Rising ecosystem water demand exacerbates the lengthening of tropical dry seasons

Abstract

The timing and length of the dry season is a key factor governing ecosystem productivity and the carbon cycle of the tropics. Mounting evidence has suggested a lengthening of the dry season with ongoing climate change. However, this conclusion is largely based on changes in precipitation (P) compared to its long-term average (P ̅) and lacks consideration of the simultaneous changes in ecosystem water demand (measured by potential evapotranspiration, Ep, or actual evapotranspiration, E). Using several long-term (1979-2018) observational datasets, we compared changes in tropical dry season length (DSL) and timing (dry season arrival, DSA, and dry season end, DSE) among three common metrics used to define the dry season: P < P ̅, P < Ep, and P < E. We found that all three definitions show that dry seasons have lengthened in much of the tropics since 1979. Among the three definitions, P < E estimates the largest fraction (49.0%) of tropical land area likely experiencing longer dry seasons, followed by P < Ep (41.4%) and P < P ̅ (34.4%). The largest differences in multi-year mean DSL (> 120 days) among the three definitions occurred in the most arid and the most humid regions of the tropics. All definitions and datasets consistently showed longer dry seasons in southern Amazon (due to delayed DSE) and central Africa (due to both earlier DSA and delayed DSE). However, definitions that account for changing water demand estimated longer DSL extension over those two regions. These results indicate that warming-enhanced evapotranspiration exacerbates dry season lengthening and ecosystem water deficit. Thus, it is necessity to account for the evolving water demand of tropical ecosystems when characterizing changes in seasonal dry periods and ecosystem water deficits in an increasingly warmer and drier climate.

Biomes of the world under climate change scenarios: increasing aridity and higher temperatures lead to significant shifts in natural vegetation

“…and more severe dry seasons (Xu et al, 2022;Agudelo et al, 2019;Arias et al, 2015), with an increase in frequency of droughts, floods and fires (Barlow et al, 2020;Lovejoy and Nobre, 2018;Marengo and Espinoza, 2016); a recent study by (Gatti et al, 2021) has demonstrated how the southeastern edge of the Amazon rainforest has already reached the tipping point, acting as a net carbon source instead of a carbon sink. These findings agree with the projections showed in our results, which now are part of a long series of studies showing alarming signs of an incoming process of savannization in the area; the feedback loop created by a disruption in the carbon cycle such as the one showed by Gatti et al (2021) could further exacerbate the savannization process.…”