Although the magnitude of climate change that will occur this century is uncertain, all climate models indicate that the planet will warm. The suite of global climate models deployed in IPCC (2007c) report a mean climate sensitivity1 of 3.2°C, with a standard deviation of 0.7°C (IPCC, 2007c, Table 11.2); this indicates broad agreement, with some scatter, about the effects of carbon dioxide on global mean temperature. Other largescale aspects of climate change are also robust, such as water vapor feedbacks (increasing atmospheric moisture), thermosteric sea-level rise, ocean acidification, Arctic amplification of climate warming, warming feedbacks due to reductions in seasonal snow cover, and a poleward shift of circulation systems.
Despite these confirmations of the value of climate models, a number of longstanding and emerging problems require improvements and developments in model capability. Bader et al. (2008) provide a detailed summary of weaknesses of the current generation of climate models. The next section examines some of these weaknesses and outlines several high-priority scientific frontiers that can be better addressed through advances in climate models.
GRAND CHALLENGES FOR CLIMATE MODELS
Climate change is expected to affect society in many ways, including impacts on health, infrastructure, food and water security, ecological integrity, and geopolitical stability. Climate models are essential tools to inform planning and policy development surrounding these issues, but advances are required on a number of research fronts to improve the information that climate models can provide. High-priority questions include the following:
• Climate sensitivity: How much will the planet warm this century?
• How will climate change on regional scales? How will this affect the water cycle, water availability, and food security?
• How will climate extremes change?
• How quickly will sea level rise?
• How will Arctic climate change?
• What is the potential for abrupt change in the climate system?
• How will marine and terrestrial ecosystems change?
• How will society respond to and feed back on climate change?
• Can the evolution of the climate system over the next decade be predicted?
1 Climate sensitivity: the equilibrium, global mean temperature change associated with a doubling of CO2.