DO GLACIALS START BY TROPICAL WARMING?

Frequency of paleoclimatic variations matches that of the elements of the earth's circumsolar orbit. It is therefore generally accepted that the seasonal and latitudinal redistribution of solar energy reaching the Earth controls the state of global climate. Correlation of paleoclimatic evidence with orbital changes shows that the build-up of polar ice accelerates, when due to the low obliquity, the insolation is preferably channeled to low latitudes at the expense of the high ones, and when, due to the precessional cycle, the solar beam in astronomic winter and spring becomes stronger than in summer and autumn. This is the case when the perihelion occurs after the winter solstice but prior to the summer solstice.

As a result of the insolation shift the planet loses less energy in the highly reflective high latitudes and absorbs more in the tropical oceans where the albedo is low. The heating of equatorial oceans is highly sensitive to the wave of solar radiation in transitional seasons. It increases when the energy income to the ocean is strong in spring and weak in autumn. Models show that in suchcase the El Nino warm anomalies become more frequent than the cold La Nina. Since the shift in the strength of solar beam toward boreal spring favors earlier establishment of snow fields in the high latitudes, the temperature gradient between the tropics and the poles, which drives the weather systems in middle latitudes, is invigorated. As a result the transport of water vapor to high latitudes increases, leading to the build-up of ice on land. Because the warming oceans in low latitudes form the largest portion of the planet, it is not only possible, but highly probable that the areally averaged global surface temperature during the first several millennia of the glacial ice build-up was increasing and consequently, that the glacials started with global warming.