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Weather is notoriously unpredictable.
From one moment to the next, any of dozens of atmospheric variables
can change to create a new weather event. In contrast, climate
descriptions, which identify average and normal temperatures and
precipitation levels, tend to be perceived as stable, at least
over time scales that humans can easily relate to. However, that
hasn't always been the case.
This video segment adapted from NOVA
describes climate data that suggest the Earth
has undergone dramatic climate shifts in relatively
short spans of time.
Environmental conditions are constantly
in flux. Many of these changes may escape our notice: Temperatures
rise and fall throughout the day, humidity and air pressure fluctuate,
and clouds form and dissipate. However, these same variables can
combine to create phenomena that are readily observable, such
as wind, rain, snow, and thunderstorms. These relatively short-term
environmental changes, which might occur over periods of hours,
days, weeks, or seasons, are collectively referred to as weather.
Climate describes environmental
conditions over much longer periods of time than weather forecasts
and reports. These long-term environmental analyses characterize
a specific geographic location's temperature and precipitation
averages and ranges. Anomalous high and low readings are absorbed
by these averages, resulting in a reliable estimate of expected
Indeed, the global climate is, by definition,
more stable than local weather. But climate is also constantly
changing. In fact, research conducted over the last 20 years or
so describes dramatic shifts in climate in Earth's distant past.
These shifts occurred over a period of a decade or less, rather
than over thousands of years as scientists once thought was necessary.
Scientists began studying evidence
of climate change, especially the role of ice ages in Earth's
geologic history, more than a century ago. During the most recent
ice age, the Pleistocene, average global temperatures were about
5°C or more below present temperatures. This and other ice
ages detected in the geological record were set in motion by gradual
changes in the Earth's tilt, rotation, and orbit over thousands
of years. Despite the gradual nature of these changes, Earth's
climate appears to respond rapidly once certain boundary conditions
are set in place.
Layers of ice analyzed from Greenland
ice cores provide a chronology detailing the rapid onset of ice
age conditions. They show average continental surface temperatures
rising and falling dramatically in just a few years, rather than
over the course of hundreds or thousands. For example, between
43,000 BC and 8,000 BC, average global temperatures fluctuated
periodically by as much as 20°C (36°F) or more. In contrast,
climate changes since 8,000 BC have been characterized by temperature
shifts of just 4°C (7°F) or less.
Many climatologists think these events
resulted from changes in heat energy transfer by ocean currents
from the tropics to the higher latitudes, caused by a decrease
in salinity. For example, computer models suggest that around
13,000 years ago, the Gulf Stream waters, which warm northwestern
Europe, might have been altered or halted dramatically by influxes
of fresh water from melting glaciers. However, scientists do not
understand the specifics of how a decrease in the rate of energy
transfer by the ocean currents from the tropics to the higher
latitudes translates to changes in regional and global climate.
To learn about the role greenhouse
gases may play in global warming, check out Global
Warming: The Physics of the Greenhouse Effect.
To learn more about the role ocean
currents have played in climate change, check out Great
Ocean Conveyor Belt: Part I and Great
Ocean Conveyor Belt: Part II.
To learn about the role orbital cycles
have played in Earth's climate, check out Natural Climate
Change in Djibouti, Africa.
(Note: for the three
links above, please scroll to the bottom of that page, click
on the same links there, and register (free) to view the material.)
Questions for Discussion
- Explain the relationship between climate and weather
using examples from the video.
- Explain why floods, hurricanes, and tornadoes are aspects of weather, not climate.
- The video points out how dramatic climate changes
have been in the past. Do we know the possible triggers of these
rapid shifts? Is it possible that we may experience one of these
dramatic shifts in our lifetime?
- The graph of average temperatures shows that today's
temperatures are higher than they were 10,000 to 40,000 years
ago. It also shows that today's temperatures have been dropping
in the past centuries. However, what doesn't show on this graph
is that temperatures have been increasing over the past several
decades. What do most scientists believe is contributing to
this increase in temperature?
Source: NOVA "Warnings from the Ice"