Paleoclimatology

Paleoclimatology involves the reconstruction of past climates prior to the instrumental period. Paleoclimatology focuses on: (1) describing past climates, (2) understanding the natural and anthropogenic processes responsible for these patterns, and (3) using this knowledge of past climates and climate dynamics to identify and understand potential responses to climatic forcing. Detailed views of the modern climate can be obtained using available instrumental records and documentary or historical sources of information, but their short temporal resolution and/or sparse spatial coverage limits their ability to capture the variability that is inherent in the climate system. Paleoclimatology, therefore, extends instrumental and historical records further back in time, enabling the capture of a greater range of variability. This enhances not only our understanding of climate variability in terms of mean conditions, extremes, and states, but also improves insight into the dynamic forces controlling the operation of the climate system.

Examples of research areas that interest paleoclimatologists include: abrupt climate change, hydrological variability, land-cover change, sea-level rise, and modeling the potential response of the climate system to natural and anthropogenic forcing. Paleoclimate research methods involve the testing of specific hypotheses such as how to identify the forcing mechanism(s) or drivers responsible for specific climate events that occurred in the past (for example, the Dust Bowl, Little Ice Age, mid-Holocene aridity in central North America, and the Younger Dryas). One important field of paleoclimate research centers on identifying potential surprise behavior in the climate system, the mechanisms responsible for these nonlinear responses, and ultimately the impacts that may result.

Paleoclimatologists extract paleoclimatological data from a plethora of natural archives, or proxies, such as corals, ice cores, marine and lake sediment, tree rings, and spleothems. For example, ice cores recovered from alpine glaciers located in South America, Africa, and Central Asia and high latitude ice sheets located in Greenland and Antarctica have provided detailed records of atmospheric trace gas concentrations (CO2, CH4), temperature (O18, δD), storminess (dust), and volcanic eruptions (SOx) extending back over hundreds of thousands of years. The European Project for Ice Coring in Antarctica (EPICA) recently completed an ice-drilling project at Dome C, Antarctica that provided a climate record spanning the last 740,000 years. The data from Dome C suggests that a tight coupling between trace greenhouse gases and Antarctic temperature variations has existed for the last four glacial cycles (420,000 years).

Paleoclimate research is increasingly utilizing global and regional climate models to generate simulations of past climates and evaluate the role feedbacks play in the different climate subsystems (atmosphere, ocean, land surface, sea ice, and land ice) at various spatial and temporal scales. A recently completed study, the Paleoclimate Modeling Intercomparison Project (PMIP), systematically assessed the ability of the current generation of general circulation models (GCMs) to simulate past climates that differed significantly from present climate conditions. The output from these models was directly compared to biophysical and geochemical proxy records to evaluate how well models can simulate past conditions. Studies such as PMIP demonstrate that the current generation of general circulation models (GCMs) can simulate known past climatic conditions and events with skill, strengthening their ability to predict future climate change.

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