conditions and of the magnitude of various forcings extend back about 150 years at best. Comparisons of observed surface temperatures with those simulated using reconstructions of the past forcings have yielded important insights into the roles of various natural and anthropogenic factors governing climate change. The shortness of the instrumental record and of accurate monitoring of climate forcings, however, limits the confidence with which climate change since preindustrial times can be attributed to specific forcings. Proxy records obtained from ice cores, sediments, tree rings, and other sources provide a critical tool for extending knowledge of both climate forcings and climate response further back in history. Improved historical radiative forcing reconstructions will require new understanding of physical process to better understand the relevance of available historical observations and their relationship to the actual forcings. For example, in the case of solar forcing, separate physical connections must be made of the solar magnetic activity with irradiance and with the near-Earth space environment, which modulates galactic cosmic rays that produce cosmogenic isotopes—the only long-term archive of solar activity and a crucial component of long-term Sun-climate research.

The lack of proxy climate data in certain key regions (e.g., large parts of the tropical Pacific and the extratropical oceans) is a major limitation. Such regional information is important in evaluating the potential roles of changes in modes of climate variability, such as ENSO, in past climate changes. Experiments employing fully coupled land-ocean-atmosphere models to study regional past climate change are just now under way. For comparison with model simulations, greater historical knowledge should be sought for a broad array of climate system parameters including the hydrological cycle (e.g., droughts, rainfall, streams), modes of variability (e.g., ENSO, annular modes), land use, the stratosphere, and ozone.

RECOMMENDATIONS:

• Seek greater historical knowledge for a broad array of climate system parameters including the hydrological cycle (e.g., droughts, rainfall, streams), modes of variability (e.g., ENSO, annular modes), land use, the stratosphere, and ozone.

• Improve proxy records of past radiative forcing (e.g., indicators of solar and volcanic forcing).

• Enhance physical understanding of how these proxy records relate to the forcings (e.g., relationship between solar activity, irradiance, and cosmogenic isotopes).

• Undertake “data archeology” projects to recover long-term instrumental records of climate variables of the past few centuries.

• Continue to develop high-quality, high-resolution (or well-dated,



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