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.
Part of a series on the |
Carbon cycle |
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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:
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.[citation needed]
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 (XCO2) 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.[18] 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 XCO2, space-based observations may also be compared with ground-based observations such as those from the TCCON.[19]
Instrument/satellite | Primary institution(s) | Service dates | Approximate usable daily soundings |
Approximate sounding size |
Public data | Notes | Refs |
---|---|---|---|---|---|---|---|
HIRS-2/TOVS (NOAA-10) | NOAA (U.S.) | July 1987– June 1991 |
100 × 100 km | No | Measuring CO2 was not an original mission goal | [20] | |
IMG (ADEOS I) | NASDA (Japan) | 17 August 1996– June 1997 |
50 | 8 × 8 km | No | FTS system | [21] |
SCIAMACHY (Envisat) | ESA, IUP University of Bremen (Germany) | 1 March 2002– May 2012 |
5,000 | 30 × 60 km | Yes[22] | [23] | |
AIRS (Aqua) | JPL (U.S.) | 4 May 2002– ongoing |
18,000 | 90 × 90 km | Yes[24] | [25][26] | |
IASI (MetOp) | CNES/EUMETSAT (ESA) | 19 October 2006 | 20-39 km diameter | Yes (only a few days)[27] | [28] | ||
GOSAT | JAXA (Japan) | 23 January 2009– ongoing |
10,000 | 10.5 km diameter | Yes[29] | First dedicated high precision (<0.3%) mission, also measures CH4 | [30][31] |
OCO | JPL (U.S.) | 24 February 2009 | 100,000 | 1.3 × 2.2 km | N/A | Failed to reach orbit[32] | |
OCO-2 | JPL (U.S.) | 2 July 2014– ongoing |
100,000 | 1.3 × 2.2 km | Yes[33] | High precision (<0.3%) | [34] |
GHGSat-D (or Claire) | GHGSat (Canada) | 21 June 2016– ongoing |
~2–5 images, 10,000+ pixels each |
12 × 12 km, 50 m resolution image |
available to selected partners only | CubeSat and imaging spectrometer using Fabry-Pérot interferometer | [35] |
TanSat (or CarbonSat) | CAS (China) | 21 December 2016– ongoing |
100,000 | 1 × 2 km | Yes (L1B radiances)[36] | [37][38] | |
GAS FTS aboard FY-3D | CMA (China) | 15 November 2017– ongoing[39] |
15,000 | 13 km diameter | No | [40][41] | |
GMI (GaoFen-5, (fr)) | CAS (China) | 8 May 2018– ongoing[42] |
10.3 km diameter | No | Spatial heterodyne | [43][44] | |
GOSAT-2 | JAXA (Japan) | 29 October 2018– ongoing[45] |
10,000+ | 9.7 km diameter | Yes (L1B radiances)[46] | Will also measure CH4 and CO | [47] |
OCO-3 | JPL (U.S.) | 4 May 2019– ongoing[48] |
100,000 | <4.5 × 4.5 km | Yes[49] | Mounted on the ISS | [50] |
MicroCarb | CNES (France) | expected 2022 | ~30,000 | 4.5 × 9 km | Will likely also measure CH4 | [51] | |
GOSAT-3 | JAXA (Japan) | expected 2022 | |||||
GeoCARB | University of Oklahoma (U.S.) | expected 2023 | ~800,000 | 3 × 6 km | First CO2-observing geosynchronous satellite, will also measure CH4 and CO | [52][53] |
In addition to the total column measurements of CO2 down to the ground, there have been several limb sounders that have measured CO2 through the edge of Earth's upper atmosphere, and thermal instruments that measure the upper atmosphere during the day and night.
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: