Satellite observations of carbon dioxide: Why are they important; and what CO2 data from different NASA missions tell us
NASA GES DISC provides access to current satellite archive of atmospheric CO2 data while awaiting launch of OCO-2
CarbonTracker (carbontracker.noaa.gov) xCO2 column average over the entire atmospheric column for the year 2010.
Advances in CO2 Observations from AIRS and ACOS, by J. Wei, A. Savtchenko, B. Vollmer, T. Hearty, A. Albayrak, D. Crisp, and A. Eldering, IEEE Geoscience and Remote Sensing Letters, 11(5), doi: 10.1109/LGRS.2013.2281147 (click to access the manuscript)
Natural processes, such as photosynthesis and respiration, volcanic eruptions, chemical reactions, and dissolution have kept carbon dioxide (CO2) and the other major greenhouse gases (e.g., methane (CH4), nitrous oxide (N2O), and ozone, (O3)) in stable concentrations for thousands of years. Starting with the industrial era in the late 18th century, the burning of fossil fuels for energy, population growth, and the intensification of agriculture and deforestation have caused the concentrations of aforementioned greenhouse gases to increase at unprecedented rates (Forster et al. 2007).
Among all greenhouse gases emitted by human activities, CO2 produces the largest radiative forcing – and, thus, the greatest potential influence on Earth's climate. Radiative forcing is used to assess and compare the anthropogenic and natural drivers of climate change. The concept of radiative forcing was formulated in early studies of the climate response to changes in CO2, and has proven to be particularly applicable to the assessment of the climate impact of long-lived greenhouse gases (Ramaswamy et al. 2001). Models and observations agree that climate change, signified by increasing global temperatures, melting polar ice caps and glaciers, and rising sea levels, is closely related to the accelerated rates of accumulation of greenhouse gases in the atmosphere. CO2 is also a very stable gas, and it may take hundreds of years for natural processes to return CO2 concentrations to pre-industrial levels.
Because of its extremely long atmospheric lifetime, CO2 measured locally will generally approximate global concentrations. The rising trend of CO2 concentrations (Fig. 1a), measured at Mauna Loa (Tans and Keeling 2012), therefore represents well the increase in global atmospheric CO2 concentrations. Figure 1b shows the location of the Mauna Loa Observatory on the island of Hawaii in the North Pacific Ocean, where the CO2 record has been compiled.
Figure 1. (a, top) CO2 concentrations (monthly averages) at Mauna Loa, Hawaii, in the past 55 years (Tans and Keeling, 2012). (b, bottom) The Mauna Loa observatory is located on the slopes of Mauna Loa volcano on the island of Hawaii, in the central North Pacific Ocean. (Map courtesy of Wikipedia Commons.)
Local measurements, however, are not sufficient to understand the regional sinks and sources of CO2 at the surface. Neither can local measurements provide an accurate global picture of accumulations of CO2 in the mid-troposphere. The oceans constitute a net sink for CO2 by dissolving about two gigatons (GT) of CO2 per year, which is about 1/3 of the anthropogenic CO2 pumped into the atmosphere (Raven and Falkowski 1999). While this is a fortunate net result for Earth’s climate, large regions of the ocean surface are nevertheless releasing CO2 into the atmosphere, indicating the importance of tracking and understanding the dynamics of sources and sinks of CO2 at regional scales.
To address these issues, researchers require ground-based, airborne and spaceborne measurements. Even though ground and airborne measurements of CO2 have existed since 1958, these data contain large spatial gaps, which can only be rectified by observations of Earth's atmosphere from space. The complexity of making accurate atmospheric CO2 observations from space, however, has made them possible only very recently.
Complementary satellite-derived CO2 records from the (Atmospheric Infrared Sounder (AIRS), onboard NASA’s Aqua satellite, and from the TANSO-FTS instrument, onboard the Japanese GOSAT (Greenhouse gases Observing Satellite – IBUKI in Japanese) have been available since 2002 and 2009, respectively. Later this year, NASA is preparing to launch the second Orbiting Carbon Observatory (OCO-2), a follow-up spacecraft to OCO-1, which was unfortunately lost at launch in 2009.
As a result of a NASA and Japanese Space Agency (JAXA) collaboration, radiances acquired by TANSO-FTS have been input to OCO algorithms, in preparation for the OCO-2 mission. Both agencies independently offer their results on atmospheric CO2 concentrations for the benefit of researchers around the world. This NASA and JAXA collaborative effort is known as the Atmospheric CO2 Observations from Space (ACOS) project and is managed by the Jet Propulsion Laboratory (JPL).
The Goddard Earth Sciences Data and Information Services Center (GES DISC) at NASA Goddard Space Flight Center (GSFC) is archiving and distributing all AIRS and ACOS data, and will be the source of upcoming releases of CO2
data from OCO-2. All of these data records are searchable by various criteria (temporal, spatial, event), via the Mirador search engine (http://mirador.gsfc.nasa.gov/
), and are available online for free download by FTP, and also with more advanced methods such as OPeNDAP (Open-source Project for a Network Data Access Protocol).
While it is easy to acquire, open, and read AIRS and ACOS data files, caution needs to be exercised when analyzing and interpreting these data, as is always the case with sensors working in different spectral bands and utilizing different technologies (Fig. 2 and Table 1). AIRS and TANSO-FTS are two very different instruments, with different sensitivities to various parts of the atmospheric column resulting from their spectral differences. In contrast to TANSO-FTS (the ACOS retrievals), AIRS CO2 retrievals utilize the 15 mm region of the spectrum (Fig. 2), where strong absorption does not allow the retrieval of information from surface layers. Moreover, because ACOS algorithms use reflected sunlight, these CO2 measurements (and those from the OCO-2) are all obtained during daytime only, while the AIRS measurements are derived from both daytime and nighttime observations.
Figure 2. The mid-infrared spectrum of clean air in a 24 m cell. Main absorption bands of target gases CO2, carbon monoxide (CO), and water vapor (H2O) are shown. [After Howard (1959) and Goody and Robinson (1951)].
Table 1. Summary of AIRS and GOSAT/ACOS instruments and data retrieval properties.
CO2 Retrieval Spectra
15 mm (VPD algorithm)
1.6 mm (ACOS algorithm total column)
The vertical sensitivity of AIRS and ACOS retrievals is demonstrated in Fig. 3, where AIRS is seen to be most sensitive to mid-tropospheric CO2 at 5-7 km (500-400 hectoPascals, hPA), whereas ACOS is more representative of the column CO2 and surface-atmosphere fluxes.
Figure 3. Approximate vertical sensitivity to CO2 of AIRS (red curve) and ACOS (blue curve) retrievals. (Adapted from Crisp et al. 2004)
The dynamics of CO2 in the mid-troposphere are quite different from its dynamics in the total atmospheric column or in the atmospheric surface layers. Fig. 4 is adapted from Olsen and Randerson (2004), and demonstrates that, while strong diurnal and synoptic variations are present at 991 and 911 hPa, respectively, mid-tropospheric CO2 at 526 hPa is apparently decoupled from short-term variability. Furthermore, the column CO2 variability, unlike that occurring at 526 hPa, is clearly influenced by the synoptic event which takes place between days 152-161.
Figure 4. Variability of the CO2 mixing ratio in different atmospheric layers (adapted from Olsen and Randerson (2004)).
These are just a few of the difficulties hindering an improved understanding of the coupling between surface fluxes of CO2 and accumulation in the atmospheric column and thus, the achievement of a consistent picture of global CO2 distributions derived from various models and satellites. The difficulties in reconciling satellite observations and model analysis of global atmospheric CO2 concentrations are illustrated in Fig. 5, which shows examples of the 2010 average of AIRS mid-tropospheric CO2, ACOS xCO2, and the NOAA global CO2 modeling and assimilation system CarbonTracker xCO2 (CT2011). Overlain 2010 MERRA (Modern-Era Retrospective Analysis for Research and Applications) wind fields indicate the prevalent mid-to-upper tropospheric circulation at 500 hPa (Fig. 5a) and near-surface air circulation at 950 hPa (Fig. 5b). The large-scale patterns of the AIRS mid-tropospheric CO2 distribution agree well with the general circulation of the atmosphere. However, a belt of enhanced CO2 surrounding the globe in the Southern Hemisphere that can be seen in the AIRS mid-tropospheric CO2 is not observed in the ACOS xCO2 nor in the CarbonTracker xCO2. This difference requires further investigation.
In addition to differing vertical sensitivity, another difference between ACOS and AIRS data is that ACOS data are heavily weighted by retrievals over land and, hence, by the Northern Hemisphere, whereas AIRS data feature almost complete global coverage (Fig. 5). Moreover (as previously mentioned) because ACOS measurements are based on the visible part of the spectrum, all data are obtained during the daytime, while the AIRS measurements are derived from both daytime and nighttime observations. The amplitude of diurnal (24-hour) variation in CO2 near the surface can rival that of seasonal variation.
Figure 5. Global CO2 distribution from (top left) AIRS mid-tropospheric CO2 and overlain Modern Era Retrospective-analysis for Research and Applications (MERRA) 500 mb wind; (top right) ACOS xCO2 and overlain MERRA 950 mb wind; (bottom left) CarbonTracker xCO2 between 5–7 km; and (bottom right) CarbonTracker xCO2 column average over the whole column. (The results shown have not been convolved with the ACOS averaging kernels.) (Click any figure for full-size; note that the top two figures are very large at full-size).
CO2 data users should be mindful of these differences between AIRS and ACOS retrievals of CO2, and how the differences might influence their studies. For instance, ACOS data exhibit larger seasonal variability of global CO2 mixing ratios than that of AIRS (Fig. 6). Given the previously described differences in the retrievals, such a difference should be expected.
Figure 6. Time-series of globally averaged CO2 mixing ratios from AIRS Version 5 (2002-2012) and ACOS Version 2.9 (2009-2012). A subset of the ground observations record at Mauna Loa Observatory (1982-2012) (Tans and Keeling, 2012) is given for reference. (Click on the figure for a much larger full-size version).
In September 2012, the AIRS suite of instruments completed its 10th year of global observations, and the mission is still in good health. Along with its climatological records of infrared energy leaving the Earth, vertical atmospheric profiles of water vapor and temperature, cloud properties, and methane, ozone, and CO concentrations, AIRS also simultaneously and uniquely provides information on mid-tropospheric CO2. This decadal record unambiguously displays the rising planetary concentrations of this greenhouse gas.
To address the complexities in understanding the fluxes of CO2 in the global atmosphere and the importance of monitoring atmospheric CO2, NASA and scientists around the world can now anticipate the launch of the Orbiting Carbon Observatory (OCO-2) in 2014. GES DISC is prepared to undertake the new challenges in fully supporting OCO-2 and, by already distributing decadal records from AIRS, it will become a convenient one-stop hub for users to acquire global atmospheric CO2 data from satellites.
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Article by Jennifer Wei and Andrey Savtchenko, peer review by AIRS team at the NASA GES DISC. Editorial review by Bill Teng, Web preparation and editing by James Acker.
The GES DISC is a NASA earth science data center, part of the NASA Earth Science Data and Information System (ESDIS) Project.