Paleoclimate Research: Climate of Past Centuries and Millennia

Global scale climate change has been detected and attributed to anthropogenic forcing. But how well are current climate models ready to project or predict regional climate over the coming decades and beyond? Two fundamental issues are limiting this necessary next step. First, regional climate generally exhibits much more variability and records are often too short to confidently identify the source of the variations. Second, emphasis in climate model development has mostly been put on reproducing the mean annual cycle, but the next challenge is to faithfully reproduce the range of variability.

Caspar Ammann's research focuses on the necessary next steps of extending the instrumental records through the development of improved high resolution climate reconstructions and of strategically designing climate model studies to understand the processes that are dominating the sub-continental scale of climate variability.

Using a new collection of high resolution sulfate records in polar ice cores, novel volcanic forcing series are being developed that are more accurate in estimating the magnitude of past forcing, but for the first time also quantify the uncertainty of eruption occurrence and magnitude. The distribution of frequency and magnitudes are then simulated and possible future volcanic forcing scenarios can be derived, an aspect of future climate variability largely ignored thus far.

A fundamental problem in identifying what part of climate variability is externally forced arises from the large uncertainty in existing reconstruction methods and series. Amman's collaborations with a multi-institution team of paleoclimatologists and statisticians is developing a new way of reconstructing climate using Bayesian Hierarchical Models as the framework. This allows for a more complete exploitation of the available climate record through inclusion of records with extremely different characteristics as well as explicit physical constraints. Additionally, the community program of the Paleoclimate Reconstruction (PR) Challenge is setup to test the accuracy of regional reconstructions and to guide the paleo reconstruction communities in developing more adequate forward models for their proxies. Using climate model output, a systematic intercomparison of the existing reconstruction approaches is performed and a double-blind setup will allow the community to identify where the next efforts need to be put in.

Directly embedded in these regional climate analyses are comparisons between real world and model-based regional climate signals that serve as a fundamental test of climate models ability to reproduce the forced aspects of regional climate. Collaborations across ESSL are used to identify what model configurations are necessary to capture the necessary processes. Using a suite of simulations with increasing model complexity from the standard CCSM towards a more complete representation of the climate system of WACCM (vertical extent, coupled chemistry), climate response to the repeated solar cycle and to the injection of volcanic aerosol are investigated.