Space-based measurements of carbon dioxide

Artist's conception of OCO-2, the second successful high precision (better than 0.3%) CO₂ observing satellite.

Space-based measurements of carbon dioxide (CO₂) are used to help answer questions about Earth's carbon cycle. There are a variety of active and planned instruments for measuring carbon dioxide in Earth's atmosphere from space. The first satellite mission designed to measure CO₂ was the Interferometric Monitor for Greenhouse Gases (IMG) on board the ADEOS I satellite in 1996. This mission lasted less than a year. Since then, additional space-based measurements have begun, including those from two high-precision (better than 0.3% or 1 ppm) satellites (GOSAT and OCO-2). Different instrument designs may reflect different primary missions.

Purposes and highlights of findings

There are outstanding questions in carbon cycle science that satellite observations can help answer. The Earth system absorbs about half of all anthropogenic CO₂ emissions.^[1] However, it is unclear exactly how this uptake is partitioned to different regions across the globe. It is also uncertain how different regions will behave in terms of CO₂ flux under a different climate. For example, a forest may increase CO₂ uptake due to the fertilization or β-effect,^[2] or it could release CO₂ due to increased metabolism by microbes at higher temperatures.^[3] These questions are difficult to answer with historically spatially and temporally limited data sets.

Even though satellite observations of CO₂ are somewhat recent, they have been used for a number of different purposes, some of which are highlighted here.

• Megacity CO₂ enhancements were observed with the GOSAT satellite and minimum observable space-based changes in emissions were estimated.^[4]
• Satellite observations have been used for visualizing how CO₂ is distributed globally,^[5] including studies that have focused on anthropogenic emissions.^[6]
• Flux estimates were made of CO₂ into and out of different regions.^[7][8]
• Correlations were observed between anomalous temperatures and CO₂ measurements in boreal regions.^[9]
• Zonal asymmetric patterns of CO₂ were used to observe fossil fuel signatures.^[10]
• Emission ratios with methane were measured from forest fires.^[11]
• CO₂ emission ratios with carbon monoxide (a marker of incomplete combustion) measured by the MOPITT instrument were analyzed over major urban regions across the globe to measure developing/developed status.^[12]
• OCO-2 observations were used to estimate CO₂ emissions from wildfires in Indonesia in 2015.^[13]
• OCO-2 observations were also used to estimate the excess land-ocean flux due to the 2014–16 El Niño event.^[14]

Challenges

Remote sensing of trace gases has several challenges. Most techniques rely on observing infrared light reflected off Earth's surface. Because these instruments use spectroscopy, at each sounding footprint a spectrum is recorded—this means there is a significantly (about 1000×) more data to transfer than what would be required of just an RGB pixel. Changes in surface albedo and viewing angles may affect measurements, and satellites may employ different viewing modes over different locations; these may be accounted for in the algorithms used to convert raw into final measurements. As with other space-based instruments, space debris must be avoided to prevent damage.

Water vapor can dilute other gases in air and thus change the amount of CO₂ in a column above the surface of the Earth, so often column-average dry-air mole fractions (X_CO2) are reported instead. To calculate this, instruments may also measure O₂, which is diluted similarly to other gases, or the algorithms may account for water and surface pressure from other measurements.^[15] Clouds may interfere with accurate measurements so platforms may include instruments to measure clouds. Because of measurement imperfections and errors in fitting signals to obtain X_CO2, space-based observations may also be compared with ground-based observations such as those from the TCCON.^[16]

List of instruments

There have been other conceptual missions which have undergone initial evaluations but have not been chosen to become a part of space-based observing systems. These include:

• Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS)^[39]
• Geostationary Fourier Transform Spectrometer (GeoFTS)^[40]

References