ES49CH13_Schuur ARI 26 September 2018 13:58 Annual Review of Ecology, Evolution, and Systematics Ecological Response to Permafrost Thaw and Consequences for Local and Global Ecosystem Services Edward A.G. Schuur and Michelle C. Mack Center for Ecosystem Science and Society, and Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011, USA; email: [email protected] Annu. Rev. Ecol. Evol. Syst. 2018. 49:279–301 Keywords The Annual Review of Ecology, Evolution, and carbon, nutrient, climate, vegetation change, disturbance, Arctic Systematics is online at ecolsys.annualreviews.org ecosystem, boreal ecosystem https://doi.org/10.1146/annurev-ecolsys-121415- 032349 Abstract Copyright c 2018 by Annual Reviews. The Arctic may seem remote, but the unprecedented environmental changes All rights reserved occurring there have important consequences for global society. Of all Arc- tic system components, changes in permafrost (perennially frozen ground) are one of the least documented. Permafrost is degrading as a result of cli- Access provided by University of Alaska - Fairbanks on 02/04/19. For personal use only. Annu. Rev. Ecol. Evol. Syst. 2018.49:279-301. Downloaded from www.annualreviews.org mate warming, and evidence is mounting that changing permafrost will have significant impacts within and outside the region. This review asks: What are key structural and functional properties of ecosystems that interact with changing permafrost, and how do these ecosystem changes affect local and global society? Here, we look beyond the classic definition of permafrost to include a broadened focus on the composition of frozen ground, including the ice and the soil organic carbon content, and how it is changing. This ecological perspective of permafrost serves to identify areas of both vul- nerability and resilience as climate, ecological disturbance regimes, and the human footprint all continue to change in this sensitive and critical region of Earth. 279 ES49CH13_Schuur ARI 26 September 2018 13:58 1. INTRODUCTION The Arctic may seem distant and remote, but the unprecedented environmental changes occurring there have important consequences for global society. Temperature rise is amplified in northern high-latitude regions, occurring 2.5 times faster than the whole Earth (IPCC 2013), with important impacts on the frozen places of the planet (the cryosphere). Changing sea ice affects global climate, ecosystems and fisheries, and ocean transportation; shrinking ice sheets and glaciers influence water supplies and sea level; degrading permafrost (perennially frozen ground) impacts infrastructure, ecosystem services, and global climate (AMAP 2017). Well-developed satellite observing systems that record sea ice extent daily (NSIDC 2018) and the Greenland ice sheet mass monthly (Wiese et al. 2016) have observed record-breaking low measurements over the last several decades. In contrast, no record of permafrost change with similar spatial or temporal resolution has been made (Biskaborn et al. 2015, Brown & Romanovsky 2008), despite the potential for significant impacts within and outside high-latitude regions. This review focuses on this lesser-known cryospheric domain to address a key societal question: What are key structural and functional properties of ecosystems that interact with changing permafrost, and how do these interactions affect local and global society? Permafrost is perennially frozen ground (rock, soil, ice) that underlies natural ecosystems and human communities in high-latitude areas of Earth. Permafrost is narrowly defined by temper- ature (<0◦C for at least 2 consecutive years), essentially marking the long-term phase change from liquid water to ice. As a result, permafrost plays a role in defining both the physical struc- ture of ground as well as modulating resource availability to organisms, including people, liv- ing on permafrost. In turn, permafrost itself is modified by the overlying ecosystem. The plant community, its associated surface soil organic layer, and other physical and biotic effects on ecosystem energy balance help define where and when permafrost persists (Shur & Jorgenson 2007). Ecological and geomorphological processes are highly sensitive to the phase change between water and ice. This transition point makes the structure and function of permafrost ecosystems unique and also vulnerable to major change in a warming climate. Here, we look beyond the classic definition of permafrost to include a broadened focus on the composition of permafrost and how it is changing. In particular, the ice content and the soil organic carbon in permafrost are key regulators of ecosystem function and provide important services to local people as well as to global society (Schuur et al. 2008). Temperature may classically define permafrost, but ecosystems and people also depend on other factors and resources such as water and nutrients, the availability of which is modulated by frozen ground. Temperature, water, and nutrients are examined within an ecosystem context to highlight aspects of permafrost ecosystems that make their ecological function unique. Access provided by University of Alaska - Fairbanks on 02/04/19. For personal use only. Annu. Rev. Ecol. Evol. Syst. 2018.49:279-301. Downloaded from www.annualreviews.org Permafrost is not static. It depends on regional and global climate conditions, the dynamics of ecosystems through ecological succession, and the impacts of people living within permafrost ecosystems. This review outlines the triggers of changing permafrost in the context of changing northern high-latitude regions as well as the implications for ecological succession and the fu- ture state of permafrost ecosystems including the organisms that live there. Finally, the review concludes with the impacts of current and projected ecosystem change on the needs of peo- ple living on permafrost as well as on global society. Indeed, the effects of permafrost change reach far outside the northern high latitudes to affect the entire Earth system. Over the last decade and more, the study of permafrost has grown from its geophysical roots to more fully realize the interplay between geophysical and biological processes that define ecosystem function. It is the hope of these authors that this ecological perspective of permafrost serves to identify areas 280 Schuur · Mack ES49CH13_Schuur ARI 26 September 2018 13:58 of both vulnerability and resilience as climate, ecological disturbance regimes, and the human footprint all continue to change in this sensitive and critical region of Earth. 2. WHAT DEFINES PERMAFROST AND WHERE IS IT? 2.1. Temperature Frozen ground can be considered a three-part system with (a) the active layer at the surface, which thaws each summer and refreezes in the winter; (b) the transient layer located below, which is ice rich compared with the active layer and has likely alternated in and out of permafrost over timescales of decades to centuries; and (c) permafrost, which is deepest, perennially frozen over timescales of centuries to millennia, and can contain massive blocks of ice interspersed with frozen soil (Shur & Jorgenson 2007). The thickness of the active layer ranges from tens of centimeters to several meters depending on the local climatic conditions as well as the thickness of the soil organic layer and structure of vegetation that develops during ecological succession and insulates the permafrost (Nelson et al. 2004). The active layer is the zone featuring heightened biological activity during the short growing season experienced by these ecosystems. Permafrost itself can extend several to hundreds of meters in depth, but the state of near-surface permafrost, defined as ∼0–3 m depth (Lawrence & Slater 2005), is often a much more useful metric relative to ecosystems and people living on top of frozen ground. The permafrost zone occupies 24% of the exposed land surface in the Northern Hemisphere, where most of it is located (entire area is 22.8 × 106 km2) (Brown et al. 2002; Zhang et al. 2000) (Figure 1). Within the Northern Hemisphere, 47% of the permafrost zone is classified as “con- tinuous permafrost,” defined as having >90% of the land surface underlain by permafrost approx- imately –15 to –2◦C. Another 19% is classified as “discontinuous permafrost,” where 50% to 90% of the land surface is underlain by permafrost. The remaining 34% of the total permafrost zone is split between “sporadic” and “isolated” permafrost, where 10% to 50% and <10% of the land surface is underlain by permafrost, respectively. The latter three categories have relatively warmer permafrost close to the freezing point, approximately –2 to 0◦C, interspersed by soils without per- mafrost. When permafrost temperature is relatively warm, ecosystem properties related to vegeta- tion structure, snow accumulation, and the soil organic layer, as well as human and natural distur- bances that affect them, exert relatively more influence on permafrost distribution ( Jorgenson et al. 2010). Although the majority of permafrost is terrestrial, an additional 3 × 106 km2 area of shallow continental shelf under the Arctic Ocean has the potential for permafrost (Brown et al. 2002). This undersea permafrost initially formed on land when sea levels were lower during the last glacial period (Strauss et al. 2013, Walter et al. 2007). Submergence with seawater over Access provided by University of Alaska - Fairbanks on 02/04/19. For personal use only. Annu. Rev. Ecol. Evol. Syst. 2018.49:279-301. Downloaded from www.annualreviews.org