Conventional wisdom states that modern technology has created a global warming, and it is going to continue without end..  Not necessarily so, says William F. Rudiman, a retired profesor of paleontology in his book Plows, Plagues & Petroleum.  The return of the Ice Age is imminent, and we are simply holding it back until we are going to run out of fuel.

Ice ages are caused by variations in the Earth’s orbit that alter the amount of sunlight reaching Canada, Siberia, and other Northern Hemisphere areas during the brief arctic summer.  During the high sunlight cycle, there’s enough warmth to melt the previous winter’s snows.  During cold one’s there isn’t, and snow gradually accumulates into glaciers.  These orbital variations occur in three well-understood cycles:

1. A 41,000 year variation in the tilt of the Earth’s axis.

2. A 26,000 year precession of the Earth’s orbit around the Sun.

3. A 100,000 year variation in the eccentricity of the Earth’s orbit.

During the past ice age all three cycles combined to plunge our planet into the icehouse.  By some 11,000 years ago, when the glaciers were in full retreat, solar radiation reached its glacier-melting peak. Afterwards, glaciers continued to melt, just as summer days get warmer even after June 21, the summer solstice.  Eventually, the decreasing solar input would result in falling temperatures, and start the next ice age.

These changes are reinforced by changes in the Earth’s atmosphere, particularly regarding two importan greenhouse gases, carbon dioxide and methane, that trap solar heat.  Of these, carbon dioxide would be dissolved in the ice-free oceans, and its concentration would fall.  Methane, which is produced in marshes and wetlands, would also diminish, as land dries out.

All this appears to have been tha case during the last million years.  Methane levels fluctuate with the three sunlight cycles, peaking when the sunlight is strongest, and declining when it is weakening.  This pattern held until some 5,000 years ago, but then something went awry, and they began to rise.  What could be the cause?

Although begun some 10,000 years ago, about 5,000 years ago agriculture began to spread, as shown by grain and vegetable pollen particles trapped in sediments.  People were cutting down forests to make room for farming.  The decaying vegetation released greenhouse gases and raised temperatures, offsetting the cooling effect of the orbital cycles.  Since then, we have had the most stable climate period in the last million years.

The next few centuries might be a bit warm, but eventually we are going to run out of coal and oil.  Then the atmosphere will start purging itelf of the extra greenhouse gases, and temperatures are going to return to those of the natural orbital cycle – those of the coming Ice Age.