Drought indices overestimate many drought impacts of global warming, even when CO2 effects on vegetation are eliminated
Abstract
Recent studies have shown that while Earth System Model (ESM) future projections for offline dryness indices like the PDSI, SPEI, and Aridity Index are almost ubiquitously negative and severe, the same ESMs' actual runoff, deep-layer soil moisture, and evaporative-fraction projections are instead dominated by regional signals of varying sign, implying that the offline indices are globally misleading under climate change. Observed historical trends also show this "index-impact gap". Most studies have attributed this gap to the offline indices' omission of CO2-driven stomatal closure, which decreases transpiration and thus increases deep soil moisture and runoff. Yet, other work has suggested that the offline indices are incorrect under climate change for more basic, non-CO2-related reasons, such as neglect of VPD-driven stomatal closure, inability to account for intensifying rain rates, rectification of the seasonal cycle, or reliance on potential evaporation.
In this study, we show that the index-impact gap is still largely present even in special ESM runs that switch off all physiological effects of CO2 on plants. Specifically, topsoil moisture, PDSI, SPEI, Aridity Index, and near-surface relative humidity still undergo very widespread declines with global warming in these runs, while runoff generation, deep soil moisture, and evaporative fraction still have a more equivocal, two-sided response (though the gap is somewhat smaller than in fully coupled experiments.) This suggests that CO2-plant effects are not in fact the main reason why drought indices overestimate hydrological impacts of climate change. Future land-model experiments will clarify the true cause(s) of the global discrepancy between drought indices and impacts.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H13E..02S
- Keywords:
-
- 1812 Drought;
- HYDROLOGY;
- 1817 Extreme events;
- HYDROLOGY;
- 1847 Modeling;
- HYDROLOGY;
- 1880 Water management;
- HYDROLOGY