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R. J. Stouffer
,
V. Eyring
,
G. A. Meehl
,
S. Bony
,
C. Senior
,
B. Stevens
, and
K. E. Taylor

Abstract

The Coupled Model Intercomparison Project (CMIP) is an ongoing coordinated international activity of numerical experimentation of unprecedented scope and impact on climate science. Its most recent phase, the fifth phase (CMIP5), has created nearly 2 PB of output from dozens of experiments performed by dozens of comprehensive climate models available to the climate science research community. In so doing, it has greatly advanced climate science. While CMIP5 has given answers to important science questions, with the help of a community survey we identify and motivate three broad topics here that guided the scientific framework of the next phase of CMIP, that is, CMIP6:

  1. How does the Earth system respond to changes in forcing?

  2. What are the origins and consequences of systematic model biases?

  3. How can we assess future climate changes given internal climate variability, predictability, and uncertainties in scenarios?

CMIP has demonstrated the power of idealized experiments to better understand how the climate system works. We expect that these idealized approaches will continue to contribute to CMIP6. The quantification of radiative forcings and responses was poor, and thus it requires new methods and experiments to address this gap. There are a number of systematic model biases that appear in all phases of CMIP that remain a major climate modeling challenge. These biases need increased attention to better understand their origins and consequences through targeted experiments. Improving understanding of the mechanisms’ underlying internal climate variability for more skillful decadal climate predictions and long-term projections remains another challenge for CMIP6.

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V. Eyring
,
N. R. P. Harris
,
M. Rex
,
T. G. Shepherd
,
D. W. Fahey
,
G. T. Amanatidis
,
J. Austin
,
M. P. Chipperfield
,
M. Dameris
,
P. M. De F. Forster
,
A. Gettelman
,
H. F. Graf
,
T. Nagashima
,
P. A. Newman
,
S. Pawson
,
M. J. Prather
,
J. A. Pyle
,
R. J. Salawitch
,
B. D. Santer
, and
D. W. Waugh

Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.

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