Snow droughts are a new way to understand changes in snowpack and subsequent runoff. Globally, we do not have a good understanding of the drivers of snow droughts or how those drivers have changed historically. Here, we identify what has been the dominant driver of global snow droughts in mountain ranges, how it shifted historically, and what similarities exist in similar snow types. We explore this in all global mountain ranges, ones that are highly dependent on winter precipitation for summer water, and two regional case studies in the Cascade Range and the Himalayan Mountains. We found that in both the northern and southern hemispheres, dry snow droughts (driven by precipitation) are the most common. In both the northern and southern hemisphere, more mountain ranges shifted to having temperature be the main driver of snow droughts in the historical record. In the northern hemisphere, tundra, boreal, prairie, and ice snow type areas had the most area with dry snow droughts. In the southern hemisphere, all snow types except for tundra had the most area with temperature as the main driver of snow droughts. With this global, multivariate methodology, we were able to identify common drivers and patterns of historical snow droughts that exist across similar geographical areas (i.e., northern and southern hemisphere and mountain ranges) and snow type areas. More research is needed to better understand snow droughts, their drivers, and the risk they pose regionally to food and water security.

The fertile Anatolian lands in Turkey, supporting about 80 million people, rely on abundant water resources. The Kızılırmak River basin in Anatolia is vulnerable to global warming, mainly due to snowmelt in its headwaters. Quantifying the upper watershed’s climate sensitivity is crucial for assessing water availability. Instead of using Global Climate Model (GCM)-driven projections, a sensitivity-based approach was employed with the Variable Infiltration Capacity (VIC) hydrologic model to assess the region’s hydrological vulnerability to potential future climatic changes. Considering the consistent projections of increasing temperature (T) over this region in GCMs, the system was perturbed to examine gradients of a more challenging climate, characterized by warming and drying conditions. The sensitivity of streamflow, snowpack water equivalence, and evapotranspiration to T and Precipitation (P) variations under each perturbation or “reference” climates was quantified. Results indicate that streamflow responds to T negatively under all warming scenarios. Streamflow responding to P increases nonlinearly as P decreases in the reference climates. These results suggest that there will be heightened difficulty in managing water resources in the region if it undergoes both warming and drying due to the following setbacks: 1) water availability will shift away from the summer season of peak water demand due to the warming effects on the snowpack, 2) annual water availability will likely decrease due to a combination of warming and lower precipitation, and 3) streamflow sensitivity to hydroclimatic variability will increase, meaning that water managers will likely need to plan for a system that is more sensitive to weather variations.

Fire regimes are influenced by both exogenous drivers (e.g., increases in atmospheric CO2; and climate change) and endogenous drivers (e.g., vegetation and soil/litter moisture), which constrain fuel loads and fuel aridity. Herein, we identified how exogenous and endogenous drivers can interact to affect fuels and fire regimes in a semiarid watershed in the inland northwestern U.S. throughout the 21st century. We used a coupled ecohydrologic and fire regime model to examine how climate change and CO2 scenarios influence fire regimes over space and time. In this semiarid watershed we found that, in the mid-21st century (2040s), the CO2 fertilization effect on vegetation productivity outstripped the effects of climate change-induced fuel decreases, resulting in greater fuel loading and, thus, a net increase in fire size and burn probability; however, by the late-21st century (2070s), climatic warming dominated over CO2 fertilization, thus reducing fuel loading and fire activity. We also found that, under future climate change scenarios, fire regimes will shift progressively from being flammability to fuel-limited, and we identified a metric to quantify this shift: the ratio of the change in fuel loading to the change in its aridity. The threshold value for which this metric indicates a flammability versus fuel-limited regime differed between grasses and woody species but remained stationary over time. Our results suggest that identifying these thresholds in other systems requires narrowing uncertainty in exogenous drivers, such as future precipitation patterns and CO2 effects on vegetation.

Although natural disturbances such as wildfire, extreme weather events, and insect outbreaks play a key role in structuring ecosystems and watersheds worldwide, climate change has intensified many disturbance regimes, which can have compounding negative effects on ecosystem processes and services. Recent studies have highlighted the need to understand whether wildfire increases or decreases after large-scale beetle outbreaks. However, observational studies have produced mixed results. To address this, we applied a coupled ecohydrological-fire regime-beetle effects model (RHESSys-WMFire-Beetle) in a semiarid watershed in the western US. We found that surface fire probability and fire size decreased in the red phase (0-5 years post-outbreak), increased in the gray phase (6-15 years post-outbreak), and depended on mortality level in the old phase (one to several decades post-outbreak). In the gray and old phases, surface fire size and probability did not respond to low levels of beetle-caused mortality (<=20%), increased during medium levels of mortality (>20% and <=50%), and remained elevated but did not change with mortality (during the gray phase) or decreased (during the old phase) when mortality was high (>50%). Wildfire responses also depended on fire regime. In fuel-limited locations, fire typically increased with increasing fuel loads, whereas in fuel-abundant (flammability-limited) systems, fire sometimes decreased due to decreases in fuel aridity. This modeling framework can improve our understanding of the mechanisms driving wildfire responses and aid managers in predicting when and where fire hazards will increase.