The natural causes of normal climate variations include changes in solar activity, volcanic activity, variations in Earth’s orbit, and the role of the oceans. Among these, the variations in Earth’s orbit is the major driver of glacial and interglacial periodicity. It is important to note that the climate change we are experiencing today is a level of variation that far exceeds the normal climate variations caused by these natural causes.
Learn more about the sun’s fascinating sunspots.
Solar activity determines the amount of solar radiation that the sun emits. Sunspots are storms on the sun’s surface that are accompanied by intense magnetic activity; the storms and magnetic activity affect the output of solar radiation. There is an 11- to 22-year cycle of sunspots, which causes the total solar irradiance to vary within the cycle, and affects Earth’s climate. However, the variation in solar radiation caused by sunspot cycles is relatively small compared to total solar output (~0.1%) and far too low to cause the temperature changes observed by climate scientists today.
Volcanoes emit large amounts of ash that can remain in the atmosphere for long time periods, blocking solar radiation and effectively lowering the solar input to Earth, which causes a cooling period. The long period of cooling between 1500 and the end of the 1800s known as the “Little Ice Age” is now considered to be the result of a sizable increase in world-wide volcanic activity. There is some evidence for at least four major volcanic eruptions that initiated this cooling period.
Milutin Milankovitch, a Serbian astronomer and mathematician, suggested that variations in Earth’s orbit affect both the amount and the distribution of sunlight received at the Earth’s surface, which directly impacts the warming of Earth. These variations are called the “Milankovitch Cycles” (Figure 12) and are caused by three forces;
- changes in the shape of Earth’s orbit around the sun (eccentricity),
- the tilt of Earth on its axis (obliquity), and
- the wobbling of the Earth’s axis (precession).
Milankovitch’s theory explains the timing of the past ice ages and major continental glaciations according to paleoclimatology studies, but these cycles occur over tens of thousands of years and longer, and cannot account for the rapid temperature changes observed in the last few decades.
The Role of the Oceans in Moderating the Climate
The oceans cover 70% of the earth’s surface and because of their great depth and the high specific heat capacity of water, oceans retain much more heat than land surfaces. The 2013 IPCC report indicates that 90% of the net energy increase in the climate system between 1971 and 2010 is stored in the Earth’s oceans; with 60% being stored in the upper ocean (0-700 meters depth), and 30% being stored in depths below 700 meters.
There is a natural interactive 3-6 year cycle that involves both the oceans and the atmosphere that has be termed the El Niño-Southern Oscillation (ENSO). ENSO has a major impact on regional climate, often with disastrous consequences. Every few years, it brings flooding to some areas and drought to others. It is believed that as more heat becomes stored in the waters of the ocean, the ENSO effect will become more extreme. This means that ocean storms, including typhoons and hurricanes, will become more intense and frequent. Read more about El Niño at the National Oceanic and Atmospheric Administration webpage. As the examples above demonstrate, natural factors do affect Earth’s climate. However, the changes in climate that have taken place since the 1900s do not fit the patterns of natural variability in climate as caused by these natural factors alone.