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CLIVAR/PAGES scientific goals

A number of key scientific issues were identified by the intersection. They are categorized into overarching cross-cutting issues, in addition to 4 more specific topical issues.

1) Overarching cross-cutting issues

- Forward modeling of proxy data, which means including the processes that produce proxy records directly within Earth System Models (ESMs) is considered of fundamental importance to further improving model-paleodata comparisons.

- Reducing uncertainties in proxy reconstructions (and data synthesis in general) are important for improving climate modeling targets and for understanding the intrinsic variability and forced response of the climate system.

- Calibration of proxies against variability seen in the instrumental period is a prerequisite for improved synthesis of proxyand observation-based approaches, and requires interaction between paleoclimatologists and climatologists.

2) Climate variability over the last few millennia

Well-dated, high-resolution proxy reconstructions and model simulations incorporating estimates of natural and anthropogenic forcings for the last 2 ka offer opportunities to assess the natural decadal to centennial variability and forced responses in conditions similar to present. Despite progress in recent years, however, important uncertainties and caveats exist with regard to both empirical reconstructions and model estimates.

Upcoming activities will therefore focus on advancing process-based comparisons of models and data through an enhanced appreciation of forward modeling of specific proxies and at specific sites, including the role of downscaling from large-scale model simulations. The Paleoclimate Reconstruction Challenge and the regional PAGES 2k Network will both play key roles in driving the science of this issue.

3) North Atlantic circulation changes

Interactions among the ocean, atmosphere, and sea ice are the likely cause of decadal to multi-decadal and centennial variability in the Atlantic meridional overturning circulation (MOC), with attendant impacts on spatial patterns of temperature and precipitation. Thus improved understanding of MOC variability may serve to improve the climate projections in these regions. Uncertainties in model parameterizations and the response of the climate system to anthropogenic forcings make projections of future MOC behavior unclear. Since multiple proxy records reflect MOC changes and their climatic impact, MOC variability is an excellent showcase for the value of using forward models of paleo-proxies, specifically ocean proxies, water, carbon and nitrogen isotopes, atmospheric chemistry, dust and sea salt aerosols.

The Intersection will support synthesis activities focused on data-model integration, particularly those that seek to improve mechanistic understanding of multidecadal variability and its impacts on, in particular, hydrology.

4) Hydrological changes and interactions with the land surface

Recent observations indicate that the tropical realm is expanding with increasing occurrence of drought in the sub-tropics. This trend is projected to continue under IPCC AR4 scenarios. There is also considerable evidence suggesting terrestrial climate variability is strongly influenced by hydrological and biospheric interactions and feedbacks. This is particularly relevant to high-latitude regions and the tropics, where it has been shown that feedbacks between the monsoon and land surface conditions have significantly influenced climate variability on all timescales.

ESMs that incorporate these feedbacks are now being used for future climate change prediction and need to be rigorously tested against the paleoclimate record. The emphasis of the Intersection will primarily be on initiating and supporting data synthesis activities concerned with data-model interaction. Particular emphasis lies on forward modeling of climate proxy data with relevance to low latitude changes in hydrology.

5) Tropical cyclones, extreme precipitation events

For some extreme events (e.g., tropical cyclones, droughts and floods), there is some theoretical basis for expecting changes in their occurrence and/or intensity, associated with changes in the background climate state. However, it is the nature of extreme events that they are rare, and so the observational record is often sparse. By targeting specific proxies (paleo-tempestology) or by increasing the appreciation of long documentary records available in Europe, US East Coast, Japan, China and Korea, an improved basis for the characterization of some extreme events could be developed.