Page 44 - Earth's Climate Past and Future

20 PART I • Framework of Climate Science

FIGURE 2-4 Ocean drilling
(A) Hundreds of ocean sediment cores
are archives of past climatic changes.
(B, C, D) The longest cores have been
retrieved by drilling operations on the
JOIDES Resolution, run by the
international Ocean Drilling Program.
(A: National Paleoclimate Data Center,
NGDC, Boulder, CO. B, C, D: Ocean
Ocean cores Drilling Program, Texas A&M
University.)

A

B C D

surface (Figure 2–4A). Older and more deeply buried sheets several kilometers thick (Figure 2–5). Ice core
sediments have been retrieved by the JOIDES Resolution, archives contain many kinds of climatic information,
a ship capable of drilling into and recovering sediment although it is limited geographically to the few regions
sequences several kilometers thick (Figure 2–4B–D). where ice exists (Figure 2–6).
Because the deep ocean is generally a quiet place Ice recovered from the Antarctic ice sheet now dates
with relatively continuous deposition, it yields climate back more than 700,000 years, and ice from the Green-
records of higher quality than most records from land, land ice sheet dates back 120,000 years. Future ice drilling
where water, ice, and wind actively erode deposits. Some is likely to extend these records even further back in time.
deep-sea sediments are subject to disturbances such In contrast, most small glaciers that exist in mountain val-
as dislodgment from steep slopes, physical erosion and leys (even in the tropics) record only the last 10,000 years
reworking by currents on the sea floor, and chemical or less of climate change. Deposition rates range from a
dissolution by corrosive water in the deeper basins. few centimeters per year in the coldest and driest areas to
Despite these problems, many ocean basins have been meters per year in less frigid, wetter regions.
sites of continuous sediment deposition for tens of mil- Other Climate Archives In areas of sufficient rain-
lions of years. Deposition of sediments is usually much fall, groundwater percolating through soil and bedrock
slower in the ocean than on land, but rates are higher in dissolves and redeposits limestone (calcite, or CaCO ) in
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regions that receive influxes of sediments eroded from caves. These deposits contain records of climate over
nearby continents, in sediments beneath productive sur- intervals that can extend back several hundred thousand
face waters, and in regions high above the corrosive bot- years.
tom waters in the deepest ocean. Trees are valuable climate archives for the interval of
Glacial Ice At the very cold temperatures found at the last few tens, hundreds, or (in exceptional cases)
high latitudes and high altitudes, annual deposition of thousands of years. The outer softwood layers of many
snow can pile up continuous sequences of ice that range kinds of trees are deposited in millimeter-thick layers
in thickness from small mountain glaciers tens to hun- that turn into hardwood. These annual layers are best
dreds of meters thick to much larger continent-sized ice developed in mid-latitude and high-latitude regions