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Significant advances in the field of paleoceanography have both sharpened the focus of paleoclimate and ocean history research on classic problems and initiated new research directions. The classic problems driving our long-range research program include: (1) relationships between sea level, ice volume, and climate change; (2) interactions between atmospheric carbon dioxide, climate, and its biospheric and geospheric regulation; (3) long-term changes in ocean chemical composition and geochemical fluxes as related to geological and biological evolution; (4) solar and magnetic field variability and their role in climate change and affect on cosmogenic nuclides; and (5) changes in modes of ocean circulation in relation to climate change and the evolution of oceanic basins.

Work proceeding along these paths has led to two important shifts in how we view climate that seem to cut across all time scales: (1) the growing realization that substantial changes in atmospheric CO 2 are likely to have played a large role in both long-term and short-term climate change, and (2) the discovery that nonlinear interactions in the ocean-climate system may have played a key role in determining the sensitivity of climate to both internal and external forcing. Furthermore, these nonlinear interactions can shift climatic variance to both higher and lower frequency oscillations.

In recent years, work on these classical themes has uncovered ''bombshells" that have rattled prevailing views: (a) tropical sea-surface temperatures may have been 5°C cooler during glacial maxima, in contrast to CLIMAP reconstructions with stable tropical SST; (b) evidence for cool tropics and low equator-pole thermal gradients also is found for the late Cretaceous and Eocene, again countering prevailing beliefs; (c) ice core isotope paleothermometers appear to have understated glacial cooling at high latitudes by a factor of two; (d) transitions between glacial and interglacial states can occur in only a few decades, and (e) in the late Paleocene (~56 million years ago), there was a sudden input of isotopically light carbon into the ocean-atmosphere system accompanied by global warming lasting for no more than a few thousand years. These findings have invigorated ocean-climate investigations and forced us to reexamine many traditional assumptions.

In the future, we must emphasize the search for a better understanding of processes that affect climate change and cause variability in the climate-ocean system. Through this effort we hope to gain a better understanding of the coupling of the ocean-climate system through the entire range of the Earth's climatic spectrum (Figure 3).

Examples of specific science questions that will drive research in paleoclimates include:

FIGURE 3 Estimate of relative variance of climate over all wavelengths of variation, from those comparable to the age of the Earth to about one hour. Shaded area represents total variance on all spatial scales of variation. Strictly periodic components of variatior are represented by spikes of arbitrary width. Modified from Mitchell, Jr., J.M. 1976. An overview of climate variability and its causal mechanisms. Quaternary Research 6:481-493, with permission from Academic Press, Inc.

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