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The Atmospheric Chemistry and Climate Initiative Activity
“Scenarios: Sensitivities & Uncertainties”

Paris, 20 June 2009

The simulations performed for the CMIP3 (Coupled Model Intercomparison Project) activity in support of the IPCC AR4 have provided a tremendously useful resource for exploring issues of climate sensitivity, historical climate and climate projections. However, the radiative forcings imposed in both the simulations of the 20 th century and the future projections varied from model to model due to varying assumptions about emissions, differences in the behavior of physical processes affecting short-lived species that were included, and differences in which processes and constituents were included at all. Furthermore, the CMIP3 archive does not include diagnostics of radiative forcing from aerosols, ozone, or greenhouse gases other than CO 2 . Hence it is not straightforward to understand how much of the variation between simulated climate in the models results from internal climate sensitivity and how much results from differences in the forcings.

To characterize the radiative forcing from individual species in the CMIP5/AR5 simulations, and to diagnose causes of inter-model differences, we have created a set of simulations to be performed by the chemistry/aerosol modeling community to compliment those performed directly under the CMIP5 project. This Atmospheric Chemistry and Climate Model Intercomparison Project (ACC-MIP) consists of several sets of simulations that have been designed to facilitate useful evaluation and comparison of the AR5 transient climate model simulations. Two sets of simulations are designed to be run on the AR5 timeline, while two others are lower priorities to examine additional sensitivities that may not be completed until after the AR5 period.

A spreadsheet has been created with detailed specifications for the ACC-MIP outputs, largely based on fields archived for HTAP, AeroCom, and/or CCMVal. This can be downloaded from the HTAP/AC&C Wiki page ( ; “Atmospheric Chemistry & Climate (AC&C) Initiative Experiment Descriptions”).

This meeting will be used to discuss and finalize these plans and possible future with the modeling community. In particular, we encourage participants to prepare a few slides describing specific scientific questions that they hope to tackle under this framework as a basis for discussions.

Download Agenda, Meeting Location, and Background Information Here