Carbon dioxide concentrations are transported around Earth by the general circulation of the atmosphere. Based on data acquired by AIRS, the Atmospheric Infrared Sounder on NASA's Aqua Satellite.
Measuring carbon from space
7 April 2011
Understanding changes in the global atmospheric concentration of carbon dioxide (CO2) is as much a political imperative as a scientific one. But are these demands realistic with the instruments available? Paul Palmer, Hartmut Bösch and Andy Kerr explore the science and politics of measuring CO2 from space.
For both scientists and policy-makers, one of the biggest challenges is finding out where and how CO2 is released and absorbed by natural and human-influenced land and ocean processes.
Inventories of emissions from each country suggest that approximately two thirds of human-induced greenhouse gas (GHG) emissions arise from fossil-fuel emissions, and one third from agriculture, forestry and other changes in how land is used. This includes around 18 per cent from deforestation.
Based on accurate measurements at ground level, we know that on average less than half of the CO2 emitted by human activities remains in the atmosphere. The rest is apparently being taken up by the world's oceans, plants and soils.
However, there are substantial uncertainties associated with these natural components of the carbon cycle - where is carbon being taken up, how much, and will this situation last? Without understanding these natural fluxes - the difference between the amount emitted and the amount absorbed - we cannot reliably predict the future climate, nor can we establish a robust emission verification scheme. Knowing how much each country is emitting is critical if we are to reduce global GHG emissions.
The main focus of activities under the UN Framework Convention on Climate Change (UNFCCC) has been to manage GHG emissions from the production and use of energy. Emissions from agriculture, forestry and other land uses have traditionally been the poor relation, receiving less attention because of the temporary nature and complexity of the way carbon is stored in terrestrial ecosystems - a process known as 'sequestration'. However, the international policy framework aims to increase the sequestration of CO2 by encouraging beneficial changes in land use.
Central to these efforts is the need to measure, report and verify carbon emissions and sequestration from land-use projects and policies in different countries, so we can compile national emissions inventories and comply with emission reduction market mechanisms. Currently, emissions are calculated by 'inputs', based on how the land is being used, rather than from measuring levels of emissions in the atmosphere.
The current network of carbon dioxide measurements available. Data provided by the Global Monitoring Division of the US National Oceanic and Atmospheric Administration's Earth System Research Laboratory. Click image to enlarge.
The importance of reducing emissions from deforestation has been largely ignored until recently, when the REDD (Reducing Emissions from Degradation and Deforestation) agenda took centre stage in the run-up to the UNFCCC meeting in Cancun in November 2010.
The debate focuses on how to value carbon stored in forests, offering financial incentives to slow down deforestation. But this depends on being able to make robust measurements to ensure countries are following the rules.
Recent analysis of satellite observations has reported that annual deforestation rates over the Amazon have dropped repeatedly since a peak in 2004. Since that peak the mean annual reduction is approximately 4000km2, equating to an annual reduction of 117 million tonnes of CO2.
Scientists and policy-makers both need to measure CO2. But how good is our current observing system? The existing network of carbon-cycle measurements, taken with flasks, continuous sensors, tall towers and aircraft flights, provides an excellent measure of how global carbon fluxes vary over time, in different places and from season to season.
However, these measurements are mostly taken over North America, Europe and the remote oceans, whereas vital, vulnerable carbon stores like the tropics and the boreal zone - the subpolar forests and tundra that cover much of Canada and Russia - are essentially unobserved.
Shifting our strategy
This means these regions remain poorly understood, and we urgently need dense and frequent observations to improve our understanding of the global carbon cycle. There is a compelling need for a radical shift in our measurement strategy, and satellite observations will be important in making that shift.
The technology needed to measure the tiny variations in atmospheric CO2 caused by differences in its uptake and release at ground level has progressed rapidly, with sensors already flying on a number of satellites. Compared to the ground-based network, these space-based observations monitor the global atmosphere frequently (though they are less precise).
Current space-borne instruments work by measuring either thermal emission or reflected sunlight from the Earth's surface and atmosphere, in the infrared (IR) parts of the electromagnetic spectrum where atmospheric CO2 is absorbed.
Sensors measuring thermal infrared (IR) emission, such as NASA's AIRS and TES, and the European IASI instruments, are most sensitive to changes in CO2 in the mid-upper troposphere (the lowest 6-10km of Earth's atmosphere). But they are less sensitive nearer the surface, so they provide limited information about how the land is emitting and absorbing CO2.
In contrast, sensors measuring reflected sunlight in the shorter IR wavelengths are most sensitive to CO2 in the lower troposphere. Three sensors of this kind have been launched so far: the European Space Agency (ESA) SCIAMACHY in 2002, Japan's GOSAT in 2009 and NASA OCO 2009, which failed to reach orbit due to a problem with the launch vehicle.
SCIAMACHY provided the first tantalising glimpse of the total amount of CO2 in the troposphere - known as the tropospheric column - and how this varies around the globe, but the instrument was not optimised for CO2.
GOSAT was the first successfully-launched sensor dedicated to measure CO2 in the lower troposphere. Initial data are promising and improvements are still being made. OCO-2, a near exact copy of OCO, is scheduled for launch in 2013, and several European concepts are being considered that will provide the necessary sensitivity to CO2 near the surface. They will let us estimate carbon release and uptake over areas smaller than a continent and at weekly or monthly timescales.
The ongoing efforts to measure CO2 accurately in the lower atmosphere reflect the technical challenge of measuring minute changes in CO2. The variations of interest are of the order of a few parts per million (ppm - roughly equivalent to 1 per cent of the tropospheric column), against a background level of about 385ppm; consequently CO2 measurements from satellites need to be extremely precise.
It is these small variations in CO2 that we can use, via computer models of the exchange of gases between land and atmosphere, and of how gases move around the atmosphere, to estimate how much CO2 is being absorbed and emitted across a whole region.
Whilst current and planned instruments are a major step forward for improving models of the carbon cycle, the data they provide do not yet satisfy politicians' needs. International legislation requires us to measure annual CO2 emissions to within one tonne in order to calculate emission inventories by states, or to calculate emissions from individual emission- reduction projects.
Nevertheless, these developments do open the door to independent assessments of the impact on global emissions of regional changes in land use, such as deforestation in Southeast Asia, central Africa and South America.
To address concurrently the science and policy questions that require knowledge of carbon fluxes on a wide range of temporal and spatial scales, and with limited financial resources, we must strive both for improved ground-based networks and for satellite systems with denser and more frequent sampling over the regions we know least about.
A space-borne instrument that orbited only above the tropics, for instance, would provide unprecedented coverage of tropical land ecosystems, and would contribute significantly to verifying emissions from human activities as part of international efforts such as REDD.
Such a mission would also complement the global survey orbits adopted by many other satellite instruments, and also the ground-based measurement networks which are mostly outside the tropics. Such efforts need to be integrated with existing surface and aircraft measurements of trace gases and land-surface properties. Watch this space.
Professor Paul Palmer is a member of the School of GeoSciences at the University of Edinburgh, Dr Hartmut Bösch is a member of the Department of Physics and Astronomy at the University of Leicester, and Dr Andy Kerr is Director of the Edinburgh Centre on Climate Change. Email: email@example.com
Interesting? Spread the word using the 'share' menu on the top right.