Why do the blue, green and red lines go in the same direction at the same time?
This is a complicated graph, but extremely interesting. The data are from the 3600 meter-long Vostok ice core, which gave climate scientists an unprecedented look into the history of Earth's climate. The red line is temperature. You can see that carbon dioxide and methane are correlated with temperature. When these greenhouse gases are high, temperature is high. This holds true for the 440,000 years revealed in the core.
Causes of Long-term Climate Change
Many processes can cause climate to change. These include changes:
- In the amount of energy the Sun produces over years.
- In the positions of the continents over millions of years.
- In the tilt of Earth's axis and orbit over thousands of years.
- That are sudden and dramatic because of random catastrophic events, such as a large asteroid impact.
- In greenhouse gases in the atmosphere, caused naturally or by human activities.
The amount of energy the Sun radiates is variable. Sunspots are magnetic storms on the Sun’s surface that increase and decrease over an 11-year cycle (Figure below). When the number of sunspots is high, solar radiation is also relatively high. But the entire variation in solar radiation is tiny relative to the total amount of solar radiation that there is, and there is no known 11-year cycle in climate variability. The Little Ice Age corresponded to a time when there were no sunspots on the Sun.
Sunspots on the face of the Sun.
Plate tectonic movements can alter climate. Over 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. When the continents separate, heat is more evenly distributed.
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.
Plate motions trigger volcanic eruptions, which release dust and CO2 into the atmosphere. Ordinary eruptions, even large ones, have only a short-term effect on weather (Figure below). Massive eruptions of the fluid lavas that create lava plateaus release much more gas and dust, and can change climate for many years. This type of eruption is exceedingly rare; none has occurred since humans have lived on Earth.
An eruption like Sarychev Volcano (Kuril Islands, northeast of Japan) in 2009 would have very little impact on weather.
The most extreme climate of recent Earth history was the Pleistocene. Scientists attribute a series of ice ages to variation 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:
1. The shape of the Earth's orbit changes slightly as it goes around the Sun. 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 greater difference in solar radiation between winter and summer.
2. The planet wobbles on its axis of rotation. 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. This results in chilly summers and warmer winters.
3. The planet’s tilt on its axis varies between 22.1o and 24.5o. Seasons are caused by the tilt of Earth's axis of rotation, which is at a 23.5o angle now. When the tilt angle is smaller, summers and winters differ less in temperature. This cycle lasts 41,000 years.
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.
Changes in Atmospheric Greenhouse Gas Levels
Since greenhouse gases trap the heat that radiates off the planet’s surfaces, what would happen to global temperatures if atmospheric greenhouse gas levels decreased? What if greenhouse gases increased? A decrease in greenhouse gas levels decreases global temperature and an increase raises global temperature.
Greenhouse gas levels have varied throughout Earth history. For example, CO2 has been present at concentrations less than 200 parts per million (ppm) and more than 5,000 ppm. But for at least 650,000 years, CO2 has never risen above 300 ppm, during either glacial or interglacial periods (Figure below).
CO2 levels during glacial (blue) and interglacial (yellow) periods. Are CO2 levels relatively high or relatively low during interglacial periods? Current carbon dioxide levels are at around 400 ppm, the highest level for the last 650,000 years. BP means years before present.
Natural processes add and remove CO2 from the atmosphere.
- Processes that add CO2:
- volcanic eruptions
- decay or burning of organic matter.
- Processes that remove CO2:
- absorption by plant and animal tissue.
When plants are turned into fossil fuels, the CO2 in their tissue is stored with them. So CO2 is removed from the atmosphere. What does this do to Earth’s average temperature?
What happens to atmospheric CO2 when the fossil fuels are burned? What happens to global temperatures?
- The positions of continents, the sizes of oceans and the amount of volcanic activity that takes place are all ways that plate tectonics processes can affect climate.
- Milankovitch cycles affect the way Earth relates to the Sun due to the shape of the planet's orbit, its axial tilt, and its wobble.
- Atmospheric greenhouse gas levels correlate with average global temperatures.
Use these resources to answer the questions that follow.
- What do Milankovitch cycles initiate?
- How often do Milankovitch cycles produce a warm period lately?
- What is the Holecene?
- When should we enter another ice age?
- What is happening to climate now?
- How do Milankovitch cycles affect global temperatures?
- How do plate tectonics processes affect global climate?
- How are atmospheric greenhouse gas levels correlated with global temperatures?
- What are carbon dioxide levels now? How often in the past 650,000 years have they been that high?