Page 508 - Environment: The Science Behind the Stories

changes happening in our climate today. To understand composition, greenhouse gas concentrations, temperature
paleoclimate, scientists have developed ingenious methods trends, snowfall, solar activity, and even (from trapped soot
to decipher clues from thousands or millions of years ago by particles) frequency of forest fires and volcanic eruptions dur-
taking advantage of the record-keeping capacity of the natu- ing each time period. By extracting ice cores from Antarc-
ral world. Proxy indicators are types of indirect evidence that tica, scientists have now been able to go back in time 800,000
serve as proxies, or substitutes, for direct measurement and years, reading Earth’s history across eight glacial cycles (see
that shed light on past climate. The Science behind The STory, pp. 508–509.)
For example, Earth’s ice caps, ice sheets, and glaciers Researchers also drill cores into beds of sediment beneath
hold clues to climate history. In frigid areas over the poles and bodies of water. Sediments often preserve pollen grains and
atop high mountains, snow falling year after year for millen- other remnants from plants that grew in the past (as we saw
nia compresses into ice. Over the ages, this ice accumulates with the study of Easter Island; pp. 24–25). Because climate
to great depths, preserving within its layers tiny bubbles of influences the types of plants that grow in an area, knowing
the ancient atmosphere (Figure 18.6). Scientists can examine what plants were present can tell us a great deal about the
the trapped air bubbles by drilling into the ice and extracting climate at that place and time.
long columns, or cores. The layered ice, accumulating season Tree rings provide another proxy indicator. The width
after season over thousands of years, provides a timescale. of each ring of a tree trunk cut in cross-section reveals
By studying the chemistry of the ice and the bubbles in each how much the tree grew in a particular growing season. A
layer in these ice cores, scientists can determine atmospheric wide ring means more growth, generally indicating a wet-
ter year. Long-lived trees such as bristlecone pines can pro-
vide records of precipitation and drought going back several
thousand years. Tree rings are also used to study fire history,
since a charred ring indicates that a fire took place in the
region in that year.
In arid regions such as the U.S. Southwest, packrat
middens are a valuable source of climate data. Packrats are
rodents that carry seeds and plant parts back to their middens,
or dens, in caves and rock crevices sheltered from rain. In arid
locations, plant parts may be preserved for centuries, allowing
researchers to study the past flora of the region.
Researchers gather data on past ocean conditions from
coral reefs (pp. 449–450). Living corals take in trace elements
and isotope ratios (p. 42) from ocean water as they grow, and
they incorporate these chemical clues, layer by layer, into
growth bands in the reefs they build.
Proxy indicators often tell us information about local or
regional areas. To get a global perspective, scientists need to
combine multiple records from various areas. Because the
number of available indicators decreases the further back
in time we go, estimates of global climate conditions for
the recent past tend to be more reliable than those for the
(a) Ice core distant past.

Direct measurements tell us about the present

Today we measure temperature with thermometers, rainfall
with rain gauges, wind speed with anemometers, and air pres-
sure with barometers, using computer programs to integrate
and analyze this information in real time. With these tech-
nologies and more, we document in detail the fluctuations in CHAPTER 18 • Glob al Cli M aT e Chan G e
weather day-by-day and hour-by-hour across the globe. As a
result, we have gained an understanding of present-day cli-
mate conditions in every region of our planet.
(b) Micrograph of ice core We also measure the chemistry of the atmosphere and
the oceans. Direct measurements of carbon dioxide concen-
trations in the atmosphere reach back to 1958, when scientist
Figure 18.6 Scientists drill deep into ancient ice sheets and
remove cores of ice. Dr. Gerald Holdsworth of the University of Charles Keeling began analyzing hourly air samples from
Calgary (a) extracts information about past climates from an ice a monitoring station at Hawaii’s Mauna Loa Observatory.
core. Bubbles trapped in the ice (b) contain small samples of the Here, unpolluted, well-mixed air from over vast stretches
ancient atmosphere. of ocean blows across the top of Earth’s most massive 507

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