Regional atmospheric cooling and wetting effect of permafrost thaw‐induced boreal forest loss

Abstract

In the sporadic permafrost zone of North America, thaw-induced boreal forest loss is leading to permafrost-free wetland expansion. These land cover changes alter landscape-scale surface properties with potentially large, however, still unknown impacts on regional climates. In this study, we combine nested eddy covariance flux tower measurements with satellite remote sensing to characterize the impacts of boreal forest loss on albedo, eco-physiological and aerodynamic surface properties, and turbulent energy fluxes of a lowland boreal forest region in the Northwest Territories, Canada. Planetary boundary layer modelling is used to estimate the potential forest loss impact on regional air temperature and atmospheric moisture. We show that thaw-induced conversion of forests to wetlands increases albedo: and bulk surface conductance for water vapour and decreases aerodynamic surface temperature. At the same time, heat transfer efficiency is reduced. These shifts in land surface properties increase latent at the expense of sensible heat fluxes, thus, drastically reducing Bowen ratios. Due to the lower albedo of forests and their masking effect of highly reflective snow, available energy is lower in wetlands, especially in late winter. Modelling results demonstrate that a conversion of a present-day boreal forest-wetland to a hypothetical homogeneous wetland landscape could induce a near-surface cooling effect on regional air temperatures of up to 3-4 °C in late winter and 1-2 °C in summer. An atmospheric wetting effect in summer is indicated by a maximum increase in water vapour mixing ratios of 2 mmol mol . At the same time, maximum boundary layer heights are reduced by about a third of the original height. In fall, simulated air temperature and atmospheric moisture between the two scenarios do not differ. Therefore, permafrost thaw-induced boreal forest loss may modify regional precipitation patterns and slow down regional warming trends.

Higher climate warming sensitivity of Siberian larch in small than large forest islands in the fragmented Mongolian forest steppe

“…This pattern is attributable to the long winter in Mongolia's forest‐steppe region, which largely determines mean annual temperature that lies below freezing point in many regions of the country (Hauck, Dulamsuren, & Leuschner, ). The generally lower temperatures in the grassland‐dominated forest steppe compared to the forest‐dominated landscape may result from higher wind speeds (Kelliher, Leuning, & Schulze, ) and from the higher albedo of grasslands than of boreal forests especially in winter (Li et al., ; Helbig et al., ). Furthermore, the higher aerodynamical roughness of forests compared to grasslands causes a greater potential to heat the air (Baldocchi & Vogel, ).…”

Minor contribution of overstorey transpiration to landscape evapotranspiration in boreal permafrost peatlands

“…Mean annual air temperature has increased since the 1970s, contributing to accelerated permafrost thaw increasing from 0.19% (of total basin area) per year (1970–2000) to 0.58% per year (2000–2015; Chasmer & Hopkinson, ). The increasing rate of permafrost disappearance has caused the expansion of bogs and fens at the expense of forested peat plateaus at Scotty Creek and across the southern Taiga Plains ecozone (Baltzer et al, ; Chasmer & Hopkinson, ; Helbig, Pappas, & Sonnentag, ; Helbig, Wischnewski, et al, ). Similar landscapes to Scotty Creek are widespread throughout the Canadian and Russian boreal forest (Olefeldt et al, ), with a reported 21% of the global boreal forest classified as lowland forest overlying permafrost or thick overburden (Helbig, Pappas, & Sonnentag, ).…”

“…Along its southern limit, permafrost is relatively warm and thin (Smith, Burgess, Riseborough, & Mark Nixon, ). In recent decades, warming air temperatures have accelerated the rate of permafrost thaw (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, ; Chasmer & Hopkinson, ; Grosse et al, ; Quinton, Hayashi, & Chasmer, ), impacting forest composition, structure, and function (Baltzer et al, ; Jorgenson et al, ; Lara et al, ; Sniderhan & Baltzer, ), and thus carbon, water, and energy fluxes (Grosse et al, ; Helbig, Wischnewski, et al, ; Turetsky, Wieder, & Vitt, ). Thawing sporadic discontinuous permafrost, that is, permafrost occurring on <50% of the landscape, can induce surface subsidence, thereby replacing forests on peat plateaus with permafrost‐free peatlands (Baltzer et al, ; Jorgenson et al, ; Lara et al, ).…”

Increasing canopy photosynthesis in rice can be achieved without a large increase in water use—A model based on free‐air CO2 enrichment

“…The temporal evolutions of potential temperature (θ m ), mixing ratio of water vapor ( w m ), and mixing ratio of CO 2 ( c m ) within the PBL were simulated with the land surface model coupled with a simple 1‐D clear‐sky PBL model (e.g., Baldocchi & Ma, ; Helbig et al., ). The coupled model also provides T c when simulated θ m , w m , and c m were used for meteorological forcing for the next time step ( T c, P B L ).…”