Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake

From Nature Communications.

"Abstract

Terrestrial ecosystems play a significant role in the global carbon cycle and offset a large fraction of anthropogenic CO₂ emissions. The terrestrial carbon sink is increasing, yet the mechanisms responsible for its enhancement, and implications for the growth rate of atmospheric CO₂, remain unclear. Here using global carbon budget estimates, ground, atmospheric and satellite observations, and multiple global vegetation models, we report a recent pause in the growth rate of atmospheric CO₂, and a decline in the fraction of anthropogenic emissions that remain in the atmosphere, despite increasing anthropogenic emissions. We attribute the observed decline to increases in the terrestrial sink during the past decade, associated with the effects of rising atmospheric CO₂ on vegetation and the slowdown in the rate of warming on global respiration. The pause in the atmospheric CO₂ growth rate provides further evidence of the roles of CO₂ fertilization and warming-induced respiration, and highlights the need to protect both existing carbon stocks and regions, where the sink is growing rapidly.

Introduction

The oceans and the terrestrial biosphere remove about 45% of the CO₂ emitted by human activities each year¹. The rate of CO₂ uptake is not constant, however, and varies greatly from year to year in response to changes in the atmosphere (for example, El Niño events, volcanic eruptions and natural climate variability). The largest component of the year-to-year variability in CO₂ uptake is contributed by processes on land^2,3. Any increase or decrease in terrestrial uptake thus generates a feedback to the atmosphere⁴, which affects the growth rate of atmospheric CO₂, and the rate at which the climate warms.

Over the past 50 years, the amount of CO₂ absorbed by the oceans and terrestrial biosphere annually has more than doubled^1,5,6,7,8. The enhanced carbon sink has been attributed to increased ocean⁹ and terrestrial uptake^1,6,7,8,10, and has occurred despite an increase in the severity and intensity of regional disruptions to ecosystems in recent years, such as extensive droughts, wildfires and insect damage^11,12,13,14. On land, reports suggest a decline in the tropical sink^15,16, increased plant mortality^17,18 and decreased plant productivity due to droughts and extreme events^19,20. In contrast, others report that elevated CO₂ has led to increased photosynthesis⁸ and a greening of the biosphere^21,22. The relative contributions of the different processes involved in the terrestrial sink enhancement remain unquantified. Global warming over vegetated land notably slowed since the start of the twenty-first century²³, while atmospheric CO₂ concentrations continue to rise, providing an opportunity to test the relative roles of various processes in the enhancement of terrestrial carbon uptake, and examine the implications of enhanced carbon uptake for the growth rate of atmospheric CO₂.

Here we use extensive ground observations of earth–atmosphere CO₂ exchange, atmospheric CO₂ observations and satellite observations of vegetation, along with an ensemble of 10 prognostic dynamic global vegetation models (DGVMs), and a diagnostic process-based modelling approach, to examine the causes of the long-term enhancement of terrestrial carbon uptake and consequences for the growth rate of atmospheric CO₂. Our analysis suggests that enhanced carbon uptake is due to the combined effects of rising CO₂ on photosynthesis (the CO₂ fertilization effect) and, in the past decade, a slowdown in the rate of warming on global respiration. The continued enhancement of the terrestrial carbon sink during the recent slowdown in global warming led to a pause in the atmospheric CO₂ growth rate, and a decline in the fraction of anthropogenic emissions that remains in the atmosphere.

Results

Slowing of the growth rate of atmospheric CO₂

Changes in the residual terrestrial carbon sink affect the proportion of anthropogenic emissions that remain in the atmosphere (the airborne fraction), and thus the growth rate of atmospheric CO₂. Our analysis suggests that the airborne fraction increased steadily from the 1960s to the 1990s (1.8% per year, P=0.03; Fig. 1b), albeit with large interannual variability reflecting year-to-year variability in the terrestrial sink⁴. Since the start of the twenty-first century, however, the airborne fraction has been declining (−2.2% per year, P=0.07; Fig. 1b), despite the rapid increase in anthropogenic emissions (Fig. 1b). Changes in the airborne fraction are reflected in the atmospheric CO₂ growth rate. For the three decades from the start of the measurement record in 1959, the atmospheric CO₂ growth rate increased from 0.75 to 1.86 p.p.m. per year (Fig. 1a). However, for the period 2002–2014 there has been no significant increase in the growth rate of CO₂ (Fig. 1a and Supplementary Fig. 1). The decline in the airborne fraction since the start of the twenty-first century has therefore been sufficiently large as to result in a pause in the rate of increase of the atmospheric CO₂ growth rate (Fig. 1a). Atmospheric growth rates have deviated significantly from predictions of a linear model of atmospheric CO₂ concentrations and anthropogenic emissions since 2002 (Supplementary Fig. 1), suggesting a nonlinear increase in the global sink strength."