CFusion/NCEO Working Note

A UKresearch programme exploitingcarbon dioxide measurements from the NASA Orbiting Carbon Conservatory (OCO) and the JAXA Greenhouse gas Observing SATellite (GOSAT)

Authors: Paul Palmer, Mathew Williams and Shaun Quegan.

1 Introduction

The twentieth century saw a rapid rise in atmospheric CO2 concentrations, due largely to anthropogenic activity. This trend is set to continue in the twenty-first century with increased emissions from deforestation, fossil fuel burning and cement manufacture due to increased global economic activity. The current century is likely to see a range of responses from land and ocean carbon pools to the associated climate warming (e.g., net release of carbon from peat reservoirs due to melting of permafrost). It is therefore critical to have a capability to monitor and quantitatively understand the rate of change in land and ocean carbon pools.

Observed variations in atmospheric CO2 concentrations measure the net surface-atmosphere exchange. Inferences on the surface fluxes giving rise to these concentrations currently rely on inversing models of atmospheric transport and mixing, combined with CO2 concentrations measured by the ground-based network. This ill-posed problem requires regularization, typically provided by prior CO2 concentrations derived from estimates of surface CO2 fluxes inferred from in situ data, and has several problematic features: a) the sparseness of the data network, particularly over tropical landmasses; b) biases introduced by inaccurate modelling of atmospheric transport and mixing processes; c) covariance between surface fluxes and boundary layer meteorology (rectification artefacts). Diffusive atmospheric transport amplifies these uncertainties. Recent improvements in the sampling density from ground-based and aircraft-based instrumentation have improved flux estimates over northern hemisphere landmasses. However, the tropical landmasses, thought to be large sinks of CO2, remain one of the most under-sampled regions in the world for carbon cycle research. Space-borne measurements of CO2 have the potential to fill these gaps, providing global coverage and daily observations. Although less accurate than in situ measurements and challenging to use, these data seem certain to offer new quantitative insights into the working of the global carbon cycle.

NERC is well placed to contribute significantly to the exploitation of CO2 data from two new space-based missions to be launched in 2008: the NASA Orbiting Carbon Observatory (OCO) and the JAXA Greenhouse gas Observing SATellite (GOSAT). OCO and GOSAT are the first missions dedicated to measuring column CO2 with sufficient precision to test current quantitative understanding of the global carbon cycle. UK involvement with these new data is a natural progression from pioneering UK work on interpreting space-based CO2 measurements from SCIAMACHY and AIRS, and builds on decades of UK research on carbon cycle measurements, satellite remote sensing, biomass burning, terrestrial and ocean biosphere modelling, atmospheric transport and data assimilation. The UK’s contribution to these missions should also include ground-based and aircraft measurements of CO2 and other trace gases as part of the calibration and validation activities critical to the success of OCO and GOSAT. The expertise and leadership embodied within the National Centre for Earth Observation (NCEO) and its research partners places the UK in an enviable position as regards reaping science and applications benefit from use of these instruments. Failure to work effectively with these data is likely to put the UK far behind internationally in both carbon cycle research and our ability to provide quantitative estimates of carbon fluxes.

The document is divided into sections related to the major steps within the work-plan in Figure 1, and includes a set of recommendations intended to allow the UK to take a leading international role in exploiting the OCO and GOSAT data for C cycle research and monitoring. Section 2 concerns the science questions to be addressed using OCO and GOSAT. Section 3 describes the OCO and GOSAT missions and the associated data products. Section 4 outlines calibration/validation activities in support of OCO and GOSAT. Section 5 deals with surface flux and atmospheric transport models. Section 6 outlines the use of data assimilation to recover surface flux maps of CO2 from OCO and GOSAT observations. Section 7 concerns improvement in source attribution by combining XCO2 data with in situ and other remotely sensed data. Section 8 is a summary of UK capabilities and international collaborations in support of OCO and GOSAT.

Figure 1: Schematic of workplan for processing and interpreting CO2 measurements from OCO and GOSAT.

2 Outstanding Questions and Uncertainties in Carbon Cycle Science

Global coupled carbon-cycle climate models disagree on the strength of carbon-climate coupling (Friedlingstein et al 2006), reflecting fundamental gaps in our knowledge of C cycling. These differences between models are clear indicators of major uncertainty in predictions of the future of the Earth system. Most information about the C cycle is gathered at either global scales or local scales (e.g. eddy flux towers). Regional datasets, lying between these scales, are sparser and more uncertain(by regional, we mean observations that sample the seasonal activity of an ecoregion, such as the Amazon basin, Siberian taiga, Alaskan tundra and African deciduous woodlands (~106 km2)). Lack of regional data is one of the major issues preventing severe testing and subsequent winnowing of the coupled models.

Atmospheric inventory and fossil fuel burning estimates provide strong constraints on the C cycle at the global scale (Table 1). At the regional scale the constraints are weaker. This is related to sparsity of the CO2 network and fire records; equatorial and tropical ecosystems are particularly poorly observed. The largest weaknesses relate to: a) quantifying carbon emissions from fires in terms of long-lived carbon stores lost to the atmosphere and global and regional flux estimates, b) regional distribution of CO2 over land and ocean, particularly over tropical ecosystems, c) land use change and deforestation, again particularly within the tropics, and d) the magnitude of air-sea fluxes of CO2. Given the precision requirements on the XCO2data from OCO and GOSAT, these data are best employed to constrain surface fluxes of CO2due to, e.g.,fire and biospheric release and uptake.

Constraints on Global C Budget /
Data
/
Strength of Constraint (uncertainty)
Atmospheric C inventory and its rate of change
/ Atmospheric concentration record / Strong (<5%)
C emissions from fossil fuels and cement manufacture /
Carbon Dioxide Information Analysis Centre (CDIAC)
/ Strong (<5%)
Anthropogenic C in the oceans / Ocean surveys (GEOSECS, WOCE, ongoing repeat sections) / Strong (<5%)
Patterns in atmospheric CO2 and inverse modeling of atmospheric transport / Atmospheric CO2 records (NOAA/ESRL and other networks) / Intermediate (~50%)
Air-sea partial pressure differences / Atmospheric CO2 and ocean surface pCO2 /
Intermediate for magnitudes (~50%)
Good for spatio-temporal patterns
Fire emissions /
Ground-based forest and peat records
Atmospheric records (CO2, CO, 13CO2, CH4, COS, O2/N2)
Global Fire Emission Database / Intermediate (~50%)
Deforestation (mainly tropics) / Remote sensing and atmospheric records (CO2) /
Weak (~100 %)
Forest changes / Forest censuses /
Strong in N America, Amazon, Europe. Elsewhere weak or missing.
Soils
/ Inventories / Weak
Peat
/ Inventories / Weak

Table 1. Characterizing the strength of various constraints on the global C budget (M. Gloor).

The poor constraints at the regional scale give rise to a number of key outstanding science questions associated with the carbon cycle:

  • What is the relative importance of the land-biosphere CO2 fluxes over different latitudes? There is still argument about the location of hypothesized terrestrial C sinks.
  • Which processes control land-biosphere fluxes in different ecoregions, how do these processes operate and how will they respond to climate change?
  • How much carbon is emitted from biomass burning, deforestation and land use change?
  • What is the magnitude and distribution of air-sea exchange of CO2?
  • How much will large stores of C in northern high latitudes contribute to future carbon emissions?

The overarching objective of this document is to define a UK programme to resolve these questions using novel measurements of atmospheric CO2 from OCO/GOSAT. This programme will provide the tools to exploit OCO/GOSAT data and combine these with state-of-the-art models and surface and aircraft observations. The outcome will be improved understanding of processes that control the distributions of CO2 and ensuing improvements in attribution and prediction of atmospheric CO2. Such a programme will focus a wide range of NERC expertise on unlocking the potential of the OCO and GOSAT missions, thus enhancing the scientific return of NERC-funded carbon cycle research and the profile of NERC in the international arena. In this document we outline the science, the existing UK expertise, and the current gaps and opportunities associated with using CO2 measurements from space.

3 The OCO and GOSAT Missions

Both missions are due to be launched in late 2008, acquiring spectroscopic measurements of CO2 and O2 allowing the column integrated CO2 dry air mole fraction, XCO2, to be estimated at precisions of 1 ppm (0.3% of column) on regional scales (8ox10o).

OCO

The OCO mission is dedicated to measuring space-based CO2 columns at sufficient precision to identify CO2 sources and sinks on regional scales over the globe and quantify their variability over the seasonal cycle.It has a 2-year design lifetime with launch scheduled for December 2008. OCO will join the EOS Afternoon Constellation (A-Train), flying in a sun-synchronous polar orbit with a constant 1:26 p.m. local solar time flyover, a 16-day (233 km altitude orbit) repeat cycle and near global sampling.

The OCO instrument will measure near-infrared, high-resolution spectra of the O2 A-Band and two CO2 bands in nadir and glint geometry, thus providing high signal-to-noise observations over land and ocean. It will acquire 4-8 cross-track observations with 3 Hz sampling frequency, resulting in 200-400 observations per degree of latitude along-track and 7 to 14 million observations every 16 days. The horizontal resolution of each measured scene will be less than 10 km2, even at large solar zenith angles.

Calibrated Level 1 spectra and noise estimates of all soundings will be available from NASA. The spectra will first be analyzed with a fast semi-analytic algorithm to obtain cloud and aerosol information as well as an estimate of XCO2. A subset (~2% of all soundings) of the cloud-free spectra will then be analyzed with a full-physics, optimal-estimation algorithm that will provide XCO2 retrievals together with error estimates and averaging kernels, as well as estimates of all other retrieved parameters, such as surface pressure and aerosol optical depth.

GOSAT

The Japanese GOSAT mission is led by JAXA, the National Institute for Environmental Studies (NIES) and the Ministry of the Environment (MOE), and will be launched in 2008 with a nominal lifetime of four years. Its main objective is to halve current sub-continental scale CO2 annual flux estimation errors. GOSAT will be launched into a sun-synchronous orbit at 666 km height and an equator crossing time of 1 pm, similar to that of OCO. As compared to OCO, GOSAT offers both CO2 and CH4 with greater coverage of the Earth, but at slightly lower spatial resolution (the issue of resolution is discussed further below).

GOSAT will operate in nadir and glint modes, using a Fourier transform spectrometer (FTS) and a 4-channel camera with 1 km resolution (ch1: 0.38 μm, ch2: 0.67 μm, ch3: 0.87 μm, and ch4: 1.61 μm) for cloud detection. The FTS will also have 4 channels, three in the near infrared and one in the thermal infrared. Channel 1 (0.75-0.78 μm) will cover the 0.76 μm O2 A-band at short wavelengths, channel 2 (1.56-1.72 μm) the 1.6 μm CO2 and CH4 bands, and channel 3 (1.92-2.08 μm) the 2 μm CO2 band at a maximum resolution of 0.2 cm-1. The TIR band (5.5-14.3 μm) will cover some of the long wavelength CO2 and CH4 bands, plus ozone and water vapour. The spatial resolution is 10 km2 on the ground. The estimated signal-to-noise ratio is 300. GOSAT can measure at night using the TIR channels, and will monitor the polarization of the reflected and emitted radiation from the earth, providing additional information about clouds and aerosols. The “operational” GOSAT data products will be total column densities for CO2 and CH4, derived primarily from the NIR data, as well as atmospheric profiles derived mainly from the TIR data. The NIR and TIR will be used synergistically to improve the retrievals.

Effective temporal and spatial resolution

The temporal and spatial resolutions quoted for the OCO and GOSAT sensors are for a single observation. However, the important quantities from the point of view of using the data are their effective resolutions in time and space after averaging or compositing the data. It is not obvious from the available literature what these resolutions will be and what will be their finest possible values, and we need greater clarity about this.

Data access

OCO Level 1 and Level 2 data will be delivered to a NASA Distributed Active Archive Center (DAAC) for open distribution no later than 6 months and 9 months after launch, respectively. Members of the OCO science team will have early access to the data for calibration and validation purposes. NASA Headquarters plans to release a NASA Research Announcement (NRA) for participation in the OCO science team within the NASA Roses 2008 program to support cal/val activities. Such announcements usually require a designated PI for each proposed investigation, but often allow multiple investigators on each team. A successful proposal to this NRA by a team from NCEO would provide early access to OCO data.

The ESA third-party agreement is not yet finalized, but ESA will keep us informed on progress.

An invitation to jointhe GOSAT science team will be announced at the EGU in April.

Recommendations

3.1 The UK should identify individuals or groups charged with acting as the UK points of contact with OCO and GOSAT, who will know how to access the datasets and have detailed knowledge of their properties, especially instrument and retrieval errors properties, sampling and averaging issues. We must if possible ensure participation in the GOSAT science team.

4 Calibration and validation

The unprecedented precision required for the total CO2 column measurements makes extensive cal/val imperative if OCO and GOSAT are to meet their mission objectives. Expertise within the UK will allow NERC to make a major contribution to cal/val, and thus benefit from early access to OCO and GOSAT data, as well as science team hardware and software knowledge that will greatly improve our understanding of the data.

Ground-Based Instruments

The OCO team has installed a small network of Fourier Transform spectrometers (FTS) that observe the solar spectrum after passage of the direct beam through the atmospheric column. This network, called the Total Carbon Column Observing Network (TCCON, is dedicated to making accurate and precise measurements of CO2 and O2 in support of OCO and GOSAT. Figure X shows the sites of current, future and possible sites within TCCON.

The FTS instruments have been placed at locations that provide information on aerosols (e.g. Atmospheric Radiative Measurements sites [ because space-based retrievals of CO2 are sensitive to optical depth and aerosol scattering. The satellite instruments will be pointed at a specific ground-site and measure an atmospheric spectrum using scattered sunlight and/or thermal emission; at the same time, the ground-based FTS will measure a solar absorption spectrum in the same spectral interval, but at higher spectral resolution. The vertical columns of CO2 and CH4 measured by the satellite and the FTS will then be compared using the same atmospheric forward model and the same spectroscopic parameters.

Mention should also be made of the NOAA ESRL flask network ( that currently provides the ground-based observations supporting atmospheric inversion. It seems likely that OCO and GOSAT will be used alongside the flask data in any scientifically mature carbon data assimilation scheme.

Aircraft campaigns

The OCO science team has recently prepared a science case for the Carbon Observatory Validation Experiment (COVE). This will deploy in situ sensors on two aircraft: a high-altitude platform that can reach the tropopause (ER-2 or WB-57B) and an aircraft that will routinely measure trace gases in the boundary layer, focusing on CO2 and other species measurable by the FTS (CH4, CO, O3). The high-altitude platform will also carry the GOSAT airborne simulator, a cloud imager and aerosol lidar, and a sun photometer capable of measuring in the near IR. These in situ concentration data will be used to sample atmospheric profiles of key species from the surface to the mid-stratosphere, with sampling anchored at the FTS sites at Park Falls, WI and the Oklahoma ARM site. The GOSAT simulator will provide a view of the spectra observed by the space-borne platforms, but with the geometry controlled by the experiment. The aerosol and cloud data will inform level-1 retrieval algorithm developers and testers. COVE will likely take advantage of aircraft deployments already planned for late 2009 or early 2010.

Recommendations

4.1 Build on UK FTS strengths to establish FTS measurements at one or more sites within the UK and possibly at the NERC-fundedCape Verde atmospheric observatory ( The latter would be particularly important, since there are no confirmed equatorial FTS deployments in TCCON, despite the importance of tropical ecosystems in determining global CO2 concentration.

4.2 Use NCEO influence to raise the priority of OCO and GOSAT cal/val in NCAS activities, including inclusion in the EUFAR proposal, and formulate plans to link aircraft measurements to ground-based data, flux modelling and satellite observations of CO2 and related species.

5 Relating atmospheric column CO2 concentration measurements to surface fluxes

Relating surface CO2 fluxes to global 3-D concentrations of CO2 mole fraction in either forward or inverse mode requires prior estimates of surface fluxes (often derived from a model) together with an atmospheric transport model (here under the more general banner of chemistry-transport models [CTMs]).