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

===== 5.2.1.4.2 Interannual variability in land–atmosphere CO <sub>2</sub> exchange =====

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The AR5 stated that the interannual variability of the atmospheric CO <sub>2</sub> growth rate is dominated by tropical land ecosystems. A set of new satellite measurements applied to assess the variability of the tropical land carbon balance since AR5 ([[#Ciais--2013|Ciais et al., 2013]]) confirm this statement, including satellite column CO <sub>2</sub> measurements, estimating the recent anomalous land–atmosphere CO <sub>2</sub> exchange induced by El Niño at continental scale (e.g., J. [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ; [[#Palmer--2019|Palmer et al., 2019]]), and L-band vegetation optical depth, estimating tropical above-ground biomass carbon stock changes ([[#Fan--2019|Fan et al., 2019]]). In addition, based on ''medium evidence'' and ''medium agreement'' between studies with DGVMs and atmospheric inversions, semi-arid ecosystems over the tropical zones have a larger contribution to interannual variability in global land–atmosphere CO <sub>2</sub> exchange than moist tropical forest ecosystems (''low'' to ''medium confidence'') ([[#Poulter--2014|Poulter et al., 2014]] ; [[#Ahlstrom--2015|Ahlstrom et al., 2015]] ; [[#Piao--2020|Piao et al., 2020]]).

Understanding the mechanisms driving interannual variability in the carbon cycle has the potential to provide insights into whether and to what extent the carbon cycle can affect the climate (carbon–climate feedback), with particular interests over the highly climate-sensitive tropical carbon cycle (e.g., [[#Cox--2013|Cox et al., 2013]] ; [[#Wang--2014|X. Wang et al., 2014]] ; [[#Fang--2017|Fang et al., 2017]] ; [[#Jung--2017|Jung et al., 2017]] ; [[#Humphrey--2018|Humphrey et al., 2018]] ; [[#Malhi--2018|Malhi et al., 2018]] ; see [[#5.4|Section 5.4]]). Consistent findings from studies with atmospheric inversions, satellite observations and DGVMs (e.g., [[#Malhi--2018|Malhi et al., 2018]] ; [[#Rödenbeck--2018|Rödenbeck et al., 2018]]) lead to ''high confidence'' that the tropical net land CO <sub>2</sub> sink is reduced under warmer and drier conditions, particularly during El Niño events. Interannual variations in tropical land-atmosphere CO <sub>2</sub> exchange are significantly correlated with anomalies of tropical temperature, water availability and terrestrial water storage (X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] ; [[#Jung--2017|Jung et al., 2017]] ; [[#Humphrey--2018|Humphrey et al., 2018]] ; [[#Piao--2020|Piao et al., 2020]]), whose relative contribution are difficult to separate due to covariations between these climatic factors. At continental scale, the dominant climatic driver of interannual variations of tropical land-atmosphere CO <sub>2</sub> exchange was temperature variations (Figure 5.11; [[#Piao--2020|Piao et al., 2020]]), which could partly result from the spatial compensation of the water availability effects on land-atmospheric CO <sub>2</sub> exchange ([[#Jung--2017|Jung et al., 2017]]).

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[[File:067816dc3276215cf4b949a44f046544 IPCC_AR6_WGI_Figure_5_11.png]]

'''Figure 5.11 |''' '''Interannual variation in detrended anomalies of the net land CO''' <sub>2</sub> '''sink and land surface air temperature during 1980–2019.''' Correlation coefficients between the net land CO <sub>2</sub> sink anomalies and temperature anomalies are show on the right bar plots. The net land CO <sub>2</sub> sink is estimated by four atmospheric inversions (blue) and 15 Dynamic Global Vegetation Models (DGVMs) (green), respectively ([[#Friedlingstein--2020|Friedlingstein et al., 2020]]). Solid blue and green lines show model mean detrended anomalies of the net land CO <sub>2</sub> sink. The ensemble mean of DGVMs is bounded by the 1– σ inter-model spread in each large latitude band (North 30°N–90°N, Tropics 30°S–30°N, South 90°S–30°S) and the globe. The ensemble mean of atmospheric inversions is bounded by model spread. For each latitudinal band, the anomalies of the net land CO <sub>2</sub> sink and temperature (orange) were obtained by removing the long-term trend and seasonal cycle. A 12-month running mean was taken to reduce high-frequency noise. The bars in the right panels show correlation coefficients between the net land CO <sub>2</sub> sink anomalies and temperature anomalies for each region. ** indicates P&lt;0.01; * indicates P&lt;0.05. The grey shaded area shows the intensity of El Niño–Southern Oscillation (ENSO) as defined by the Niño 3.4 index. Two volcanic eruptions (El Chichón and Mount Pinatubo) are indicated with light blue dashed lines. Temperature data are from the Climatic Research Unit (CRU), University of East Anglia ([[#Harris--2014|Harris et al., 2014]]). Anomalies were calculated following [[#Patra--2005|Patra et al. (2005)]] , but using a 12-month low-pass filter and detrended to obtain interannual variations. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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