Final remarks

Abstract

The climate of the Earth has oscillated through wild variations with long, extended ice ages interspersed by relatively short interglacial periods of warmth. The last ice age peaked around 20,000 ybp and the current interglacial (Holocene) commenced about 10,000 ybp. The historical record shows that sharp transitions from extreme cold to warm have occurred in time periods of centuries or even decades. The Earth’s climate appears to be delicately balanced between these extremes and can tip one way or another when perturbed. If the polar areas are sufficiently cold, they can trap out water vapor from the atmosphere, forming large ice sheets that will usurp a significant fraction of the world’s water resources. The ice sheets provide positive reinforcement of cooling trends by reflecting incident solar irradiance. However, heat is delivered to the polar areas via solar energy, ocean currents, and to a lesser extent from air masses, providing a balance in which limited glaciation occurs in polar areas. When the solar irradiance or the ocean currents vary, the heat input to the polar areas changes, and this can lead to expansion or contraction of polar glaciation. There are various theories that attempt to explain major climate changes based on changes in the Sun, changes in the Earth’s orbit about the Sun, or changes in ocean currents, but none of these is entirely satisfactory, and it is difficult to test these theories with data. The past ∼10,000 years (the Holocene) has been a rather lengthy period of relatively benign climate with occasional moderate fluctuations.