Frontiers in Understanding Climate Change and Polar Ecosystems: Report of a Workshop (2011)

Jeff Severinghaus, Scripps Institution of Oceanography, University of California San Diego

Ice cores provide unique records of polar climate and atmospheric composition over the past 800,000 years. In particular, ice cores have revealed that abrupt changes in climate, occurring in as little as a decade, have occurred more than 23 times in the last 100,000 years and are very likely a persistent feature of the Pleistocene glaciations. These abrupt changes display a marked hemispheric asymmetry, with large changes in the Arctic of up to 10 degrees C but almost no change in the Antarctic. The Antarctic records, by contrast, are characterized by gradual change of no more than 2 degrees C per thousand years. As such, ecosystems in the Arctic have been subjected to at least 200 and probably >500 abrupt events that must have had profound biological consequences due to the extreme rapidity of the change. It is therefore quite likely that genomes in the Arctic bear witness to this history, with adaptations for resilience to environmental change. The Antarctic, however, is quite a different story. Because of the long-term climate stability of this region, genomes are likely to have shed such resilience genes, because there is a cost to their maintenance. Thus the Antarctic biota may be less resilient to the anticipated warming of the next few centuries than their Arctic counterparts.

Glenn Patrick Juday, School of Natural Resources and Agricultural Sciences, University of Alaska Fairbanks

The northwestern North American Arctic and boreal regions are centers of biodiversity at the species and genetic levels, almost certainly because of the continuous availability of unglaciated refugia during past periods of rapid climate change. Specifically identifying these basic biodiversity resources will need to be a priority if management for their survival during a time of rapidly shifting climate is to be successful. Recent Arctic warming has reversed cooling of 2000+ years duration that was orbitally driven. Several aspects of the altered climate regime are in the process of causing apparently insurmountable challenges to the survival of much of dominant vegetation where it occurs today, while simultaneously creating suitable climatic conditions where some of these species are largely absent today. Alaska has a ~100-year instrument-based climate record. The combination of temperature and precipitation at low elevations in central Interior Alaska in the early 21st century has approached or exceeded lethal limits for the currently dominant tree species. For example, natural distribution limits of white spruce in North America do not include areas with precipitation below about 280 mm, the current 100-year mean at Fairbanks. The combination of increased July temperature and unchanged annual precipitation in central Alaska now exceeds the previous limits that characterized white spruce distribution. Spruce budworm has now developed outbreak potential on Alaska and northern Canadian forests. Alaska birch have been stressed to near lethal levels across low-land Interior regions twice in the last decade from acute drought injury. Aspen leaf miner is causing widespread tree death, and dieback. The current wildland fire and insect outbreak regimes, both directly temperature related, have been disturbing the forest at a rate that will not allow the recent age structure of forests to appear again as long as the new disturbance rate is maintained. The accelerated disturbance is significantly reducing available habitat for a set of specialized older forest organisms. Recent temperature increases have improved climate suitability for black and white spruce in far western Alaska, and possibly in the far northern tundra as well. However, these areas generally have sparse tree populations, which may or may not represent the best-adapted genotypes to these new conditions, and practical challenges to migration may require a significant amount of time to be overcome by exclusively natural processes. Landscape-scale tundra fire is now a reality on the Alaska North Slope, initiating the process of mobilizing one of the Earth’s great pools of sequestered carbon into the atmosphere. The synoptic picture is consistent

with a biome shift, in which the interior boreal forest is being severely altered and eliminated from many landscape positions, and opportunities for migration upward in mountains or coastward represent the best survival prospect for elements of the boreal forest.

Sharon Stammerjohn, Ocean Sciences Department, University of California, Santa Cruz

Circumpolar averages of sea ice extent show alarming decreases in the Arctic but increases in the Antarctic, presenting a paradoxical picture of polar amplification of climate change. But, changes in circumpolar sea ice extent can be misleading in that they mask much greater regional/seasonal sea ice changes, and thus ocean-atmosphere changes that are critically important for assessing high latitude climate sensitivity (and vulnerability) to global climate change. Regionally Antarctic sea ice has decreased quite dramatically in the high latitude Southeast Pacific Ocean (SPO), while the largest Arctic sea ice decreases have been in the Western Arctic Ocean (WAO). Further, by resolving seasonal sea ice changes during sea ice advance and retreat, we find some extraordinary rates of change: (1) sea ice in the WAO (of ~1 × 10⁶ km²) is advancing 26 days later and retreating 35 days earlier, resulting in a 59-day shorter ice season duration; and (2) sea ice in the high latitude SPO (of ~0.5 × 10⁶ km²) is advancing 48 days later and retreating 35 days earlier, resulting in an 83-day shorter ice season duration. Regionally therefore, sea ice duration is decreasing faster in Antarctica. However, changes are seasonally asymmetric, polar-opposite, but globally similar in timing: fastest during austral autumn sea ice advance and fastest during boreal spring sea ice retreat (both during ~March-June). Given these seasonal asymmetric sea ice changes, we explore how solar ocean warming in spring-summer may be delaying autumn sea ice advance and/or thinning winter sea ice and/or pointing to additional ocean heat sources.

Patricia L. Yager, School of Marine Programs, University of Georgia

High-latitude marine ecosystems are changing rapidly in ways that go far beyond “global warming,” reflecting large deviations from normal for many environmental variables across a wide range of spatial

and temporal scales. Most stunning is that these reported changes are determined from direct observations. We no longer need to use model projections to argue that the high-latitude ecosystems are highly sensitive to anthropogenic climate forcing. For example, Arctic air temperatures over large regions have risen more than 3°C relative to 1968-1995 averages while some other areas have cooled (Overland et al., 2008). Similarly, some areas of Antarctica are warming faster than 0.1°C per year; other regions are cooling at a comparable rate. Sea ice extent in the Northern Hemisphere has decreased by more than 6.4 (±1.4)% per decade (Perovich et al., 2010; NSIDC¹), while the Southern Hemisphere has increased at a rate of 1 (±0.7)% per decade. At regional and seasonal scales most relevant to marine organisms, some key areas are experiencing extreme change. Faraday (West Antarctic Peninsula; WAP) air temperatures have warmed more than 5°C in the past 50 years (Ducklow et al., 2007). Consequently, the WAP region has lost significant sea ice, while other areas such as the Ross Sea have gained ice. Most critical to marine life is that the ice season duration in these two regions has diverged dramatically (Stammerjohn et al., 2008), with significantly shorter ice seasons in the WAP, Bellingshausen, and Amundsen Sea regions.

The loss of sea ice has a direct impact on marine organisms whose life histories are tightly coupled to that ice. Sea ice itself is critical habitat for hunting, feeding, breeding, and refuge for many higher trophic animals (Gradinger and Bluhm, 2004; Thomas and Dieckmann, 2010). The increased sightings of unattended walrus pups in the Beaufort and Chukchi Seas (Cooper et al., 2006) is a good example of how critical the sea ice can be. Sea ice also serves as critical substrate for microorganisms, including ice algae and other sea ice microbiota (Melnikov, 1997; Junge et al., 2001; Thomas and Dieckmann, 2010) that contribute to polar productivity, microbial food webs, and carbon flux. Microbial production and degradation of climate-active gases such as carbon dioxide, dimethylsulfide, and organohalogens are also tightly coupled to the sea ice.

Changes in air temperature and sea ice cover also have profound effects on the polar ocean. Where summertime sea ice has been lost, ocean surface temperatures have risen by more than 3 degrees in some areas (Proshutinsky et al., 2009) and springtime cloud cover has increased relative to the 1982-2004 average (Schweiger, 2004; Liu et al., 2007). Wind speeds have also varied from the norm (e.g., Francis et al., 2005), with deviations up to 2 m/s. Precipitation increases due to warmer air temperatures over land also impact the marine ecosystem. Annual river discharge to the Arctic, already high relative to the size of the ocean, has

increased (Peterson et al., 2002; Shiklomanov, 2009). While there are no overland rivers flowing to the Southern Ocean from Antarctica, melting glaciers contribute freshwater to the ocean. This contribution is increasing significantly as some glaciers (e.g., Pine Island, Smith, and Thwaites) are thinning at a rate of more than 9 meters per year (Pritchard et al., 2009). Sea surface temperature and salinity, cloud cover, and wind-driven mixing all contribute to stratification and the availability of light and nutrients in the surface layer, impacting phytoplankton and associated ecosystems.

In the water column, primary productivity depends on the availability of light and nutrients, which is often determined by the presence of a sea ice margin (Sullivan et al., 1988). But ecosystem structure and the fate of a bloom depend on more than just the total quantity of production, however. Phytoplankton community structure varies according to position through the marginal ice zone. Also critical is the timing of the bloom and how it is coupled in time to the life history of higher trophic levels. For regions where we have long-term observations, polar ecosystems are clearly responding to climate changes (Grebmeier et al., 2006; Ducklow et al., 2007). The dramatic changes in WAP sea ice cover have led to significant decreases in chlorophyll in the north and increases to the south (Montes-Hugo et al., 2009). As a result, krill populations are declining over two-fold in some areas and are being replaced by salps (Atkinson et al., 2004). Penguin and marine mammal populations are also changing in response to changes in their prey (Ducklow et al., 2007).

The climate impacts on pelagic productivity are multi-faceted (e.g., Boyd and Doney, 2003) and it seems hard to predict what polar productivity is likely to be in the future. Data sets across larger areas are uncommon, forcing us to generalize from short-term or regional observations. In one large-scale (2-year) study, changes in sea ice cover were linked to increased primary productivity attributed to a longer growing season (Arrigo et al., 2008). Without good measurements of ice algal production from satellites, however, it is difficult to know whether polar productivity is indeed higher than it used to be. The open-water areas that satellites can observe are increasing, and where there is open water, we tend to observe higher productivity. Model results suggest, however, that the increase will be short lived because of nutrient limitation set up by increased stratification (Lavoie et al., 2010; Cai et al., 2010). Shifts in the structure of the bloom and associated herbivore populations, however, may lead to a greater fraction of production being exported (Lavoie et al., 2010).

The shallow shelves of the Arctic have tight benthic-pelagic coupling and an unusually high proportion of pelagic production has historically fueled a substantial benthic food web (Petersen and Curtis, 1980). Changes in the quantity and timing of sea-ice and pelagic primary production are predicted to cause a shift toward the pelagic food web (Bluhm

and Gradinger, 2008). Observations supporting this idea are reductions in certain benthic invertebrate fauna (Grebmeier et al., 2006), declining dabbling duck populations (e.g., Lovvorn et al., 2003), increasing planktivorous bowhead whale populations while benthic-feeding gray whales are shifting their feeding grounds (Moore et al., 2002; 2003), and the spread of pelagic-feeding pollock and other fish populations into the Beaufort Sea (Rand and Logerwell, 2010). In addition to the obvious worries about how these shifts will impact ecosystem function, local Alaskan communities are concerned about the predictability of ice conditions and its impact on their subsistence hunting and quality of life.

Climate driven changes to ecosystem structure will no doubt impact biogeochemical fluxes and could very well impact the flux of CO₂ between the oceans and the atmosphere (Sarmiento et al., 2004; Doney et al., 2009) and polar oceans are particularly important because of the close connections between the high-latitude and deep oceans (e.g., Sarmiento and Toggweiler, 1984). Ultimately the air-sea exchange of CO₂ depends on the relative rates of upwelling (of high-CO₂ water) and net community production with an efficient biological pump. While a proposed global biogeography of particle flux is still in its early stages, many suspect that the efficiency of the biological pump is very closely linked to ecosystem structure. The above-described shift from krill to salps in the WAP region, for example, could have a profound impact on the regional carbon flux since salps “package” carbon differently and produce sinking particles much more effectively than krill. A similar situation is observed in the Arctic (Wassman et al., 2008). Yet, the role of physical processes may overwhelm any biological response, as it seems to do in the Antarctic Polar Front region. Here, where upwelling rates are high, CO₂ tends to flux out of the ocean (Takahashi et al., 2009). A key question is which way the balance between upwelling and biological drawdown will shift in the future (Lovenduski et al., 2008; Lovenduski and Ito, 2009; Le Quéré et al., 2007).

Although we can confidently say that marine ecosystems respond to climate drivers, it often seems as though we have very little confidence in our ability to predict future polar marine ecosystems under climate change. We are not in any position yet to put the polar marine biosphere into large-scale climate system models. How then do we move ahead? Since the themes of this workshop are (1) thresholds and tipping points, (2) polar amplification, and (3) ecosystem connectivity and resilience, I conclude my talk by suggesting that we consider whether Complexity Theory (e.g., Lewin, 1999) may help guide our thinking or plan our field programs. Complex systems are dynamical (non-steady state), with multiple interacting components and multiple stable states. They typically exhibit rapid phase transitions (“tipping points”), cross-scale effects (connectivity), and cascade behavior. Additional properties of interest are similarity

at multiple scales, adaptive behavior, and emergent properties (the whole is greater than the sum of the parts). Feedbacks like “polar amplification” are emergent properties. Studying complex systems requires modeling, which means that field programs need to coordinate with the modelers first to find out what they really need us to measure. And while reductionism may still be a useful approach, it will very likely not be enough to understand complex systems. Field programs, then, must emphasize mechanistic understanding (a good example of this would be to look beyond community structure to actual gene expression). We must try to define and then measure the emergent properties with high spatial and temporal resolution (e.g., Schofield et al., 2010). We should also look for generalizablility across regions. Complex systems can often be described with simple rules; we just need to figure out what those rules are.

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