CCT3 - Rationale and Goals


Rationale

Physically based Earth System models (ESM) provide a valuable tool to assess conceptual models of past climate changes derived from paleodata. Conversely, data-based reconstructions of past climate variations provide critical testbeds for ESM, which are usually optimized to capture present-day climate. Hence, combining data-based reconstructions and paleo-modeling offered a promising way to fully comprehend past climate dynamics and accordingly benefit forecasting simulations through improved model skill.

Numerical modeling played a crucial role in the research underlying all PAGES Foci. The models employed not only included ESM but also those for interpreting proxies at a local scale, as well as human-climate interaction models. The models in use were frequently developed outside the paleoresearch community and optimized for capturing modern processes. Accordingly, ESM did typically not deal with a number of issues that are important to paleo-modeling and therefore were promoted by CCT 3:

- Forward modeling of proxies

- Objective comparison of proxy data and modeling results

- Optimizing selection of sampling locations

- Requirement for modules of reduced complexity allowing for longer integration times

- Modules for Earth System components operating at geological timescales

These topics were best addressed in a paleo-framework that included modelers as well as specialists dealing with the intricacies of paleo-proxies and proxy-based reconstructions.


Goals

The specific goals for CCT 3 were threefold:

1) Fostering the development of strategies for proxy modeling

Focus was on proxies with wide spatial coverage and for which large data collections already exist (e.g., stable isotopes reflecting the physical and biogeochemical processes in the Earth System). Strategies for including proxies such as aerosols or pollen were also developed.

2) Devising methods for the objective comparison of proxy data and modeling results

In present-day climatology, data assimilation has become an important tool for quantifying the state of the climate system based on observations. However, these methods are not readily available for the sparsely distributed paleodata. Successful strategies for assimilation of paleodata could also help scientists working on proxy-based reconstructions to optimize the selection of sampling locations and will be promoted by this CCT. Additionally, proxy-based reconstructions are often widely spaced and associated with uncertainties that are not well constrained. The lack of suitable statistical methods for comparing such data with model output was addressed by the CCT. Finally, strategies for downscaling the output of global models to regional scales are still immature and were also promoted by the CCT.

3) Promoting the development of comprehensive ESM families

The long timescales associated with geological processes often require integration times that are significantly longer than those used for generating future climate scenarios. To meet this requirement and also rely on the same type of models used in the IPCC community, models should be offered with a range of resolutions and system components suitable for long paleoclimate experiments.