Ice Ages

In the controversy over global warming, some people have suggested that human-induced warming might be a good thing if it kept us out of the next Ice Age.  Here are some related data--the graph below shows how Earth's temperature has changed over the last half million years, and also how the ice volume has changed.  Note that as the temperature goes up, the ice volume goes down, as expected.

Temperature changes over the last 450,000 years

Temperature changes over the last 450,000 years, from oxygen isotope ratios in ice cores; note the periodicity of approximately 100,000 years in the brief "interglacials;" (image credit: Global Warming Art; data is from Petit, J.R., et al., Nature 399: 429-436)

In this graph, time moves backwards to the left, and the zero of temperature changes corresponds to the present day.  The blue and green lines show temperature determinations from two different Antarctic ice cores.  These temperatures are determined from oxygen-16 and oxygen-18 ratios in ice cores (see Sidebar below).

Note the rapid increase of temperature as Earth emerges from each of the four ice ages shown, and how regularly--about every 100,000 years--these rapid rises occur. Also, note the short duration of the interglacials (the periods in between the ice ages)--and we are fortunate to be living in one now. During the most recent ice age, up to 32% of Earth's surface was covered. (See image of polar ice)


Ice coverage in the Northern Hemisphere, 18,000 years ago and present-day (image credit: Mark McCaffrey NGDC/NOAA)

Ice coverage in the Northern Hemisphere, 18,000 years ago and present-day (image credit: Mark McCaffrey NGDC/NOAA)

The abrupt ending of these ice ages suggests powerful feedback mechanisms. Here are two possibilities:

  • Some warming melts part of the glaciers, exposing rock and dirt, which absorbs far more radiation than the highly reflective surface of ice, leading to more warming.
  • Some warming drives carbon dioxide out of solution in the oceans, and this carbon dioxide can lead to warming through the greenhouse effect.

Backing up considerably further, the graph below shows the temperature record for about the last 5.5 million years. Note that in this graph, going back in time is to the right, and the zero of temperature changes corresponds approximately to the present day. Cycles of 100,000 years and 41,000 years are labeled. This graph was obtained from ratios of oxygen-16 to oxygen-18 concentrations in sediment cores.

Ice coverage in the Northern Hemisphere, 18,000 years ago and present-day (image credit: Mark McCaffrey NGDC/NOAA)

Ice coverage in the Northern Hemisphere, 18,000 years ago and present-day (image credit: Mark McCaffrey NGDC/NOAA)

Relative concentration of the isotopes oxygen-16 and oxygen-18 in water molecules is known as a "proxy" for the average temperature.  The idea comes from kinetic theory.  Since each water molecule contains an oxygen atom, and oxygen-18 is more massive than oxygen-16, a water molecule containing oxygen-18 has the lower average velocity. When seawater evaporates, the water vapor therefore becomes enriched in oxygen-16.  When water vapor condenses to form water, or freezes to form ice, the opposite process occurs--the oxygen-16 is depleted, and the lower the temperature, the stronger the depletion.  Consequently, a measurement of the ratio of the concentrations of these two oxygen isotopes in part of an ice core can lead to a determination of the temperature when that part was formed.

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