- Climate is a System
- Measuring Climate
- Greenhouse Gases and Global Temperature
- Natural Causes of Climate Change
- Summary and Additional Resources
Image above: Eruption of the Augustine Volcano (Alaska) on March 27, 2006; image by Cyrus Read for the USGS (public domain).
In this section we address climate changes caused not by human activities, but by natural forces within and outside of Earth’s climate system.
As we ask and answer the question of why climate changes, we must simultaneously consider the temporal scale of our discussion, that is, the extent of time over which changes occur.
Table: Some common causes of climate change in Earth's history and their temporal scale.
|Climate Change Cause||Scale of Change|
|Position in the solar system||billions of years|
|Heat generated by the sun||billions of years|
|Evolution of photosynthesis, other biological impacts||millions to billions of years|
|CO₂ input from volcanism||millions of years|
|CO₂ removal from weathering||millions of years|
|Movement of tectonic plates||millions of years|
|Shape of Earth’s orbit around the sun (eccentricity)||hundreds of thousands of years|
|Tilt of Earth’s axis relative to the sun (obliquity)||tens of thousands of years|
|“Wobble” of Earth’s axis (precession)||tens of thousands of years|
Earth has been in existence for 4.6 billion years, and life has been visibly thriving on it, in one form or another, for most of that time. Thus, what has happened in the last 100 years is only a tiny part of the history of Earth and its life and climate. Some causes of climate change have tremendous influence, but are only apparent over a million years or more. Others are smaller, but their impacts are seen more readily over shorter time scales, in decades or hundreds of years.
On the scale of millions of years, climates change because of plate tectonic activity. Plate tectonics, the mechanism that moves the continents across the globe and forms new ocean floor, has many effects on global climate. Plate tectonic activity, for example, causes volcanism, and extended 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 interior of the Earth and cools on the surface. In underwater volcanic activity, new rock can displace ocean water and increase global sea level, which changes the way the oceans distribute heat, and further impacts global climate. For example, the Cretaceous Period, from 145 million to 66 million years ago, was a particularly warm period in Earth’s history, in part due to the high amounts of greenhouse gas emission from volcanism, and was also a time of higher global sea level.
Plate tectonics also impacts climate on the scale of millions of years due to the changing location of the continents. Climate on land is heavily influenced by ocean currents, so global climate is significantly different when the continents are close together (as in the supercontinent Pangea, which came together approximately 250 million years ago) versus when they are more widely separated, as in modern times. Also, land masses in the equatorial regions have a different impact on climate than continents in higher latitudes because of how heat is distributed from equatorial regions north- and southward around land masses. Therefore, the position of plates over time has had significant impacts on past global climate.
On the scale of hundreds of thousands of years, climates change because of periodic oscillations of the Earth’s orbit around the sun, called astronomical or Milankovitch cycles.
The sun is the source of most incoming energy on Earth; it is this solar energy over a given area and time known as insolation that controls the energy that drives Earth’s climate. Climate models indicate that a relatively small change in the amount of heat retained from the sun can have a lasting impact on Earth’s temperature.
Because nearly all of Earth’s atmospheric energy is ultimately derived from the sun, it makes sense that the planet’s position and orientation 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 yearly path.
In addition, the axis of the Earth (running from pole to pole) is not vertical with respect to the sun, but is currently tilted approximately 23.5°. Earth’s tilt is responsible for the seasons, which various parts of the world experience differently. It is summer in the Northern Hemisphere during the part of the year that it is tilted toward the sun and receives the sun’s rays more directly; conversely, when Earth is on the other side of its orbit and the Southern Hemisphere is tilted toward the sun, it is summer in the Southern Hemisphere.
Earth’s orbit also changes on a longer time scale. Milankovitch Cycles describe how the position of Earth changes over time in predictable patterns of alternations of the proximity and angle of Earth to the sun, and therefore have an impact on global climate. These are: eccentricity, obliquity, and precession.
Eccentricity, the change of Earth’s orbit from a round orbit to an elliptical one, which occurs on a 100,000-year cycle. When Earth’s orbit varies between more circular and more elliptical (i.e. more extreme eccentricity), the length of the seasons change.
Eccentricity, caused by gravitational forces from other planets in our solar system, changes the shape of the orbit on a 100,000-year cycle from a circular to a more elliptical shape. Animation by NASA/JPL-Caltech (public domain).
Obliquity is the tilt of the Earth on its orbital axis, which can range from 22-24.5° from vertical, and occurs on a roughly 41,000-year cycle. The tilt of the Earth impacts how much insolation is absorbed by the planet at different latitudes.
Obliquity is the change of the angle of Earth’s axis, which ranges from 22° to 24.5° from normal, and occurs on a 41,000-year cycle. Animation by NASA/JPL-Caltech (public domain).
Precession, commonly called “wobble,” because it is the small variation in the direction of Earth’s axis as it points relative to the fixed stars in the galaxy. Because of precession, the point in Earth’s orbit when the Northern Hemisphere is angled toward the sun changes over a cycle of approximately 26,000 years.
Precession, commonly called the “wobble” of Earth’s axis, affects the positions in Earth’s orbit at which the Northern and Southern Hemispheres experience summer and winter. Precession changes on an approximately 26,000-year cycle. Animation by NASA/JPL-Caltech (public domain).
These three variables interact with each other in ways that can be very complex, but are predictable mathematically. For example, the influence of the shape of the orbit on Earth’s climate depends very much on the angle of tilt that Earth is experiencing at the time. The orbital variations described by Milankovitch Cycles are predictable based on the known laws of planetary motion. Confirmation of their climatic effects, however, comes from the geological record, where, in deep-sea sediment cores stretching back further than 5,000,000 years, scientists have found indications of temperature fluctuating in ways similar to what Milankovitch Cycles would predict.
These oscillations primarily affect the subtly varying amount of sunlight received over the course of the year and the distribution of that sunlight across latitudes. Glacial intervals can occur when, in part as a result of these orbital variations, high latitudes receive less summer sunlight, so that their cover of ice and snow 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.
100,000-year temperature cycles. Ice age temperature changes for the last 450,000 years in this diagram are represented as differences of temperature (in °C) from a modern baseline. These differences are called temperature anomalies. The graph shows abrupt temperature spikes approximately 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 temperature changes correlate with changes in the shape of Earth’s orbit (due to Milankovitch Cycles). According to this pattern, Earth should now (during this interglacial period) be 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 graph produced by Robert A. Rohde) (CC BY-NC-SA 4.0 license).
Understanding these cycles has turned out to be critical to understanding natural variation in the Earth’s climate system over the past several million years and beyond.
Recent Natural Climate Change
On the scale of millennia (thousands of years), climates during the last glacial-interglacial cycle have been influenced by cyclic events such as Heinrich events. Heinrich events occurred approximately every 7,000-13,000 years and are evidenced by sediment layers on the northern Atlantic Ocean floor, deposited by the melting of huge ice sheets with small rocks and debris contained in them. Scientists believe that these were caused by large icebergs that were released from Canada that, after floating into warmer waters, melted and released large quantities of freshwater into the North Atlantic. This changed ocean circulation because the large, quick releases of freshwater are less dense than the seawater, decreasing the density of the ocean surface and diminishing the sinking of dense water that drives ocean circulation. These large, abrupt releases of freshwater caused a switch from glacial to interglacial types of ocean current patterns.
On the scale of human experience and history (centuries to decades), climates change for a number of reasons. Some are cyclic, and others are the culmination of small changes in topography, land use, and other factors that occur in this relatively short span of time. Two examples of changes on this scale are the Younger Dryas event and the Little Ice Age. The Younger Dryas event was a 1,200-year interval of colder temperatures that punctuated a warming trend that began approximately 13,000 years ago. Scientists have ascertained that a shift from warming to cooling happened over the course of only a few decades, and brought back glacial climate characteristics such as mountain glaciers in New Zealand and intense windstorms in Asia. One hypothesis suggests that the Younger Dryas was triggered by an ice dam breaking and sending large amounts of freshwater into the northern Atlantic Ocean, reducing flow of warm Gulf Stream water into the area. Other hypotheses have been offered to explain the Younger Dryas, including one postulating icebergs breaking off of an Arctic ice sheet and floating southward, again sending large amounts of freshwater into the North Atlantic. One thing seems certain—the Younger Dryas is an example of how a single event can reverse or significantly change global climate within a matter of decades.
The Little Ice Age occurred between approximately the years 1200 and 1800 CE and followed a time in history called the Medieval Warm Period, which peaked approximately 1,000 years ago. The difference in temperature between the Medieval Warm Period, which allowed the Viking people to inhabit Greenland, and the Little Ice Age, which kept Icelandic fishermen frozen in port for up to three months per year from the 1600s through 1930, was only approximately 1°C (1.8°F) globally.
Many factors affect weather and climate on the scale of a few years, and some of these can be cyclic or nearly so. One of the most important of these is El Niño. El Niño is a climate pattern that occurs across the tropical Pacific Ocean every 3-7 years, characterized by warming ocean surface temperatures and accompanying major shifts in precipitation in the Americas and ocean circulation in the eastern Pacific.
The sun also plays a role in a short-term climate cycle through its frequency of solar flares, or sunspots, which increase and decrease on an 11-year cycle. When solar flares occur more frequently, the sun has a larger number of “spots” on its surface and emits more solar energy, which increases the intensity of energy (irradiance) that the Earth receives from the sun. Direct measurements of solar output since 1978 show a rise and fall over the 11-year sunspot cycle, but there is no overall up- or downward trend in the strength of solar irradiance that might correlate with the temperature increase that Earth has experienced. Similarly, there is no trend in direct measurements of the sun’s ultraviolet output or in cosmic rays. Thus, even though solar irradiance is the primary energy that heats our planet, because sunspots have shown no major directional increases or decreases in their recorded history, they do not appear to be related to the current, directional change in global climate.
The Carbon Cycle
The element carbon plays a crucial role in the way that the Earth works. Because of its ability to readily form up to four bonds with other elements and other particular chemical properties, carbon constitutes the basic building block of living things as well as major constituents of the atmosphere, crust, and oceans. Individual carbon atoms combine with other elements in a variety of ways as they move between these various Earth systems in a series of steps known as the carbon cycle. Understanding the role of CO2 in the Earth’s climate starts with understanding how carbon behaves in this cycle.
The carbon cycle (a very simplified view). Every living thing contains carbon. When animals exhale, CO2 is emitted into the atmosphere. It is absorbed by plants through the process of photosynthesis and gets incorporated into their structures. When plants and animals die, the carbon in their bodies gets incorporated into sediments, which might eventually become rocks in the Earth’s crust, where it usually remains for millions of years. The extraction and burning of this carbon in the form of fossil fuels emits CO2 into the atmosphere, and some of it becomes incorporated into carbon sinks like oceans and forests. Omitted from this figure are biological processes in the oceans, volcanoes, weathering of rocks, and the formation of limestone. Image created by Jonathan R. Hendricks for PRI's [email protected] project (CC BY-NC-SA 4.0 license).
CO2 that enters the atmosphere from volcanoes is approximately balanced (in the absence of humans) by removal of CO2 from the atmosphere by two processes. One is long-term burial of organic matter—that is, the products of photosynthesis not recycled. This occurs when, for example, dead phytoplankton sink to the bottom of the ocean and are covered by sediment. The other is chemical weathering, or the breakdown of rocks at the surface by chemical change. During chemical weathering, water reacts with minerals in rocks and CO2 from the atmosphere. The CO2 is thus removed from the air and transferred into other compounds, which eventually become stored in the sediments that accumulate in the ocean and ultimately become part of sedimentary rocks.
Evolutionary changes in organisms throughout geologic time have had a strong effect on the global carbon cycle. The evolution of organisms capable of photosynthesis, over 3 billion years ago, drew CO2 out of the atmosphere and created the first significant amounts of atmospheric oxygen. The first appearance of large animals in the early Cambrian, 540 million years ago, and the evolution of land plants in the Devonian, approximately 380 million years ago, accelerated the cycling of carbon and its burial in sediments. It is widely believed that the evolution of land plants led to a significant drop in CO2 concentrations in the atmosphere and caused the widespread glaciation of the Carboniferous Period (360 to 295 million years ago).
The carbon cycle functions on a variety of time scales. A single atom of carbon that you exhale (as part of a molecule of CO2) will on average remain in the atmosphere for hundreds of years, before being absorbed by a plant or other photosynthesizing organism. When the plant dies, that carbon atom could in a few weeks or months be taken up by another plant, or oxidize back into CO2 and re-enter the atmosphere, or it might be buried in the Earth’s crust and remain there for millions of years. When we burn fossil fuels—oil, natural gas, and coal extracted from the Earth—we very quickly release carbon into the atmosphere from sources that took millions of years to form.
The Earth’s surface is like a jigsaw puzzle. It is made up of many huge pieces, or plates, which slide around the globe very slowly, at about the rate that your fingernails grow. The continents are embedded in these tectonic plates.
A world map with all of the individual tectonic plate boundaries highlighted. The plates move around like puzzle pieces over the globe. Image by the USGS (Wikimedia Commons; public domain).
Where these plates come together or move apart, earthquakes, mountain building, and many other geologic processes can occur. Plate movement is thought to be driven by the Earth’s internal heat, as convection currents from the lower mantle heat the rock above, lowering its density and pushing it upward.
Plate movement can significantly affect climate over millions of years, in several ways. The position of a plate on the globe, and of any continents that might be on top of it, is one determinant of whether that continent will experience glaciation or tropical temperatures. For example, if the plate that now holds North America and Greenland were shifted a bit to the north, North America might now be covered in a continental ice sheet. Instead, only Greenland is covered in ice because it is positioned farther north today. Plate movement also affects climate because when two plates come together, volcanoes often result, adding CO2 to the atmosphere when they erupt. When plates move apart, in a process known as seafloor spreading, hot magma is often released directly into the ocean, bringing CO2 with it.