Hasn't Earth warmed and cooled naturally throughout history?

Author: 
January 23, 2014

Yes, Earth has warmed and cooled naturally throughout its history. For example, increases or decreases in the Sun's brightness would have caused short-term warming and cooling. Also, major volcanic eruptions can cause short-term cooling. For example, in 1991, Mt. Pinatubo erupted with such force it injected sulfate gases and particles into the stratosphere, above where rain clouds form. There, these reflective aerosol particles lingered for almost a year and spread around the globe. Pinatubo's volcanic particle plume scattered and reflected so much sunlight back to space that it actually caused Earth to cool by about 0.9°F (0.5°C) that year.

A single bloom of Mountain avens in an alpine valley

A single bloom of the alpine wildflower Dryas octopetala in a high valley in the Orjen Mountains of Montenegro. This tundra flower gave its name to the Younger Dryas, a cold period that briefly interrupted the warming that had begun following the end of the last ice age. Climate scientists think the episode was triggered by changes in the great system of currents in the Atlantic Ocean as a result of meltwater from Northern Hemisphere ice sheets. Image by P. Cikovac, used under a Creative Common license.

Additionally, Earth has experienced longer term cold periods ("ice ages") and warm periods ("interglacials") on 100,000-year cycles for at least the last million years. Going from an ice age to an interglacial and back again, Earth's average temperature changed anywhere from 7°F to 12.5°F (4-7°C). These fluctuations in global average temperature happened because gradual, ongoing changes in Earth's orbital mechanics changed our planet's tilt relative to the sun.

The gradual shift in Earth's tilt changed where and how much sunlight fell on the Northern Hemisphere. Thus, there was a slight increase in the amount of sunlight shining where most of our planet's landmass is located, which was just enough to nudge our world in a warming direction. As Earth began to transition from an ice age to an interglacial, other factors (known as "climate feedbacks") in the climate system came into play and added to the warming. For example:

  • the large ice sheets on North America, Europe, and Asia shrank, which changed the land cover from a mostly bright white reflector to a dark green or brown absorber of solar energy and added to the warming;
  • global cloudiness may have declined, which would have allowed more sunlight to reach the surface and add to the warming;
  • as the land and oceans warmed, they released more carbon dioxide and methane (heat-trapping gases) which added to the warming; and
  • lightning-triggered wildfires probably grew more frequent, and burned over larger areas, thereby accelerating the conversion of forest biomass into carbon dioxide and methane gas, which added to the warming.

As Earth's orbital mechanics tilted the Northern Hemisphere away from the sun, these processes slowed or reversed, leading to ice ages. These processes explain why Earth has warmed and cooled on roughly 100,000-year cycles for at least the last 1 million years.

References

Hansen, J., M. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall (2007): "Climate change and trace gases." Phil. Trans. R. Soc., v365. DOI:10.1098/rsta.2007.2052