Could Climate Projections Be Verifiable?

ABSTRACT Verification for weather prediction is highly developed and has guided, motivated, and documented improvements in recent decades. In comparison, verification is far less developed for climate projections. Verification is an important part of the scientific process for testing and identifying ways to improve our understanding, and for establishing credibility and trust. In contrast to weather forecasts, climate projections are not initialized with an estimate of the state of the atmosphere or climate system as an initial condition; instead, the goal is to solve a boundary condition problem of estimating the climate state given a set of greenhouse gas and aerosol emissions and other forcings that are external to the model components. Because the primary use of climate projections is around future changes, which are often cast as anomalies relative to a baseline state, verification should include or even focus on the responses of the climate to external forcing, rather than just evaluation of the baseline climatology. Verification of climate projections is a challenge because (1) the relevant lead times are long, resulting in very small sample sizes, and (2) climate projections are conditional on the forcing pathway, which determines the boundary conditions, so they depend on additional information about the future external to the projection itself. Some existing activities contain elements of verification, such as the evaluation of climatological characteristics and the comparison of simulations of the past historical forcing with observed trends. Three concepts from weather forecast verification that present opportunities for advancing verification of climate projections are discussed: representativeness, forecast skill, and system design for verifiability.