II. Greenhouse Gas Emissions: Sources and Trends
Greenhouse gases are generally divided into two categories:
(1) the three
principal greenhouse gases (carbon dioxide, nitrous oxide, and methane) and (2) other gases (primarily,
hydrofluorocarbons [HFCs], perfluorocarbons, and sulfur hexafluoride).
12
Based on overall emission levels
and global warming potential,
13
CO
2
is by far the most important GHG, accounting for 85 percent of total
U.S. GHG emissions in 2002.
14
Methane and nitrous oxide account for almost 14 percent. The three
 other gases  account for less than 2 percent of total U.S. GHG emissions when weighted by their 
100 year global warming potential.
15
Residential, commercial, and industrial buildings are responsible for 43 percent (658 MMTC) of
U.S. CO
2
emissions the GHG most focused on in this report.
16
Among the other two principal GHGs,
buildings are responsible for an estimated 7 percent of methane (an estimated 10 MMTC equivalent from
construction and demolition debris in landfills and 2 MMTC equivalent from the incomplete combustion
of wood in fireplaces and cookstoves) and 8 percent of nitrous oxide (0.3 MMTC equivalent, principally
+
from fireplaces and woodstoves).
17
Among the three  other gases,  only HFCs are significantly related to buildings. HFC emissions
are increasing because of their use as replacements for CFCs, halons, and other ozone depleting
chemicals that damage the earth's stratospheric ozone layer and are being phased out under the Montreal
Protocol. In particular, HFCs are used as refrigerants in refrigeration, chillers, and automobile air
conditioning, and as blowing agents in insulation. In 2002, the United States emitted an estimated 
23 MMTC equivalent of HFCs and an additional unknown amount of CFCs and hydrochlorofluorocarbons
+
(HCFCs) that will eventually be replaced. With the exception of automobile air conditioning and a few
other minor uses, the majority of these emissions are from applications in buildings.
18
Figure 3 shows a breakdown of the CO
2
emissions generated by the U.S. building sector, by energy
source.
19
Emissions from electricity consumption dominate in both residential and commercial buildings,
accounting for 71 percent of CO
2
emissions. Direct combustion of natural gas (e.g., in furnaces and water
6
+
Towards a Climate Friendly  
Built Environment
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