DEEPENING: Climate Change
Has global climate changed in the past?
The global climate changes very slightly when averaged over timescales
such as decades, but can change perceptibly on the timescales of
centuries and millennia.
We also know that there have been dramatic changes on longer timescales
(10,000 years and more) as the earth has entered and emerged from
ice ages.
Such indication of the earth's temperature have been deduced from
ice cores and ocean sediment samples.
What causes climate change?
Drastic climatic changes leading to ice ages are thought to be
driven by small natural changes in the earth's orbit around the
sun, which alter the amount and seasonal distribution of solar energy
we receive (the Milankovitch theory).
Because we can predict these orbital changes we can conclude that
the next ice age is likely to start in perhaps 5,000 years and reach
its furthest limits in about 60,000 years.
There are other causes of climate change.
For example, volcanic eruptions can send sunlight-absorbing dust
clouds high into the atmosphere; there is evidence that the output
from the sun itself varies slightly; and long-term changes in the
interaction between the atmosphere and the oceans may account for
much of the variation seen over the past few thousand years.
The North European "little ice age" between the 16th and
the 19th centuries is one recent example of climate variation: there
was widespread freezing of rivers and canals in winter and mountain
glaciers reached their greatest extent.
Accurate measurements of temperature across the globe have been
collected for only about a hundred years and these show that the
earth's average temperature has risen by about 0.5°C since 1880.
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