Greenhouse Gases (GHGs)
Greenhouse Gases (GHGs) are gases that trap heat in the Earth’s atmosphere. As solar radiation passes through the atmosphere and reaches the surface, it is absorbed and re-radiated primarily as infrared radiation. GHGs prevent nearly 90% of this radiation from being radiated back to space, absorbing and re-emitting it to warm the lower atmosphere and planetary surface. Without this ‘greenhouse effect’, Earth would be a stark, cold and uninhabitable planet for the majority of life forms. The GHGs that contribute most to the greenhouse effect are water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), ozone (O3), and Chlorofluorocarbons (CFCs). 1)
Anthropogenic (human) release of GHGs leads to the ‘enhanced greenhouse effect’, where more of the Sun’s heat is trapped in the atmosphere which in turn heats the planet further. This effect causes global temperatures to rise, known as global warming. Since the mid-1800s, human activities have significantly increased the atmospheric GHG concentration, contributing to the 1.18°C increase in average global temperature. The Intergovernmental Panel on Climate Change (IPCC) concluded in their Sixth Assessment Report that the increase in CO2, CH4, and N2O in the atmosphere since the industrial revolution is the result of human activities. They also stated that human influence is the principal driver in many changes in the atmosphere, ocean, cryosphere and biosphere. 1) 2)
Spaceborne sensors are capable of monitoring GHGs in the atmosphere and identifying their compositions. Measurements can also be made by airborne and ground-based sensors, however these are not capable of making repeat observations across the globe. This ability provides insight into atmospheric composition dynamics, and remotely sensed GHG data can be used in atmospheric models to estimate GHG sources and sinks. 1)
Three broad satellite types can be identified: 19)
- Global GHG Mappers: Typically undertaken by national space agencies, these missions are the most important sources of information for tracking GHG emission and removal from both natural and anthropogenic sources and sinks on spatial scales spanning from large urban areas to nations. These sensors typically do not have the spatial resolution to attribute emissions from individual facilities, but are the only sensors with the precision and accuracy needed to track fluxes from diffuse sources of CO2 from the biosphere and oceans, and of methane from natural wetlands and from agricultural lands. Data from Global GHG Mappers are being used in national inventory development and to assess the completeness and accuracy of country reporting to the UNFCCC (United Nations Framework Convention on Climate Change).
- Facility Scale Plume Monitors: A wide range of public, commercial and NGO groups are operating or planning missions (often smallsats) in this category, with many of them targeted at helping the fossil fuel industry address emission reductions from their operations, particularly for CH4 extraction and transport. Many of the relevant sensors are multi-purpose hyperspectral imagers, which can track intense plumes of CO2 and CH4. Sensors assigned to this category must have a spatial resolution of 1 square km or finer, but do not require the precision, accuracy or coverage of the Global GHG Mappers.
- Operational Sounders: Sounder instruments have been used for decades in support of operational weather forecasting. A number of sounder instruments have the ability to measure CH4 (to about ~20 ppb accuracy) and/or CO2 (to about 2-5 ppm accuracy) in the upper troposphere or above. These measurements are useful for monitoring the transport of greenhouse gases into the upper troposphere and their impact on the climate, but generally provide little insight into surface sources and sinks of CO2 and CH4.
Most prevalent GHGs
Carbon Dioxide (CO2)
Carbon dioxide is the largest contributor to global warming, and its global atmospheric concentration is at the highest levels ever recorded. This GHG is present in the atmosphere from both anthropogenic release like burning fossil fuels and deforestation, as well as through natural processes like respiration, volcanic activity and ocean outgassing. Carbon dioxide has a residence time of 300 to 1000 years, and humans have increased the amount of CO2 in the atmosphere by 47% since the industrial revolution. It is held out of the atmosphere in plants thanks to photosynthesis, but deforestation releases their stores back into the atmosphere. 1)
Methane is much less abundant than carbon dioxide in the atmosphere, but it has a 100-year global warming potential 80 times that of CO2. ‘Global warming potential’ is a measure of how much energy a gas can absorb, and thus how much it warms the Earth. Approximately 15 years ago, researchers observed an increase in atmospheric methane, which in recent years has accelerated into a surge. Of the 1.18°C increase in global temperatures since the industrial revolution, approximately a third of it is due to methane. Atmospheric methane’s growth has been increasing more in recent years, and scientists are still trying to figure out why. 1) 3) 5) 8)
Approximately 60% of global CH4 emissions are due to anthropogenic release, with the main sources coming from the oil and gas industries, agriculture, landfills, wastewater treatment, and mining. Fossil fuels emit an estimated 110 million tonnes of methane every year. Despite CH4’s relatively short residence time (approximately nine years 4)), its higher global warming potential than CO2 and constant anthropogenic replenishment make methane management highly important. However this short residence time means that cutting down on methane emissions could slow the short-term pace of global warming. 3)
Unearthing the methane mystery will evaluate the possibility for a ‘methane bomb’ - a feedback loop where a warmer planet emits more methane naturally, ramping global temperatures up further. Scientists are searching the Earth for methane sources with the aid of satellite data. We know the increase in methane emission comes from microbial sources, like wetlands, cattle and landfills, and satellite observations are being used to identify how culpable each of these sources are. The sources of the increased methane emissions may be natural, but they are fuelled by a climate warmed by humans. 8)
Satellites are capable of identifying methane ‘ultra-emitters’ - sources emitting more than 25 tonnes or more of the GHG per hour. ESA’s Tropospheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5P (Precursor) mission observed more than 1800 ultra-emitting events, some releasing hundreds of tonnes of GHGs per hour with plumes spanning hundreds of kilometres. 13)
Nitrous Oxide (N2O)
Nitrous Oxide is a GHG that is approximately 100 times more potent per point than carbon dioxide over 100 years, despite only occupying approximately six percent of global GHG emissions. This gas is emitted through fossil fuel combustion, industrial activities, agricultural soil management, and manure management. N2O is also a stratospheric-ozone-depleting substance (ODS). 1) 6) 14)
Water vapour (H2O)
This potent GHG is responsible for most of the Earth’s greenhouse effect, as it is the most abundant GHG in the atmosphere. It absorbs heat radiated from Earth’s surface in the lower atmosphere, and as the atmosphere warms it can hold more water vapour, which leads to further absorbed heat and the creation of a feedback loop, just like with the ‘methane bomb’. Water vapour is not monitored to the same degree as the other GHGs, as it is not directly emitted by anthropogenic release. 1) 14)
CFCs are GHGs with massive global warming potential. They can be tens of thousands more powerful than CO2 at warming the Earth, as well as being an stratospheric ODS. The production and consumption of CFCs became regulated in 1987 by the United Nations Montreal Protocol treaty, shortly after the discovery of the Antarctic ozone hole in 1985. 1)
Ozone is a GHG found in both the stratosphere (upper atmosphere) and troposphere (lower atmosphere). Stratospheric ozone protects life on Earth from the Sun’s UV radiation, but tropospheric ozone is a powerful GHG and air pollutant that can affect humans with respiratory illnesses. Stratospheric ozone is formed between natural chemical reactions between UV light from the Sun and oxygen in the atmosphere, whereas tropospheric ozone is formed primarily through the reactions between volatile organic compounds (VOC), also known as hydrocarbons, and nitrogen oxide gases. Fossil fuel combustion is a primary source of VOCs that lead to tropospheric ozone production. 1)
The applications of remotely sensed satellite data are immense, creating a vast range of data products for monitoring GHGs. This section will cover the most widely used products.
Satellites can measure the concentration of GHGs on a global scale, enabling near-real time monitoring of anywhere on Earth. GHGSat provides an emissions intelligence platform, SPECTRA, that shows concentrations of methane around the world with high spatial and temporal resolutions. The map is based on data from the Tropospheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5P mission.
The map offers key insights into current emission events, including the emission source, location and rate. Many layers and filters can be applied to the map, including methane hotspots, flares, predictions, and data from GHGSat satellites and other third party missions. 15)
Greenhouse gas concentration maps are one of the most valuable tools for monitoring and combating climate change, by identifying regions with high GHG levels, and also tracking GHG movement though the atmosphere over time.
Satellite observations can be used to identify GHG emission rates, sources, hotspots and sinks. These maps are important for pinpointing the sources of GHG emissions and evaluating their size and potential impacts on the climate. Long-term repeat satellite observations are used to define emission rates, by measuring the outputs from sources over time. 16)
Climate Trace’s emission map is a global GHG emissions map that estimates emission rates at their sources. The tool pinpoints emission source (CO2, CO2e, CH4, N2O), type (e.g. oil and gas, agriculture, forestry), location, and size.
‘CO2e’ stands for ‘carbon dioxide equivalent’, and represents the impact of a GHG in terms of CO2’s global warming potential. The measure was created by the UN Intergovernmental Panel on Climate Change (IPCC) in order to make the effects of different GHGs comparable. 17)
These maps are also used to identify ‘hotspots’ - areas of high GHG emission rates. Satellites like ESA’s Copernicus Sentinel missions are capable of detecting individual methane plumes from sources like leaks in natural gas pipelines. The release of methane during natural gas transportation was previously accredited to small, unintended emissions by defective installations. But thanks to high resolution satellite data, scientists are observing frequent and intentional methane release, known as ‘venting’.
The environmental intelligence company, Kayrros, combined data with both Copernicus Sentinel-5P and Sentinel-2 missions, with artificial intelligence (AI) algorithms to detect 13 ultra-emitting methane events along the Yamal-Europe pipeline.
Air Quality Monitoring
Another use of GHG satellite data is for monitoring air quality and pollution. Satellites in geostationary orbits like NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission and ESA’s Copernicus Sentinel-4 mission, or constellations of low-Earth orbiting (LEO) satellites like the GHGSat Constellation can provide a comprehensive and near-real time view of air quality on a global scale. Monitoring air quality is imperative for monitoring anthropogenic GHG emissions as well as for public health and environmental policy-making. Repeat observations of air quality will allow scientists to observe how pollution episodes evolve over time, and how they interact with weather. 18)
NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission is a spaceborne instrument mission onboard the Intelsat 40e satellite
Global GHG Mappers
Copernicus: Sentinel-5P (Precursor)
Sentinel-5P is an atmospheric satellite mission operated by ESA and the Netherlands Space Office (NSO) that serves both wide area coverage and facility scale GHG monitoring. Launched in October 2017, the mission succeeds the EnviSat mission and is the first Copernicus mission dedicated to monitoring the atmosphere. With its Tropospheric Monitoring Instrument (TROPOMI), Sentinel-5P performs atmospheric measurements with high spatio-temporal resolution for monitoring air quality, ozone & UV radiation, methane, and climate monitoring & forecasting. Sentinel-5 will be launched as an instrument onboard the Meteorological Operational - Second Generation (MetOp-SG) satellites from 2024.
Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) Mission
CO2M is an ESA mission that will be the first mission to measure carbon dioxide in the atmosphere specifically from anthropogenic release. The European Union (EU)-backed mission is part of the Copernicus Sentinel Expansion, and will measure CO2, CH4 and N2O with high spatial resolution to accurately measure anthropogenic emissions. CO2M is a planned constellation of three satellites: CO2M-A, -B, and -C, launching in December 2025, March 2026, and late 2026, respectively.
Greenhouse gases Observing Satellite (GOSAT) / Ibuki
The Greenhouse gases Observing Satellite (GOSAT) constellation of JAXA (Japan Aerospace Exploration Agency) consists of three spacecraft, GOSAT-1, GOSAT-2, and GOSAT-GW (Water Cycle). GOSAT is a Global Change Observation Mission (GCOM), launched in January 2009, to monitor GHG sources and sinks on a sub-continental scale with its Fourier Transform Spectrometer (FTS) and Cloud and Aerosol Imager (CAI). Launched in October 2018, GOSAT-2 succeeds GOSAT as a mission dedicated to monitoring GHGs in the atmosphere including CO2, CH4, O3, CO, NO2, and H2O. The third mission in the constellation, GOSAT-GW, which plans to launch in March 2024, will further monitor GHGs as well as improve our understanding of the water cycle.
Methane Remote Sensing Lidar Mission (MERLIN)
MERLIN is a planned collaborative minisatellite climate mission between the German (DLR) and French (CNES) space agencies, which aims to provide global coverage of methane measurements with a lidar instrument.
OCO (Orbiting Carbon Observatory)
OCO is a series of NASA missions dedicated to the study of atmospheric CO2 and O2 sources and sinks over time, with OCO’s (failed) launch in February 2009, OCO-2 succeeding in July 2014, and OCO-3 launched to the International Space Station (ISS) in May 2019.
Aqua is a collaborative climate satellite mission between NASA, JAXA, and the National Institute for Space Research, Brazil (INPE), launched in May 2002. The mission studies Earth’s interconnected processes and water cycle, measuring CO2 and CH4 in the middle troposphere, and the atmosphere’s response.
FengYun-3D and -3H
Of the FengYun-3 (FY-3) CMA/NSMC (China Meteorological Administration/National Satellite Meteorological Center) satellite constellation, FY-3D and -3H are key missions for global GHG mapping. FY-3D launched in Nov 2017, and the next-generation FY-3H plans to launch in 2024. Both missions target CO2 and CH4 concentrations in the global atmosphere, each carrying a Greenhouse Gas Absorption Spectrometer (GAS) capable of measuring CO2, CH4, CO, & N2O in Near-Infrared (NIR) & Shortwave-Infrared (SWIR).
Facility Scale Plume Monitors
GHGSat of GHGSat Inc. is a satellite constellation that quantifies GHG emissions from point sources around the world. Launched from June 2016 with the demonstrator, GHGSat-D, the constellation of nine satellites - with plans for more at the end of 2023 - focuses on detecting CH4 emissions from anthropogenic sources to provide actionable insights for carbon-intensive industries to mitigate emissions.
EnMAP (Environmental Monitoring and Analysis Program)
EnMAP is a German hyperspectral satellite mission launched in April 2022 that aims to monitor the Earth’s surface and atmosphere, for applications including the assessment of ecosystem statuses and responses to environmental changes like anthropogenic CO2 release.
Global Hyperspectral Observation Satellite (GHOSt) Constellation
Launched from April 2023, the commercial constellation of hyperspectral microsatellites by Orbital Sidekick Inc. (OSK) plans to gather point-source GHG emission data globally, providing valuable insights for a diverse set of customers.
Carbon Mapper is a planned nonprofit satellite mission that aims to pinpoint, quantify and monitor CO2 and CH4 emissions at the facility-scale level, promoting global accessibility and transparency of the GHGs in our atmosphere.
EMIT (Earth surface Mineral dust source InvesTigation)
EMIT is a NASA mission onboard the ISS since July 2022 that measures the composition of minerals in Earth’s atmosphere to investigate how they affect our climate.
CO2Image is a German Aerospace Centre (DLR) mission planned to launch in 2026, and aims to monitor facility-scale CO2 emissions while complementing Copernicus GHG missions like CO2M.
Aurora, the precursor mission to the GHOSt constellation, is a hyperspectral imaging satellite launched in June 2021 that aims to measure point source GHG emissions including CO2, CH4, and N2O, providing real-time insights into their customers’ emissions.
PRISMA is an Italian Space Agency (ASI) hyperspectral imaging satellite that launched in March 2019, aiming to provide a global observation capability for monitoring natural resources and atmospheric characteristics like the carbon cycle, as well as quantifying point source GHG emissions.
Aqua AIRS (Atmospheric Infrared Sounder) instrument
Among the Aqua mission’s suite of instruments is AIRS, an infrared sounder that makes global measurements of Earth’s atmosphere. Observations between AIRS, the Advanced Microwave Sounding Unit (AMSU), and the Humidity Sounder for Brazil (HSB) onboard Aqua are together used to study the effects of increased GHG emission, looking at land surface temperatures and infrared spectral emissivity.
Meteorological Operational satellite-Second Generation (MetOp-SG)
MetOp-SG is a planned constellation of six meteorological satellites, succeeding the MetOp series from 2024. The ESA and EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) mission will acquire diverse measurements over land, ocean and atmosphere for long-term climate and weather forecasting. The constellation will consist of two satellites: MetOp-SG-A and -B.
MetOp consists of three meteorological satellites of ESA and EUMETSAT, MetOp-A launched in 2006 and retired in 2021, followed by MetOp-B and -C launched in 2012 and 2018 respectively. MetOp-B and -C acquire diverse land, ocean and atmospheric measurements with their Infrared Atmospheric Sounder Interferometer (IASI) instruments, for weather nowcasting & forecasting, and climate applications. Profiling the middle atmosphere to categorise CO2, CH4, and N2O concentrations allows MetOp to contribute data toward global climate and GHG monitoring efforts.
Suomi National Polar-orbiting Partnership (NPP)
Suomi NPP is a weather satellite launched in October 2011 to advance Earth system science and support operational weather forecasting, with measurements made over land, ocean and atmosphere including CO2 and CH4 concentrations in the middle troposphere.
SciSat-1 / ACE (Science Satellite/Atmospheric Chemistry Experiment)
SciSat-1 is a Canadian Space Agency (SCA) mission launched in August 2003 that houses the ACE-Fourier Transform Spectrometer (ACE-FTS), a limb-scanning infrared spectrometer that has measured the change in atmospheric composition of 44 gases as a function of latitude, longitude and altitude for 20 years.
SciSat-1 is a Canadian Space Agency (SCA) mission launched in August 2003 that has measured the change in atmospheric composition of 44 gases as a function of latitude, longitude and altitude for 20 years, with the ACE-Fourier Transform Spectrometer (ACE-FTS).
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).