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Table 1.1 Global warming potential (GWP) – relative to CO₂ – as well as atmospheric concentration and lifetime of selected greenhouse gases.

      Source: Data are based on CDIAC (2010).

Gas	Chemical formula	Recent tropospheric concentration	GWP (100 yr time horizon)	Atmospheric lifetime (yr)
Carbon dioxide	CO2	≈386 ppm	1	100
Methane	CH4	≈1800 ppb	25	12
Nitrous oxide	N2O	≈320 ppb	298	114
Ozone	O3	≈34 ppb	N/A	h/d
CFC-11	CCl3F	≈240 ppt	4750	45
CFC-12	CCl2F2	≈536 ppt	10 900	100
CFC-113	CCl2FFClF2	≈76 ppt	6 130	85
HCFC-22	CHClF2	≈200 ppt	1 810	12
HCFC-141b	CH3CCl2F	≈20 ppt	725	9
HCFC-142b	CH3CClF2	≈20 ppt	2 310	18
Halon 1211	CBrCIF2	≈4 ppt	1 890	16
Halon 1301	CBrCIF3	≈3 ppt	7 140	65
HFC-134a	CH2FCF3	≈52 ppt	1 430	14
Carbon tetrachloride	CCL4	≈88 ppt	1 400	26
Methyl chloroform	CH3CCl3	≈10 ppt	146	5
Sulfur hexafluoride	SF6	≈7 ppt	22 800	3200

For a much longer time frame, data from the Vostok ice core provide an insight into the variations in CO₂ levels for the past four glacial–interglacial cycles. In January 1998, the collaborative ice-drilling project between Russia, the United States, and France at the Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3623 m (Petit et al. 1999). Data from this project are displayed in Figure 1.1. One conclusion from the authors was that present-day atmospheric burdens of CO₂ seem to have been unprecedented during the past 420 000 years. Note that the mean resolution for the CO₂ measurements was about 1500 years.

1.3 Natural Accumulations and Emissions of CO₂

There are many examples of natural emissions of CO₂ to the atmosphere. Forest fires and grass fires are natural phenomena, which release large amounts of CO₂ to the atmosphere. On the other hand, after such fires, new trees and grass will grow and eventually bind more or less the same amount of CO₂ as was released. One can say that as long as the total amount of biomass on a global basis is constant over a longer period of time, the combustion, decay, and growth of biomass is balanced with respect to CO₂. In this respect, the use of biomass for energy purposes can be regarded as CO₂ neutral. Consequently, if the amount of biomass is increasing through an expansion of the area covered by forests, this contributes to reducing the amount of CO₂ in the atmosphere. In reality, there is a significant ongoing reduction in biomass globally by man-made forest fires and wood cutting, which contributes to about 20% of the annual increase in greenhouse gas emissions.

Figure 1.1 Vostok ice core data for 420 000 years. Atmospheric CO₂ concentrations from trapped gas bubbles.

      Source: Petit et al. (1999). Reproduced with permission of Springer Nature.

      There are also a number of examples of seepage of CO₂ from the ground. From many of the ground seepages, the flux is so small that it is hardly noticeable. Others have a high flux so that the ground-level concentration can cause death of plants through ‘root anoxia’ and even pose risks to human health and safety. At concentrations above about 2%, CO₂ has a strong effect on respiratory physiology, and at concentrations above 7–10%, it can cause unconsciousness and death. Exposure studies have not revealed any adverse health effect from chronic exposure to concentrations below 1% (Fleming et al. 1992).

      One important aspect to note about CO₂ is that it has a higher molecular weight than air, meaning that it is denser than air. As a consequence, the release of CO₂ at a low point in the terrain, with little or no wind, can cause the concentration at that point to increase to a very high level, as the CO₂ will not be transported vertically by buoyancy.

      Seepage of CO₂ from the ground has a relevance to CO₂ capture and storage because stored CO₂ may leak out to the atmosphere. The impact on human health from releases of CO₂ can be severe. Some examples are Lake Monoun, Lake Nyos, and Lake Kivu, which are the only three lakes in the world known to be saturated

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