sponse of the climate system to the forcing; and (3) the internal (stochastic) variability of the climate system. In the seasonal-to-decadal prediction context, uncertainties of types 2 and 3 are relevant, but, in addition, there are also uncertainties in the initial conditions of the climate system. The latter arise due to observational errors and errors in the assimilation systems used to generate the initial conditions.
Uncertainty in Future Climate Forcing
The energy balance of Earth provides the engine that powers the planet’s climate. That energy balance in turn is shaped by, among other things, the composition of Earth’s atmosphere, which is being altered by emissions of greenhouse gases, aerosols, and short-lived species. Future climate forcing will be shaped by
• emissions of greenhouse gases, aerosols, and short-lived species into the atmosphere;
• processes that control the composition of the atmosphere, such as atmospheric chemistry, terrestrial and marine components of the carbon cycle, and nitrogen cycles; and
• climate processes, including interactions among the atmosphere, ocean, land, and cryospheric systems.
The future of each of these processes is subject to important uncertainties. Human emissions of greenhouse gases, aerosols, and short-lived species are sufficiently large (and growing) that they are significantly changing the composition of the atmosphere. Historical emissions of carbon dioxide (CO2) and other greenhouse gases (GHGs) from fossil fuel use and industrial processes are reasonably well known. Emissions of CO2 and other compounds resulting from land-use and land-cover change are smaller and less well measured. Future projections of all these sources of anthropogenic emissions will be subject to important uncertainties.
The annual global emissions of CO2 can vary by more than an order of magnitude in nonclimate policy intervention scenarios (see, for example, Reilly et al., 1987, 2001; Scott et al., 1999). However, the cumulative nature of the carbon cycle means that variation in the concentration of CO2 in the atmosphere is more constrained. Factors that influence the scale of future anthropogenic emissions include the scale of economic activity, the technologies with which human societies generate and use energy, and the public policy environment in which human activities are conducted. Hence, predicting emissions of GHGs and aerosols requires being able to predict how the entire human world will develop in the future, a truly daunting task fraught with multiple profound uncertainties.