Gt—gigaton. One billion metric tons.
CO2e—carbon dioxide equivalent, a measure used to compare heat-trapping gases based on their warming potential. The CO2e value of carbon dioxide is 1.
Heat-trapping gases—also commonly referred as “greenhouse gases.” The term “heat-trapping gas” more clearly captures the impact of these gases in the atmosphere and will be used throughout this Guide.
The principal heat-trapping gases emitted by human activities are carbon dioxide (CO2), methane (CH4), nitrous oxide (NOx) and fluorinated gases (such as HFCs and SF6 ). Of these, carbon dioxide is by far the most important, with roughly 76% of the total warming impact of these gases globally. Methane is the second most important, with roughly 16% of the warming impact, followed by nitrous oxides with 6% and fluorinated gases with 2%.
Carbon dioxide emissions are caused mainly by burning coal, oil and gas. Some industrial processes, including cement production, also release carbon dioxide, as does deforestation. Methane and nitrous oxide emissions come from agriculture, the energy sector and elsewhere. The fluorinated gases are used mostly in air-conditioning and refrigeration.
The unit “CO2 equivalent” (CO2e) is a measure used to compare heat-trapping gases based on their warming potential. One molecule of methane traps roughly 28 times more heat than a carbon dioxide molecule over a 100-year period, for example. Methane is therefore often assigned a CO2e value of 28. One molecule of nitrous oxide traps roughly 265 times more heat than a carbon dioxide molecule over a 100-year period. Nitrous oxide is therefore often assigned a CO2e value of 265. The leading fluorinated gases have CO2e values in the hundreds and thousands.
 Intergovernmental Panel on Climate Change (IPCC) Working Group III, Climate Change 2014: Mitigation of Climate Change—Summary for Policymakers (Fifth Assessment Report) at p.6.
 IPCC, Climate Change 2014: Synthesis, Fifth Assessment Report at p.87; G. Myhre et al., Chapter 8: Anthropogenic and Natural Radiative Forcing in IPCC, Climate Change 2013: The Physical Science Basis at p.714.