Robust ecological drought projections for drylands in the 21st century

Abstract

Dryland ecosystems may be especially vulnerable to expected 21st century increases in temperature and aridity because they are tightly controlled by moisture availability. However, climate impact assessments in drylands are difficult because ecological dynamics are dictated by drought conditions that are difficult to define and complex to estimate from climate conditions alone. In addition, precipitation projections vary substantially among climate models, enhancing variation in overall trajectories for aridity. Here, we constrain this uncertainty by utilizing an ecosystem water balance model to quantify drought conditions with recognized ecological importance, and by identifying changes in ecological drought conditions that are robust among climate models, defined here as when >90% of models agree in the direction of change. Despite limited evidence for robust changes in precipitation, changes in ecological drought are robust over large portions of drylands in the United States and Canada. Our results suggest strong regional differences in long‐term drought trajectories, epitomized by chronic drought increases in southern areas, notably the Upper Gila Mountains and South‐Central Semi‐arid Prairies, and decreases in the north, particularly portions of the Temperate and West‐Central Semi‐arid Prairies. However, we also found that exposure to hot‐dry stress is increasing faster than mean annual temperature over most of these drylands, and those increases are greatest in northern areas. Robust shifts in seasonal drought are most apparent during the cool season; when soil water availability is projected to increase in northern regions and decrease in southern regions. The implications of these robust drought trajectories for ecosystems will vary geographically, and these results provide useful insights about the impact of climate change on these dryland ecosystems. More broadly, this approach of identifying robust changes in ecological drought may be useful for other assessments of climate impacts in drylands and provide a more rigorous foundation for making long‐term strategic resource management decisions.

Forest density intensifies the importance of snowpack to growth in water‐limited pine forests

“…Forest density increases competition for water resources as well as the volume of SWE and timing of snowmelt under the forest canopy. Although climate change has uncertain long‐term projections for many components of drought, other aspects, particularly seasonal moisture dynamics with high ecological relevance, display reasonably consistent responses because of a tight link to temperature (Bradford et al 2020). In the context of rising temperature and declining snowpack in the 21st century, these results suggest that moderating forest density may be particularly important in hot‐dry, water‐limited forests.…”

“…Looking forward, continued temperature increases are consistent features of climate projections, meaning that impacts to snowpack may only increase (Collins et al 2013 b , Knutti and Sedlacek 2013, Polade et al 2014, Trenberth et al 2015, Diffenbaugh et al 2017, Prein et al 2017). Although the impact of climate change on many aspects of the hydrologic cycle is still uncertain (Collins et al 2013 a , Padrón et al 2019, Bradford et al 2020), the close link between temperature and snowpack implies that declining snowpack is expected to continue, and loss will be relatively extreme in lower elevation and/or lower latitude forests, where snowpack is particularly vulnerable and also where drought dynamics are particularly important (Demaria et al 2016, Scalzitti et al 2016).…”

Onset and propagation of drought into soil moisture and vegetation responses during the 2012–2019 drought in Southern California

“…If these sorts of drought occur in the future and if they intensify, it is expected to lead to dramatic shifts in soil moisture towards a drier baseline for extended periods. This outcome could potentially leave grassland species diversity and productivity severely compromised, due to increased susceptibility to encroachment of invasive species and expansion of shrublands, thus limiting native species regeneration (Bradford et al, 2020;Peters et al, 2010;Reynier et al, 2017). Earlier and longer dry seasons with increased temperatures and increased evaporative losses and soil drying also reduce surface evaporative cooling and increase the possibility of more intense summer heatwaves (Lian et al, 2020).…”

“…With climate change projected to impact the temperate and precipitation regimes in California and much of the Southwestern U.S., the frequency and magnitude of droughts and drought-like conditions is expected to increase (Bradford et al, 2020;Diffenbaugh et al, 2015). Under a more severe emission scenario of RCP 8.5, the frequency of extreme dry years is projected to almost triple and temperatures are projected to increase by up to 4°C throughout California (Pierce et al, 2018;Thorne et al, 2016).…”

“…Under a more severe emission scenario of RCP 8.5, the frequency of extreme dry years is projected to almost triple and temperatures are projected to increase by up to 4°C throughout California (Pierce et al, 2018;Thorne et al, 2016). Precipitation projections remain uncertain (Pierce et al, 2018, Bradford et al, 2019, but given the degree of already existing aridity in the Southwest, even relatively modest changes to precipitation intensity and frequency would create conditions much more conducive to prolonged drought periods. The increased occurrence of extended dry periods would slow down the recovery from moisture deficit inherited from prior drought conditions, raising the potential for soil moisture deficits to accumulate to unprecedented levels (Diffenbaugh et al, 2015).…”