Energy > Climate Change
Is the Earth’s warming just natural variability?
This article is part of our series focused on the causes and consequences of a warming planet
Could the recent warming of the Earth be caused by natural variability in the Earth’s climate? After all, the Earth’s temperature and climate have been changing for millions of years, long before humans were on the Earth.
However, just because past warming was caused by natural factors does not mean that human activity is not responsible for the current warming. Using this logic, we would conclude that forest fires cannot be caused by humans because forest fires occurred before humans were on the Earth.
Many of the temperature changes experienced in the past were only regional. For example, the warm temperatures that tempted the Vikings to settle Greenland and visit North America were done during a relatively warm period. However, at the same time, Siberia experience a period of unusually cold weather. Taken together, these two anomalies offset each other and the temperature of the Earth did not change. More recent examples are El Niño and La Niña where the temperature of the Pacific Ocean off the coast of South America warms and cools.
To find the cause for the Earth’s warming, we need to look at the evidence. Although scientists have identified the various causes of Earth’s warming in its long history, they have found little evidence to support the natural variability argument for the current warming of the planet.
For example, if we look over the one thousand years before 1800 when there was little influence by humans, we see nothing similar to the magnitude or rate of warming experienced in recent decades. So, if the current warming is due to natural variability, there was no sign of it during this previous period, and no known cause for such natural variability since 1800.
Looking even further back in time, there is some evidence that periods of rapid warming occurred over the last 20,000 years as the earth moved out of the ice age. But these changes were accompanied by identified, rapid natural changes in atmospheric and oceanic circulation patterns. No such changes are occurring today.
If natural variability is the cause of the recent warming, it would most likely be driven by changes in ocean currents that warm the ocean surface. If this were occurring, the ocean surface would be warming faster than the land surface. However, we are experiencing the opposite. Land areas are warming significantly faster than oceans.
Therefore, we conclude from the scientific evidence that the contribution of natural variability to the warming of the planet over recent decades is small at best compared to the influence of humans to the Earth’s warming.Furthermore, individuals posing the natural variability argument for global warming implicitly assume that the climate variability will be mild with little impact on human activity. However, past climate changes have been powerful and would cause great damage to human civilization. And, these natural forces would be very difficult to control.
See the Ag Decision Maker website for more from this series.
Don Hofstrand, retired extension agriculture specialist, firstname.lastname@example.org
Reviewed by Dr. Eugene Takle, retired professor emeritus Iowa State University