accuracy, precision, and continuity in instrument characteristics and processing methods needed to permit the resolution of small but significant longer-time-scale climate variations that can be buried within the much larger diurnal or seasonal signals. Other such data have yet to be monitored. Particularly important are external forcings (e.g., solar variability) and critical variables (e.g., water vapor and moisture and energy fluxes).
Over the last few decades, satellites have provided some of the most helpful operational data. These data are especially useful for increasing spatial coverage and determining otherwise difficult-to-observe climate variables such as snow and ice distribution, which has been shown to be a major influence on the climate and the sensitive thermohaline circulation.
Research Data. The aforementioned types of operational data will not be sufficient alone; they will need to be supplemented with focused data collected specifically for climate-oriented research. These latter data sets will be critical for formulating, improving, and validating specific processes in theoretical and numerical models that are necessary to our understanding of the climate system and the specific mechanisms that control it. For instance, the ocean is severely undersampled with respect to time except in specific coastal and island locations. Establishing and maintaining globally distributed observational systems to provide spatial and time-series measurements of velocity, temperature, and salinity will be central to producing a legacy for monitoring, understanding, and predicting future climate variability.
Model Simulations. Decade-to-century-scale modeling activities are going on at universities, government laboratories, and other research centers in many countries. The complementary and overlapping results obtained at these institutions offer excellent opportunities for intercomparison among model results and validation against instrumental data sets. Proper documentation, archiving, and data-management procedures for model simulations are essential, however, since the quality and continuity are a concern with model-derived information just as with observational data.
The workshop and the papers in this volume show that the earth's climate is always changing, and that gradual changes, periodic variations, and sudden shifts are all characteristic of this natural propensity for change. The Climate Research Committee concludes that the climate fluctuations of the last few millennia were as varied and extensive as any of those observed over the last few decades, though modern climate seems to be intriguingly close to the warmest limits of our poorly documented record of natural variability over the past several thousand years (see Jones and Briffa's paper in Chapter 2). Modeling studies do suggest that climate is very sensitive to relatively small perturbations in key locations. For example, subtle changes in the air/ sea/ice interaction of the high-latitude North Atlantic may lead to abrupt changes in ocean circulation and Northern Hemisphere climate. If one of these disproportionate responses were triggered by an anthropogenic effect, climate could be altered more rapidly than the past record would lead us to expect.
The magnitude of the climate changes indicated by records of natural variability, and the rapidity with which they have taken place, suggest that society should expect significant climate change even if anthropogenic influence is minimized. We need to understand and be able to predict this change so that we can adapt to it or modify our contribution to it. Given our recent advances in documenting past climate change, monitoring modern climate processes, and improving process and coupled atmosphere-ocean models, the study of natural climate variability represents an area of great scientific opportunity—one that is important not only for guiding policy, but for understanding how our present biological and geochemical environment evolved and learning to predict how it may respond to natural variations or anthropogenic changes.