Atmospheric CO2 concentrations are expected to continue increasing due to ongoing fossil fuel emissions, leading to innumerable climate impacts. These impacts are largely associated with the enhanced greenhouse effect, but recent work highlights that plant CO2 physiological responses can also strongly influence the climate. This study explores the impacts of plant responses on hydrological cycling at 2x preindustrial CO2 concentrations by analyzing simulations that separate plant physiology from climate responses to CO2 using the Community Earth System Model (CESM) versions 1 and 2. We find that CESM2 leaf area growth increases canopy evaporation, which offsets transpiration declines from reduced stomatal conductance, and dampens changes in precipitation, evapotranspiration, and runoff compared to CESM1 and a configuration of CESM2 with fixed leaf area. CESM2 better captures present-day leaf area magnitudes compared to observations but potentially over-estimates leaf area-CO2 sensitivity, highlighting the ongoing need to reduce plant allocation-related biases.

Comprehensive land models are subject to significant parametric uncertainty, which can be hard to quantify due to the large number of parameters and high model computational costs. We constructed a large parameter perturbation ensemble (PPE) for the Community Land Model vesion 5.1 with biogeochemistry configuration (CLM5.1-BGC). We performed more than 2000 simulations perturbing 211 parameters across six forcing scenarios. This provides an expansive dataset, which can be used to identify the most influential parameters on a wide range of output variables globally, by biome, or by plant functional type. We found that parameter effects can exceed scenario effects and that a small number of parameters explains a large fraction of variance across our ensemble. The most important parameters can differ regionally and also based on the forcing scenario. The software infrastructure developed for this experiment has greatly reduced the human and computer time needed for CLM PPEs, which can facilitate routine investigation of parameter sensitivity and uncertainty, as well as automated calibration.

Plant stomata mediate the fluxes of both carbon and water between the land and the atmosphere. The ratio between photosynthesis and stomatal conductance (gs), or intrinsic water-use efficiency (iWUE), can be directly inferred from leaf or tree-ring carbon isotope composition. In many Earth system models, iWUE is inversely proportional and controlled by a parameter (g1M) in the calculation of gs. Here we examine how iWUE perturbations, setting g1M to the 5th (low) and 95th (high) percentile for each plant type based on observations, influence photosynthesis using coupled Earth System model simulations. We find that while lower iWUE leads to reductions in photosynthesis nearly everywhere, higher iWUE had a photosynthetic response that is surprisingly regionally dependent. Higher iWUE increases photosynthesis in the Amazon and central North America, but decreases photosynthesis in boreal Canada under fixed atmospheric conditions. However, the photosynthetic response to higher iWUE in these regions unexpectedly reverses when the atmosphere dynamically responds due to spatially differing sensitivity to increases in temperature and vapor pressure deficit. iWUE also influences the photosynthetic response to atmospheric CO2, with higher and lower iWUE modifying the total global response to elevated 2x preindustrial CO2 by 6.4% and -9.6%, respectively. Our work demonstrates that assumptions about iWUE in Earth system models significantly affect photosynthesis and its response to climate. Further, the response of photosynthesis to iWUE depends on which components of the model are included, therefore studies of iWUE impacts on historical or future photosynthesis can not be generalized across model configurations.

Terrestrial processes control land-to-atmosphere fluxes of water, energy, and carbon and are also influenced by climate. Feedbacks in the land-atmosphere coupled system can therefore potentially modulate changes in land surface water fluxes. Prior work has focused on one specific aspect of land-atmosphere coupling modulated by soil moisture, while largely ignoring other ways in which coupling between the land and atmosphere could alter climate. We use a novel experimental design of paired perturbed parameter ensembles in an Earth system model to isolate the extent to which land-atmosphere feedbacks modulate the global water cycle by perturbing land parameters spanning a wide range of terrestrial processes which affect evapotranspiration. We find support for the traditional soil moisture-precipitation coupling in some dry or transitional dry-to-wet regions where evapotranspiration is soil-moisture limited. However, we also find that land-atmosphere feedbacks have a dampening effect on evapotranspiration in wet regions. The effect in wet regions is more widespread and more robust than the impact in dry regions which is consistent with first principles but surprisingly has not been quantified in prior work. By adopting a more holistic definition of land-atmosphere coupling, our analysis provides insights into where and how land-atmosphere feedbacks modulate terrestrial processes, posing a challenge to the widespread practice of developing and evaluating land models in an uncoupled configuration and then deploying them to understand and predict terrestrial processes in a coupled context.

Photosynthetic eukaryotic algae survived the Neoproterozoic Snowball Earth events, indicating that liquid-water refugia existed somewhere on the surface. We examine the potential for refugia at the coldest time of a snowball event, before CO2 had risen and with high-albedo ice on the frozen ocean, before it became darkened by dust deposition. We use the Community Earth System Model to simulate a “modern” Snowball Earth (i.e., with continents in their current configuration), in which the ocean surface has frozen to the equator as “sea glaciers”, hundreds of meters thick, flowing like ice shelves. Despite global mean surface temperatures below -60°C, some areas of the land surface reach above-freezing temperatures because they are darker than the ice-covered ocean. With low CO2 (10 ppm) and land-surface albedo 0.4 (characteristic of bright sand-deserts), 0.1 percent of the land surface could host liquid water seasonally; this increases to 12 percent for darker land of albedo 0.2, characteristic of polar deserts. Narrow bays intruding from the ocean to these locations (such as the modern Red Sea) could provide a water source protected from sea-glacier invasion, where photosynthetic life could survive. The abundance of potential refugia increases more strongly in response to reducing the land albedo than to increasing the CO2, for the same global radiative forcing.

Soils are a major source of nitrogen oxides, which in the atmosphere help govern its oxidative capacity. Thus the response of soil nitric oxide (NO) emissions to forcings such as warming or forest loss has a meaningful impact on global atmospheric chemistry. We find that the soil emission rate of NO in Amazonia from a common inventory is biased low by at least an order of magnitude in comparison to tower-based observations. Accounting for this regional bias decreases the modeled global methane lifetime by 1.4% to 2.6%. In comparison, a fully deforested Amazonia, representing a 37% decrease in global emissions of isoprene, decreases methane lifetime by at most 4.6%, highlighting the sensitive response of oxidation rates to changes in emissions of NO compared to those of terpenes. Our results demonstrate that improving our understanding of soil NO emissions will yield a more accurate representation of atmospheric oxidative capacity.