Atmospheric concentrations of carbon dioxide (CO2) and methane (CH4), both components of the carbon cycle, have increased since the beginning of the Industrial Revolution by 42% and 150% respectively. Together they accounted for over 80% of the human-caused increase in the Earth’s radiative forcing due to greenhouse gases (GHGs) in 2014 (www.esrl.noaa.gov/gmd/aggi/aggi.html), and therefore are the leading causes of human-induced climate change. These two gases have very unique properties. CO2 emissions has a very long atmospheric “residence time” once emitted: 40% of today’s CO2 emissions will remain in the atmosphere in 100 years and about 20% will still be airborne in 1000 years (Ciais et al. 2013). Thus, anthropogenic CO2 emissions cause, to a large degree, irreversible climate change at human time-scales, which justifies a precautionary approach to addressing climate change – given the risks to human wellbeing associated with a warmer world. Second, the CH4 molecule is a much more powerful GHG than CO2 (~28 times on the 100 year horizon) but its shorter residence time (~9 years), makes it a good target for efficient mitigation actions on human timescales. Further, CH4 also impacts ozone chemistry. The Global Carbon Project (www.globalcarbonproject.org) works towards the construction of a more robust global carbon budget and in-depth trend analyses, and has provided annual updates for the past 10 years for CO2, and more recently the first global CH4 budget.
Global Carbon Budget – CO2
The global carbon budget, here referring to the human perturbation budget of CO2, includes all changes in the annual CO2 fluxes of carbon sources and sinks that result from the direct or indirect effects of CO2 emissions from human activities, mostly fossil fuel combustion, cement production, and land-use changes. This perturbation determines the changes in the atmospheric CO2 concentration given that we know concentrations were stable at around 278 ppm for thousands of years prior to the Industrial Era. Figure 1 shows the carbon budget for the decade of 2004-2013 (Le Quéré et al. 2015), with emissions from fossil fuels of 8.9±0.4 GtC yr-1 (Gigatons or billion tons), and emissions from land-use change of 0.9±0.5 GtC yr-1. Of those emissions: 4.3±0.1 GtC yr-1 had accumulated in the atmosphere, and increased the GHG effect; 2.6±0.5 GtC yr-1 was removed by the oceans; and 2.9±0.8 GtC yr-1 was removed by terrestrial ecosystems. Fossil fuel emissions accelerated and grew at a rate of 2.3% per year on average in the past decade – compared to 1.1% during the 1990s, and are now about 60% higher than those in the early 1990s when international negotiations to address climate change began. Emissions from land-use change seemed to be more stable or declining (in particular in Brazil) during that decade, albeit with large uncertainties associated with its component fluxes. It is worth noticing that due to the smaller land- use change fluxes and, to a larger extent, due to the strong growth in fossil fuel emissions over the past decade, the fraction of anthropogenic CO2 emissions contributed by land-use change is now around 10% of the total, down from 36% in 1960.
Figure 3: Global tree cover based on MODIS data (M. Hansen, UMD
Figure 1: Global carbon budget (CO2) for the decade 2004 - 2013
Credit: Le Quéré et al. 2015
Land and ocean sinks are responding to the increasing atmospheric CO2 by increasing the CO2 flux being removed from the atmosphere. Natural carbon sinks remove over half of all anthropogenic carbon emissions, thus providing an economic subsidy worth hundreds of billions of dollars annually if an equivalent sink had to be created by currently available financial instruments and technologies (Canadell and Raupach 2011). However, the sink growth is slower than the growth of fossil fuel emissions indicating a loss of sink efficiency and inability of the sinks to keep pace with the fast emissions growth (Raupach et al. 2014). The collective dynamics of all sources and sinks has led to current high atmospheric CO2 concentrations, now with months-long incursions over 400 ppm and an annual average of 397 ppm in 2014.
Global Carbon Budget – CH4
Methane is the second most important GHG – contributing 20% of warming by the well-mixed GHGs. Despite a pause in atmospheric growth, perhaps contributed by better management in the agricultural and energy sectors, growth resumed in 2007 and has since continued. The causes of the new growth are uncertain but likely candidates are a series of wet years in the tropics causing increased wetland emissions and by the rapid growth of the energy sector in Asia. Today, the atmospheric mixing ratio of methane has surpassed 1800 ppb, 1.5 times its pre-industrial value. Anthropogenic emissions in the CH4 budget now represent about 2/3 of the global methane emissions due to livestock, rice paddies, landfills, and human-caused biomass burning – with significant contributions by leaks from natural gas and oil, coal extraction and distribution. Natural emissions of CH4 are dominated by natural wetlands and other continental water bodies, with smaller contributions from geological natural venting, wildfires, and termites. 90% of the global sink for atmospheric CH4 is oxidation by hydroxyl radicals (OH), mostly in the troposphere, with additional minor sinks in soils, stratosphere and marine boundary layers. Figure 2 shows the budget for the decade of 2000- 2009 (Kirschke et al. 2013; Ciais et al. 2013). Atmospheric inversions of CH4 concentrations taken from observational networks estimate total global emissions of 548 Tg (Teragrams) of CH4 yr-1 (526–569) and a global sink of 540 (514–560) Tg CH4 yr-1. The mismatch between the sources and sinks leads to an atmospheric growth rate of +6 Tg CH4 yr-1. Interestingly, a combination of national statistics and land based inventories with biogeochemical models show larger global CH4 emission fluxes and emission trends, not consistent with the atmospheric constraints, and thus highlighting the need for independent observation networks to better constrain national and international based reporting.
Figure 2: Global methane budget 2000-2009 Credit: Kirschke et al. 2013
Observing the Perturbation of the Carbon Cycle
Strategies to stabilise the climate require understanding and managing the carbon cycle to control anthropogenic GHG emissions, possibly increase the magnitude of CO2 sinks on land, and preserve high and vulnerable carbon stocks such as tropical forests and soils (Canadell and Schulze 2014). The global sink cannot be understood without elucidating the contribution of each region. Some land and ocean regions serve as sinks, others as net sources. The fluxes of CO2 vary across seasons and years, and the processes that explain the sink magnitude and variability of CO2 (and CH4) fluxes differ between regions.
Today’s global observational networks are able to track, to some degree, changes in concentrations of carbon species, flux exchanges between the land/oceans and the atmosphere, and a number of key indicators of change of the world’s land and ocean productivity as climate, CO2, nitrogen and other quantities change (Ciais et al. 2014).
Atmospheric observations are a key component of a global carbon cycle observing system because, when dense enough atmospheric data can be collected, it is possible to use transport models for quantifying all the fluxes that emit or absorb CO2 and CH4 at the surface of the Earth. The atmospheric approach is comprehensive because the atmosphere sees all fluxes impacting the carbon balance of a region. Measurements of fluxes at local scale over specific ecosystems, or ocean regions are more informative about specific processes and fine scale details of fluxes, and they are complementary to atmospheric observations because they help to attribute net fluxes to underlying processes and components.
The current observations of atmospheric mixing ratios of CO2 and CH4 include: surface networks continuously monitoring atmospheric concentration of GHGs; observations over the ocean made on ships and moorings; aircraft vertical profile measurements and balloon observations in some locations; and a few instrumented commercial aircraft. Satellite remote sensing of column CO2 and CH4 mixing ratio with global coverage offers very promising options to complete atmospheric observations over regions with low surface network density – although they are not yet a substitute for in-situ observations because of less accurate performance and as yet insufficient coverage of cloudy regions.
The SCIAMACHY (2002–2012), and GOSAT (since 2009) satellite missions enabled a first mapping of column CO2 and CH4 mixing ratio but they have not yet delivered enough data to deliver revolutionary knowledge of the carbon cycle, compared to existing in-situ observation assets. The Orbiting Carbon Observatory-2 mission (since 2014), thanks to its high resolution and target mode allowing comparison of space-borne data against ground based column CO2 (TCCON), appears to be promising enough to deliver new column CO2 data that will reduce the uncertainty of CO2 fluxes, in particular over mid-latitude and tropical continents (Ciais et al. 2014).
On land, observations include the monitoring of land cover change, eddy-covariance carbon exchange between land and atmosphere (FLUXNET), and a number of remote sensed vegetation greening indices that are related to the land’s gross primary productivity (Figure 3). CO2 observations over the ocean are made by ships and moorings, which are used to develop global climatologies and inter-annual variability of carbon exchanges between ocean and the atmosphere; the Surface Ocean CO2 Atlas (SOCAT) aims to synthesize these data (Bakker et al. 2014) and revealed that the tropics and southern hemisphere oceans are under-sampled. Observations of changes in carbon concentration in the ocean interior are made from repeat surveys, and provide a synoptic view of the uptake of carbon over decades.
This ensemble of observations is completed by statistical approaches to estimate anthropogenic emissions based, for instance, on energy statistics for CO2 emissions from the combustion of fossil fuels (Figure 4) and on livestock statistics for CH4 emissions from cattle.
Figure 4: Trend in Growing Season Integrated NDVI (surrogate of Annual Gross Productivity) between 1982 and 2011
Credit: Ranga B. Myneni
Enhanced Policy-relevant Global Carbon Observing System
Despite significant efforts in recent years to develop in- situ, airborne, remote sensing observations and modelling, current capabilities need to be financially consolidated over the long-term (Houweling et al., 2012) and further developed in order to address the Earth system’s emerging properties in response to human perturbation, and to support observational capabilities required by climate change policies. The density of observation is still insufficient to accurately quantify the distribution and variability of fluxes in each region, so that models used for future projections cannot be improved and benchmarked against present-day observations.
At the Earth system level, there is the need to:
– follow and evaluate human emissions of CO2 using observations independent from energy use statistics used in current inventories;
– monitor the variability and changes in the natural land and ocean fluxes which affect excess atmospheric CO2;
– be able to provide early detection information on emissions hot spots (e.g. methane from permafrost, hydrates, leaks from the energy sector);
– document CH emissions from natural wetlands given 4 their dominant role in the global budget and large uncertainties;
– monitor emissions pathways to zero for climate stabilisation and the symmetry (or otherwise) of the carbon cycle under negative emissions as required by the low carbon pathways; and
– address all carbon greenhouse gases and N2O, the latter a gas growing in importance as land-use intensification continues in order to meet growing demands for food, bioenergy and the emerging bioeconomy (Canadell and Schulze 2014).
The carbon observing system needs to be enhanced from a research-based set of observation arrays to a policy- relevant carbon observing system with capabilities to monitor, report and verify policy effectiveness at national and sub-national scales. This will enable nations, provinces, and local municipalities to implement and evaluate the effectiveness of policies that reduce emissions or create sinks of CO2 and CH4. Uncertainties in inventories need to be dramatically reduced in order to support such effective policies (Ciais et al. 2014).
Remote sensing offers the advantage of dense spatial coverage at a global scale. A key challenge is to bring remote-sensing measurements to a level of long-term consistency, high spatial resolution and accuracy so that they can be efficiently and consistently combined in models and inventories with ground-based data in order to reduce uncertainties. Bringing tight observational constraints on fossil fuel related emissions and land-use change emissions is the biggest challenge for deployment of a policy-relevant integrated carbon observation system and one that could begin to be achieved with upcoming and proposed satellite missions. This requires in-situ and remotely sensed data at much higher resolution and density than currently achieved for natural fluxes, including over small land areas (cities, industrial sites, power plants), as well as the inclusion of specific tracers of individual emission processes such as radiocarbon of CO2 for fossil fuels, isotope 13C in CO2 for land fluxes, or 13C in CH4 to separate flux sources (biogenic, thermogenic, and pyrogenic), carbon monoxide to track combustion-related emissions of CO2 and ethane to methane emissions from the gas industry. Additionally, a policy-relevant carbon monitoring system needs to provide mechanisms for reconciling regional top-down (atmosphere- based) and bottom-up (surface-based) flux estimates across the range of spatial and temporal scales relevant to mitigation policies (Ciais et al. 2014). Uncertainties for each observation data-stream should be assessed. The success of such a system will rely on long-term commitments to monitoring, on the generalisation of the monitoring of “process” tracers, on improved international collaboration to fill gaps in the current observations, on sustained efforts to improve access to the different data streams and to make databases interoperable, and on the calibration of each component of the system to agreed- upon international scales.
A number of specialist greenhouse gas observing satellite missions are in orbit or planned by CEOS agencies
Josep Canadell (CSIRO), Philippe Ciais (LSCE), Corinne Le Quéré (Univ. East Anglia), Philippe Bousquet (LSCE)