Climate Changes 5: Climate

Which natural process has caused the fewest changes to global climates over time?

changing ocean tides

What is the cause of global warming, the gradual increase in global temperatures being observed today?

increasing levels of carbon dioxide and other greenhouse gases

Global Warming

As a result of increases in carbon dioxide levels, as well as other greenhouse gases, global temperatures have increased. This increase is called global warming. During the twentieth century, Earth's average surface temperatures increased about 1.0°C. Scientists predict that by the year 2100, temperatures could increase by more than another 5°C.

Volcanic Eruptions

As you can see in the figure, volcanic eruptions can emit large volumes of ash and dust into Earth's atmosphere. Volcanic eruptions also send minute particles containing sulfur into the air. If the volume of these very fine particles called aerosols is great enough, it can cause short-term changes in Earth's surface temperature. Volcanic ash, dust, and sulfur-based aerosols in the air increase the amount of solar radiation that is reflected back into space. This causes Earth's lower atmosphere to cool. But over longer time scales, volcanic eruptions can raise global temperatures by adding gases like carbon dioxide to the atmosphere, increasing the greenhouse effect.

Which natural processes can cause climate change?

Changes in global ocean circulation, plate tectonics, and volcanic eruptions.

Earth's Orbital Motions

Changes in the shape of Earth's orbit and the tilt of Earth's axis of rotation, which is illustrated in the figure, affect global climates over intermediate time scales. Earth's orbit is always elliptical. But over 100,000-400,000 year periods, the path becomes more and then less elliptical. This change in shape brings Earth closer to and then farther from the sun. This affects global climates: Earth is warmer when it is closer to the sun. The tilt of Earth's orbit also changes with respect to the rest of the solar system, with a 100,000 year period. The tilt and direction of Earth's axis of rotation change over periods of about 20,000-40,000 years. The tilt of Earth's axis changes by about 3 degrees. This affects the severity of the seasons. When Earth's axis is less tilted, the temperature difference between summer and winter is less.

Ocean Circulation

El Niño is a change in ocean circulation that causes parts of the eastern tropical Pacific Ocean to become warmer than usual. These changes in ocean circulation also can result in short-term climate fluctuations. For example, some areas that are normally arid receive large amounts of rain during El Niño. Also, some regions that receive abundant precipitation may experience dry periods when the ocean circulation patterns change.

Plate Tectonics

Geographic changes in Earth's land and oceans due to plate tectonics cause changes in climate over very long time scales. Oceans open and close, changing ocean circulation patterns. Large supercontinents like Pangaea develop monsoon climates, as in Asia. But most important, mountain building cools global climates by removing carbon dioxide from the atmosphere. For example, Earth has been cooling over the past 50 million years because of the collision of India and Asia that created the Himalayas. In this process, mountains weather and erode quickly, and the sediments wash down rivers and into the ocean where they form carbonate rocks such as limestone. The carbon in limestone begins as carbon dioxide in the atmosphere. Weathering removes carbon dioxide from the atmosphere and buries it on the seafloor. Before India collided with Asia, there was three times as much carbon dioxide in the atmosphere as there is now.

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Solar Activity

In general, the sun has been giving off increasing amounts of energy over its lifetime. Over short time scales, fluctuations in the amount of solar radiation can change global climates. An increase in sunspots appears to correspond with warm periods in Europe and North America. Fewer sunspots seem to be correlated with cooler periods. For example, the "Little Ice Age" in Europe during the 1600s occurred when there were very few sunspots. This process is not understood and is still being investigated.

Natural Processes That Change Climate

Many different natural processes can cause climates to change. Some act over very long time scales, and some over much shorter time scales. All occur together, making climate change a very complex process.

Evidence of Past Climate Change 2

Misplaced fossils are another way scientists can tell the climate of a region has changed. Fossil corals and other marine organisms have been found in abundance in parts of Europe not covered by water. However, these fossils show that a shallow, tropical sea must have once covered this region. Coal deposits in Antarctica also indicate a change in the climate. Coal deposits were created when giant plants that lived hundreds of millions of years ago in areas of tropical, swampy forests died and formed layers at the bottom of the swamp.

Human Impact on Climate Changes

Natural processes have certainly contributed to many climatic changes throughout Earth's 4.6- billion-year history. These processes will also cause some of the future shifts in Earth's climates. But human activities have contributed and will contribute to global climatic change.

How do changes in Earth's land cause climate change?

The sediment formed by the erosion of mountains end up in the ocean. These sediments use carbon dioxide in the atmosphere and form carbonate rocks such as limestone, thus removing carbon dioxide from the atmosphere.

Global Warming 2

Scientists base their predictions about global warming on computer models of change in Earth's atmosphere called climate models. Climate models are complex computer programs. These models involve huge amounts of data on temperature, precipitation, and other variables. But climate models cannot describe Earth's atmosphere completely. For this reason, the results from a model are always an approximation. What follows is one prediction of how temperature increases could affect Earth. Warmer surface temperatures increase evaporation rates. This, in turn, increases the amount of water vapor in the atmosphere. Water vapor is an even more powerful absorber of radiation emitted by Earth than is carbon dioxide. Therefore, more water vapor in the air will magnify the effect of carbon dioxide and other gases.

Evidence of Past Climate Change 3

Scientists drill down into glaciers or polar ice caps to obtain samples called ice cores. Dust, air bubbles, sea salts, and layers of volcanic ash and soot from forest fires all get trapped in the ice. Once back at the laboratory, scientists can carefully melt or crush the ice cores to release these relics of the past and analyze them. By feeding the data found while studying these features into a computer, scientists can map and model ways in which the climate has changed in the past and predict how it might change in the future.

Evidence of Past Climate Change

Scientists use several methods to reveal Earth's climate history. These methods include the study of glacial striations, fossils, and ice cores. As glaciers move, they trap and drag rocks beneath them. The grooves that result in the underlying landscape are called glacial striations. Glacial striations have been found in areas of the world that are not currently covered in glaciers, but which must have been covered in ice sheets at some point.

Global Warming 3

Temperature increases will also cause sea ice to melt. Ice reflects more incoming solar radiation than liquid water does. The melting of the ice will cause a substantial increase in the solar energy absorbed at the surface. This, in turn, will magnify the temperature increase created by higher levels of greenhouse gases. The melting of sea ice and ice sheets will also cause a global rise in sea level. This will lead to shoreline erosion and coastal flooding. Warmer oceans could also lead to stronger and more frequent storms, including hurricanes. Scientists also expect that weather patterns will change as a result of the projected global warming. More intense heatwaves and droughts in some regions and fewer such events in other places are also predicted. Hotter, more arid conditions have already led to more forest fires in the western United States.

The Greenhouse Effect

The greenhouse effect is a natural warming of both Earth's lower atmosphere and Earth's surface. The major gases involved in the greenhouse effect are water vapor, carbon dioxide, and methane. These greenhouse gases, as they are often called, are transparent to solar radiation, and therefore much of this energy reaches Earth's surface. Most of this energy is then reradiated skyward. The greenhouse gases are good absorbers of Earth's radiation. These gases maintain warm temperatures in the lower atmosphere, making Earth habitable for living things. Without the greenhouse effect, Earth would be much too cold to support most types of life presently living on Earth. But an increase in the greenhouse effect could prove devastating. Studies indicate that human activities for the past 200 or so years have strengthened the greenhouse effect. As you can see in the figure, carbon dioxide levels in the air have risen at a rapid pace since about 1850. Much of this greenhouse gas has been added by the burning of fossil fuels. The clearing of forests also contributes to an increase in carbon dioxide because this gas is released when vegetation is burned or when it decays.

How has the greenhouse effect contributed to global warming?

The greenhouse effect is a natural warming phenomenon that is necessary to keep the Earth habitable for living things. However, as a result of an increase in levels of greenhouse gases such as carbon dioxide, global temperatures have increased.

Over the past 200 years, which human activity has led to a global temperature increase?

burning fossil fuels

How can volcanic eruptions cause the climate to become cooler?

Volcanic ash, dust, and aerosols increase the solar radiation reflected into space.