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Natural Climate Variability on Decade-to-Century Time Scales
by the cooperative efforts of dynamic meteorologists and physical oceanographers, sustained by long-term scientific coordination and by relatively healthy and stable governmental support, both national and international. The effort required to shed light on the global system's variability on decade-to-century time scales is proportionately greater, and atmospheric scientists, other oceanographers, glaciologists, biologists, and ecologists will be needed to join those meteorologists and oceanographers thus far engaged. The problem is further complicated by the fact that both natural and anthropogenic effects occur on the time scales of interest, and are hard to separate. It is thus important to establish an understanding of the natural variability, so that it can serve as a baseline against which possibly anthropogenic effects can be gauged. The NRC workshop papers were contributions toward this goal.
FINDINGS FROM THE WORKSHOP PAPERS
Essays at the beginning of each section of this volume discuss our progress in atmospheric observations, atmospheric modeling, ocean observations, ocean modeling, coupled systems, and climate proxy data. They set the stage for the 42 papers included. The preliminary results of these explorations of natural climate variability summarized in this volume are interesting and encouraging; the Climate Research Committee has extracted four main findings from them.
First, the relatively short instrumental record of climate (the last 50 to 100 years) does not represent a stationary or steady record. The papers in this volume show that climate's natural propensity for change has manifested itself through periodic variations, sudden shifts, gradual changes, and changes in variability. Furthermore, such changes do not appear to be unique to this century, as proxy records such as ice cores and tree rings attest. Climate fluctuations over the past few millennia or so will need to be analyzed in greater detail to establish a baseline against which future variations can be gauged.
Second, we are not yet certain why these changes in climate occur. Models must be used to test our hypotheses and to increase our understanding of the climate system. Models of the atmosphere, the ocean, and the coupled atmosphere-ocean system are beginning to yield insights into the causes of natural climate variations. For example, recent ocean modeling studies suggest that significant changes in the deep-water circulation may occur over time scales of decades to centuries, and that these changes might critically affect climate. A second ocean model finding is that the thermohaline circulation can oscillate between quasi-steady 'equilibrium' modes.
Third, the Climate Research Committee feels that systematically combining observations and models, and ensuring the long-term continuity and sufficient quality of the data, will be critical to the assessment of climate variability and of the models that are used for climate simulation and prediction. The observations permit us to initialize, force, and diagnose models, providing reassurance that we are simulating the real world. Models not only serve as the measure of our understanding and the means of predicting, but are now good enough to help guide observation, monitoring, and data-management programs.
Finally, additional data are needed to supplement and expand the currently sparse and sporadic record of past natural climate variability. Proxy data, historical records, current operational data, research data, and model simulations can all contribute significantly to our understanding. In some cases they are available but under-utilized; in others they must be obtained through special programs or refinement of existing collection programs. Consistent data quality and uniform data-management practices are essential, and all climate data should be standardized and made available to researchers worldwide.
The comfort and livelihood of future generations on this planet demand the vigorous pursuit of new insights into the characteristics of decade-to-century-scale climate variability. Significant benefits will be realized if this research yields reliable prediction capabilities. Such benefits have already been achieved for prediction on interannual time scales. In countries such as Peru, successful predictions of El Niño have enabled fisheries and agricultural communities to introduce adaptive measures in order to minimize its negative impacts. Evidence to date suggests that the magnitude of climate change is often proportional to the time scale over which it occurs, so that over the longer scales the potential benefits of accurate prediction could be even greater. The four recommendations below for making credible prediction methods a reality are discussed in detail in Chapter 7.
Criteria must be established to ensure that key variables are identified and future observations are made in such a way that their results will yield the most useful data base for future studies of climate variability on decade-to-century time scales. Assurances of continuity and high data quality are essential.
Modeling studies must be actively pursued, using a variety of models, in order to improve our skill in simulating and predicting the climate state, and to assess potential modes of variability. Closer links between models and observational studies will be necessary. Continuing efforts must be made to assimilate the data sets in dynamical models, so that ultimately the added value of dynamical consistency and dynamical interpolation may be realized.