HIST103: World History (1600–Present)

Plate Tectonics

Tectonic plates are moving around the earth's surface because of convection within the mantle. This is the driving force of mountain building, earthquakes, and volcanoes around the planet. However, the movement of tectonic plates can also alter regional and global climates. Over millions of years, as seas open and close, ocean currents may distribute heat differently across the planet. For example, when all the continents are joined into one supercontinent (such as Pangaea), nearly all locations experience a continental climate. When the continents separate, heat is more evenly distributed.

Tectonic plate movement.

Plate tectonic movements may help start an ice age. When continents are located near the poles, ice can accumulate, which may increase albedo and lower global temperature. Low enough temperatures may start a global ice age. A recent scientific study by scientists at MIT in 2019, titled Arc-continent collisions in the Tropics Set Earth's climate state, suggests that the last three major ice ages occurred because of plate tectonic activity near the equator.

Tectonic plate motions can trigger volcanic eruptions, which release dust and carbon dioxide into the atmosphere. Ordinary eruptions, even large ones, have only a short-term effect on weather. Massive eruptions of the fluid lavas that create lava plateaus release much more gas and dust and can change the climate for many years. This type of eruption is exceedingly rare; none has occurred since humans have lived on Earth.

Plate tectonics and seafloor spreading.

Milankovitch Cycles

The most extreme climate of recent Earth history was the Pleistocene. Scientists attribute a series of ice ages to variations in the Earth's position relative to the Sun, known as Milankovitch cycles. The Earth goes through regular variations in its position relative to the Sun:

The shape of the Earth's orbit changes slightly as it goes around the Sun, called eccentricity. The orbit varies from more circular to more elliptical in a cycle lasting between 90,000 and 100,000 years. When the orbit is more elliptical, there is a more significant difference in solar radiation between winter and summer.

Circular Eccentricity

Elliptical Eccentricity

The Earth wobbles on its axis of rotation, called precession. At one extreme of this 27,000-year cycle, the Northern Hemisphere points toward the Sun when the Earth is closest to the Sun. Summers are much warmer, and winters are much colder than now. At the opposite extreme, the Northern Hemisphere points toward the Sun when it is farthest from the Sun, resulting in cool summers and warmer winters.

Precession

The planet's tilt on its axis varies between 22.1 degrees and 24.5 degrees, called obliquity. Seasons are caused by the tilt of Earth's axis of rotation, which is at a 23.5^o angle now. When the tilt angle is smaller, summers and winters differ less in temperature. This cycle lasts 41,000 years.

Obliquity

When these three variations are charted out, a climate pattern of about 100,000 years emerges. Ice ages correspond closely with Milankovitch cycles. Since glaciers can form only over land, ice ages only occur when landmasses cover the polar regions. Therefore, Milankovitch cycles are also connected to plate tectonics.

Sun Variation

The amount of energy the Sun radiates is relatively constant over geologic time but slightly fluctuates over the decades. Part of this fluctuation occurs because of sunspots, magnetic storms on the Sun's surface that increase and decrease over an 11-year cycle.

The natural cycle of sunspots over the past few hundred years.

When the number of sunspots is high, solar radiation is also relatively high. However, the entire variation in solar radiation is tiny relative to the total amount of solar radiation. There is an 11-year cycle in climate variability. The Little Ice Age corresponded to a time when there were no sunspots on the Sun.

Sunspots

Changes in Atmospheric Greenhouse Gas Levels

Climatic data from ice core drillings, rings within coral reefs and trees, ocean and lake sediments, and other sources indicate that when greenhouse gasses increase in the atmosphere, global temperatures rise. When greenhouse gases decrease in the atmosphere, global temperatures fall. In 1958, the National Oceanic and Atmospheric Administration (NOAA) began measuring carbon dioxide levels in real-time. What direct measurements of carbon dioxide in the atmosphere indicate is that every year, the concentration of the gas increases globally every six months and decreases six months later.

This has mostly to do with the continents in the northern hemisphere, where the majority of the continents and trees are located. During the warmer months, the trees in the northern hemisphere begin photosynthesizing by taking carbon dioxide out of the atmosphere and using sunlight to create chlorophyll. This causes global greenhouse gases to decrease for six months. When the northern hemisphere experiences fall and winter, the trees stop photosynthesizing and become dormant, causing global greenhouse gases to increase. However, even though carbon dioxide levels increase and decrease every year, the global trend is that carbon dioxide levels are growing every year. Current measurements from NASA indicate that carbon dioxide levels are at 411 ppm, the highest the Earth has seen in nearly a million years.

Direct measurements of carbon dioxide by NOAA.

Recently, NASA has created ultra-high-resolution computer models, giving scientists a stunning new look at how carbon dioxide in the atmosphere travels around the globe.

Greenhouse gas levels have varied throughout Earth's history. For example, carbon dioxide has been present at concentrations less than 200 parts per million (ppm) and more than 5,000 ppm. However, for at least 650,000 years, carbon dioxide has never risen above 300 ppm during either glacial or interglacial periods. Natural processes add (volcanic eruptions and the decay or burning of organic matter) and remove absorption by plants, animal tissue, and the ocean) carbon dioxide from the atmosphere. When plants are turned into fossil fuels, the carbon dioxide in their tissue is stored with them. So carbon dioxide is removed from the atmosphere.

This graph from NASA, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution.

Fossil fuel use has skyrocketed in the past few decades. More people want more cars and industrial products, releasing vast quantities of carbon dioxide into the atmosphere. Burning tropical rainforests to clear land for agriculture, a practice called slash-and-burn agriculture also increases atmospheric carbon dioxide. By cutting down trees, they can no longer remove carbon dioxide from the atmosphere. Burning the trees releases all the carbon dioxide stored in the trees into the atmosphere.

There is now nearly 40 percent more carbon dioxide in the atmosphere than there was 200 years ago, before the Industrial Revolution. About 65 percent of that increase has occurred since the first carbon dioxide measurements were made on Mauna Loa Volcano, Hawaii, in 1958. Carbon dioxide is the most important greenhouse gas that human activities affect because it is so abundant. However, other greenhouse gases are increasing as well. The main greenhouse gases include:

• Water vapor (36-70 percent of the total) is the most abundant and powerful greenhouse gas on the planet and is part of the hydrologic cycle.
• Carbon dioxide (9-26 percent of the total) is released from the burning of fossil fuels.
• Methane (4-9 percent of the total) is released from raising livestock, rice production, and the incomplete burning of rainforest plants.
• Tropospheric ozone (3-7 percent of the total) is from vehicle exhaust; it has more than doubled since 1976.