We all know that skillful weather prediction is very difficult, and after a week or so, little or no skill remains. Yet, the IPCC made, and policymakers are accepting, forecasts of climate decades from now as skillful.
Part of this confusion is due to the fact that even professional societies are presenting conflicting definitions. For example, the American Meteorological Society (AMS) definition of “climate change” is
“(Also called climatic change.) Any systematic change in the long-term statistics of climate elements (such as temperature, pressure, or winds) sustained over several decades or longer. Climate change may be due to natural external forcings, such as changes in solar emission or slow changes in the earth’s orbital elements; natural internal processes of the climate system; or anthropogenic forcing.”
yet the same AMS defines the climate system as the
“system, consisting of the atmosphere, hydrosphere, lithosphere, and biosphere, determining the earth’s climate as the result of mutual interactions and responses to external influences (forcing). Physical, chemical, and biological processes are involved in the interactions among the components of the climate system.”
This later definition fits with the 2005 National Research Council definition of the climate system as shown in the figure below from that report.
Using this figure, it is clear that short-term (up to a week or so) weather prediction is a subset of the atmosphere part of the above the figure. Seasonal weather prediction is also a subset of this figure, where ocean and land components are required, along with the atmosphere, for skillful predictions. Even in this case, aspects of these components, such as sea surface temperature, are usually assumed to be static over the time period of the prediction.
However, for multi-decadal climate predictions, all components of the climate system must be predicted. Even for decadal-long average values of climate variables (such as for a global average surface temperature) an understanding of all of the natural and human climate forcings and feedbacks is still required.
Thus, in answer to the question on this weblog, weather is a subset of climate. Climate prediction is necessarily much more difficult than weather prediction. While the sensitivity of the climate system can be estimated from models by changing one or more components of the climate forcings (e.g., by inserting more CO2 into the atmosphere), such as completed for the 2007 IPCC report, these are not climate predictions. To use the results of the IPCC models for regional assessments of climate impacts, such as the claim of more droughts in the future (e.g., see and see), is scientifically flawed and thus is misleading policymakers.