Vegetation greenness and land carbon-flux anomalies associated with climate variations: a focus on the year 2015

Abstract

Abstract. Understanding the variations in global land carbon uptake, and their driving mechanisms, is essential if we are to predict future carbon-cycle feedbacks on global environmental changes. Satellite observations of vegetation greenness have shown consistent greening across the globe over the past three decades. Such greening has driven the increasing land carbon sink, especially over the growing season in northern latitudes. On the other hand, interannual variations in land carbon uptake are strongly influenced by El Niño-Southern Oscillation (ENSO) climate variations. Marked reductions in land uptake and strong positive anomalies in the atmospheric CO 2 growth rates occur during El Niño events. Here we use the year 2015 as a natural experiment to examine the possible response of land ecosystems to a combination of vegetation greening and an El Niño event. The year 2015 was the greenest year since 2000 according to satellite observations, but a record atmospheric CO 2 growth rate also occurred due to a weaker than usual land carbon sink. Two atmospheric inversions indicate that the year 2015 had a higher than usual northern land carbon uptake in boreal spring and summer, consistent with the positive greening anomaly and strong warming. This strong uptake was, however, followed by a larger source of CO 2 in the autumn. For the year 2015, enhanced autumn carbon release clearly offset the extra uptake associated with greening during the summer. This finding leads us to speculate that a long-term greening trend may foster more uptakes during the growing season, but no large increase in annual carbon sequestration. For the tropics and Southern Hemisphere, a strong transition towards a large carbon source for the last 3 months of 2015 is discovered, concomitant with El Niño development. This transition of terrestrial tropical CO 2 fluxes between two consecutive seasons is the largest ever found in the inversion records. The strong transition to a carbon source in the tropics with the peak of El Niño is consistent with historical observations, but the detailed mechanisms underlying such an extreme transition remain to be elucidated.

Changes in the Response of the Northern Hemisphere Carbon Uptake to Temperature Over the Last Three Decades

“…These findings are counterintuitive given that net primary productivity outside the peak growing season is relatively small (NBP being negative for both seasons) and soil respiration rates are expected to increase with temperature (Bond‐Lamberty & Thomson, ; Lloyd & Taylor, ), opposite to what is observed. Focusing on processes driving the uptake of CO 2 , vegetation greenness has increased prominently in autumn and winter since 2000 in central and eastern Siberia, Eastern North America, and parts of Europe as shown in a recent study (see Figure 1 in Yue et al, ), indicating vigorous vegetation activity outside the peak growing season in the mid‐to‐high latitudes in the recent decades. Indeed, using normalized difference vegetation index (NDVI) observed from the Moderate Resolution Imaging Spectroradiometer instruments Collection 6 (Didan, )—a proxy for vegetation greenness, we found, in general, positive correlations between ΔNDVI and ΔT in the midlatitudes and mid‐to‐high latitudes during autumn and winter (Figure S4).…”

Climate drivers of the terrestrial carbon cycle variability in Europe

“…Dunn et al 2007, Jongen et al 2011, Nijp et al 2015 and for NPP, albeit showing a weaker relation and with a strong radiation control (Nemani et al 2003). The boundary between the temperature and water-controlled zones lies around 45°N (Yi et al 2010, Yue et al 2017, and can be seen as a transition zone where many sites are co-limited by temperature and dryness.…”

Variability in a four-network composite of atmospheric CO<sub>2</sub> differences between three primary baseline sites

“…Global carbon cycle models generally attribute short-term variations in atmospheric CO 2 to exchanges with the terrestrial biosphere (Le Quéré et al, 2018;Rödenbeck et al, 2018;Yue et al, 2017) and implicitly assume model atmospheric transport is correct on all time frames. While the models have demonstrated an impressive ability to predict mid-to-highlatitude CO 2 variations influenced by weather, it is less clear that short-term variations in IH exchange (of a magnitude sufficient to influence hemispheric growth rates) have been adequately captured.…”

“…The 2015-2016 El Niño was stronger and has also been associated with unprecedented behaviour in the global carbon cycle (elsewhere attributed to the terrestrial biosphere anomalies, e.g. Yue et al, 2017). However, Frederiksen and Francey (2018;FF18), argued that the unprecedented strength in the Hadley circulation increased IH exchange (reduced IH CO 2 difference) late in 2016, overwhelming the earlier reduced eddy exchange linked to the strong 2015-2016 El Niño.…”