An Answer to the Question:
So, OK....what caused the previous ice ages and glacial retreats, all occurring before fossil fuels had been invented?
Mark Chandler
The ice age oscillations during the past 2.6 million years are controlled by cyclic variations in the Earth's orbit called "Milankovitch Cycles". They effect climate because they alter the amount and distribution of solar energy that hits the Earth (a pretty concise explanation can be found at http://en.wikipedia.org/wiki/Milankovitch_cycles). Although fossil fuels didn't exist, carbon dioxide and other greenhouse gases actually do oscillate along with the orbital cycles (in response to things like changing vegetation and warming oceans, which absorb or release more carbon dioxide). In fact, measurements of gas bubbles in ice cores from Antarctica and Greenland showed that, over the last couple of million years, there is a close relationship between climate warming and increasing carbon dioxide (see Figure 1). Since 1958 we have had direct measurements of atmospheric carbon dioxide levels, which are shown in figure 2 (they actually catch air in canisters and measure the chemical constituents).
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Figure 1: (CLICK ON IMAGE TO ENLARGE) Measurements of carbon dioxide (CO2) levels and temperature change taken from air bubbles in an ice core from Vostok, Antarctica. The measurements span approximately the last 400,000 years. The oscillations in the curve are related to the waxing and waning of ice ages during the Pleistocene epoch.
| | Figure 2: (CLICK ON IMAGE TO ENLARGE) The increase in carbon dioxide since 1958 is known as the Keeling Curve, named after Charles Keeling who first measured the CO2 on a continuos basis and discovered the rapid increase. The small oscillations are related to the seasonal cycle in which Northern Hemisphere plants increase and decrease foliage.
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The measurements show that the levels of greenhouse gases coming from fossil fuel burning have been climbing steadily since the 1950's and are now nearly 40% higher than the previous WARMEST period during the ice ages (the levels are 105% higher than the coldest periods). Global temperatures have also risen steadily during the later part of the 20th century (see figure 3) and there are many signs that even the relatively small warming (about 1.5 degree F) is beginning to have recognizable impacts (record low Arctic ice amounts, permafrost melting in Alaska and Russia, increases in the intensity of floods and droughts). Although some impacts might be positive (longer growing seasons) most will probably be bad because human infrastructure is often closely tied to the climate (examples: roads and pipelines in Alaska are anchored in permafrost and are falling apart (at great expense to the government and industry) as the permafrost melts; misquito borne diseases that used to be found only in the tropics are appearing in the mid-latitude regions). Btw, most scientists don't seem to think that the recent increase in hurricanes is related to global warming, but they don't hesitate to warn that hurricanes ARE strengthened by warmer ocean temperatures and thus should be expected to increase in strength and number in the future.
| Figure 3: (CLICK ON IMAGE TO ENLARGE) Increase in global annual average temperatures during the past century is about 0.8oC (1.44oF).
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