fundamentally through a lack of understanding of the natural climate variability that underpins modern climate.

While significant progress has been made in the short-term prediction of important atmospheric phenomena such as ENSO, relatively little is understood about potential changes in the frequency and magnitude of this and other important climate systems (e.g., monsoons, North Atlantic hurricanes) on decadal and greater scales, despite evidence that such events did vary in frequency and magnitude over such periods as the LIA.

Holocene climate and environmental response have been sufficient to create significant stresses for emerging civilizations. Human impact on the chemistry of the atmosphere and on the land-ocean environment and climate has also been extremely significant. However, the complex interplay of human activity and environmental and climatic change still holds many unanswered questions (see Box 6.2).

Focus on the Past 250,000 Years

The interglacial climate we currently enjoy is known from paleoclimate records that cover the past 1 million years to be relatively rare. These records also demonstrate that interglacial climates appear coincident with the relatively rapid dissipation (several hundred to thousands of years) of glacial-age ice sheets and end more gradually as ice sheets re-encroach. Pioneering studies link the cadence of these glacial/interglacial cycles to insolation changes produced by changes in the Earth's orbit (Milankovitch cycles), yet not all details of event phasing or event frequency can be explained by these theoretically calculated insolation changes.

Ice core records from Antarctica have demonstrated the close linkage between temperature and the greenhouse gases CO2 and CH4. However, issues of phasing, particularly for CO2, remain less well understood. Furthermore, despite their importance in climate forcing, changes in the concentration of greenhouse gases cannot fully explain documented changes in temperature.

Rapid climate change events recently developed from the central Greenland ice cores and since found in marine and terrestrial sediments punctuate the slower pattern of ice sheet growth and decay. From the highly resolved and well-dated Greenland ice cores comes evidence that many of these rapid climate change events occur in relatively predictable cycles of ~6,000 years (Heinrich events, first revealed from marine sediments as massive discharges of icebergs) or as massive atmosphere-ocean reorganizations that occur with ~1,500-year frequency (Dansgaard/Oeschger cycles).

Although globally distributed and dramatic in magnitude and timing, important characteristics of the rapid climate change events are still not clearly understood. These events were documented through the investigation of well-dated continuous records; to understand the phasing of these events from region to



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