Mitigation of climate changes by carbon sequestration in biomass under elevated atmospheric CO2 concentration– facts and expectations
Natural transformations of carbon compounds in the environment favor its retention in terrestrial and aquatic ecosystems, while the human activities mostly lead to the accumulation of C in the atmosphere, in the form of C02. The problem with anthropogenic emissions concerns mainly the imbalance of the C'02 release and assimilation rates by terrestrial and aquatic ecosystems.
The anthropogenic emission influences the upper part of natural carbon cycle that occurs between the atmosphere, oceans, and terrestrial ecosystems, but it docs not involve the carbon exchange with the geologic reservoirs. The exchange with the upper part of the carbon cycle occurs on the timescales ranging from sub-daily to millennia, while the exchange with the lower part needs the longer timescales (Archer et al. 2009). Therefore, there the return of a huge pool of coal to the zones from which they were released as a result of human activities, is impossible within a reasonable time scale.
The imbalance related to the rates of C02 assimilation and release processes leads to an increase of the atmospheric C accumulation. The value of this parameter rose from 3.3 ± 0.1 GtC yr'1 in 1980s (IPCC 2001), by 4.1 ± 0.1 GtC yr'1 in 2000-2005 (IPCC 2007) to 4.7 ± 0.1 GtC yr'1 for the years 2007-2016 (Le Qucrc et al. 2018). The anthropogenic emissions resulting from industrial development are considered as the main reason of this phenomenon. Until the start of the industrial revolution in the 18th century, the global C02 emissions were at the level of 0.011 Gt C02 yr' '. From 1760 onward, it began to slowly increase, reaching a 25-fold higher value after 100 years. A significant increase in C02 emissions occurred after the Second World War. By the 1960s, emissions had increased about 28 times compared to the value of 1860. During the following years, emissions grew further, reaching a short-time stabilization of 35.69 GtC02 yr'1 in 2014 and 2015. Unfortunately, the growth was observed again in 2016-2018 (Figure 1).
An increase in the C02 emissions from anthropogenic sources is related to the increase in the atmospheric C02 concertation (Figure 1). Since the pre-industrial era, when the C02 concentration was 280 ppm, it has increased by 1.5 times compared to the present time. An evident acceleration of C02 accumulation in the atmosphere has been observed since the 1950s. According to the data from Earth System Research Laboratory (ESRL), the recent monthly mean C02 concentration measured at Mauna Loa Observatory (Hawaii) in August 2019 was 408.53 ppm. This value is similar to the average C02 concentration measured for 2018. However, this is the lowest month value noted in 2019, and the others ranged from 408.54 to 414.66 ppm.
This continuous increase of the C02 concentration in the atmosphere, reaching 2-3 ppm per year (ESRL date), on average, indicates that the initiatives undertaken in recent years to reduce the C02 emissions arc still insufficient. It is necessary to intensify two-way activities, not only aimed at reducing emissions, but also at increasing the rate of C02 removal from the
Figure 1. Time-depended changes of the anthropogenic C02 emission (based on the data of Le Quere el al., 2018) and the atmospheric C02 concentration (ESRL data).
atmosphere. For the latter, the carbon sequestration in terrestrial ecosystems, including biomass, which is a natural carbon sink, is a prospective solution. It could be expected that due to the intensification of photosynthesis induced by the increase in substrate concentration, i.e. C02, biomass growth will be stimulated, which will lead to increased C02 absorption (Cel et al. 2016). As a result, the alleviation of atmospheric carbon increase will be achieved. However, this phenomenon is not so obvious, due to a number of factors, such as an increase in temperature due to the greenhouse effect, changes in evapotranspiration, caused by the physiological changes in plants and an increase in temperature, etc. However, the extremely important is the human activity. Further destruction of natural ecosystems, such as tropical forest or savannas may nullify the positive effects of the stimulating activity of the environmental factors, even if they occur.
The authors analyzed the results obtained during the examination of different plant species under different conditions, regarding the impact of increased C02 concentration in the atmosphere on the accumulation of carbon in biomass, and on the other phenomena that may affect the plant growth. The purpose of this review was to assess the reality of expectations for self- accelerating mitigation of greenhouse effect by enhancement of photosynthesis due to the increased availability of C02.