Frontier Questions in Climate Change and Polar Ecosystems
The goal of the Polar Research Board’s workshop was to bring together a diverse group of scientists to identify key research frontiers at the intersection of polar ecosystems and global climate change. “Frontiers” in this context signifies those cutting edge ideas and research needs that will take the science forward into the coming decades. Workshop participants were asked to consider: Where does the science need to go next? What has been accomplished and what are the future questions to be answered? What are the next big innovative topics in this area of scientific research?
Through presentations and discussions, the workshop participants identified five key questions that represent forward-looking opportunities:
Will a rapidly shrinking cryosphere tip polar ecosystems into new states?
What are the key polar ecosystem processes that will be the “first responders” to climate forcing?
What are the bi-directional gateways and feedbacks between the poles and the global climate system?
How is climate change altering biodiversity in polar regions and what will be the regional and global impacts?
How will increases in human activities intensify ecosystem impacts in the polar regions?
The list is not intended to be unique or exhaustive and, indeed,
relevant work is already occurring within the science community, as described in the examples and case studies in Chapter 1.
WILL A RAPIDLY SHRINKING CRYOSPHERE TIP POLAR ECOSYSTEMS INTO NEW STATES?
Many of the workshop participants emphasized the need to quantify both the vulnerability and resilience of the polar ecosystems, including local communities and populations, in response to the rapidly shrinking cryosphere, and to understand the connectivity between the cryosphere and the global system. Changes in air temperature and precipitation patterns are altering the structure of the cryosphere, the hydrological cycle, fire regimes, and permafrost melting in the terrestrial system. Warming atmospheric and seawater temperatures over the western Arctic (Chukchi Sea and Canada Basin) and the western Antarctic Peninsula have dramatically reduced sea ice cover, changing air-sea interactions regionally and their connectivity to the global system.
The polar regions are poised to lose biodiversity as the result of air, sea, and land temperature changes and seasonal-to-total melting of sea ice, glaciers, and permafrost. Changes in biodiversity can be expected to result in altered biogeochemical processes, which can affect the overall production of the system. For example, a shift in dominance from krilleating Adelie penguins to fish-eating seals can alter the net efficiency of biogeochemical processing. If the dominant higher trophic animal is eating higher on the food chain (fish-eating seals) versus feeding lower on the food chain (krill-eating penguins), the system is less efficient as more total energy is used to get the same base level of food to the top predator, requiring more food at the base of the food chain.
Other impacts of a shrinking cryosphere include changes to subsistence life styles, resource exploration, and tourism. Coastal erosion is increasing as sea ice retreats and open water can degrade coastal regions and negatively impact human habitation. Increased potential for resource access and extraction may be realized as the open water season increases in length (Arctic Council, 2009). Traditional hunting methods and sites are changing with changes in weather, the landscape, and resource availability (e.g., Ford et al., 2008). Understanding ecosystem changes with climate forcing, their complexities, vulnerabilities, and feedbacks are considered important research frontiers in a world that continues to warm. Workshop participants stressed the important goal of coupling climate models with biogeochemical models in order to identify potential tipping points and associated tipping elements, transformational processes,
and thresholds within polar regions to ultimately develop strategies to minimize and/or adapt to the impact of climate change on ecosystem services and processes.
A tipping point describes a critical threshold reached in a nonlinear system, where a small perturbation to the system can cause a shift from one stable state to another (see Box 1.1). The global climate system is a nonlinear system and there are several possible tipping points that could potentially be reached this century as a result of human-induced activities. These have been referred to as “policy-relevant” tipping points (Lenton et al., 2008; see Figure 2.1 for examples). Abrupt climate change can be considered a sub-type of tipping point, where a climate system response is faster than the cause itself (NRC, 2002). Lenton et al. (2008) describes “tipping elements” as large-scale components of the Earth system (at least subcontinental in scale) that may pass a tipping point. The transition of the tipping element in response to forcing can be faster, slower, or no different in rate than the cause, and can be either reversible or irreversible. Although variable in nature, the inherent common property of these tipping elements is that they exhibit “threshold-type behavior in response to anthropogenic climate forcing, where a small perturbation at a critical point qualitatively alters the future fate of the system” (Lenton et al., 2008). A large proportion of defined tipping elements have direct relevance to polar regions, not only because these areas are warming more rapidly than any other place on Earth, but also because these tipping elements typically involve amplifying ice-albedo and greenhouse gas feedbacks that are specific to high-latitude regions.
Declining seasonal sea ice and the disappearance of the Arctic perennial sea ice pack, as well as the shrinking Greenland and West Antarctic ice sheets are processes of particular concern to the workshop participants because of their inevitability and/or severity of impacts and the potential for tipping points to be reached. Additional processes with potential tipping points of concern include dieback of the boreal forest, a northward shifting treeline into tundra regions, CO2 and CH4 release from carbon-rich permafrost soils, and release of marine methane hydrates from subsea permafrost. Recent work has been put forth advancing the ability to anticipate and forecast an approaching tipping point in the Earth’s climate system, where an initial slowing down in response to a perturbation is commonly experienced (e.g., Dakos et al., 2008). Advances in modeling and forecasting an approaching tipping element may enable us to further understand whether these critical thresholds and their repercussions can be avoided (i.e., mitigation) and/or whether they can be tolerated (i.e., adaptation).
WHAT ARE THE KEY POLAR ECOSYSTEM PROCESSES THAT WILL BE THE “FIRST RESPONDERS” TO CLIMATE FORCING?
Workshop participants discussed the role of Arctic and Antarctic polar regions in highly coupled systems, with strong links between land, ocean, ice, atmosphere, and humans. These individual components cannot be fully understood independently of one another, as a perturbation to one system component will likely cause cascading effects throughout the entire polar system. For example, current regional and global models have not been able to accurately capture patterns of recent Arctic change (e.g., sea ice decline) and pathways for model improvements are currently sought. Some of the workshop participants emphasized the importance of understanding and quantifying the system interactions (rather than simply the isolated components) to accurately predict polar ecosystem response to climate forcing. Models that address the complex interactions between living organisms and their environment (i.e., a focus on “biocomplexity”) are critical to understanding how climate change influences ecosystem processes. Developing these models in concert with observational studies is essential to developing predictive tools that are useful to policymakers and have benefits for society. As such, these models can be used to support judgments to create adaptive systems of decision making.
In the terrestrial realm, major uncertainties in current modeling capabilities include the ability to quantify shifts and feedbacks associated with ecosystem disturbances (e.g., fires, logging, insect infestation), migrations of flora and fauna, coastal erosion, and hydrological and carbon-related impacts of warming and permafrost degradation. Major ice-albedo and greenhouse gas feedbacks may be associated with these changes as well. These feedbacks have the potential to drastically alter predicted outcomes if they are not modeled properly. For example, it is estimated that ~1024 Pg C is currently locked away in the top 0–3 meters of permafrost soils (which amounts to twice the current atmospheric carbon pool) (Schuur et al., 2008). However, with warming and permafrost thaw, this pool of carbon may be reintroduced to the contemporary carbon cycle through release of significant CO2 and CH4 to the atmosphere through decomposition and methanogenesis of organic carbon. Major uncertainties surrounding the rates of change in these scenarios of permafrost thaw, the magnitude of released CO2 and CH4 to the atmosphere, as well as whether climate forcing will result in wetter or drier landscapes, need to be resolved if the overall impact and the direction of feedbacks to the polar and global climate system is to be assessed critically. Improved modeling capabilities and understanding of system interactions are not only essential to improve
the ability to predict polar ecosystems responses to climate, but, because of globally significant feedbacks, are also essential to improve knowledge of the overall polar and global climate system as well.
The ice-free continental ecosystems of the Antarctic are microbially dominated and are driven to a large extent by the summer production of liquid water and the distribution of biotic and abiotic matter by wind. To accurately predict ecosystem responses to warming in both the Antarctic and Arctic, models need to include coupled information on surface energy balance, hydrology, and biological/biogeochemistry. Accurate models should also emphasize connectivity among the components and utilize (i) high resolution digital elevation data (e.g., from LIDAR) so that the spatial reference of each modeled segment is properly connected to the others, (ii) high resolution remote sensing (Quickbird and Worldview) data to classify the status of every control volume, (iii) data from field observations/experiments to determine biogeochemical cycling rates and food web/populations dynamics, (iv) stoichiometry of material and energy transformation within each control volume, and (v) matter and energy transfer at the surface via aeolian or water transport. High resolution meteorological and stream hydrology records can provide direct input to the latter components. These spatially explicit process-based models can further include the presence of particular genes, microbes, or nutrients at any point in the landscape, which will allow prediction of the role of wind versus water in promoting growth and movement of biological components of the ecosystem.
The strong dependence of Arctic and Antarctic polar marine organisms on seasonal sea ice provides the principal connection between ecosystems and climate in these regions. Better understanding of how polar marine ecosystems respond to climate change requires improved models of coupled atmosphere-ocean-ecosystem dynamics at local, regional, and global scales, informed and driven by new observations. Better prediction of polar ecosystem changes requires new models of coupled global-scale atmosphere-ocean circulation that simulates the teleconnections between lower-latitude climate variability and high latitude responses of atmospheric pressure and wind fields, ocean circulation, and sea ice. A good example is the sea ice of the southwest Pacific Ocean/Bellingshausen Sea sector of the Southern Ocean, which exhibits strong covariability with the El Niño-Southern Oscillation (ENSO). Annual advance and retreat of sea ice and the resulting duration and extent are strongly modulated by the interactions of ENSO with the Annular Modes that modify wind speed and direction in spring and fall. Forcing of the Southern Annular Mode
by the combined and interacting anthropogenic effects of the ozone hole and greenhouse warming also indicate how better understanding and improved predictive capabilities rest on comprehensive coupled modeling tools.
In general, current models do a poor job simulating high latitude sea ice variability and these shortcomings hamper realistic simulations of atmospheric and oceanic circulation. In addition, deep water formation will be strongly affected by accurate representations of areas of new sea ice formation, which in turn are affected by ice shelf-ocean and continental shelf-ocean interactions, all of which are poorly resolved (or coarsely parameterized). Circumpolar sea ice extent can be reasonably reproduced, but resolving regional and seasonal sea ice variability is still a challenge. Sea ice and snow thickness, rafting/ridging, snow-ice flooding, and melt-ponding are all a challenge even for higher-resolution regional sea ice models, largely because of the lack of data, both for evaluation purposes and for forcing the sea ice models (e.g., accurate regional resolution of winds and upper ocean data). Accurately simulating open water areas (leads, polynyas) is another challenge, which will ultimately affect the prediction of ocean-atmosphere heat fluxes and the consequent strong positive feedbacks. Lack of skill in representation of sea ice dynamics also affects modeling of ocean vertical mixing processes that are critical to plankton dynamics. New observations of these sea ice properties and ocean dynamics at all scales are critical to improved model development and understanding.
Marine ecosystem models have reasonable utility in representing bulk phytoplankton distributions (chlorophyll), for which remotely-sensed data are available, and for which we have reasonably good understanding of the fundamental biophysics underlying the ecology. Even so, detailed modeling of community dynamics like the observed transition from diatom-dominated to cryptophyte-dominated communities along the Western Antarctic Peninsula is still a frontier area. Modeling the potential impacts of lower trophic level species changes on marine carbon cycling, such as the impact of a shift to smaller phytoplankton species production observed in the Western Amerasian Arctic with increased freshwater content (Li et al., 2009), is critical to forecasting potential large-scale ecosystem response to climate forcing. Until the details of lower trophic level community response to climate change are better understood, mechanistic modeling of the upper trophic levels cannot progress. Further up the foodchain, the behavior of individual predators becomes paramount and modeling of functional groups as chemical reactors (as with bulk chlorophyll) is inadequate, instead requiring more detailed information on lower trophic species composition. The divergence of characteristic time and space scales between the base and apex of foodwebs from days
and meters to decades and ocean basins also remains a large challenge facing climate-ecosystem models.
WHAT ARE THE BI-DIRECTIONAL GATEWAYS AND FEEDBACKS BETWEEN THE POLES AND THE GLOBAL CLIMATE SYSTEM?
Workshop participants emphasized that polar regions and lower latitude ecosystems are parts of a coupled Earth system. For many ecosystem processes, changes in one component elicit responses from other components, which can further alter other system components. This cascade of bi-directional connectivity makes atmosphere-ecosystem interactions among the most complex in the natural world (NRC, 2007). These responses and interactions become even more complex with the involvement of human actions as broad ecosystem drivers. Given the complexity of these interactions and the feedbacks involved, participants stressed that breakthroughs will require effective collaboration among a wide range of sciences and long-term ecosystem monitoring, as well as involvement of multiple funding agencies.
Studies to date have shown unequivocally that climate change has produced many direct regional impacts at the poles (IPCC, 2007b). Polar regions are expected to be primary drivers of the global climate system because of the strong modification of the surface-energy budget through snow and ice cover, which is tightly coupled to the global circulation of the atmosphere and the ocean. The global implications and associated feedbacks of these polar impacts are difficult to define, and require long-term on-site monitoring and experimentation, in concert with coupled modeling efforts, to resolve. Participants noted that such efforts should focus on the construction of scenarios that cross many scales, a dynamic that we currently have little quantitative knowledge of.
Workshop participants discussed a number of processes and phenomena (including those identified in Anisimov et al. ) that may have bi-directional feedbacks on the global system:
Atmospheric variation: Changes in the polar energy sink region exert a strong influence on the mid- and high-latitude climate by modulating the strength of the sub-polar westerlies and storm tracks (Dethloff et al., 2009). Disturbances in the wintertime Arctic sea-ice and snow cover may induce perturbations in the zonal and meridional planetary wave-train from the tropics over the mid-latitudes into the Arctic. Consequently, Arctic processes can feed back on the global climate system via an atmospheric wave bridge between the energy source in the tropics and the energy sink in the polar regions.
Sea level rise: Of the many potential processes that influence sea level, melting of polar glaciers and ice sheets is perhaps the most tightly linked to atmosphere. Melting and direct ice discharge of the Greenland and Antarctic ice sheets would produce about 7 and 61 meters of sea-level rise, respectively (IPCC, 2001). The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would also contribute significantly to sea level. Rising sea levels would cascade through the world’s tightly connected economic and political systems producing catastrophic global impacts.
Ocean circulation: The increase in freshwater input to the sea could influence ocean circulation producing wide spread global impacts (Lemke et al., 2007). Models have indicated that arctic polar warming and moistening are important on a global scale because associated enhancement of sea-ice melting and freshwater inflow to the Arctic Ocean plays a critical role in controlling the deep convection and ocean meridional circulation, which in turn affects global climate (Kug et al., 2010). In addition to tidal processes and melting of Antarctic glacial ice, changes in atmospheric circulation patterns have also been attributed to the increased upwelling of Circumpolar Deep Water on continental shelves bordering the West Antarctic Ice Sheet (WAIS) (e.g., Thoma et al., 2008). In turn, these atmosphere-ocean and related sea changes have been implicated in amplifying the warming trend over the WAIS (Steig et al., 2009). Unfortunately, details of the actual mechanisms are lacking, emphasizing the need to better resolve ocean processes in particular, and pointing to a need for increased ocean observations.
Albedo: Surface albedo has long been recognized as one of the key surface parameters in climate models through its direct effect on the energy balance (Dethloff et al., 2006). Observed changes in snow, ice, and vegetation cover are all producing changes in surface albedo. Holland and Bitz (2003) have suggested that the rapid loss of snow and sea-ice in certain areas of the Arctic can produce feedbacks that can affect climate change over larger scales.
Arctic terrestrial carbon flux: Some models indicate that, in the next century, terrestrial ecosystems will act as a carbon sink (Stephens et al., 2007; Baker, 2007). However, there are large uncertainties due to the complexity of the processes and it is also possible that melting permafrost and the associated increased carbon emissions will lead to positive climate forcing (Sitch et al., 2007).
Biome shifts and migration patterns: Species that migrate between low and high latitudes may be significantly influenced by changing polar ecosystem dynamics (Alerstam et al., 2007; Wilcove and Wikelski, 2008). The rapid climate warming occurring in Alaska
has led to drastic changes in forest ecosystems. Such changes can lead to potential shifts in bird and large mammal migration patterns. Likewise, changes in ocean pH, temperature, and circulation patterns can reach thresholds that will eventually alter plankton distribution and the migration patterns of marine fish and mammal populations. Once these thresholds are reached, biodiversity at the species and genetic levels will almost certainly be altered.
Methane hydrates: Methane hydrates are known to be abundant in marine sediments, particularly those associated with the continental shelves of the Arctic (Kvenvolden, 1988). As ocean temperatures warm, either directly or as the consequence of altered circulation patterns, the hydrates can become unstable and release significant amounts of methane, a potent green house gas, to the atmosphere (Sloan, 2003; Maslin, 2004). This marine efflux of methane can exacerbate warming at the global scale. For example, Shakhova et al. (2010) have recently suggested that atmospheric release of a small amount of methane from the East Siberian Arctic Shelf could lead to abrupt warming.
Southern Ocean biological production: Regional and global models indicate that heat transport and associated stratification of the Southern Ocean will change in response to climate forcing (Ganachaud and Wunsch, 2000; Boning et al., 2008). In concert with the prediction of amplified ocean acidification in south polar waters, it can be expected that these changes in the physical environment will influence the species composition and rate of primary production in the Southern Ocean. Such changes may alter the production of methane sulfonic acid, a potent cloud nucleator, to the atmosphere, and change the sequestration of atmospheric carbon dioxide and transport to the deep ocean. Such physical and biochemical processes influencing changes in biological production are also being studied regionally in the Arctic Ocean.
HOW IS CLIMATE CHANGE ALTERING BIODIVERSITY IN POLAR REGIONS AND WHAT WILL BE THE REGIONAL AND GLOBAL IMPACTS?
The rapid warming of the Arctic is potentially leading to rapid shifts in productivity, habitat, and biodiversity that are likely to have profound implications for northern ecosystems and for the globe. Macroecology, the subfield that deals with the study of relationships between organisms and
their environments at large spatial scales to characterize and explain patterns of abundance, distribution, and diversity (Brown and Maurer, 1989), will likely bring an important perspective to understanding regional and global impacts. Understanding pattern and process in macroecology presents a considerable methodological challenge, as the scales of interest are simply too large for the traditional ecological approach of experimental manipulation to be possible or ethical (Blackburn and Gaston, 2003; Blackburn, 2004).
Alaska populations of boreal plants and animals contain mixtures of Eurasian species and genes, making Alaska a center of boreal biodiversity from the global perspective. The boreal forest is distinctive in being dominated by conifers from the large landscape perspective. Most of the boreal conifer tree species can attain long life spans, and if they survive to old age, they become a specialized habitat for a set of highly adapted plant (e.g., arboreal lichens, mosses) and animal (e.g., woodpeckers, cavity-nesting bird) species. Human inhabitants are also dependent on a number of critical forest resources in the Arctic (e.g., Usher et al., 2005 and references therein).
Boreal conifers play a key role in enabling fire to propagate across landscapes. In the past, warm temperature anomalies that trigger or promote boreal forest disturbance events, such as fire and tree-killing insect outbreaks, were infrequent. However, in the warmer climate of recent decades disturbances triggered by warm temperatures have occurred so frequently and severely that a substantial reduction in older forest has occurred already (ACIA, 2005). An inescapable consequence of the recent rapid warming and other anthropogenic changes (e.g., increased trade and travel) in the far north is the introduction of an increasing number of species from the south (or from the southeast in the case of Alaska), where species richness is greater (ACIA, 2004). In addition, a principal risk for boreal forest is that climate change appears to be happening so rapidly that a continued shift in the location of areas with a climate optimum for forest growth could outpace tree migration rates (Davis and Shaw, 2001). If so, tree dispersal rates and habitat availability as controls over forest migration will not have sufficient time to operate for the successful movement of all gene types and species. Consequently, these forests may be among the Earth’s most susceptible ecosystems with respect to the loss of genetic and species diversity due to climatic change. Thus, a challenge for science and resource management is to identify the diversity of adaptive genetic types present in key boreal species. If genetic biodiversity diminishes, future human uses and opportunities in the boreal forest are likely to be reduced, and ecosystem services, including sequestration of carbon, are likely to be less effective.
As in the terrestrial case, there is an increase in subarctic species moving northward into the Arctic, with the potential for increased species competition and major ecosystem reorganization. The biodiversity of polar oceans is structured to a large extent by cold temperatures. The Antarctic Ocean has had low, stable temperatures for at least 8 million years, whereas the Arctic Ocean has been cold for only the last ~2.5 million years. In response, organisms in Antarctic waters appear to have lost much of their physiological ability to adjust to increased temperatures (Peck, 2005) compared to Arctic species. For example, the Antarctic notothenoid fishes, which are the most stenothermal animals known, die of heat death at temperatures above 4 °C (Somero and DeVries, 1967). Consequently, some workshop participants theorized that Antarctic marine species might be more susceptible to the effects of regional and global climate change than Arctic species.
In light of the regional warming trends observed in the polar marine environment, it is important to consider marine biodiversity in the context of long-term evolutionary processes in which the genetics of the organisms is modified in ways that allow them to adapt to the temperature environment and short-term pulsed events. Genomic approaches to identify the types of genetic mechanisms that provide organisms with the abilities to adapt to environmental change and, conversely, to understand what types of genetic limitations exist in stenotolerant organisms that possess very limited abilities to tolerate and acclimate to temperature changes, are needed to fully understand the effects that climate change will have on polar marine biodiversity.
HOW WILL INCREASES IN HUMAN ACTIVITIES INTENSIFY ECOSYSTEM IMPACTS IN THE POLAR REGIONS?
Workshop participants commented on the possibility of increased human activity in the polar regions as a result of greater access and more open water days. Until the recent economic downturn, ecotourism was increasing significantly. Shipping across northern routes has started and is expected to increase as the number of ice-free days increases. Potential impacts from such activities include disturbance to wildlife and cultural resources from tourists, oil spills, discharge of gray and black water (sewage) from cruise ships, as well as the potential for invasive species and diseases into these remote and previously difficult to access regions. Natural resource development in the Arctic is likely to be one of the key drivers of marine activity in the future (Arctic Council, 2009). Approximately 13 percent of the world’s undiscovered oil may be found in the Arctic (Gautier et al., 2009) and oil and fuel spills are among the most significant
threats in both polar regions. As increased open water allows additional time for transit, the chances of oil and fuel spills increases. Additional risk comes from the unpredictable nature of storms and ice, some of which are large enough to sink or damage ships.
In the Arctic, tourism has occurred since the early 1800s. The earliest Arctic tourists were individuals attracted to abundant fisheries, exotic wildlife species, and remote regions. Today, with improved access and technology allowing more comfortable travel and easier access, these numbers have rapidly increased. In fact, tourism has become the largest human presence in many regions of the Arctic (UNEP, 2007). There are serious concerns that tourism is promoting environmental degradation in the polar regions in both the Arctic and Antarctic by putting extra pressures on land, wildlife, water, transportation, and other basic necessities. There are also cultural and social impacts to consider in the Arctic. Examples include inappropriate visitor behavior that violates traditional customs and disturbance of cultural sites or removal of cultural objects. Conversely, there may be positive local economic impacts from the tourist industry (e.g., job creation and the use of local transport, accommodations, and eating establishments).
In the Antarctic, tourism has grown rapidly in recent years with approximately 45,000 visitors to the region during the 2007-2008 season (IAATO, 2008), up from less than 10,000 per year during the 1990s (IAATO, 1997). Large cruise ship tourism, as well as small boat cruising and landings make up the majority of activities with impacts on the polar regions. These visits occur at the most sensitive time for the region, the polar summer, when resident species are present and tending young, feeding, and fledging. There has also been a recent upswing in the use of Antarctica and the Arctic as sites for “extreme adventure” trips and “climate tourism” (tourists who wish to see a region and its species before potential extinction caused by climate change) often requiring detailed planning and logistical support. Smaller expeditions may not plan adequately and may resort to “humanitarian” requests for aid from shipping or nearby national bases when they encounter problems. There are also impacts incurred by scientific researchers, however, these impacts tend to be more constrained to the areas surrounding the research stations.