Editing IPCC:AR6/WGI/Chapter-5 (section)

=== FAQ 5.1 | Is the Natural Removal of Carbon From the Atmosp here Weakening? ===

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''For decades, about half of the carbon dioxide (CO'' 2 '') that human activities have emitted to the atmosphere has been taken up by natural carbon sinks in vegetation, soils and oceans. These natural sinks of CO'' 2 ''have thus roughly halved the rate at which atmospheric CO'' 2 ''concentrations have increased, and therefore slowed down global warming. However, observations show that the processes underlying this uptake are beginning to respond to increasing CO'' 2 ''in the atmosphere and climate change in a way that will weaken nature’s capacity to take up CO'' 2 ''in the future. Understanding of the magnitude of this change is essential for projecting how the climate system will respond to future emissions and emissions red'' ''uction efforts.''

Direct observations of CO <sub>2</sub> concentrations in the atmosphere, which began in 1958, show that the atmosphere has only retained roughly half of the CO <sub>2</sub> emitted by human activities, due to the combustion of fossil fuels and land-use change such as deforestation (FAQ 5.1, Figure 1). Natural carbon cycle processes on land and in the oceans have taken up the remainder of these emissions. These land and ocean removals or ‘sinks’ have grown largely in proportion to the increase in CO <sub>2</sub> emissions, taking up 31% (land) and 23% (ocean) of the emissions in 2010–2019, respectively (FAQ 5.1, Figure 1). Therefore, the average proportion of yearly CO <sub>2</sub> emissions staying in the atmosphere has remained roughly stable at 44% over the last six decades, despite continuously increasing CO <sub>2</sub> emissions from human activities.

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'''FAQ5.1, Figure 1 |''' '''Atmospheric carbon dioxide (CO''' <sub>2</sub> ''') and natural carbon sink''' '''s.''' '''(Top)''' Global emissions of CO <sub>2</sub> from human activities and the growth rate of CO <sub>2</sub> in the atmosphere; '''(middle)''' the net land and ocean CO <sub>2</sub> removal (natural sinks); and '''(bottom)''' the fraction of CO <sub>2</sub> emitted by human activities remaining in atmosphere from 1960 to 2019. Lines are the five years running mean, error bars denote the uncertainty of the mean estimate. See Table 5.SM.6 for more information on the data underlying this figure.

On land, it is mainly the vegetation that captures CO <sub>2</sub> from the atmosphere through ''plant photosynthesis'' '','' which ultimately accumulates both in vegetation and soils. As more CO <sub>2</sub> accumulates in the atmosphere, plant carbon capture increases through the ''CO'' 2 ''fertilization effect'' in regions where plant growth is not limited by, for instance, nutrient availability. Climate change affects the processes responsible for the uptake and release of CO <sub>2</sub> on land in multiple ways. Land CO <sub>2</sub> uptake is generally increased by longer growing seasons due to global warming in cold regions and by nitrogen deposition in nitrogen-limited regions. Respiration by plants and soil organisms, natural disturbances such as fires, and human activities such as deforestation all release CO <sub>2</sub> back into the atmosphere. The combined effect of climate change on these processes is to weaken the future land sink. In particular, extreme temperatures and droughts as well as permafrost thaw (see FAQ 5.2) tend to reduce the land sink regionally. In the ocean, several factors control how much CO <sub>2</sub> is captured: the difference in CO <sub>2</sub> partial pressure between the atmosphere and the surface ocean; wind speeds at the ocean surface; the chemical composition of seawater (that is, its ''buffering capacity'' ), which affects how much CO <sub>2</sub> can be taken up; and the use of CO <sub>2</sub> in photosynthesis by seawater microalgae. The CO <sub>2</sub> -enriched surface ocean water is transported to the deep ocean in specific zones around the globe (such as the Northern Atlantic and the Southern Ocean), effectively storing the CO <sub>2</sub> away from the atmosphere for many decades to centuries. The combined effect of warmer surface ocean temperatures on these processes is to weaken the future ocean CO <sub>2</sub> sink.

The ocean carbon sink is better quantified than the land sink, thanks to direct ocean and atmospheric carbon observations. The land carbon sink is more challenging to monitor globally, because it varies widely, even regionally. There is currently no direct evidence that the natural sinks are slowing down, because observable changes in the fraction of human emissions stored on land or in oceans are small compared to year-to-year and decadal variations of these sinks. Nevertheless, it is becoming more obvious that atmospheric and climate changes are affecting the processes controlling the land and ocean sinks.

Since the land and ocean sinks respond to the rise in atmospheric CO <sub>2</sub> and to human-induced global warming, the absolute amount of CO <sub>2</sub> taken up by land and ocean will be affected by future CO <sub>2</sub> emissions. This also implies that, if countries manage to strongly reduce global CO <sub>2</sub> emissions, or even remove CO <sub>2</sub> from the atmosphere, these sinks will take up less CO <sub>2</sub> because of the reduced human perturbation of the carbon cycle. Under future high-warming scenarios, it is expected that the global ocean and land sinks will stop growing in the second half of the century as climate change increasingly affects them. Thus, the total amount of CO <sub>2</sub> emitted to the atmosphere and the responses of the natural CO <sub>2</sub> sinks will both determine what efforts are required to limit global warming to a certain level (see FAQ 5.4), underscoring how important it is to understand the evolution of these natural CO <sub>2</sub> sinks.

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