B53E-01 INVITED
The Importance of Ecosystem Thresholds in Assessing Safe Concentrations of Greenhouse Gases
There is a major strategic challenge in the public debate about global environmental change related to concentrations of greenhouse gases in the atmosphere that might lead to environmentally, socially, and economically unacceptable impacts. This project takes one approach to this problem: avoiding "dangerous anthropogenic interference" and "allowing ecosystems to adapt." But these phrases implicitly assume that the influences of climate change are likely to be gradual and that there will be substantial time for natural resources to adapt or for managers to cope with change. The current state of the science suggests that something quite different may be in the offing. The Intergovernmental Panel on Climate Change (IPCC) and other assessments of possible impacts now agree on two important points. One is that there is already well-documented evidence of the biological and ecological consequences of climate change – in the behavior of migratory birds, in corals bleached from the influence of warming ocean temperatures, in the loss of glaciers to warming air temperatures, and in the loss of sea grass beds to sea level rise. The second is that ecological systems may not in fact change gradually. Modeling studies and the historical record both suggest that changes in ecosystems can be rapid, large, and sometimes irreversible, i.e., there are thresholds that, once crossed, will present serious coping challenges to humans. Moreover, as suggested in a recent National Academy of Sciences (NAS) workshop on "Understanding and Responding to Multiple Environmental Stresses," dealing with threshold responses that may lead to sudden and dramatic change in societal or environmental structure and function will also require that we develop ways to proceed with decision-making despite the many uncertainties associated with thresholds. These observations present serious challenges to the modeling frameworks used in integrated assessment. Not only do the models have to characterize the dynamic behavior of ecosystems as they cross thresholds, but they also have to represent adaptation strategies that are promoted to cope with such sudden or irreversible changes. A major challenge in the discussion over the implications of tipping points and thresholds in natural resources and management systems is what lessons there are for debates over targets for concentrations of greenhouse gases in the atmosphere. Are there levels of greenhouse gases that would protect against ecosystems exhibiting tipping point behavior, for example? How does uncertainty in our knowledge of either the resources or the climate system influence margins of safety? What models and analytical tools are available for conducting the analyses that are needed to address these questions. The JGCRI's suite of integrated assessment models provide a systematic way of simulating different emissions and concentration scenarios that can then be used to investigate the climate triggers for ecological tipping points and thresholds.
B53E-02 INVITED
Climate Warming in Antarctica is Triggering Changes in Biodiversity and Terrestrial Ecosystems
Antarctica climate changes relating to ice and ocean currents have global impacts, but changes on terrestrial ecosystems in the Antarctic are less well known. This is partially due to the small area of exposed land, the apparent isolation, and lack of permanent residents. However, low diversity ecosystems, such as Antarctic polar deserts, are expected to be more vulnerable to global changes and are located in regions that are likely to see some of the greatest climate changes. Evidence is accumulating that terrestrial regions of Antarctica are experiencing substantial but variable responses to climate change and human disturbance. In the McMurdo Dry Valleys and in the rapidly warming Antarctic Peninsula region, temperature changes have a rippling effect that control habitat dynamics, species, carbon cycling, especially since these ecosystems are situated on a threshold between frozen and liquid water. Direct anthropogenic effects, including tourism and invasive species are also changing terrestrial communities but the magnitude and duration is dependent on numerous interacting factors. Global change scenarios incorporating species abundance, species traits, community change and monitoring of changes in biogeography will be important for determining alterations to ecosystem processes such as nutrient cycling.
B53E-03
A Climate Change Threshold for Forest Dieback in the African Sahel
Increases in human greenhouse gas emissions to the atmosphere have increased global sea surface temperatures. Reinforced by a reduction in vegetation cover in the African Sahel, warmer sea surface temperatures have reduced rainfall in the Sahel by up to 30% in the 20th Century. In Senegal, annual precipitation fell to below one standard deviation of the 148 year mean for 5 years in the period 1968-1973. Although the region had experienced high historic variability in precipitation, the 1968-1973 drought crossed a climate threshold for agriculture that caused famine and human death. Sahel, Sudan, and Guinean ecosystems also crossed a climate threshold of aridity in an abrupt, nonlinear manner. The long-term decrease in precipitation caused extensive forest dieback and a latitudinal shift of the Sahel, Sudan, and Guinean ecological zones. The range of xeric forest species has expanded and mesic species have retracted southward towards areas of higher precipitation. Field inventories of tree species richness show declines in local biodiversity across the Sahel. Analyses of 1954 and 1989 aerial photographs and 2002 1-meter resolution IKONOS satellite images of three 200 km2 areas in Senegal and Mauritania also show declines in the density of trees of height > 3 m. Forest dieback fuels three positive feedback mechanisms: reduction of the evapotranspiration inputs necessary for the northward advance of the summer monsoon rains that sustain vegetation and forestall desertification, increases in the greenhouse gas emissions that cause the reduction in rainfall, and reduction of the forest biodiversity that strengthens ecosystem resilience to long-term drought. The interaction of climate change, desertification, and loss of biodiversity, as well as the complex social, economic, and political factors that lead to forest dieback and other ecological changes in the Sahel present difficulties in monitoring and foreseeing future threshold behavior. Nevertheless, any reduction in greenhouse gas emissions and any increase in vegetation cover will reduce the risk of future catastrophic ecological change in the Sahel.
B53E-04
Thresholds controlling shifts in forest cover types in the boreal region of Interior Alaska: inter- actions between climate, fire and edaphic factors
There is a general consensus that future warming in the North American Boreal Region will cause a reduction in coniferous species common to cool, wet sites and an increase in deciduous/coniferous species found on warmer drier sites. In addition, it is believed that much of the change in forest cover will occur during secondary succession following disturbance and that the frequency of disturbance is likely to increase in response to climate warming; however, neither the rate at forest cover will change, nor the mechanisms thereof are well understood. Here, we summarize results from recent studies in Alaska that are being carried out as part of the Bonanza Creek Long Term Ecological Research Project and research being funded by the Joint Fire Science Program and NASA. We have examined factors important in regulating the change in the extent of black spruce (Picea mariana), a dominant forest type across the North American boreal region. Depth of burning of the surface organic layer is a fire severity measure that is important in regulating the post-fire environment in black spruce forests. In particular, seeds from deciduous trees have extremely low germination rates in post-fire organic soils that are greater than 3 cm deep. In addition, we found the growth of deciduous species in burned stands is inversely proportional to the depth of the remaining organic soil, with the highest growth observed on sites with exposed mineral soils. Other factors controlling seedling survival and growth include soil temperature and moisture, nutrient availability, and the fact that deciduous and coniferous species have different capabilities in absorbing different forms of soil nitrogen. These additional factors are also controlled by the amount of organic soil remaining after the fire. Finally, our research has shown that the depth of the remaining organic soil after fires is controlled both by topography and climate, with the frequency of sites with organic layers shallower than 3 cm being significantly greater during fires that occurred late in the growing season (after 20 July) compared to fires that occurred early in the growing season, a result of the influences of seasonal thawing of the ground layer on ground moisture. In addition, we found that the drier ground conditions that occur during extremely large fire years lead to a higher fraction of deep burning fires than occurs during small fire years. Thus, changes in climate can be directly related to factors regulating an important threshold for change in Alaskan black spruce forests. As the boreal forest provides a wide range of ecosystems services, particularly to Native Peoples, being able to predict the rate of change in forest cover is particularly important in developing policies that account for the effects of climate in relationship to fire management policies.
B53E-05
Few Like it Hot: Coral Reef Reponses to Elevated Temperatures and CO2
Coral reefs live within a fairly narrow envelope of environmental conditions constrained by water temperatures, light, salinity, nutrients, bathymetry and the aragonite saturation state of seawater. As documented in numerous studies, the world's coral reefs are "in crisis" as a result of human impacts on their environment. While local stresses currently dominate, coral reefs are increasingly confronted with global-scale changes due to rising greenhouse gas concentrations. These changes are rapidly modifying the environmental envelope of coral reefs through both increased thermal stress and ocean acidification. In the former case, there is a well-documented relationship between thermal stress and the response of corals that include coral bleaching, disease, and mortality. Clear tolerance thresholds exist beyond which high temperature and accumulated thermal stress have deleterious effects. However, the synergistic effects of increasing temperature and ocean acidification are not yet fully understood. At this time, there is mounting concern that decreasing pH and aragonite saturation state will cause net reef accretion to cease or become negative. The threshold at which this could occur is likely to be reached much sooner than the pH drop necessary to induce carbonate dissolution. Both the thermal and chemical limits that control coral survival and reef growth will likely be passed before 2100 assuming even conservative projections reported in the 4th Assessment Report of the Intergovernmental Panel on Climate Change. This talk will discuss these thresholds and their ramifications for ecosystems and resource management. http://coralreefwatch.noaa.gov/
B53E-06
The role of human-induced climate change in the 2005 Caribbean coral bleaching event and the implications for the future
Episodes of mass coral bleaching in recent decades have been attributed to periods of anomalously warm ocean temperatures. In 2005, the sea surface temperature (SST) anomaly in the tropical North Atlantic that contributed to the strong hurricane season caused widespread coral bleaching in the eastern Caribbean. In this study, we used the GFDL global climate models to evaluate the contribution of natural climate variability and anthropogenic forcing to the thermal stress that caused the 2005 coral bleaching event. Historical temperature data and simulations for the 1870-2000 period show that the observed warming in the region is unlikely to be due to unforced climate variability alone. Simulation of background climate variability suggests that anthropogenic warming may have increased the probability of occurrence of significant thermal stress events for corals in this region by an order of magnitude. Under scenarios of future greenhouse gas emissions, mass coral bleaching in the eastern Caribbean may become a biannual event in 20-30 years. However, if corals and their symbionts can adapt by 1 - 1.5°C, such mass bleaching events may not begin to recur at potentially harmful intervals until the latter half of the century. The delay could enable more time to alter the path of greenhouse gas emissions, although long-term "committed warming" even after stabilization of atmospheric CO2 levels may still represent an additional long-term threat to corals. These results suggest that protecting coral reefs from climate change will require both managing local pressures on reefs, in order to increase resilience to committed warming, and reducing greenhouse gas emissions.
B53E-07
Thresholds of Climate Change in Ecosystems - Synthesis and Assessment
The Global Change Research Act of 1990 (P.L. 101-606) calls for the periodic assessment of the impacts of global environmental change in the U.S. The U.S. Climate Change Science Program is helping to meet the fundamental need of providing a periodic synthesis and assessment of cumulative knowledge and the evaluation of the implication of that knowledge for scientific research and policy formulation through a series of 21 "synthesis and assessment products"(SAP). The SAPs will integrate research results focused on important issues and questions frequently raised by decision makers. SAP 4.2: Thresholds of Change in Ecosystems is addressing and synthesizing the present state of scientific understanding regarding potential abrupt state changes or regime shifts in ecosystems in response to climate change. Issues to be addressed include recognizing thresholds of climate changes; are thresholds commonly crossed in Nature; identifying appropriate ecological variables or indicators for measuring such changes; effectively ‘managing' and or adapting to thresholds and state changes; and our ability to recognize sudden changes in ecosystems with varying spatial and time scales of observation and analysis. The process followed and progress to date will be presented.