over the deglaciation.42 The former study also identified a 3,000-year lag between monsoon intensity and insolation that lasted from about 9,500 to 5,500 years ago. By the end of this period, when northern hemisphere glacial boundary conditions had disappeared, monsoon behavior responded more linearly to insolation. Further significant centennial-scale decreases in monsoon intensity occurred prior to ~6,000 years ago, when monsoon strength was enhanced relative to the present. Since abrupt changes in Arabian Sea sediment monsoon records occurred when northern hemisphere summers were significantly warmer than present,43 some researchers have speculated that future greenhouse-warmed summers 44 may offer “surprises” in monsoon behavior.

Major reorganizations in Holocene climate plus finer-scale climate fluctuations such as abrupt shifts in drought and flood frequency may be explained by a combination of climate forcings.45 For the Holocene such forcings may include (1) changes in thermohaline circulation; (2) changes in insolation, notably precession that may generate long-period El Niño-type reorganizations in moisture and temperature and changes in marine and land ice cover; (3) changes in solar output; and (4) changes in the concentrations of volcanic aerosols and dusts.46 A variety of paleorecords are available to test the impact of these forcing mechanisms, including, for example, potential proxies for solar variability derived from d14C series in tree rings and 10Be series from ice cores, CO2 from ice cores, CH4 from ice cores, and volcanic sulfate from ice cores.47

The Late Holocene (~2,000 years ago to present)
Summary of Previous Work

Although the exact timing and geographic distribution of Holocene climate change events are complex, the past 1,000 to 2,000 years offer important opportunities for unraveling the decadal- to centennial-scale and finer climate variability that influences modern climate. There is general agreement that glaciers around the world expanded during at least parts of the thirteenth through the nineteenth centuries, a period called the Little Ice Age (LIA), and that warming occurred for several centuries prior to this period,48 at least in some regions, during what is controversially called the Medieval Warm Period (MWP).

Similarities in decadal- to centennial-scale variability over the past 1,000 years are observed in a variety of paleoclimate records from, for example, China (e.g., temperature, drought, rain frequency, dust events), although differences in the timing of peak cooling differ by region.49 Furthermore, broad similarities exist between the Chinese records and records covering a wide geographic range.

New Developments

Although spatially and temporally incomplete at present, paleoclimate records provide unique environmental reconstructions for the most recent millennia. The



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