Paul Godwin

摘要

My letter about global warming ('Hit or myth?') which was featured in February's QW unleashed a maelstrom of discussion and debate. I value the opinions of all those who responded, but here I shall attempt to clarify my argument further. It was around the end of the last Ice Age, some 13,000 years ago, that a global warming process began. Not because of all those Stone Agefolk roasting mammoth meat on fossil fuel camp fires, but X because of something called the 'Milankovitch cycles,' named after Milutin Milankovitch, the Serbian astronomer who is generally credited with calculating their magnitude. They are a natural fact of planetary life that depend on the tilt of the earth's axis and its orbit around the sun. Milankovitch cycles refer to the fact that the episodic nature of the earth's glacial and interglacial periods within the last couple of million years have been caused primarily by cyclical changes in a circumnavigation of the sun. Variations in the earth's eccentricity, axial tilt and precession represent these three dominant cycles. Eccentricity is the shape of the earth's orbit around the sun. These oscillations are of prime importance to glaciation, in that they alter the distance from the earth to the sun consequently reducing or increasing the amount of radiation received on the earth's surface in different seasons.