Arctic and Boreal Carbon

Observation and modeling results synthesized in this chapter suggest that significant changes in the carbon stocks of Arctic and boreal regions may occur with impacts on the atmospheric GHG budget. These projections primarily are due to the large pools of soil carbon preserved in cold and waterlogged environments vulnerable to a changing climate. This region, which previously has sequestered large amounts of carbon for centuries to millennia, is expected to transform into a one that acts as a net carbon source to the atmosphere over the next decades to centuries in a warming climate. Indeed, Arctic and boreal systems possibly have gone through this transition already.

Carbon offsets by vegetation remain a key part of the net response of this region to warming. Rising Arctic temperatures appear to have increased plant biomass, an effect observed in the tundra over the last three decades using satellite remote-sensing tools (Frost and Epstein 2014; Jia et al., 2003; Ju and Masek 2016) and field observations (Elmendorf et al., 2012; Salmon et al., 2016). A greener Arctic has important implications for regional and global climate because of anticipated increases in atmospheric CO₂ uptake, changes in surface energy, and altered nutrient and water cycling. Despite this long-term trend toward a greener Arctic, a distinct reversal of this trend has been observed for tundra from 2011 to 2014 (Epstein et al., 2015; Phoenix and Bjerke 2016), and the long-term trend is in contrast to boreal regions that show decreased NDVI (browning; Beck and Goetz 2011). Models, in contrast, tend to show consistent increases in plant growth, both in retrospective analyses (McGuire et al., 2016) and in future forecasts. Documenting changes in biomass with repeat LIDAR measurements is an approach for producing future datasets that help validate or refute model projections of enhanced carbon uptake.

Emerging research on disturbance of permafrost soils by abrupt thaw is another knowledge gap where new information on modeling and landscape mapping is helping to describe patterns and proc­esses (Olefeldt et al., 2016). Abrupt permafrost thaw can trigger destabilization of permafrost and soils at rates much higher than predicted from changes in temperature alone. However, this disturbance occurs at specific points covering only a fraction of the landscape compared to that affected by the influence of temperature increases occurring regionally (Kokelj et al., 2017). New research is critical for highlighting the importance of this subgrid pulse disturbance at the landscape scale and for providing the process-level detail needed but currently lacking in regional- and global-scale models.

Lastly, apparent offsets in carbon flux estimates made by top-down atmospheric measurements and from bottom-up scaling of ecosystem measurements always will be hampered in this region because of the relative scarcity of study locations. New research and satellite capabilities currently focused on high-latitude ecosystems are helping to increase data coverage in this remote and understudied region and will set important baselines against which to measure future change.