The Arctic may be the most climatically sensitive region on Earth. High latitudes have experienced the greatest regional warming in recent decades and are projected to warm at a rate twice that of the global average in the coming century (Bieniek et al. 2014). The implications of such warming include permafrost thaw and deepening of the active layer, microbial decomposition of vulnerable soil organic matter, altered productivity and migration of vegetation, and changes in surface and groundwater storage (Hinzman et al. 2013). Recent literature emphasizes that the following topics are central priorities for experimental, observational, and model development research:
- Improved representation of subgrid heterogeneity related to permafrost distribution, soil carbon stocks, surface inundation, distribution of vegetation, and atmospheric forcing (Rowland et al. 2010; Aleina et al. 2013);
- Better descriptions of permafrost thaw and deepening of the active layer, and the consequences for microbial dynamics and carbon feedbacks to climate (Cahoon et al. 2012; Hodgkins et al. 2014; McCalley et al. 2014; Schuur et al. 2015);
- Increased understanding of the fundamental controls on vegetation dynamics and their representation in models, including the relationships and tradeoffs among plant functional traits, above and below ground, that enable future innovations in modeling (Epstein et al. 2004a; Wookey et al. 2009; Freschet et al. 2010; van Bodegom et al. 2014; Koven et al. 2015a);
- Enhanced understanding of the climatic and edaphic controls on shrub expansion including uncertainty analysis and new benchmarking datasets for spatial and temporal representation of shrubs in large-scale models (Myers-Smith et al. 2011; Jiang et al. 2012; Zhang et al. 2013; Frost et al. 2014; Wullschleger et al. 2014); and
- Improved characterization of hydrology at watershed to regional scales, and integration of surface and subsurface processes that drive water distribution across Arctic ecosystems, especially as a function of disturbance and landscape transitions in a changing climate (Lara et al. 2015; Natali et al. 2015).
ESMs lack many of the key processes that govern interactions between high-latitude ecosystems and climate (Koven et al. 2013a; Hayes et al. 2014; Schaefer et al. 2014). Multiple carbon, water nutrient, and energy feedbacks that occur in response to rising temperatures must be resolved if we are to improve model prediction of climate. Observations suggest that permafrost thaw is now occurring throughout the Arctic (Jorgenson et al. 2006; Romanovsky et al. 2010) and is expected to drive changes in climate forcing through biogeochemical and biophysical feedbacks. Biogeochemical feedbacks are dominated by the potential to release a large amount of currently stored carbon into the atmosphere as CO2 and CH4 (Schuur et al. 2015), whereas biophysical feedbacks include terrestrial radiation, and sensible and latent heat flux budgets that can lead to large-scale warming (Swann et al. 2010) and that are caused by, among other factors, changes in vegetation distributions (Chapin et al. 2005; Euskirchen et al. 2009). These processes will take place not only in response to warmer temperatures, but also within an environment undergoing geomorphic change and landscape reorganization (Rowland et al. 2010; Grosse et al. 2011). Thawing of ice-rich permafrost can lead to subsidence and deformation of land surfaces that can dramatically change topography, surface and groundwater hydrology, biogeochemistry, and vegetation structure on timescales of years to decades.
Although existing representations of land surface processes in ESMs describe some of the many relationships that exist among vegetation, biogeochemistry, and climate, these representations are highly uncertain and require extensive confrontation with observations to test and improve models (Bouskill et al. 2014; Fisher et al. 2014). Furthermore, many of the coupled properties and processes related to permafrost thaw, surface and subsurface interactions, and soil moisture (Vogel et al. 2009; Natali et al. 2015) are not currently explicitly represented in process models. The presence of ground ice and cryostructures, for example, and their influence on surface topography appear to be critical drivers of landscape-scale processes (Belshe et al. 2013) but cannot be resolved at even the highest resolutions presently conceived for global-scale climate models (Aleina et al. 2013; Lee et al. 2014). This underscores the importance of conducting holistic, multidisciplinary investigations of the Arctic and representation of ecosystems in models as dynamic, evolving, highly coupled systems (Slater and Lawrence 2013).
Accurate prediction of long-term ecosystem climate feedbacks in the Arctic depends in part on the ability of ESMs to represent the critical aspects of land surface climate, such as mean temperatures and precipitation, which control the formation and loss of permafrost, the water and energy balance of watersheds, the function of vegetation, and the burial and potential decomposition of soil organic material. It is encouraging to note that the recent movement toward high-resolution ESMs is providing a more realistic surface climate in the Arctic (Figure 1). Thus, the opportunity exists to match this resolution with an underlying understanding of ecosystem processes and consequences for climate.
As climate predictions improve, we can reasonably set goals for improved models of Arctic ecosystem processes. To assist in setting these goals, there is a need for high-resolution land ecosystem simulation capabilities that allow explicit representation of properties and processes at the spatial and temporal scales where they occur, and methods to incorporate the effects of these fine-scale processes into larger-scale model representations. Such high-resolution modeling can only be achieved through synthesis of new knowledge and understanding of processes emerging from mechanistic studies carried out in the field and in the laboratory. Thus, to cover this broad set of processes and model requirements, we identify five overarching science questions that will guide our research in Phase 2:
Q1. How does the structure and organization of the landscape control the storage and flux of carbon and nutrients in a changing climate?
Q2. What will control rates of CO2 and CH4 fluxes across a range of permafrost conditions?
Q3. How will warming and permafrost thaw affect above- and belowground plant functional traits, and what are the consequences for Arctic ecosystem carbon, water, and nutrient fluxes?
Q4. What controls the current distribution of Arctic shrubs, and how will shrub distributions and associated climate feedbacks shift with expected warming in the 21st century?
Q5. Where, when, and why will the Arctic become wetter or drier, and what are the implications for climate forcing?
Just as we have done in Barrow, we will use variation in the structure and organization of the Seward Peninsula landscape to guide a series of process-level investigations (Questions 1 through 3) that will be nested at scales ranging from core to plot, landscape, and watershed levels. Knowledge derived in these studies will identify mechanisms controlling carbon, water, nutrient, and energy fluxes, which will then be brought to bear on two integrative and timely questions concerning the future of the Arctic in a changing climate (Questions 4 and 5).
Our multi-scale measurement and modeling approach developed in Phase 1, and now proposed for further use in Phase 2, is motivated by three major deficiencies in current ESMs. These are (1) inadequate high-resolution representation of land surface heterogeneity, including the temporal transition of landscapes (i.e., evolution of the land surface) with projected warming, (2) distribution and fate of permafrost and associated vulnerability of stored carbon to loss back to the atmosphere, and (3) biophysical feedbacks to climate brought about by changes in vegetation dynamics and especially shrub migration along the boreal-tundra transition zone. The following sections highlight our 10-year vision for the NGEE Arctic project where we set forth a series of milestones that will lead to the resolution of the above-mentioned deficiencies. We also elaborate on Phase 1 accomplishments and outline plans for Phase 2 where we not only propose to continue our research on the North Slope, but to expand those efforts to the Seward Peninsula.
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