Page 396 - Earth's Climate Past and Future

372 GLOSSARY

sunspots Dark areas of temperatures lower than average on troposphere The layer of the atmosphere just above Earth’s
the surface of the Sun. surface (10 km or more in thickness) in which weather occurs.
sunspot cycle A natural 11-year cycle in the number of dark tundra A high-latitude or high-altitude environment in
spots visible on the face of the Sun, reliably recorded by which the ground freezes deeply in winter but thaws at the
astronomers for more than four centuries. surface in summer, permitting low-growing plants to flourish.
surge A sudden and rapid forward movement of the margin 2 x CO sensitivity The amount of warming produced by an
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of a glacier. increase in atmospheric CO levels from the preindustrial
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level (280 parts per million) to a level of 560 parts per million.
tabular iceberg A large flat-topped slab of ice produced
by calving from the seaward margin of an ice shelf. uplift weathering hypothesis The hypothesis that tectonic-
tectonic plates Divisions of the upper solid Earth (lithos- scale climate changes are caused when uplift of plateaus and
phere) that are 100 km thick and thousands of kilometers in lat- mountains alters the amount of CO removed from the
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eral extent and move as rigid units in plate tectonic processes. atmosphere by chemical weathering of fragmented rock.
termination A 10,000-year interval of rapid melting of ice upwelling The rise of cool, nutrient-rich subsurface water to
sheets that brings to an end a longer (90,000-year) interval the ocean surface to replace warm nutrient-poor surface water.
of slower ice growth.
urban heat island An urban area where asphalt and other
theory A hypothesis that has survived repeated testing. heat-absorbing surfaces absorb solar radiation during the day
thermal expansion coefficient The volumetric expansion of and radiate it back at night, keeping the area unusually warm.
seawater when it warms, and contraction when it cools, by 1
part in 7000 per degree C (for temperatures above 4°C). varves Alternating layers of dark and light (or coarse and fine)
thermal inertia The resistance of a component of the cli- sediment that accumulate in annual couplets in lakes or in
mate system to temperature change. ocean margin basins with no water turbulence near the bottom.
thermocline A layer of water in which temperature changes vegetation-albedo feedback A positive feedback that ampli-
rapidly in a vertical direction. fies an initial temperature change through vegetation changes
thermohaline flow The vertical and lateral movement of that alter Earth’s surface albedo and the absorption of solar
subsurface waters in the ocean as a result of contrasts in den- radiation.
sity caused by differences in temperature and salinity. vegetation-precipitation feedback A positive feedback that
thermostat A mechanism that senses changes in temperature amplifies an initial precipitation change through vegetation
and acts to moderate them. changes that alter the transpiration of water vapor and the
threshold A level at which a sudden change in the basic availability of moisture.
nature of a climatic response occurs. viscous Characterized by a consistency such that the material
tilt The angle between Earth’s equatorial plane and the plane in question deforms slowly under pressure (as is bedrock
of its orbit around the Sun, also equivalent to the angle under the pressure of a glacier) and only slowly regains its ini-
between Earth’s axis of rotation and a line perpendicular to its tial shape when the pressure is removed.
axis of rotation around the Sun. Also referred to as obliquity.
time-dependent model A geochemical model that tracks warm saline deep water Deep water proposed to have
changes in the rate of movement of tracers among reservoirs formed in warm salty tropical seas in the past, in contrast to
within the climate system over time. modern sources of deep water in cold polar regions.
time-series analysis A group of techniques for extracting water vapor feedback A positive feedback that amplifies
periodic signals (cycles) from complex signals and quantifying an initial change in temperature by altering the amount of water
their strength, relative timing, and correlation. vapor held by the atmosphere and changing the amount of
transform fault margin A boundary between lithospheric Earth’s back radiation trapped by the atmosphere.
plates in which the plates slide past each other. wavelength The distance between successive peaks or
transgression Relative movement of the ocean up and across troughs in a series of regular wave forms.
the margins of the land. weather Fluctuations in temperature, precipitation, and
transpiration The release of water vapor by plants into the winds on time scales of less than a year.
atmosphere.
tree rings Annual bands formed by trees in regions of seasonal Younger Dryas An interval during the middle of the last
climate, with lighter layers formed during rapid growth in the deglaciation (near 12,000 years ago) marked by slower melting
spring and darker layers at the end of growth in the autumn. of ice sheets and a major cooling in the North Atlantic region.