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

=== Contemporary Trends of Greenhouse Gases ===

<div id="h2-2-siblings" class="h2-siblings"></div>

'''It is unequivocal that the increase of CO''' <sub>2</sub> ''', CH''' <sub>4</sub> ''', and N''' <sub>2</sub> '''O in the atmosphere over the industrial era is the result of human activities''' ( ''very high confidence'' ''').''' This assessment is based on multiple lines of evidence including atmospheric gradients, isotopes, and inventory data. During the last measured decade, global average annual anthropogenic emissions of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O, reached the highest levels in human history at 10.9 ± 0.9 petagrams of carbon per year (PgC yr <sup>–1</sup> , 2010–2019), 335–383 teragrams of methane per year (TgCH <sub>4</sub> yr <sup>–1</sup> , 2008–2017), and 4.2–11.4 teragrams of nitrogen per year (TgN yr <sup>–1</sup> , 2007–2016), respectively ( ''high confidence'' ). {5.2.1, 5.2.2, 5.2.3, 5.2.4; Figures 5.6, 5.13, 5.15} .

'''The CO''' <sub>2</sub> '''emitted from human activities during the decade of 2010–2019 (decadal average 10.9 ± 0.9''' '''PgC y''' '''r''' –1 ''') was distributed between three Earth system components: 46% accumulated in the atmosphere (5.1 ± 0.02''' '''PgC y''' '''r''' –1 '''), 23% was taken up by the ocean (2.5 ± 0.6''' '''PgC y''' '''r''' –1 ''') and 31% was stored by vegetation in terrestrial ecosystems (3.4 ± 0.9''' '''PgC y''' '''r''' –1 ''')''' ( ''high confidence'' ''').''' Of the total anthropogenic CO <sub>2</sub> emissions, the combustion of fossil fuels was responsible for 81–91%, with the remainder being the net CO <sub>2</sub> flux from land-use change and land management (e.g., deforestation, degradation, regrowth after agricultural abandonment, and peat drainage). {5.2.1.2, 5.2.1.5; Table 5.1; Figures 5.5, 5.7, 5.12}

'''Over the past six decades, the average fraction of anthropogenic CO''' <sub>2</sub> '''emissions that has accumulated in the atmosphere (referred to as the airborne fraction) has remained nearly constant at approximately 44%.''' The ocean and land sinks of CO <sub>2</sub> have continued to grow over the past six decades in response to increasing anthropogenic CO <sub>2</sub> emissions ( ''high confidence)'' . Interannual and decadal variability of the regional and global ocean and land sinks indicate that these sinks are sensitive to climate conditions and therefore to climate change ( ''high confidence'' ). {5.2.1.1, 5.2.1.2, 5.2.1.3, 5.2.1.4; Figures 5.7, 5.8, 5.10}

'''Recent observations show that ocean carbon processes are starting to change in response to the growing ocean sink, and these changes are expected to contribute significantly to future weakening of the ocean sink under medium- to high-emissions scenarios.''' However, the effects of these changes are not yet reflected in a weakening trend of the contemporary (1960–2019) ocean sink ( ''high confidence'' ). {5.1.2, 5.2.1.3, 5.3.2.1; Figures 5.8, 5.20; Cross-Chapter Box 5.3}

'''Atmospheric concentration of CH''' <sub>4</sub> '''grew at an average rate of 7.6 ± 2.7 ppb y''' '''r''' –1 '''for the last decade (2010–2019), with a faster growth of 9.3 ± 2.4 ppb y''' '''r''' –1 '''over the last six years (2014–2019)''' ( ''high confidence'' ''').''' The multi-decadal growth trend in atmospheric CH <sub>4</sub> is dominated by anthropogenic activities ( ''high confidence'' ), and the growth since 2007 is largely driven by emissions from both fossil fuels and agriculture (dominated by livestock) ( ''medium confidence)'' . The interannual variability is dominated by El Niño–Southern Oscillation cycles, during which biomass burning and wetland emissions, as well as loss by reaction with tropospheric hydroxyl radical (OH) play an important role. {5.2.2; Figures 5.13, 5.14; Table 5.2; Cross-Chapter Box 5.2}

'''Atmospheric concentration of N''' <sub>2</sub> '''O grew at an average rate of 0.85 ± 0.03 ppb y''' '''r''' –1 '''between 1995 and 2019, with a further increase to 0.95 ± 0.04 ppb y''' '''r''' –1 '''in the most recent decade (2010–2019).''' This increase is dominated by anthropogenic emissions, which have increased by 30% between the 1980s and the most recent observational decade (2007–2016) ( ''high confidence'' ). Increased use of nitrogen fertilizer and manure contributed to about two-thirds of the increase during the 1980–2016 period, with the fossil fuels/industry, biomass burning, and wastewater accounting for much of the rest ( ''high confidence'' ). {5.2.3; Figures 5.15, 5.16, 5.17}

<div id="Ocean" class="h2-container"></div>

<span id="ocean-acidification-and-ocean-deoxygenation"></span>