluoride (SF ) 3,200 23,900
6
Source: Intergovernmental Panel on Climate Change. 16