12.4: Causes of Climate Change

Processes that affect Climate

Many processes can alter the climate. These include changes in the Sun's energy output over years; the positions of the continents over millions of years; the tilt of Earth’s axis; the orbit over thousands of years; and sudden, dramatic changes due to random catastrophic events, such as a significant asteroid impact.

Plate Tectonics

Plate tectonics is the scientific theory that Earth's lithosphere comprises several large tectonic plates, which have been slowly moving since 3–4 billion years ago, as illustrated in Figure \(\PageIndex{1}\). Such plate movement and location can alter the climate. Over millions to tens of millions of years, as seas open and close, ocean currents may distribute heat differently. For example, when all the continents are joined into one supercontinent (such as Pangaea), nearly all locations experience a continental climate, except for a small coastal area. When the continents separate, heat is more evenly distributed because there’s more coastal land, and even interior land is, on average, closer to the ocean.

Figure \(\PageIndex{1}\): A map of Earth's 16 principal tectonic plates, which are constantly moving, subducting, and converging as they float over the Earth's mantle. (CC BY-SA 3.0; M. Bitton via Wikimedia Commons). Alternative Text Description.

Plate tectonic movements may also help start an ice age. When continents are located near the poles, ice can accumulate, thereby increasing albedo and lowering global temperature. Low enough temperatures may start a global ice age. Plate motions can also trigger volcanic eruptions, which release dust and CO₂ into the atmosphere. Ordinary eruptions, even large ones, have only a short-term effect on the weather. Massive eruptions 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.

Solar variations

Figure \(\PageIndex{2}\): The last three solar cycles as measured in solar irradiance, sunspot numbers, solar flare activity, and 10.7 cm radio flux. Solar irradiance, i.e., the direct solar power at the top of Earth's atmosphere, is presented as both a daily measurement and a moving annual average. (CC BY-SA 3.0; Robert A. Rohde via Wikimedia Commons). Alternative Text Description.

Stars have life cycles. Some grow, some contract. The Sun continues to grow, and over the last 4.6 B years, its size and energy output have increased by approximately 33%. The amount of energy the Sun radiates is variable, as shown in Figure \(\PageIndex{2}\). Sunspots are magnetic storms on the Sun’s surface that increase and decrease over an 11-year cycle. When the number of sunspots is high, solar radiation is also relatively high. The Little Ice Age coincided with a period of solar inactivity, known as the Maunder Minimum. But the entire variation in solar radiation is tiny relative to the total amount, and there is no known 11-year cycle in climate variability. We do not see the same 11-year cycle in global temperatures. Instead, we see a steady increase in temperatures over the last century.

The Milankovitch Cycles

The most extreme climate in Earth's recent history was the Pleistocene. This time period began 2 million years ago. Scientists attribute a series of Pleistocene ice ages to variations in Earth’s orbit and tilt relative to the Sun, known as Milankovitch cycles. They are caused by changes in Earth’s orbit around the sun, which affects the seasonal distribution of incoming solar radiation. This theory was first proposed in 1938 by Serbian astronomer Milutin Milankovich, who calculated variations in Earth’s orbital parameters and linked those to past ice ages. Earth’s orbit can be described by three parameters, as illustrated in Fig. \(\PageIndex{3}\). The eccentricity E is the deviation from a perfectly circular orbit. Earth’s orbit is a slight ellipse, although it is close to circular. E varies on ~100,000-year cycles between zero and 0.06. The tilt T, or obliquity, describes the angle between Earth’s axis of rotation and the ecliptic, which is the plane of Earth’s orbit around the sun. It is currently T = 23.5°, but it varies between 22.5° and 24.5° on a 40,000-year cycle. Precession P is the wobble of Earth’s axis, like the wobble of a top. Currently, we’re closest to the sun in January, which corresponds to the axis tilted towards the left. P varies on a 23,000-year cycle and is strongly modulated by eccentricity. These variations are caused by gravitational forces from the other planets, particularly Jupiter and Saturn.

Figure \(\PageIndex{3}\): The Earth moves around the Sun in an elliptical orbit (eccentricity, E), so the distance to the Sun, and therefore the amount of solar energy reaching the Earth, varies periodically. Furthermore, since the Earth’s equatorial plane is tilted relative to its orbital plane (obliquity or tilt, T), the amount of energy received from the Sun varies with the seasons. Third, the Earth’s axis of rotation spins around the normal to its orbital plane (precession, P). (CC BY-NC 4.0; Mathematics of Planet Earth via Rutgers University). Alternative Text Description.

Milankovitch’s theory suggests that summer insolation at 65°N controls the waxing and waning of the ice sheets, as illustrated in Figure \(\PageIndex{4}\). When summer isolation is high, all the snow from the previous winter will melt away. When summer insolation is low, some snow survives, and during the following winter, more snow accumulates. In this way, an ice sheet can grow. The Northern Hemisphere is important because it contains the major landmasses and is where additional ice sheets can grow. The Antarctic ice sheet did not fundamentally change during the ice ages, although it grew somewhat larger during them and shrank a bit during interglacials. The Patagonian ice sheet was much smaller than those in the north.

Figure \(\PageIndex{4}\): Variation of Earth’s orbital parameters align with Solar Insolation, and the glacial-interglacial cycles. 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. (Public Domain; Causes of Climate Change via NASA). Alternative Text Description.

While the Milankovich theory explains the cyclicity and timing of glacial-interglacial cycles, it does not explain their amplitudes (how much global average temperatures have changed). Simulations with global climate models show that the amplitude of glacial-interglacial temperature changes can be reproduced only when CO₂ changes are accounted for. This leads us to conclude that CO₂ changes are an important (feedback) factor in determining glacial-interglacial temperature changes. However, the ultimate cause of ice-age cycles is Earth’s orbital cycles.

Volcanoes

Aerosols are small particles suspended in the air. Natural processes that deliver aerosols into the atmosphere are dust storms and volcanic eruptions. The combustion of oil and coal also releases aerosols into the atmosphere. Aerosols have two main effects on Earth’s radiative balance. They directly reflect sunlight back to space (direct effect). They also act as cloud condensation nuclei, forming more or brighter clouds that reflect more solar radiation back to space (an indirect effect). Thus, both direct and indirect effects of increased aerosol concentrations lead to surface cooling. Therefore, aerosol forcing is negative.

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Figure \(\PageIndex{5}\): A: Photograph of a rising Mt. Pinatubo ash plume on June 12, 1991 (Public Domain; Dave Harlow, USGS via Wikimedia Commons). B: A photograph from the Space Shuttle over South America taken on Aug. 8, 1991, showing the dispersion of the aerosols from the MT. Pinatubo eruption in two layers in the stratosphere above the top of cumulonimbus clouds (Public Domain; Stephen Self et al., 1993 via USGS). C: Measurements of solar radiation transmitted at Hawaii’s Mauna Loa Observatory show a reduction in solar incoming radiation during major volcanic eruptions. (Public Domain; Kokoo via Wikimedia Commons). Alternative Text Description.

On June 12, 1991, Mt. Pinatubo in the Philippines erupted spectacularly, sending an ash column 19 km (12 mi) into the atmosphere, as shown in Figure \(\PageIndex{5}\)A. Large explosive volcanic eruptions such as these can eject ash particles and gases such as sulfur dioxide (SO₂) into the stratosphere (Figure \(\PageIndex{5}\)B), where they can be rapidly distributed over large areas. In the stratosphere, SO₂ is oxidized to form sulfuric acid aerosols. However, stratospheric aerosols eventually mix back into the troposphere and are removed through precipitation or dry deposition. The lifetime of volcanic aerosols in the stratosphere is in the order of months to a few years. Estimates of the radiative forcing from volcanic eruptions depend on the eruption but vary from a few negative tenths of a watt per square meter to -3 or -4 Wm^-2 for the most significant eruptions during the last 100 years, as illustrated in Figure \(\PageIndex{5}\)C.