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Frontier 1: Anthropogenic Climate Change
Undertake the predictive science that aims to develop the adaptation decisions that must be made in response to human activity
Prepared by: G. Meehl, S. Bony, F. Zwiers and W. Cai
Human activity has changed our climate over the past century, and further change is inevitable over the next several decades, even if we take strong mitigation actions. Thus it is imperative to undertake predictive science that aims to inform the adaptation decisions that we must collectively make. This includes improving our ability to predict the future state of the climate system, including variations in the likelihood of extremes (see figure below), drought, and the availability of water, on time scales of seasons to decades and longer, and in particular, to better understand how human influences on the climate system exacerbate (or dampen) these variations on regional to global scales.
Projected changes in Atlantic hurricane frequency by category over the 21st Century based on CMIP3 climate warming scenarios
This will require us to further improve our understanding of the processes that govern the natural variability of the climate system on these scales and of how those processes are affected by human influence, and to use this understanding to provide observational constraints that can be used to improve predictions and projections.
Scientific Background and Major Challenges
Post CMIP3 paradigm shift: The CMIP3 climate change experiments coordinated by the Working Group on Coupled Modelling (WGCM) represented the end of the era of non-mitigation scenarios represented by the SRES suite with the main climate change projection time frame being near the end of the 21st Century. The paradigm shift that occurred after the publication of the IPCC AR4 involved a move toward mitigation scenarios, with implied policy actions, to better quantify various feedbacks, including carbon cycle, relevant to longer term climate change out to 2100 and beyond, as well as an enhanced focus on shorter term climate change out to about 2035. This paradigm shift grew out of the research assessed for the AR4 that recognised the need to understand and interpret observed climate changes in order to understand how much we can attribute to human activity, to internal variability, or to external forcings (natural and anthropogenic). This built on the growing need for climate science to inform adaptation and mitigation decisions.
The post-CMIP3 paradigm shift led to the formulation of a new set of climate change experiments that have become known as CMIP5. The experimental design for CMIP5 recognised the two timescales mentioned above, and that better quantification of climate change would likely involve two classes of new generation climate models to address scientific questions directly arising from processes and interactions involved with those two timescales. Therefore, CMIP5 has experiments to address what is now being called “decadal prediction” with initialised higher resolution models to study, for example, regional climate change and extremes, and longer term experiments run with either Atmosphere-Ocean General Circulation Models (AOGCMs) or the new generation of Earth System Models (ESMs) with candidate components of coupled carbon cycle, chemistry, aerosols, and dynamical vegetation added to the tradition AOGCMs.
Major Challenges: On the longer time scale, the extent to which we undertake mitigation will have a profound impact on the climate that will be experienced by our children and grand children later in this century and beyond. Thus it is imperative that we undertake science that informs policy makers about the constraints that natural variability, physical and biogeochemical feedback processes, and human activity that alters these feedback processes (such as land use change) impose upon policy choices concerning areas such as emissions, land use, and long term water use planning.
Near-term climate change experiments: For decadal prediction, the CMIP5 experiments have, as a primary focus, hindcasts to quantify decadal predictability, as well as predictions out to 2035 to address short-term climate change. One of the main science questions involves how best to initialise the ocean, and how much additional regional prediction skill (over and above un-initialised runs) can be obtained from an initialised climate model. This science question bridges the climate change problem to seasonal to interannual prediction, and decadal prediction is bringing together these two communities to address this problem. Another challenging problem related to initialisation is how much additional regional predictive skill can be obtained by resolving regional internal decadal variability mechanisms in addition to the climate change produced by commitment and changes in external forcing. The CMIP5 hindcasts involve 10-year runs initialised with the climate state every five years starting with 1960 and continuing to 2005. Then three of these 10-year runs will be extended to 30 years for starting dates of 1960, 1980, and 2005. Therefore, for the 30-year experiments, two will be hindcasts, and one (starting in 2005) will be a prediction. Since the various scenarios do not diverge much until about 2030, only one scenario will be used for the decadal predictions (RCP4.5, see below) and, consequently, these decadal predictions are most relevant to inform adaptation strategies.
Long-term climate change experiments: For the long-term experiments, four new mitigation scenarios will be used for the 21st century (2005 to 2100) and beyond to 2300. A low overshoot mitigation scenario with an approximate radiative forcing of 2.6 Wm-2 in 2100 is called “RCP2.6”, the two medium mitigation scenarios are termed “RCP4.5” and “RCP6.0”, and the high scenario is “RCP8.5”. The focus of the long-term integrations is to provide information on how feedbacks in the climate system contribute to the magnitude of climate change in the future for various mitigation strategies. Therefore, these simulations are relevant to mitigation and adaptation, with climate sensitivity in the different models contributing to the size of the feedbacks and climate change. It is on these longer timescales that sea level rise and the role of the melting of ice sheets will come into play. The combination of the various scenarios and feedbacks will provide information on possible abrupt climate change as well.
Climate change detection and attribution: The simulations leading up to the long term integrations will be run from 1850 to 2005 with observed natural (solar and volcano) and anthropogenic (GHG, aerosols, ozone) forcings for analyses relevant to climate change detection/attribution. A new aspect of these 20th Century (and 21st Century) simulations will be specified time-evolving land use change so that, for the first time, the contribution of land use change to local, regional, and global climate change can be addressed.
Understanding inter-model differences in climate change projections: Additionally, there will be several experiments to quantify the magnitude and nature of the carbon cycle feedback, and to understand the origin of inter-model differences in the climate response to a given perturbation. Some experiments will allow us to diagnose climate sensitivity and radiative forcings from coupled models. Idealised experiments (e.g. atmosphere-only experiments forced by prescribed SST perturbations, aqua-planet experiments) will make it possible to assess both the robustness and the uncertainties of the climate change response predicted by coupled models, and to better interpret the origin of inter-model differences in the simulation of clouds, precipitation and large-scale dynamics. This will help to understand the climate change response simulated by climate models, and to assess some components of this response using observations or process studies.