5.3 Human disruption of carbon cycle

Human activities have altered the natural cycles of GHGs. The natural carbon cycle has been shifted toward accumulation in the atmosphere. Human activities accounting for the majority of anthropogenic carbon compound emission include fossil fuel burning, land use change (most importantly deforestation, increasing agricultural lands) emitting CO2 and methane as well.

In the contemporary cycle of methane, natural sources of account for 40% of the annual emissions while anthropogenic emissions account for the other 60% of methane released to the atmosphere. The atmospheric abundance of methane before the industrial era has varied from lows of about 400 ppb during glacial periods to highs of about 700 ppb during interglacials based on examination of ice cores. Based on the latest analysis of WMO GAW program, in 2011, the global average abundance of CH4 is 1813±2 ppb. Present atmospheric levels of CH4 are unprecedented in at least the last 650 kyr. The growth rate of CH4 decreased from ~13 ppb/yr during the early 1980s to near zero during 1999–2006. Since 2007, atmospheric CH4 has been increasing again, with a nearly constant rate during 3 years.

Methane (CH4) sources to the atmosphere generated by human activities exceed CH4 sources from natural systems. Between 1960 and 1999, CH4 concentrations grew an average of at least six times faster than over any 40-year period of the two millennia before 1800, despite a near-zero growth rate since 1980. The human activities that produce CH4 include energy production from coal and natural gas, waste disposal in landfills, raising ruminant animals (e.g., cattle and sheep), rice agriculture and biomass burning. At the same time, wetlands and rice paddies emit about 175 Tg CH4 annually. The warmer and moister the rice field, the more methane is produced.

Waste water treatment facilities are responsible for CH4 emissions as anaerobic treatment of organic compounds in the water results in the production of methane. Decaying organic matter and anaerobic conditions cause landfills to be a significant source of methane.

In 2011, CO2 concentration in the atmosphere reached 390.9±0.1 ppm (based on WMO GAW measurements), which means an increase of 140% compared to preindustrial level. The additional CO2 emitted to the atmosphere by human activities (the anthropogenic CO2) has altered the natural carbon cycle. Since the beginning of the industrial era, human activity released large amount of carbon mainly from the longterm pools due to fossil fuel burning. Main sources of anthropogenic emissions include (i) CO2 from fossil fuel burning and cement production, newly released from hundreds of millions of years of geological storage (ii) deforestation and turning lands into agricultural fields releasing carbon, which has been stored for decades to centuries. As a result, net land-atmosphere and ocean-atmosphere fluxes have become significantly different from zero (also see red arrows in Fig. 5.5). This latter component of human induced carbon emission associated with land use change is the most uncertain part of anthropogenic emission estimations.

Arrhenius was the first who proposed the idea that CO2 emitted from fossil fuel burning and combustion can be large enough to cause global temperature increase due to the greenhouse effect. The first continuous measurement of atmospheric CO2 concentration started in 1958 at Mauna Loa Observatory Hawaii by Charles David Keeling. Keeling's measurements showed the first significant evidence of rapidly increasing carbon dioxide levels in the atmosphere. Many scientists credit Keeling's graph (Fig 5.10) with first bringing the world's attention to the current increase of carbon dioxide in the atmosphere, which, based on Arrhenius’ theory can lead to a global temperature change.

The Keeling curve

Figure 5.10: The Keeling curve. source: Wikipedia

This figure shows the history of atmospheric carbon dioxide concentrations as directly measured at Mauna Loa, Hawaii. This curve is known as the Keeling curve, and is an essential piece of evidence of the man-made increases in greenhouse gases that are believed to be the cause of global warming. The longest such record exists at Mauna Loa, but these measurements have been independently confirmed at many other sites around the world. The annual fluctuation in carbon dioxide is caused by seasonal variations in carbon dioxide uptake by land plants. Since many more forests are concentrated in the Northern Hemisphere, more carbon dioxide is removed from the atmosphere during Northern Hemisphere summer than Southern Hemisphere summer. This annual cycle is shown in the inset figure by taking the average concentration for each month across all measured years. The grey curve shows the average monthly concentrations, and red curve is a moving 12 month average.

The increase in atmospheric CO2 concentration is known to be caused by human activities because the character of CO2 in the atmosphere, in particular the ratio of its heavy to light carbon atoms, has changed in a way that can be attributed to addition of fossil fuel carbon. In addition, the ratio of oxygen to nitrogen in the atmosphere has declined as CO2 has increased; this is as expected because oxygen is depleted when fossil fuels are burned. A heavy form of carbon, the carbon-13 isotope, is less abundant in vegetation and hence in fossil fuels that were formed from past vegetation, and is more abundant in carbon in the oceans and in volcanic or geothermal emissions. The relative amount of the carbon-13 isotope in the atmosphere has been declining, showing that the added carbon comes from fossil fuels and vegetation. Carbon also has a rare radioactive isotope, carbon-14, which is present in atmospheric CO2 but absent in fossil fuels. Prior to atmospheric testing of nuclear weapons, decreases in the relative amount of carbon-14 showed that fossil fuel carbon was being added to the atmosphere.

Atmospheric CO2 concentration however increases with half of the rate of anthropogenic emissions. The fraction of anthropogenic emission that remains in the atmosphere is known as the airborne fraction. The airborne fraction, considering anthropogenic emissions to be relatively well constrained can be good indicators short and longterm changes in CO2 sink processes.

According to the original theory, the residual sink (formerly the ’missing sink’) takes up about half of the anthropogenic emission. The residual sink is the increased carbon absorbing capacity of terrestrial vegetation and oceans as a response to changing environment, increasing CO2 burden of the atmosphere. In other words, net carbon balance, i.e. NBP of terrestrial ecosystems is negative (meaning uptake). Estimation of carbon amount taken up by the residual sink greatly depends on our assumptions on land use change emissions, as the residual sink is determined using inverse modeling. Careful analysis reveals that the global uptake of anthropogenic carbon dioxide emissions by carbon sinks has doubled during the past 50 years − but the fractions of this absorbed by land and by sea remain unclear. A drawback of the net uptake of CO2 by the ocean, is ocean acidification. It makes seawater more acidic with potentially large impacts on the ocean food chain.

Although terrestrial vegetation is currently a net sink of CO2, aboveground biomass cannot increase indefinitely. Ultimately the most important storage place for organic carbon is the soil, and it holds the greatest potential to act as a longterm sink for atmospheric CO2. Changes in soil carbon storage, however will not occur in isolation from changes in other trace gas exchanges which raises important scientific questions for GHG mitigation efforts.

The appropriate knowledge of the responses these processes give to climate variability is critical to asses the effect of global change to their capacity to absorb CO2 and hence moderate atmospheric increase of CO2.

Factors affecting absorbing capacities of terrestrial vegetation and oceans: (i) direct climatic effects (precipitation, temperature, radiation changes) (ii) changes in atmospheric constituents (’CO2 fertilization’, nutrient deposition (N, P), damage caused by pollutants) (iii) land use change (deforestation, afforestation, agricultural management practices).

(i) Direct climatic effects

Climate and the carbon cycle is closely coupled, as it is best shown by glacial-interglacial cycles (Figure 5.11). During glacial periods, CO2 concentration is lower compared to interglacials suggesting the close coupling with temperature. Dissolved carbon is released from oceans as CO2 in higher temperatures and gets dissolved in lower temperatures. Atmospheric CO2 level responds to short term climate variability as well such as the El Nino-Southern Oscillation and Arctic Oscillation and even to climate perturbation from volcanic eruptions.

Ice core data for atmospheric carbon-dioxide

Figure 5.11 This graph shows the newest Ice Core data for Atmospheric CO2 from air bubbles in the ice. 230 ppm is marked as a transition level and colored "glacial periods" blue and interglacial periods yellow. There's a clear 80,000−110,000 period of repeating glacier even if they vary in quality. Human deforestation and burning of fossil fuel has raised atmospheric CO2 to over 380 ppm in the last century, well above pre-industrialized levels, and "off the scale" of this graph top.

(ii) Changes in atmosphere chemical composition

There are numerous ways for atmospheric concentration of CO2 to influence ecosystem functioning. In the FACE (Free-Air Carbon Dioxide Enrichment) experiments, the effect of elevated CO2 concentration on ecosystems was examined performing in-situ measurements. One would expect an increase in assimilation when more CO2 is available for the photosynthesis. However, this CO2 fertilization can be overruled by environmental or nutrient limitations.

(iii) Land-use change

Land cover and land use changes occur naturally due to e.g. changes in climate, or caused by human activity such as agricultural management or appearance of new diseases.

Currently 1/3 of the total anthropogenic CO2 emission is related to deforestation especially on the tropics. Emission due to land use change is currently the most uncertain part of the anthropogenic carbon emission estimations. Agricultural management practices can influence carbon sequestration by agricultural soils when applied properly. Atmosphere-ocean carbon fluxes are influenced by several factors depending on the scale of the processes involved. The effectiveness of the solubility pump is governed by the intensity of ocean circulation, sea surface temperature, salinity, ocean stratification, ice cover. The biological pump is basically determined by the amount of the sinking part of POC and DOC, influenced by nutrient supply, ocean circulation.

Table 5.2: The contemporary carbon budget. Source: IPCC Table 7.1

The global carbon budget (GtC yr–1); errors represent ±1 standard deviation uncertainty estimates and not interannual variability, which is larger. The atmospheric increase (first line) results from fluxes to and from the atmosphere: positive fluxes are inputs to the atmosphere (emissions); negative fluxes are losses from the atmosphere (sinks); and numbers in parentheses are ranges. Note that the total sink of anthropogenic CO2 is well constrained. Thus, the ocean-to-atmosphere and land-to-atmosphere fluxes are negatively correlated: if one is larger, the other must be smaller to match the total sink, and vice versa.

Carbon budget (2000–2005)

(GtC yr–1)

AR4

 

Atmospheric Increase  

4.1 ± 0.1 

Emissions (fossil + cement) 

7.2 ± 0.3 

Net ocean-to-atmosphere flux  

–2.2 ± 0.5 

Net land-to-atmosphere flux  

–0.9 ± 0.6 

Partitioned as follows 

 

Land use change flux 

n.a. 

Residual terrestrial sink 

n.a.