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. Anthropogenic (human-caused) GHG emissions are modifying the Earth’s energy balance between incoming solar radiation and the heat released back into space, amplifying the greenhouse effect and 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.3
- 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 2010.4
- Global Warming Potentials (GWPs) indicate the relative effectiveness of GHGs in trapping the Earth’s heat over a certain time horizon. CO2 is used as the reference gas and has a GWP of one.4 For example, the 100-year GWP of SF6 is 23,500, indicating that its radiative effect on a mass basis is 23,500 times that of CO2 over the same time horizon.1
- GHG emissions are typically discussed in terms of mass of carbon dioxide equivalents (CO2e), which are calculated by multiplying the mass of emissions by the GWP of the gas.5
The Main Greenhouse Gases1,2
Atmospheric Greenhouse Gas Emissions
- Since 1750, atmospheric concentrations of CO2, CH4, and N2O increased by 148%, 260%, and 123%, respectively, to levels that are unprecedented in the past 800,000 years.1,2
- Before the Industrial Revolution, the concentration of CO2 remained around 280 parts per million (ppm) by volume.6 In May 2021, the global monthly average concentration increased to 416.49 ppm, which is about 2.7 ppm higher than in 2020.7
Sources of Greenhouse Gas Emissions
- Anthropogenic CO2 is emitted primarily from fossil fuel combustion. Iron and steel production, cement production and petrochemical production are other significant sources of CO2 emissions.5
- The U.S. oil and gas industry emits 2.3% of its gross gas production annually, equivalent to 13 million metric tons of methane—nearly 60 percent higher than the U.S. Environmental Protection Agency (EPA) estimates.8
- 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 75% of anthropogenic N2O. Other significant sources include mobile and stationary combustion and livestock.5
- Hydrofluorocarbons (HFCs) are the fastest growing category of GHG and are used in refrigeration, cooling, and as solvents in place of ozone-depleting chlorofluorocarbons (CFCs).9
Emissions and Trends
- In 2019, total global anthropogenic GHG emissions were 52.4 Gt CO2e. Since 1990, annual anthropogenic GHG emissions increased by 59%.10
- GHG emissions increased by 0.57 Gt CO2e in 2019. Emissions averaged an increase of 0.4 Gt CO2e per year from 1970-2000.4,10
- Emissions from fossil fuel combustion account for a majority (74%) of global anthropogenic GHG emissions.10 In 2018, global emissions of CO2 from energy use totaled 36.9 Gt CO2.11
- From 2000 to 2018, global CO2 emissions from energy use increased 46%.11
- Since 2005, China has been the world’s largest source of anthropogenic CO2 emissions, surpassing the U.S.11
- The U.S. represents less than 5% of the world’s total population, but was responsible for 15% of total anthropogenic GHG emissions in 2019.12,13
- GHG emission in 2019 were 1.8% higher than in 1990, with an average annual growth rate of 0.1%.5
- Fossil fuel combustion is the largest source of U.S. GHGs, currently accounting for 74% of total emissions. Since 1990, fossil fuel consumption has grown at a rate of 0.1%. However, both GHG emissions and fossil fuel consumption have decreased since 2005 while GDP kept growing.5
- CO2 emissions accounted for 80% of total U.S. GWP-weighted emissions (CO2e) in 2019 and were 3% higher than in 1990.5
- The electric power industry produces 25% of total U.S. GHG emissions. Emissions from this sector have decreased 12% since 1990.5
- Transportation is the largest contributor of U.S. GHG emissions, responsible for 29% of total emissions in 2019, (23% higher than in 1990). Passenger cars and light-duty trucks accounted for 762 and 323 million metric tons CO2e, respectively, together making up 58% of U.S. transportation emissions and 17% of total U.S. emissions.5
- Urban sprawl, increased travel demand, population growth, and low fuel prices drive the growth of transportation GHG emissions.5
- Land use and forestry in the U.S. sequester a portion of CO2 in growing plants and trees, removing 12% of the GHGs emitted by the U.S. in 2019.5
- 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. EPA.14
U.S. GHG Emissions by Gas5
U.S. GHG Emissions by Sector5
Emissions by Activity
Future Scenarios and Targets
- Stabilizing global temperatures and limiting the effects of climate change require more than just slowing the growth rate of emissions; they require absolute emissions reduction to net-zero or net-negative levels.17
- Based on current trends, global energy-related CO2 emissions are anticipated to increase by 22% from 2018 to 2050.18
- Non-OECD countries’ CO2 emissions are expected to increase by 1.0% annually, while OECD countries’ emissions decline by 0.2% annually. Despite this difference, OECD countries will still have per capita emissions 2.2 times higher than non-OECD countries in 2050.18
- Under the Kyoto Protocol, developed countries agreed to reduce their GHG emissions on average by 5% below 1990 levels by 2012. When the first commitment period ended, the Protocol was amended for a second commitment period with a new overall reduction goal of 18% below 1990 levels by 2020.19
- In 2015, UNFCCC parties came to an agreement in Paris with a goal to limit global temperature rise to less than 1.5oC above pre-industrial levels, in order to avoid the worst effects of climate change.20
- Global CO2 emissions would need to decline 45% from 2010 levels by 2030 and reach net-zero by around 2050 to avoid temperature rise beyond 1.5oC.17
|1 Teragram (Tg) = 1000 Giga grams (Gg) = 1 million metric tons = 0.001 Giga tons (Gt) = 2.2 billion pounds (lbs)|
- Intergovernmental Panel on Climate Change (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.
- World Meteorological Organization (2020) WMO Greenhouse Gas Bulletin.
- 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. Environmental Protection Agency (EPA) (2021) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2019.
- IPCC (2014) Climate Change 2014: Synthesis Report. IPCC, Geneva, Switzerland.
- National Oceanic and Atmospheric Administration (NOAA) (2021) “Trends in Atmospheric Carbon Dioxide.”
- Alvarez, R., et al (2018) “Assessment of methane emissions from the U.S. oil and gas supply chain.” Science, 361: 186-188.
- Center for Climate and Energy Solutions (2021) “Short-lived Climate Pollutants.”
- PBL Netherlands Environmental Assessment Agency (2020) Trends in Global CO2 and Total Greenhouse Gas Emissions.
- U.S. Energy Information Administration (EIA) (2021) International Emissions by Fuel
- U.S. Central Intelligence Agency (2021) The World Factbook.
- Friedlingstein et al., (2020) The Global Carbon Budget 2020, Earth System Science Data.
- U.S. EPA (2016) Learn About the Greenhouse Gas Reporting Program (GHGRP).
- U.S. EPA (2021) “Emissions & Generation Resource Integrated Database (eGRID) 2019.”
- U.S. EPA (2021) The 2020 EPA Automotive Trends Report.
- IPCC (2018) Special Report: Global Warming of 1.5 C.
- U.S. EIA (2019) International Energy Outlook 2019.
- UN Framework Convention on Climate Change (UNFCCC) (2021) “What is the Kyoto Protocol?”
- UNFCCC (2021) “Paris Agreement.”