Droughts and deluges in semi-arid grassland ecosystems: Implications of co-occurring extremes for C cycling

The frequency of extreme climate perturbations including multi-year droughts and deluges (persistent, torrential rain events) is expected to increase with changing climate. Such climate extremes

climate extreme

Significant deviations or anomalies from typical weather patterns and conditions, observed over extended periods such as seasons or even longer durations.

Source: National LIbrary of Medicine Source

are known to have substantial impacts on ecosystems,

ecosystem

A natural community of plants, animals, and other living organisms and the physical environment in which they live and interact.

Source: A student's guide to Global Climate Change Source

but when these co-occur as compound climate extremes (e.g., deluges occur during droughts), their impacts are expected to exceed their independent effects. Although there is anecdotal evidence to suggest that droughts and deluges combined may lead to pulses or “hot moments” in carbon cycling processes, the ecosystem consequences of compound climate extremes are poorly known. The overall goal of our proposed research is to assess how co-occurring drought and deluge climate extremes will impact key carbon cycling processes known to be important for carbon-climate feedbacks. We will address this goal via research in the 280,000 km2 semi-arid shortgrass steppe ecoregion located at the western edge of the US Great Plains. Semi-arid regions, such as this one, respond strongly to precipitation extremes and play a dominant role in interannual variability of the global land CO2

carbon dioxide (CO2)

An important heat-trapping gas, also known as a greenhouse gas, that comes from the extraction and burning of fossil fuels (such as coal, oil, and natural gas), from wildfires, and natural processes like volcanic eruptions.

Source: NASA Source

sink.

Our proposed research will test the hypothesis that when a compound climate perturbation of an extreme deluge occurring within the backdrop of extreme drought, a combination of conditions converge (e.g., warm temperatures, abundant soil moisture, and increased soil N availability) to strongly stimulate C cycle processes, potentially resulting in “hot moments” or landscape-level “hot spots” (i.e., increases in biogeochemical processes

biogeochemical processes

The complex interactions and transformations of chemical elements and compounds that occur throughout Earth's ecosystems, such as nutrient and element cycling between living organisms, the atmosphere, hydrosphere, lithosphere, and pedosphere (soil).

Source: Source

in time or space that far exceed background levels). To test this hypothesis, we will conduct a field experiment designed to quantify the magnitude of carbon cycling responses to drought and deluge events (independently and combined) and identify the underlying mechanisms resulting in positive drought-deluge interactions that can lead to hot moments of carbon cycling. Both above- and belowground C cycle responses to climate extremes will be quantified during this 3-yr experiment. To scale-up from the plot-level experiment to the shortgrass steppe ecoregion, we will use historical climate data to quantify the regional frequency of potential drought-deluge interactions and remote sensing

remote sensing

The process of detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation at a distance (typically from satellite or aircraft).

Source: USGS Source

products (NDVI and Solar Induced Fluorescence) to estimate carbon cycling sensitivity to droughts, deluges and their combined effects and to identify hot spots in carbon cycling regionally. Concurrent with these research activities, we will simulate extreme drought, deluge and drought-deluge perturbations with DOE’s E3SM

Energy Exascale Earth System Model (E3SM)

The Energy Exascale Earth System Model (E3SM) project includes Earth System Models, simulations, and predictions to investigate energy-relevant science using code optimized for DOE's advanced supercomputers.

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Land Model (ELM). We will explicitly compare the experimental results and remotely sensed observations of drought-deluge compound climate perturbations to ELM simulations, with the expectation that the process-level understanding gained from our field experiment and remote sensing analyses can be used to constrain process representation and parameterization in ELM, and to improve Earth System projections of ecosystem carbon-cycling responses to droughts and deluges at the ecoregion scale.

This proposed research will fill an important gap in empirical and modeling efforts to assess the effects of increasing compound perturbations forecast to occur with atmospheric warming – co-occurring extreme drought and deluge events – on key carbon cycling processes, and the role that compound extremes in precipitation may play in generating and sustaining biogeochemical “hot moments” and “hot spots”. Our ModEx approach will leverage and complement ongoing development of DOE’s state-of-the-art ELM model to further improve representation of above-belowground interactions and feedbacks for semi-arid ecosystems.