Thirty-eight years of CO<sub>2</sub> fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

Abstract. Climate change is projected to increase the imbalance between supply (precipitation) and atmospheric demand for water (i.e., increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able offset increasing aridity because rising CO2 increases use efficiency. fertilization has also been cited as one of drivers widespread “greening” phenomenon. However, attributing size this effect complicated, due part a lack long-term vegetation monitoring interannual- decadal-scale climate variability. In study we asked question how much contributed towards greening. We focused our analysis on broad gradient spanning eastern Australia's woody ecosystems. Next analyzed 38 years satellite remote sensing estimates greenness (normalized difference index, NDVI) examine role ameliorating impacts. Multiple statistical techniques were applied separate CO2-attributable effects greening from changes aridity. Widespread occurred despite warming climate, vapor pressure deficit, repeated record-breaking droughts heat waves. Between 1982–2019 found that NDVI (median 11.3 %) across 90.5 % regions. After masking disturbance (e.g., fire), statistically estimated an 11.7 attributable CO2, broadly consistent with hypothesized theoretical expectation 8.6 efficiency CO2. contrast reports weakening effect, no temporal effect. conclude mitigated aridity, droughts, waves Australia. unable determine whether trees or grasses primary beneficiary CO2-induced efficiency, which implications projecting future ecosystem resilience. A more complete understanding affect non-tree needed.