The Greenhouse Effect
The greenhouse effect is a natural phenomenon that insulates the Earth from the cold of space. As incoming solar radiation is absorbed and re-emitted back from the Earth’s surface as infrared energy, greenhouse gases (GHGs) in the atmosphere prevent some of this heat from escaping into space, instead reflecting the energy back to further warm the surface. Human activities that produce GHGs (anthropogenic) amplify the greenhouse effect. Anthropogenic GHG emissions are modifying the Earth’s energy balance between incoming solar radiation and the heat released back into space, resulting in climate change.1
- There are ten primary GHGs; of these, water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are naturally occurring. Perfluorocarbons (CF6, C2F6), hydroflurocarbons (CHF3, CF3CH2F, CH3CHF2), and sulfur hexafluoride (SF6) are only present in the atmosphere due to industrial processes.2
- Water vapor is the most abundant and dominant GHG in the atmosphere. Its concentration depends on temperature and other meteorological conditions, and not directly upon human activities.1
- CO2 is the primary anthropogenic greenhouse gas, accounting for 78% of the human contribution to the greenhouse effect in 2010.3
- Global Warming Potentials (GWPs) indicate the relative effectiveness of GHGs in trapping the Earth’s heat over a certain time horizon. CO2 is typically used as the reference gas and has a GWP of one.3 For example, the 100-year GWP of SF6 is 22,800, indicating that its radiative effect on a mass basis is 22,800 times as powerful as CO2 over the same time horizon.2
- GHG emissions are typically discussed in terms of mass of carbon equivalents or carbon dioxide equivalents (CO2e), which are calculated by multiplying the mass of emissions by the GWP of the gas.4
The Main Greenhouse Gases2
Atmospheric Greenhouse Gas Emissions
- From 10,000 years ago until 250 years ago, atmospheric concentrations of N2O, CO2, and CH4 were relatively stable. During the last 250 years, concentrations of N2O, CO2, and CH4 increased by 20%, 40% and 150%, respectively.1
- Pre-Industrial Revolution, the concentration of CO2 remained around 280 parts per million (ppm) by volume.5 In 2016, the global concentration increased to 403.9 ppm, which is about 4 ppm higher than in 2015.6
Sources of Greenhouse Gas Emissions
- Anthropogenic CO2 is emitted primarily from fossil fuel combustion. Iron and steel production, natural gas systems, and cement production are other significant sources of CO2 emissions.4
- CH4 and N2O are emitted from both natural and anthropogenic sources. Domestic livestock, landfills, and natural gas systems are the primary anthropogenic sources of CH4. Agricultural soil management (fertilizer) contributes 79% of anthropogenic N2O. Other significant sources include mobile and stationary combustion, and livestock.4
- Hydrofluorocarbons (HFCs) are now used in refrigeration, cooling, and as solvents in place of ozone-depleting chlorofluorocarbons (CFCs).7
- Perfluorocarbons (PFCs) are used primarily for aluminum production, and SF6 is used as an insulator in electricity distribution equipment.7
Emissions and Trends
- In 2010, total global anthropogenic GHG emissions were 49 Gt CO2e. Since 1970, annual anthropogenic GHG emissions increased by 81%.3
- GHG emissions increased by 1.0 Gt CO2e per year from 2000 to 2010. For comparison, emissions averaged an increase of 0.4 Gt CO2e per year from 1970-2000.3
- Emissions from fossil fuel combustion account for a majority (65%) of global anthropogenic CO2 emissions.3 In 2012, global emissions of CO2 from energy use totaled 32 Gt CO2.8
- From 2000 to 2012, global CO2 emissions from energy use increased 34%.8
- Since 2006, China has been the world’s largest contributor of CO2 emissions, surpassing the U.S.9
- The U.S. represents less than 5% of the world’s total population but was responsible for 14% of total anthropogenic GHG emissions in 2014.3,4,10
- From 1990 to 2014, U.S. GHG emissions increased by 7%, at an average annual growth rate of 0.3%.4
- CO2 emissions accounted for 81% of total U.S. GWP-weighted emissions in 2014 and were 8.6% higher than in 1990.4
- Fossil fuel combustion is the largest source of U.S. GHGs, currently accounting for 76% of total emissions. Since 1990, fossil fuel consumption has grown at about the same rate as GHG emissions—both have grown slower than GDP.4
- The electric power industry accounts for about one-third of total U.S. GHGs. Emissions from this sector have increased 12% since 1990.4
- In 2014, the residential, commercial, and industrial sectors each used approximately a third of the electricity generated.4
- Transportation is the second largest contributor of U.S. GHG emissions, responsible for 26% of total emissions in 2014. Passenger cars and light-duty trucks accounted for 763 and 338 million metric tons CO2e, respectively, together making up 61% of U.S. transportation emissions and 16% of total U.S. emissions.4
- Urban sprawl, increased travel demand, and an increase in the number of vehicles are driving the growth of transportation GHG emissions.4
- Land use and forestry in the U.S. sequester a portion of CO2, removing 11% of the GHGs emitted by the U.S. in 2014.4
- As a result of 2008 federal legislation, sources that emit over 25,000 metric tons CO2e in the U.S. are required to report emissions to the U.S. Environmental Protection Agency (EPA).11
U.S. GHG Emissions by Gas4
U.S. GHG Emissions by Sector4
Emissions by Activity
Future Scenarios and Targets
- Stabilizing atmospheric CO2 concentration requires more than just slowing the growth rate of emissions; it requires absolute emissions reduction.14
- Based on current climate regulations, global energy-related CO2 emissions are anticipated to increase by 21% from 2012 to 2040.15
- Non-OECD countries’ CO2 emissions are expected to increase by 2.2% annually, significantly faster than OECD countries at 0.6% annually. Despite these increases, OECD countries will have per capita emissions 2.6 times higher than non-OECD countries in 2040.15
- Under the Kyoto Protocol, developed countries agreed to reduce their GHG emissions on average by 5% below 1990 levels by 2012. Had the U.S. ratified the Kyoto Protocol, its reduction requirement would have been to reduce its emissions by 7%.16 When the first commitment period ended in 2012, the Protocol was amended for a second commitment period; the new overall reduction goal would be 18% below 1990 levels by 2020.17
- Global CO2 emissions must be reduced by 50-85% below 2000 levels by 2050 in order to stabilize the average CO2 concentration below 400 ppm.18
Emissions Reductions Necessary to Stabilize Atmospheric CO2 Concentrations2
|1 Teragram (Tg) = 1000 Giga grams (Gg) = 1 million metric tons = 0.001 Giga tons (Gt) = 2.2 billion pounds (lbs)|
- IPCC (2013) Climate Change 2013: The Physical Science Basis. T.F. Stocker, et al.; Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Intergovernmental Panel on Climate Change (IPCC) (2007) Climate Change 2007: The Physical Science Basis. Eds. S. Solomon, et al.; Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- IPCC (2014) Climate Change 2014: Mitigation of Climate Change. O. Edenhofer, et al. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- U.S. EPA (2016) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2014.
- IPCC (2014) Climate Change 2014: Synthesis Report. IPCC, Geneva, Switzerland.
- National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory, Global Monitoring Division (2016) “Trends in Atmospheric Carbon Dioxide.”
- IPCC (2000) “Special Report on Emissions Scenarios.” Eds. N. Nakicenovic and R. Swart.
- U.S. Department of Energy (DOE), Energy Information Administration (EIA) (2013) “Total Carbon Dioxide Emissions from the Consumption of Energy.”
- United Nations (UN) (2011) “Millennium Development Goals: Indicators.”
- U.S. Central Intelligence Agency (CIA) (2016) “The World Factbook.”
- U.S. EPA (2013) Fact Sheet: Greenhouse Gases Reporting Program Implementation.
- U.S. EPA (2016) “Emissions & Generation Resource Integrated Database (eGRID).”
- U.S. EPA (2015) Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through 2015.
- National Research Council (2011) Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia.
- U.S. DOE, EIA (2016) International Energy Outlook 2016.
- UN Environment Programme (UNEP) and UN Framework Convention on Climate Change (UNFCCC) (2003) Climate Change Information Kit.
- UNFCCC (2013) “Kyoto Protocol.”
- IPCC (2007) Climate Change 2007: Synthesis Report. Eds. R. Pachauri and A. Reisinger.