Earth's climate and the cycles of warming and cooling that it goes through is one of the more fascinating subjects in earth science. It's a subject that has become fraught with passion in recent years as climatologists have repeatedly tried to warn us that humans are upsetting Earth's cycles and science deniers have done their best to shout them down. But looking back at the historical data is perhaps a little less controversial.
Scientists have recently been doing research on the period from the 1200s to the 1900s, a very cold time in the northern hemisphere that has come to be known as the "little ice age." There have been many theories over the years about what might have caused this prolonged cold period which, in many ways, shaped the culture of Northern Europe, Asia, and North America. At the time it was happening, some people blamed witches. The ignorant always find a scapegoat. But this new study may give a definitive answer to the question of why it happened. The answer, the scientists postulate, is volcanoes.
The researchers have found evidence that there was a 50-year period just before the beginning of the little ice age during which several volcanoes were very active. They repeatedly erupted, shooting ash and debris into the atmosphere where it reflected sunlight back out into space, thus the warming rays of the sun could not reach the planet and Earth cooled off. They believe this caused a chain reaction which redounded for centuries during which the northern hemisphere, in particular, shivered and seldom saw a real summer.
This is just the opposite of what is happening now as humans send pollutants into the atmosphere which create a barrier - a greenhouse cover - that traps heat back on Earth. And so the planet heats up.
Whether heating or cooling, the laws of physics apply. We cannot escape them. The prime causal factor may be volcanoes or human beings or asteroids, but however these factors present themselves, Nature's laws will prevail and Earth will again find an equilibrium. Whether there are any humans who survive to witness it is really irrelevant to Nature.