PP52A-01 INVITED 10:20h
Using the Paleorecord to Evaluate Climate-Model Performance in Projecting Changes in Climate Variability
Changes in the variability of climate are likely to have greater societal impact over time than will simple changes in the mean state of climate, because that variability directly controls the frequency of droughts, floods, and other extreme events, and both directly and indirectly (through disturbance) affects terrestrial ecosystems. Syntheses of paleoclimatic data have been used in documenting and understanding changes in the mean state of climate, such as those that occurred during the Holocene in response to orbital forcing. Comparisons of climate-model simulations with the paleodata in that case provided critical information for the task of projecting climate changes by documenting the need to include feedbacks among coupled systems when simulating a climate different from the present day. There are two approaches for comparing paleoclimatic simulations with paleodata. The inverse approach, in which paleodata are interpreted in climatic terms, has been most frequently applied for inferring past changes in the mean state of climate. There are several problems with this approach, including the observations that the distributions of biotic indicators and the amplitude of geochemical or sedimentological indicators are usually governed by climatic extremes, and that most indicators have multiple proximal controls, which are usually interrelated in a nonlinear fashion. Chronological issues make it difficult to synchronize individual records and therefore limit our ability to detect changes in teleconnection patterns or climate modes that are defined by spatial patterns. The alternative forward approach applies offline or coupled environmental submodels to produce model output that is directly comparable with paleodata. Ideally, a global network of temporally synchronized, annual- (or subannual-) resolution records would be available for comparison with model simulations, but such a network is unlikely to be developed soon. In the meantime, time-slice syntheses or individual non-synchonized records should be the targets for data-model comparisons. With the employment of appropriate tools and experimental designs, it may be possible to use time-slice data syntheses to examine the ability of models to correctly simulate changes in climate variability in addition to changes in the mean state.
http://geography.uoregon.edu/envchange/
PP52A-02 INVITED 10:35h
The Role of Global Palaeodata Syntheses in Benchmarking Climate Model Simulations
Palaeo-records from the ice, marine and terrestrial domains are providing an ever more detailed record of past changes in regional climates and environments during the Quaternary. International initiatives over the last decade have led to the creation of a number of global syntheses of palaeodata. The ability to reproduce the spatial patterns of regional changes shown in these syntheses is a crucial measure of the confidence we can have in the ability of climate models to simulate climates radically different from the present day climate. Comparisons of climate model simulations with palaeoenvironmental observations have been an integral part of the Palaeoclimate Modelling Intercomparison Project (PMIP). Data-model comparisons carried out in the first phase of PMIP enabled the community to pinpoint areas in which atmospheric general circulation models (AGCMs) were unable to reproduce the full magnitude of past climate changes and led to the recognition of the importance of ocean and land-surface feedbacks in modulating past climate changes. PMIP is currently running palaeoclimate simulations with fully coupled ocean-atmosphere and ocean-atmosphere-vegetation models. The available global datasets will be used used in a systematic fashion to evaluate these state-of-the-art models. A new focus will be on the simulation of interannual to interdecadal variability, and the stability of teleconnections associated with modes of variability. Thus, the exercise of coupled models in the second phase of PMIP poses a new challenge to the palaeodata community requiring the synthesis and analysis of records of short-term climate variability. Palaeoenvironmental benchmarking could also play a significant role in other initiatives to quantify the uncertainties in future climate prediction. Several groups are now investigating the range of uncertainties inherent in model parameterisations of key physical processes by running large ensembles of experiments in which parameter values are changed within plausible ranges. Initial results suggest that the suite of versions of the model that are capable of reproducing the modern climate nevertheless differ considerably in their projections of future climate. There is an urgent need to explore whether constraining these experiments to reproduce key states of the climate in the past could help to reduce this large uncertainty.
PP52A-03 INVITED 10:50h
Climate Sensitivity of the Last Glacial Maximum from Paleoclimate Simulations and Observations
Global coupled climate models run for future scenarios of increasing atmospheric CO2 give a range of response of the global average surface temperature. Regional responses, including the North Atlantic overturning circulation and tropical Pacific ENSO, also vary significantly among models. The second phase of the Paleoclimate Modeling Intercomparison Project (PMIP 2) is coordinating simulations and data syntheses for the Last Glacial Maximum (21,000 years before present) to allow another assessment of climate sensitivity. Atmospheric CO2 concentrations at the Last Glacial Maximum (LGM) have been estimated using measurements from ice cores to be 185 ppmv, approximately 50% of present-day values. Global, annual mean surface temperature simulated by the slab ocean version of the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM3) shows a cooling of -2.8°C for LGM CO2 levels and a warming of 2.5°C for a doubling of CO2. Slab and coupled CCSM3 simulations that include the reductions of the other atmospheric trace gases and the large ice sheets covering North America and Eurasia at LGM give cooling in agreement with proxy inferences and indicate that LGM CO2 explains about half of the global cooling at LGM. Regional signatures of the climate system to changed LGM forcing are also an important measure of climate sensitivity and results from the fully coupled version of CCSM3 will be shown.
PP52A-04 INVITED 11:05h
Advancing Into the Past: How Proxies and Models Explore Climate Before the Instrumental Record.
In recent years, advances in extracting information from the various high-resolution (annual or seasonal) paleo archives around the world have continued to extend our record of climate. Many of these records allow us to analyze and interpret not only the mean state, but also the variability in the climate system and how it changed over time. With this shift in focus comes the potential to evaluate aspects of climate variations from a more regional perspective and, thus, the records gain in relevance for decision makers involved with environmental issues. Coupled Ocean-Atmosphere Climate System Models have propelled our understanding of numerous aspects of the Earth's climate system. Using histories of external forcing factors (such as volcanic eruptions and solar activity changes), models are capable of reproducing significant parts of past climate variations on a variety of time scales. Coupled climate models can provide us with physical concepts of how certain patterns of past climate changes might have developed. It is well known that the instrumental record is too short to sample the full range of natural variations and an extension using high-resolution proxies is very valuable. Additionally, models are essential tools to study mechanisms of how climate is responding to various forcing agents. Through close comparison of long histories as reconstructed from proxies with long transient simulations forced with best estimates of external agents such as explosive volcanism, solar activity changes as well as human induced changes to the atmospheric composition or the land surface, we can not only extend the record of climate and its variations back in time, but we can also use the `power of history' to calibrate assessments of future climate changes.
PP52A-05 11:20h
Reconstructing Terrestrial Biospheric Responses and Feedbacks to Past Climatic Change
Since CLIMAP, paleoecological records have primarily served as proxies of past climatic variation. As the IPCC mission shifts from climate change detection and attribution to prediction and mitigation, paleoecological records are increasingly valuable for studying the behavior of ecological systems at levels of organization from populations to biomes, and the responses and feedbacks of these systems to atmospheric variability. The expanding availability of isotopic and other physically based paleoclimatic records provides an independent climatic framework for interpreting paleoecological data. Moreover, dynamic vegetation models coupled to general circulation models allow biosphere-atmosphere feedbacks to be assessed and model comparisons to paleodata to be carried out at ecological units ranging from plant functional types to biomes. Fossil pollen data remain the primary source of information about late-Quaternary vegetation dynamics, complemented by plant macrofossils, phytoliths, charcoal, carbon isotopes, and molecular techniques. Here we review 1) recent work mapping late-Quaternary vegetation history in North America at 1000-year intervals at the level of plant taxa, broadleaved and needleleaved woody cover, leaf area index (LAI), and biomes, 2) time-series of climate and woody cover for selected sites, and 3) the application of these datasets to providing boundary conditions and assessing general circulation and vegetation models. Biomes emerge from the individualistic behavior of plant taxa, with a widespread increase in tree cover and LAI since the last glacial maximum and rapid shifts of the prairie-forest ecotone during the Holocene. Likely feedbacks include positive hydrological feedbacks along the prairie-forest boundary, increased carbon sequestration, and enhanced albedo seasonality with the expansion of broadleaved deciduous forests.
PP52A-06 11:35h
Paleo-environmental Perspectives on Climate-change Monitoring in the National Parks of the Northern U.S. Rocky Mountains
In the face of growing visitation, encroaching development and a changing climate, the United States National Park Service has initiated a nationwide program to inventory and monitor the resources it protects. The foundation for this initiative lies in the development of baseline or reference datasets for physical and biological systems within each park unit. In a series of paleo-proxy studies from the Greater Yellowstone and Glacier National Park regions, we demonstrate that most instrumental and observational records are too short to capture a significant portion of the climatic and ecological variability that might be expected in the parks of the northern U.S. Rockies. Networks of tree-ring based temperature and precipitation reconstructions spanning the last ~1,000 yr demonstrate that the climates of these regions are not stationary. These climates are instead characterized by strong regime-like behavior over decadal to multidecadal timescales. Complimentary studies of past plant-community and landscape dynamics show how such lower-frequency variability can have a profound impact on vital park resources and amenities. In the eastern Yellowstone region, for example, persistent (20-30 yr) wet/cool periods in the 19th and early 20th centuries led to widespread recruitment of woody plants, and the legacy of these recruitment events still persists in the structure of many woodlands and forests. Studies of fossil packrat middens also suggest that at least some recent woody-plant encroachment and densification- a major management concern in the region- is related to plant late-Holocene plant migration dynamics and population processes rather than changing climate and land-use. Though the timing and effects of such events may differ, similar ecological responses to decadal/multidecadal climate variability are seen in the Glacier National Park region. In combination these studies serve to emphasize the need for careful selection of reference periods and baseline conditions used in climate-change monitoring, and this work shows the invaluable role that paleo-environmental archives can play in natural resource management. Overall, a more complete knowledge of long-duration ecological processes and lower-frequency climate variability should influence how we monitor and manage climate-change impacts throughout the northern Rockies.
PP52A-07 11:50h
Paleoclimatic Science in Planning and Decision Making
In the past two decades, national and international efforts have assessed the current understanding and consequences of climate variability and change. Two recent documents, the IPCC and CCSP, have outlined areas of research that are needed to further our understanding of the form, direction, and impacts of climate variability and change. Both documents point to the need for an increase in data networks, and modeling and process studies, including paleoclimate-based research. In addition, cutting across the key scientific issues of climate variability and change is the need to communicate scientific information so it can provide a foundation for informed planning and decision making, particularly at the regional level. The disconnect between scientific information and decision-making has been recognized and has begun to be addressed. The challenge of communicating the relevance and usefulness of climate-related data to decision makers is the focus of this presentation, and in particular, on applications of tree-ring based hydroclimatic reconstructions to water resource planning and management in Colorado. An opportunity to engage the water resource management community in considering the usefulness of paleoclimatic records was presented by recent and continuing drought conditions in the western US. In Colorado, Front Range water managers have commonly based drought planning on the premise that the most severe drought in the instrumental record, typically the 1950s drought, represents a likely worst-case scenario. Until the drought of 2002, there was no compelling need to take a more long-term look at water resource management. However, severe and in some respects, unprecedented drought in 2002 motivated many water managers to consider using tree-ring reconstructions of annual streamflow in assessing the risks of drought over time spans beyond the 20th century. Interactions with water providers have ranged from providing regionally-representative reconstructions of streamflow to municipal planners, to partnerships with major Front Range water agencies to explore the full potential of tree-ring reconstructions in water system models. Although this initial one-on-one collaborative process has been time intensive, it may be considered a pilot for future work at a broader scale, and applicable to the scientific communications goals of the IPCC and CCSP.
PP52A-08 12:05h
Using Model Simulations to Improve Interpretations of Paleoclimate Variability and Estimates of Vegetation Response to Potential Future Drought
Future climate change will involve not just changes in the mean state of the climate but also changes in climate variability and the frequency of extreme climate events. How will modern ecosystems respond to these future climate changes? Paleoenvironmental data provide an important record of past environmental response to changes in climate variability. Often, however, paleoenvironmental records do not have a sufficient temporal resolution to indicate whether an observed environmental change was due to variations in the magnitude, frequency, or duration of a particular climate change. For example, pollen records from lakes may record a vegetation shift indicating that a drought occurred at a particular time in the past, but they may not have the temporal resolution to indicate whether the observed vegetation response was the result of one large multi-year drought or frequent annual-scale droughts occurring over a period of decades. In this study we use models to simulate vegetation response to drought under a range of climate variability scenarios. We focus on drought events because they are frequently recorded in paleoenvironmental data and because the potential impacts of droughts under future climate are of particular societal concern. We use climate data from the Climate Research Unit (East Anglia, UK) 30-minute CL 2.0 data set (New et al. 2002) and the Tyndall Centre (East Anglia, UK) 30-minute TS 2.0 data set (Mitchell et al. 2003). Our climate variability scenarios include changes in the frequency, magnitude, and duration of drought events on intra-annual, inter-annual, and decadal time scales. Vegetation responses to these changes in climate variability are simulated using LPJ, a dynamic global vegetation model (Sitch et al. 2003). We focus on simulations of woodlands and grassland-forest boundaries in North America because these vegetation types are particularly responsive to changes in moisture regime, such as drought. The results of this study indicate how model experiments can be used to improve both interpretations of paleoenvironmental data and estimations of the potential environmental impacts of future changes in climate variability.