Natural systems involving dynamical and biogeochemical processes that proceed at both large and fine scales are subject to different, though overlapping, uncertainties. There is some confidence associated with descriptions of the very long term (1,000+year) processes that determine the average abundance of carbon in the atmosphere, but the forces shaping decadal to century atmospheric composition are less well understood (Kheshgi et al., 1999). Uncertainty in the carbon cycle is such that the maximum annual emissions that would limit long-term CO2 concentrations to 550 ppm are uncertain by ±20 percent (Smith and Edmonds, 2006).

Finally, there are also uncertainties in the natural forcing of the climate system, namely fluctuations in solar irradiance and aerosol emissions due to volcanic activity. Although there is some periodicity to solar irradiance that can be estimated (Lean and Rind, 2009), it is not precise, and future forcing from volcanoes is currently completely unpredictable. The latter can substantially reduce receipt of solar radiation for short periods (e.g.,1-2 years).

Uncertainty in the Climate System Response to Radiative Forcing

Climate system uncertainty is explored through the application of global and regional climate models. While most of these models are carefully constructed to incorporate many climate-related processes and are carefully evaluated, they do not necessarily respond in the same way to a given future forcing scenario. These differences are due to scientific uncertainties about how the climate system works, differences in the way various subsystems are modeled (e.g., land-surface processes), and differences in how unresolved processes are parameterized (e.g., convection). These uncertainties are explored and characterized by analyzing the results of different types of ensembles of climate model simulations. The most common is the multimodel ensemble (MME) based on simulations with different climate models that are subjected to the same future radiative forcing. These MMEs play a central role in the analyses that contribute to the Intergovernmental Panel on Climate Change (IPCC) assessments (e.g., IPCC, 2007c). There are also ensembles developed from a single climate model whose parameters are varied in systematic ways, which are referred to variously as parameter permutation experiments or perturbed physics ensembles (PPEs) (e.g., Murphy et al., 2007).

A primary integrated metric of uncertainty related to the climate system response to radiative forcing is the value of the climate sensitivity of the climate system. Equilibrium climate sensitivity is defined as the average annual change in global mean temperature that results from forcing a climate model with the radiative equivalent of doubled concentration of CO2. For many years, this sensitivity was described as a

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