Abstract
Abstract. Climate change is projected to increase the imbalance between the supply (precipitation) and atmospheric demand for water (i.e., increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able to offset increasing aridity because rising CO2 increases water use efficiency. CO2 fertilization has also been cited as one of the drivers of the widespread “greening” phenomenon. However, attributing the size of this CO2 fertilization effect is complicated, due in part to a lack of long-term vegetation monitoring and interannual- to decadal-scale climate variability. In this study we asked the question of how much CO2 has contributed towards greening. We focused our analysis on a broad aridity gradient spanning eastern Australia's woody ecosystems. Next we analyzed 38 years of satellite remote sensing estimates of vegetation greenness (normalized difference vegetation index, NDVI) to examine the role of CO2 in ameliorating climate change impacts. Multiple statistical techniques were applied to separate the CO2-attributable effects on greening from the changes in water supply and atmospheric aridity. Widespread vegetation greening occurred despite a warming climate, increases in vapor pressure deficit, and repeated record-breaking droughts and heat waves. Between 1982–2019 we found that NDVI increased (median 11.3 %) across 90.5 % of the woody regions. After masking disturbance effects (e.g., fire), we statistically estimated an 11.7 % increase in NDVI attributable to CO2, broadly consistent with a hypothesized theoretical expectation of an 8.6 % increase in water use efficiency due to rising CO2. In contrast to reports of a weakening CO2 fertilization effect, we found no consistent temporal change in the CO2 effect. We conclude rising CO2 has mitigated the effects of increasing aridity, repeated record-breaking droughts, and record-breaking heat waves in eastern Australia. However, we were unable to determine whether trees or grasses were the primary beneficiary of the CO2-induced change in water use efficiency, which has implications for projecting future ecosystem resilience. A more complete understanding of how CO2-induced changes in water use efficiency affect trees and non-tree vegetation is needed.
Highlights
Australia is the world’s driest inhabited continent
The relative increases in normalized difference vegetation index (NDVI) were comparable between the earlier Advanced Very High Resolution Radiometer (AVHRR) epoch (1982–2000) and the later MODIS epoch (2001–2019) at 5.7 % (CI = [−2.9 %, +20.3 %]) and 5.1 % (CI = [−6.4 %, +20.1 %]), respectively
In contrast to the estimates over eastern Australia from Wang et al (2020), we found no consistent evidence of a decline in the effect of CO2 on NDVI through time
Summary
Australia is the world’s driest inhabited continent. Predicting how climate change will affect ecosystem resilience and alter Australia’s terrestrial hydrological cycle is of paramount importance. In the last 2 decades, southeastern Australia experienced the two worst droughts in the observational record (2001–2009; van Dijk et al, 2013; and 2017– 2019; Bureau of Meteorology, 2019). Between these two droughts, eastern Australia experienced record-breaking rainfall in 2011 associated with a strong La Niña event. This caused marked vegetation “greening” (e.g., increased foliar cover), even in the arid interior (Bastos et al, 2013; Poulter et al, 2014; Ahlstrom et al, 2015). This greening contributed to record-breaking fires in the following year (Harris et al, 2018)
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