first people arrived in America between 15,000 and 30,000 years
ago. During that time, much of North America was covered by
great ice sheets. Some 14,000 years ago, the last ice sheet
began to melt very quickly. By 7,000 years ago, the ice was
end to the ice ages caused big changes on the Earth. The changes
caused many kinds of plants and animals to die. For example,
mastodons elephant-like animals and other large
mammals that preferred cold climates may not have been able
to live in the warmer, drier conditions.
in the 14th century, Europeans lived through
what is known as the "Little Ice Age." The Little
Ice Age lasted for several hundred years. During the Little
Ice Age, the advance of glaciers along with hard winters and
famines caused some people to starve and others to leave their
Earth has warmed about 1ºF
in the last 100 years. And the four warmest years of the 20th
century all happened in the 1990s. Periods of increased heat
from the sun may have helped make the Earth warmer. But many
of the world's leading climatologists
think that the greenhouse
gases people produce are making the Earth warmer, too.
Glaciers: a glacier is a large sheet of ice that moves
very, very slowly. Many glaciers in the world are now melting.
For example, glaciers are melting in Montana's Glacier National
Park. Some scientists think the glaciers are melting partly
because the Earth is getting warmer.
Sea Level: have you ever built a sandcastle on the beach,
close to the ocean on wet sand? If you have, you probably
know that the sandcastle won't last very long. Chances are
the waves will wash away the sandcastle as soon as the tide
comes in. The water goes higher up the beach when the tide
comes in. At most shores throughout the world, two high tides
and two low tides occur every day. But now the level of the
sea is rising, so high tides are higher than they were before.
Over the last 100 years, the level of the sea has risen about
6-8 inches worldwide. When the sea level rises, the tide goes
farther up the beach.
think the sea has risen partly because of melting glaciers
and sea ice. When some glaciers melt, they release water into
the sea and make it higher than it was before. Scientists
also think that warmer temperatures in the sea make it rise
even more. Heat makes water expand. When the ocean expands,
it takes up more space.
are not fortune-tellers. They don't know exactly what will
happen in the future. But they can use special computer programs
to find out how the climate may change in the years ahead.
And the computer programs tell us that the Earth may continue
to get warmer.
the melting glaciers, rising seas, and computer models provide
some good clues. They tell us that the Earth's temperature
will probably continue to rise as long as we continue increasing
the amount of greenhouse gases in the atmosphere.
. . .