What Natural Factors Cause Climate Change – Top view: Eruption of Mount Augustine (Alaska) on March 27, 2006; image by Cyrus Read for the USGS (public domain).
In this section we discuss climate changes caused not by human activities, but by natural forces inside and outside the Earth’s climate system.
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What Natural Factors Cause Climate Change
When we ask and answer the question of why the climate changes, we must at the same time consider the time scale of our discussion, that is, the length of time during which the changes occur.
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The earth has existed for 4.6 billion years and life has visibly roamed it, in one form or another, for most of that time. So what happened in the last 100 years is only a small part of the history of the Earth and its life and climate. Some causes of climate change have enormous influence, but are only visible for a million years or more. Others are smaller, but their impacts are more easily seen on shorter time scales, in decades or hundreds of years.
On a scale of millions of years, the climate changes due to the activity of plate tectonics. Plate tectonics, the mechanism that moves continents around the world and forms new ocean floor, has many effects on the global climate. Plate tectonic activity, for example, causes volcanism, and prolonged periods of high volcanic activity can release large amounts of greenhouse gases into the atmosphere. Volcanism also creates new rock as magma is expelled from the Earth’s interior and cools at the surface. Volcanic activity under the sea, new rocks can move ocean water and raise global sea levels, which changes the way the oceans distribute heat and further affects the global climate. For example, the Cretaceous period, from 145 million to 66 million years ago, was a particularly warm period in Earth’s history, partly due to large amounts of greenhouse gas emissions from volcanism, and it was also a time of highest global sea level.
Plate tectonics also affects the climate on a scale of millions of years due to the changing location of the continents. The Earth’s climate is strongly influenced by ocean currents, so the global climate is significantly different when the continents are close together (as in the supercontinent Pangea, which merged about 250 million years ago) and when they are further apart, as in modern times. . Also, land masses in equatorial regions have a different impact on climate than continents at higher latitudes because of how heat from equatorial regions is distributed north and south across land masses. Therefore, the position of the plates over time has had significant impacts on the global climate of the past.
On a scale of hundreds of thousands of years, climates change due to periodic oscillations of the Earth’s orbit around the sun, called Milankovitch or astronomical cycles.
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The sun is the source of most of the energy that enters the Earth; it is this solar energy in a particular area and time known as insolation that controls the energy that drives the Earth’s climate. Climate models indicate that a relatively small change in the amount of heat retained by the sun can have a lasting impact on Earth’s temperature.
Since almost all of Earth’s atmospheric energy is ultimately derived from the sun, it makes sense that the position and orientation of the planet relative to the sun would have an effect on climate. The Earth’s orbit around the Sun is not a perfect circle, but an ellipse. The distance from the Earth to the Sun changes as the Earth travels its annual path.
Also, the Earth’s axis (going from pole to pole) is not vertical with respect to the sun, but is currently tilted about 23.5°. The tilt of the Earth is responsible for the seasons, which different parts of the world experience differently. It is summer in the northern hemisphere during the part of the year that leans towards the sun and receives the sun’s rays most directly; conversely, when the Earth is on the other side of its orbit and the Southern Hemisphere is tilted towards the Sun, it is summer in the Southern Hemisphere.
The Earth’s orbit also changes on a longer time scale. The Milankovitch cycles describe how the Earth’s position changes over time in predictable patterns of alternations of the Earth’s proximity and angle to the sun, thus impacting the global climate. These are: eccentricity, obliquity and precession.
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Eccentricity, the change in the Earth’s orbit from a round orbit to an elliptical orbit, which occurs in a cycle of 100,000 years. As the Earth’s orbit varies between more circular and more elliptical (ie, more extreme eccentricity), the length of the seasons changes.
The eccentricity, caused by the gravitational forces of other planets in our solar system, changes the shape of the orbit in a cycle of 100,000 years from a circular shape to a more elliptical shape. Animation by NASA/JPL-Caltech (Public Domain).
The obliquity is the inclination of the Earth on its orbital axis, which can vary between 22 and 24.5 ° from the vertical, and occurs in a cycle of about 41,000 years. The tilt of the Earth affects the amount of insolation that the planet absorbs at different latitudes.
The obliquity is the change in the angle of the Earth’s axis, which varies between 22° and 24.5° with respect to the normal, and occurs in a cycle of 41,000 years. Animation by NASA/JPL-Caltech (Public Domain).
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Precession, commonly called “wobble”, because it is the slight variation in the direction of the axis of the Earth relative to the fixed stars of the galaxy. Because of precession, the point in Earth’s orbit when the northern hemisphere is tilted toward the sun changes over a cycle of about 26,000 years.
Precession, commonly called the “wobble” of Earth’s axis, affects the positions of Earth’s orbit in which the northern and southern hemispheres experience summer and winter. Precession changes on a cycle of about 26,000 years. Animation by NASA/JPL-Caltech (Public Domain).
These three variables interact with each other in ways that can be very complex, but are mathematically predictable. For example, the influence of the shape of the orbit on the Earth’s climate depends greatly on the angle of inclination that the Earth is experiencing at the time. The orbital variations described by the Milankovitch cycles are predictable from the known laws of planetary motion. Confirmation of its climatic effects, however, comes from the geological record, where, in deep sediment cores dating back more than 5,000,000 years, scientists have found evidence of temperature fluctuations similar to that which would be predicted by Milankovitch cycles.
These oscillations primarily affect the subtle amount of sunlight received throughout the year and the distribution of that sunlight across latitudes. Glacial intervals can occur when, partly as a result of these orbital variations, high latitudes receive less summer sunlight, so their ice and snow cover does not melt as much. Thus, the Pleistocene Ice Age record of dramatic warming followed by slow, steady cooling reflects repeated glaciations every 100,000 years or so, caused in part by Milankovitch cycles.
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100,000 year temperature cycles. Ice Age temperature changes over the last 450,000 years in this diagram are plotted as temperature differences (in °C) from a modern baseline. These differences are called temperature anomalies. The graph shows sharp spikes in temperature about every 100,000 years, each followed by slower cooling. The highest temperatures occurred just after the global climate changed from glacial to interglacial intervals. These changes in temperature correlate with changes in the shape of the Earth’s orbit (due to Milankovitch cycles). According to this model, the Earth should be (during this interglacial period) experiencing slow cooling, not warming. Image from “Climate Change Past, Present & Future: A Very Short Guide” by Allmon et al. (2010) (in turn modified from a graphic produced by Robert A. Rohde) (license CC BY-NC-SA 4.0).
Understanding these cycles has proven fundamental to understanding the natural variation of the Earth’s climate system over the past millions of years and beyond.
On the scale of millennia (thousands of years), climates during the last glacial-interglacial cycle were influenced by cyclical events such as Heinrich events. Heinrich events occurred approximately every 7,000-13,000 years and are evidenced by layers of sediment on the North Atlantic Ocean floor, deposited by the melting of huge ice sheets with rocks small and debris contained in them. Scientists believe that they were caused by large icebergs that broke off from Canada which, after floating in warmer waters, melted and released large amounts of fresh water into the North Atlantic. This changed ocean circulation because the large and rapid releases of fresh water are less dense than seawater, reducing the density of the ocean surface and reducing the sinking of the dense water it carries. the oceanic circulation. These large and sudden releases of fresh water caused a change in ocean current patterns from a glacial type to an interglacial type.
On the scale of human experience and history (centuries to decades), climates change for a variety of reasons. Some are cyclical and others are the culmination of small changes in topography, land use, and other factors that occur in this relatively short period of time. Two examples of changes on this scale are the Younger Dryas event and the Little Ice Age. The Younger Dryas event was an interval of 1,200 years
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