Orbital Changes

The Milankovitch Theory explains the 3 cyclical changes in Earth’s orbit and tilt that cause the climate fluctuations that occur over tens of thousands of years to hundreds of thousands of years. You have learned about these orbital changes in the Temperature over Time module. These fluctuations include changes in the shape (eccentricity) of Earth’s orbit every ~100,000 years, the tilt (obliquity) of Earth’s axis every ~41,000 years, and the wobbling (precession) of Earth’s axis about ~23,000 years. Milankovitch proposed that glacial periods began when the three cycles align to favor an extended period of more solar radiation in the winter and less solar radiation in the summer at a latitude of 65°N. These conditions for the northern latitudes favor somewhat higher temperatures, but also more water vapor in the air – causing more snowfall. A relatively cool summer for the northern latitudes favors less melting of winter snow and glacier formation.

The figure above shows the alignment of each of the orbital changes to the glacial and interglacial periods. The interplay of the three orbital cycles affects the amount of solar radiation received at different latitudes over the year. The amount of solar radiation reaching the Northern Hemisphere at 65°N seems to control the advance and retreat of glaciers and ice sheets.

Volcanic Eruptions

Volcanic eruptions discharge carbon dioxide, but they may also emit aerosols, such as volcanic ash or dust, and sulfur dioxide. Aerosols are liquids and solids that float around in the air. They may also include soot, dust, salt crystals, bacteria, and viruses. Aerosols scatter incoming solar radiation, causing a slight cooling effect. Volcanic aerosols can block a percentage of sunlight and cause a cooling that may last for 1-2 years.

The year 1816, often referred to as the “year without a summer, occurred after the violent eruption of Indonesia’s Mount Tambora. This was possibly the largest known eruption in the history of human civilization. Snow fell in the northeastern United States and Canada in June, causing regional losses of crops, food shortages, and increased mortality. Relatively cold years also followed other famous volcanic eruptions (such as the 1883 eruption of Krakatau also in Indonesia and 1991 eruption of Mount Pinatubo in the Philippines).

In violent eruptions, volcanoes release ash particles and sulfur dioxide (SO₂) into the stratosphere. The larger particles settle after a few days while the sulfur dioxide combines with water vapor to from sulfuric acid (H2SO₄) and sulfate particles, known together as sulfurous aerosols. Winds transport these sulfurous aerosols around the planet in easterly or westerly directions. For this reason, volcanoes that erupt at lower latitudes (closer to the equator) are more likely to cause hemispheric or global cooling. Volcanoes that erupt at higher latitudes (closer to the poles) are less likely to cause cooling because the sulfurous aerosols are confined to wind patterns surrounding the poles.

Variation in Solar Radiation

The total amount of solar radiation varies by very small amounts. The energy emitted by the sun only varies by 1.3 W/m². This change in solar radiation is related to the number of sunspots. Sunspots are darker areas on the sun’s surface. A sunspot develops where an intense magnetic field weakens the flow of gases that transport heat energy from the sun’s interior. Sunspots appear dark because their temperature is lower than the surrounding area.

Approximately every 11 years, the number of sunspots changes from a maximum number to a minimum number. The sun emits slightly more radiation during active periods of sunspots. Because the sunspots are suppressing heat, the heat flows to surrounding areas causing these regions to be brighter than normal, radiating more heat. While more sunspots may contribute to warmer global climate, less sunspots appear to be associated with a cooler global climate. About 300 years ago, there was a period of reduced solar activity. This was called the Little Ice Age.

Movement of Crustal Plates

As tectonic plates move over geological timescales, landmasses are carried along to different positions and latitudes. These changes affect global circulation patterns of air and ocean water and the climate of the continents. One form of evidence for plate tectonics and an example of how plate tectonics affects climate is the location of coal mines. Coal mines were formed over millions of years ago in tropical areas, yet are found at higher latitudes today. You also learned in the Temperature Over Time module that, since the industrial revolution, the Northern Hemisphere has warmed more than the Southern Hemisphere. This is because the Northern Hemisphere has a larger percentage of Earth’s landmass compared to ocean than the Southern Hemisphere. Remember that landmasses warm faster than oceans due to the high heat capacity of the oceans.

El Niño-Southern Oscillation (ENSO)

El Niño-Southern Oscillation (ENSO) is an oscillation of the ocean and atmosphere system in the tropical area of the Pacific Ocean that affects global weather. Normally the southeast trade winds blow across the tropical Pacific Ocean toward the west. Every 3 to 10 years, the southeast trade winds weaken, allowing the warm water to flow further eastward toward South America. This warmer current of water typically reaches the western coast of South America near Christmas and has become known to the Peruvian fishermen as El Niño (for the Christ child).

El Niño is also known as the warm-water phase of the ENSO. El Niño causes the water temperature off of South America to be warmer. An El Niño warm-water phase changes global weather patterns. South America experiences wetter than average weather, while North America experiences mild but stormier winter weather. During an El Niño warm water phase, there are fewer and less intense hurricanes in the Atlantic Ocean because the rising warmer air over the eastern Pacific Ocean causes more wind shear and hurricanes are not able to form in the Caribbean Sea. Sometimes, after an El Niño phase subsides, a colder-than-normal water phase, known as La Niña, results.

Take a look at the weekly sea surface temperature totals for the past 52 weeks.