Climate Changes over Past Millennia Peter deMenocal (Lamont-Doherty Earth Observatory)

Overview Understanding natural climate variability: –Historical climate changes. –Examples of social adaptations to change. Climate changes over past millennia: –Large and persistent (century-scale) climate shifts. –Climate changes affect land and oceans. –Modest forcing … large response. Cultural responses to past climate change: –The Akkadian empire.

Historical climate variability Instrumental records of natural climate variability only extend back years. Short-lived, regional climate excursions. People can, and do, readily adapt to interannual- decadal changes. This apparent stability influences current climate change policy debates. –Is this accurate?

Pre-Instrumental climate variability Paleoclimate: Reconstruct past climate changes using tree rings, ice cores, lake and ocean sediments. –Objective: Place modern (and future) climate changes within the context of past centuries and millennia. The present is NOT a guide to the past. –Climate changes during recent millennia were larger, global (?), and more persistent. Climate changes over past millennia were large and widespread.  linked to weak (solar) forcing.

US Drought (based on tree ring records)

The 1930s Dust Bowl Six year drought ( ) across Great Plains and West. Large societal impact due to wanton farming practices and over-capitalization. Cost over $1 billion in 1930’s dollars, federal relief programs. US was better prepared for a longer drought in 1950s.

The “Great Drought” AD The “Great Drought” spanned ~26 years. Longest unbroken drought. Abrupt depopulation of the American SW by ancestors of the Pueblo (Anasazi). Abrupt and widespread abandonment. Mesa Verde, CO

Holocene Climate The Holocene represents the present warm period (last ca. 12,000 years). It’s “Our Time”, spanning the development of agriculture and the emergence of complex cultures. –How stable was it? –How large were the changes?

What we thought ten years ago... “… the extreme stability of the Holocene” (Dansgaard et al., 1993) cold warm Greenland ice core

European Ice Advances Known for some time that there were large changes in European glaciers Little Ice Age ( AD) advances (below) Modest glacial advances every yrs LIA trim line Val d’Hérens, Switz.

Over the last few years we have learned: Holocene was unstable. Persistent century-scale cooling/drying events, recurred every years, shifting within decades. These instabilities occurred during warm and cold (glacial) times. Surface temperature changes (±1-2°C). Deep ocean circulation changes also involved. Appear to be linked to solar irradiance changes.

Deep-sea sediment cores R/V Ewing (Columbia’s research ship) Sediment coring device

Lamont-Doherty deep-sea cores About 22,000 cores!

Paleoclimate indicators Ice-rafted grains Microfossil assemblages Example: Cooling - Land cooling (icebergs) - Ocean cooling

Holocene Subpolar N. Atlantic Bond et al., 1997

Climate of the past millennia - Multicentury warm/cool intervals - Excursions were ±1-2°C - Widespread impacts - Climate shifts within decades KEY: Large responses to modest forcing. Bond et al., 2001

“Winter Landscape” Bruegel (1565)

Mechanisms of century-scale climate variability –Solar irradiance: “Little Ice Age”: About -0.25% reduction of incoming solar energy (visible) Several percent variability across UV band. It is believed that LIA reductions in radiation were ultimately responsible for cooling. –Volcanism also important (Tambora eruption:1816)

Solar Variability: Century & millennial-scale “pulsing” of Sun Only ~0.25% variability of incoming radiation (visible)

N. Atlantic IRD and Solar Irradiance Bond et al., 2001

Some Observations Holocene climate changes are examples of how weak forcing can lead to strong climate responses. Large (>instrumental), widespread, natural changes at socially-relevant timescales. Apparent forcing was only ~0.3% reduction in solar input (~4 W/m 2 ). Projected 2x CO 2 radiation forcing (~AD 2050) equivalent to ~4-5 W/m 2...

Cultural Responses to Holocene Climate Change Paleoclimate records document large climate changes which persisted for many centuries to millennia. –Climate transitions can be very abrupt. –Regional to global (?) extent. What impact did climate change have on complex societies existing at the time? Examples: –Akkadian Empire (ca yrs BP) –Classic Maya Empire (ca yrs BP)

Akkadian Imperial Collapse (4200 yrs BP) First empire imperialized Mesopotamia between yr BP. Imperialization linked productive rainfed (semiarid) agriculture of northern Mesopotamia (Sumer) with south. Collapse occurred near 4170±150 yr BP (Weiss et al., 1993). Collapse was previously attributed to political disintegration. Sargon of Akkad

Tell Leilan, NE Syria Weiss et al. (1993) excavated this former Akkadian imperial town. Their results suggested rapid abandonment due to onset of aridity. At right, a ~600m 2 excavated residential occupation with roadway.

Deep-Sea Sediment Record of Mesopotamian Climate Cullen et al. (2000) tested the Weiss et al. (1993) claim using the deep-sea sediment record to reconstruct changes in Mesopotamian climate. –Late Holocene aridity record should be preserved in deep-sea sediments.

Mesopotamian Dust Transport Dust storm over Mesopotamia (May, 2000) Same dust storm, 10 days later, over the Gulf of Oman

Climate Change and Akkadian Collapse Cullen et al. (2000)

What can be learned from these ancient cultural responses to prolonged drought? Complex societies are neither powerless nor infinitely adaptive to climate change. Detailed paleoclimate records document changes in climate which far surpassed modern variability. –Large deviations from modern conditions –Deviations were long-lived (many centuries) However, other social factors in each case may have contributed to observed collapse. –Climate was at least a significant contributing factor Collapse occurred even though cultures had large buffering capacities.

Conclusions Modern and ancient cultures thrived in marginal environments with large interannual variability. Modern and ancient cultures (regrettably) plan for the future based on historical climate variability. Only ancient cultures experienced the onset and persistence of century-scale drought. In these two examples, complex cultures collapsed following the onset of large-scale drought which persisted for centuries. Empirical paleoclimate data can place archeological records of social change within environmental contexts.