CO2 – Carbon dioxide

Fate of anthropogenic CO2 emissions (2006-2015) (Sources: CDIAC, NOAA-ESRL, Houghton et al., 2012, Giglio et al., 2013, Le Quere et al., 2016, Global Carbon Budget 2016:

What is CO2?

CO2, named carbon dioxide or carbonic gas, is an important greenhouse gas (after H2O) with a very long lifetime (~ 100 years). It accounts for the largest proportion of the ‘trace gases’ and is currently responsible for 60% of the ‘enhanced greenhouse effect’ based on fossil-fue combustion activities (petrol, coal, gas). It is thought that it’s been in the atmosphere for over 4 billion of the Earth’s 4.6 billion year geological history and in much larger proportions (up to 80%) than today.

How is atmospheric CO2 produced?

Atmospheric CO2 – Carbon dioxide comes from a number of natural and anthropogenic sources. The main natural sources include the decay of plants, volcanic eruptions and waste products from animal respiration. Anthropogenic sources are related to burning fossil fuels releasing the carbon dioxide stored millions of years ago and cement manufacture. Fossil fuels are used for vehicles (petrol, diesel and kerosene), heat homes, businesses and power factories. Deforestation releases the carbon stored in trees and also results in reduction of CO2 removal from the atmosphere.

Atmospheric CO2 sinks are photosynthesis by plants and dissolution in oceans. Photosynthesis is the main motor of the CO2 exchanges between atmosphere-ocean-vegetation (hundreds of billions of tons). These removals occur over a long-time range, approximately 100 years. The amount of CO2 taken out of the atmosphere by plants is almost perfectly balanced with the amount released into the atmosphere by respiration and decay.

Changes as a result of human activities have a large impact on this delicate balance. Consequently, human activity has become the main and dominating contributor to the atmospheric CO2 increase: the primary agent being the enhanced combustion of fossil fuel. On the last million year, and before the industrial revolution, atmospheric CO2 concentrations varied between 170 and 270 ppm. The concentrations of CO2 in the atmosphere are estimated to be at their highest level since the past 56 million years (Myr) (Mclnerney et al., 2011). After the industrial revolution, the current level of CO2 has increased by nearly 40%,  more in the northern hemisphere where more fossil fuel burning occurs,  from an average of 270 ppm, in pre-industrial times, to over 410 ppm today. The global average mixing ratio of CO2 – Carbon dioxide is still rising at about 2 ppm per year (Ciais et al., Geo Carbon Strategy, 2010).

The Carbon Dioxide Information Analysis Center (CDIAC) database (Marland et al., 2007) shows global emissions from fossil fuels and cement have grown from 6.2 Gt C in 1990, the base year for commitments under the Kyoto Protocol, to 7.2 Gt C in 2001 and 8.4 Gt C in 2006. Rapid growth over the last five years has been dominated by economic growth in developing countries (Raupach et al., 2007).

Perturbation of the global carbon cycle caused by anthropogenic activities globally averaged for the decade 2006-2015 (GtCO2 / yr) ( Sources: CDIAC, NOAA-ESRL, Le Quere et al., 2016, Glbal Carbon Budget 2016:

In addition, land-use change used to be the dominant source of annual CO2 emissions until ~1950. It nowadays contributes for about 12.5% to the total CO2 emissions (Houghton et al., 2012). CO2 emissions induced by land-use change are mainly dominated by deforestation. They can vary over space and time, depending on how the land is used and on the local climate, topography, and soil and vegetation properties. Currently greenhouse gas emissions from land-use change are the highest in tropical areas of South America, South-East Asia, and to a lesser extent, Africa.

CO2 emissions
Total global emissions by source as a function of years (Sources: CDIAC, Houghton et al., 2012, Giglio et al., 2013, Le Quere et al., 2016, Global Carbon Budget 2016:

Power plants, most notably coal-fired power plants (PPs), are among the largest CO2 emitters (DoE and EPA, 2000). As the world coal reserves are estimated at 930 Gt coal (Shindell and Faluvegi, 2010), it can be expected that CO2 emissions of coal-fired PPs will continue for many decades – probably with significantly growing emissions as the construction of coal-fired PPs is increasing rapidly for example in China and India (Shindell and Faluvegi, 2010). Due to the high population density associated with ground transportation, residence and industry, anthropogenic CO2 emissions are large within cities (Pataki et al., 2006; Breon et al., 2015). The emitted CO2 is transported in the atmosphere and results in elevated CO2 concentration above and downwind of cities.

The sinks have continued to grow with increasing emissions, but the on-going climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO2 in the atmosphere (Global Carbon Budget 2016).

Who are the largest CO2 emitters?

Although such a question sounds easy, the way to answer it is not straightforward. Numbers only give the messages that we want to hear. If we distribute the CO2 emissions per country, China is first (10 Gt CO2 / yr), then the USA (5.2 Gt CO2 / yr), European Union (4.6 Gt CO2 / yr), India (2.4 Gt CO2 / yr), Russia (1.8 Gt CO2 / yr) and Japan (1.2 Gt Co2 / yr) (Breon et al., Atlas du climat, 2015).

However, if we normalise the emissions per inhabitant, Qatar is then 1st (40.4 Gt CO2 / yr), the USA (16.5 ), European Union (9.1 Gt CO2 / yr), China (6.2 Gt CO2 / yr), France (5.7 Gt CO2 / yr), Chile (4.2 Gt CO2 / yr), Afghanistan (0.2) and Lesotho (0.01 Gt CO2 / yr) (Breon et al., Atlas du climat, 2015).

The first approach is the basis used for international discussions on the ways to mitigate greenhouse gas emissions. The second highlights that the only solution to limit the warming climate is to change our life styles.

Why shall we observe atmospheric CO2?

CO2 – Carbon dioxide is an important greenhouse gas. The thermal infrared radiation, emitted by the Earth surface, is in part absorbed by the greenhouse gases and then re-emitted towards the ground (~ about 95%). Without this natural process, our planet temperature would be too low (-18 deg C. on average instead of +15 deg. C). However, due to the high concentrations since the last century, and its strong greenhouse effect, atmospheric CO2 is the main contributor today of our climate change and continuous increase of global temperature. In spite of some politician and society polemics, the scientists have largely demonstrated without any doubts that the anthropogenic activities, initiated during the industrial revolution, based on fossil-fuel combustion, are the causes of this large amount of atmospheric CO2 which then, as a consequence, drives our temperature increases due to a reinforcement of the greenhouse effect (IPCC, 2014).

Accurate assessment of anthropogenic CO2 emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change (Le Quere et al., 2016).  Our understanding has large gaps and global satellite data can help to improve our understanding.

The net carbon flux induced by land-cover change is the most uncertain term in the global carbon budget, not only because of uncertainties in rates of deforestation and forestation, but also due to uncertainties in the carbon density of the lands actually undergoing change (Houghton et al., 2012). Furthermore, there is an urgent need to anticipate how terrestrial biosphere, i.e. one of the main carbon sinks, will respond to our changing climate. This is much more uncertain compared to the ocean according to the large spread of model results (IPCC, 2013).

In addition to land-cover change, uncertainties remain in carbon fossil-fuel emission. In many countries national legislation requires regular reporting of CO2 emissions to limit the on-going warming climate (e.g. DoE and EPA, 2000). Emission reporting is also required by the Kyoto protocol ( Current CO2 emission reporting is mostly based on economical and technical information (e.g. amount and type of fuel burned, PP thermal efficiencies, CO2 conversion factors) (DoE and EPA, 2000) but typically not on directly measured CO2 emissions. Another requirement of the Kyoto protocol is independent verification of the reported emissions – a requirement difficult to be met globally due to lack of appropriate observations. The uncertainty of the reported anthropogenic CO2 emissions varies by sector and country. They are assumed to vary on average by about 3–5% for the USA to 15–20% for China (Gregg et al., 2008), which became the largest national source of CO2 emissions during 2006.

Ackermann and Sundquist (2008) compared PP emission data bases and found that the absolute difference of the emissions of individual coal-fired PPs in the USA is typically about 20%. They conclude that several independent approaches are needed to reliably estimate how much CO2 individual PPs emit. To improve global emission monitoring and reporting, the use of satellites has been recommended (NRC, 2010). Currently however this is not possible as none of the existing or planned satellites has been built for such an application. Knowledge about the distribution and strength of individual strong emission sources is also very relevant to better constrain the separation of sources and sinks in the Northern Hemisphere. For example, the most recent estimates of EU fossil fuel emissions for 2000 are an order of magnitude larger than the European ecosystem carbon sink (Ciais et al., 2010). As a result even small uncertainties in the budget and the distribution of fossil fuel emission sources introduce substantial errors in the overall carbon budget derived from atmospheric inversions, when spatial resolution is increased from continental to regional, national or urban carbon scales.

Thus in spite of the current knowledge of the magnitude and distribution of fossil fuel combustion from inventories in Europe, there is a clear need for much more accurate knowledge about the magnitude and the temporal and spatial variability of the anthropogenic emissions. For other countries and continents a similar or even worse situation is expected. This is recognized by the International Geosphere- Biosphere Program (IGBP) Global Emissions Inventory Activity (GEIA) project ( and this is one important motivation of research groups to produce high resolution fossil fuel maps (Gurney et al., 2009; Rayner et al., 2010; Ciais et al., 2010; Oda and Maksyutov, 2010).

To conclude, an accurate knowledge of regional-scale anthropogenic emissions is paramount for designing well-informed mitigation policies. New knowledge can also support the definition of sharing principles for efforts to reduce emissions that are acceptable to all nations, and for monitoring their effectiveness over time (ESA, CarbonSat – Report for mission selection – An Earth Explorer to observe greenhouse gases, 2015).

A typical CO2 satellite map?

The XCO2 product (i.e. near-surface sensitive column-averaged dry air mole fractions of CH2) as derived from SCIAMACHY and GOSAT satellite sensors for 3 latitude bands as a function of years. Figure generated by Dr. Michael Buchwitz, University of Bremen (IUP-UB), through the ESA GHG project (Source:
Averaged XCO2 product from OCO-2 for the period November 21-December 29, 2014. Elevated CO2 concentrations are evident over the region of biomass burning in central Africa, and over land masses in the northern latitudes, where the plant life has become dormant in autumn and has ceased to absorb CO2. The limits on OCO-2 observations at high latitudes to the north and south are imposed by the required Sun angle for data acquisition (Source:

Some reference CO2 satellite missions / products?

Time series of CO2 data from European satellites, with boundary layer sensitivity, started in 2002 with the launch of SCIAMACHY on-board ENVISAT which measured at a spatial resolution of 30 km x 60 km (spatial resolution available for measurements in the 1.54 am CO2 band).

Presently, the only current missions dedicated to CO2 are: the JAXA (Japan) Greenhouse Gases Satellite (GOSAT), NASA Orbiting Carbon Observatory-2 (OCO-2) (Crisp et al., 2004) and the Chinese mini-satellite Tansat. Future mission concepts are under study in Europe for improving the precision of XCO2, as well as the spatio-temporal coverage and spatial resolution. This is needed in order to better quantify natural surface fluxes at regional scale, and to address more challenging objectives (including the possibility to measure strong anthropogenic emission sources).

The proposed missions have been: CarbonSat (candidate for ESA Earth Explorer 8) (Bovensmann et al., 2010, ESA, 2015), Microcarb (CNES) and the potential Sentinel-7 (Europe).


More information?

  • The ESA “Requirements for CO2 monitoring by Sentinel-5” project here
  • The ESA LOGOFLUX project devoted to evaluate the greenhouse gas (GHG) surface flux estimate capabilities from the CarbonSat mission here
  • An update of the global CO2 budget and trends by the Global Carbon project here
  • ESA, CarbonSat – Report for mission selection – An Earth Explorer to observe greenhouse gases, 2015, here
  • The Orbiting Carbon Observatory-2 (OCO-2) mission website here & here
  • The Orbiting Carbon Observatory-3 (OCO-3) mission website here
  • The Greenhouse Gases Observing Satellite website here
  • ESA GHG-CCI – GreenHouse Gases Climate change Initiative here