The highest temperatures stimulated extreme methane production in wetlands, which are the atmosphere's primary natural source of this greenhouse gas.ĬO2 concentrations, which fluctuated in cycles over the past 350,000 years, varied in response to precession as well as to shifts in the tilt of the earth's rotational axis and in the shape of its orbit. Methane concentrations rose and fell over the past 250,000 years in near harmony with the precession-induced ups and downs of solar radiation in the Northern Hemisphere. Summer warmth in the Northern Hemisphere peaks once every 22,000 years, when the yearly northern summer coincides with the earth's closest passage to the sun and the Northern Hemisphere receives the most intense sunlight. The Northern Hemisphere then receives the least summer sunlight, because the earth is farthest from the sun. Summer heat bottoms out 11,000 years later, after the earth's axis has shifted (precessed) to the opposite position. Like a toy top about to fall, the earth's axis traces imaginary circles in space, making one revolution every 22,000 years. Wobble in the earth's axis of rotation, known as precession, is one of the three orbital cycles that account for sunlight variations in the Northern Hemisphere. Although scientists do not fully understand why, global concentrations of these greenhouse gases respond mainly to changes that occur during summer in the Northern Hemisphere, the time of year when the North Pole is pointed most directly at the sun.
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