How are
people changing
the climate?

Basics

1. Man-made climate change?

What is happening to the climate?

Temperature readings taken around the world show that the Earth’s mean temperature has increased by 0.6°C over the last 100 years.

Some of this warming, especially that which took place early in the 1900s, could be from natural causes – such as changes in the amount of sunlight that reaches the Earth (solar radiation). But the evidence suggests that most of the temperature increase over the last 30–50 years has been caused by man-made emissions of greenhouse gases.
 

1. TEMPERATURE CHANGES: The red line shows how scientists believe the average temperature on Earth has changed over the last thousand years – measured in degrees Celsius below or above the temperature in 1990. The gray area shows how much higher or lower scientists think the real temperatures may have been. (You can see that they are more uncertain about the temperatures the further back in history they go.)  For the years 1000-1860, scientists have tried to reconstruct the temperature in the Northern Hemisphere by studying tree rings, corals, ice samples and written documents. (They have too little information about temperatures in the Southern Hemisphere before 1860.) From 1860-2000, the temperature has been measured using thermometers all over the world. The curves for 2000-2100 show how scientists believe the climate could change in the future, depending on emissions of greenhouse gases in the future (see next unit). Source: IPCC (click to enlarge, 54 kB)

There are many different kinds of greenhouse gases. Most of the manmade contribution to the greenhouse effect is due to emissions of carbon dioxide (CO₂). It occurs naturally in the atmosphere, but the concentration of CO₂ has increased dramatically since before the Industrial Revolution. This increase is mostly, if not completely, caused by people. Other greenhouse gases such as methane (CH₄), have also increased. In addition, we see that the atmosphere now contains several greenhouse gases that do not occur naturally in the atmosphere, and can only have come from human activity (read more about observed changes in the atmosphere’s concentrations of greenhouse gases here). The concentration of the important greenhouse gas water vapor has also increased. Increased water vapor in the air is not a direct result of emissions of water vapor. Instead, it is an indirect result of emissions of other greenhouse gases. Higher temperatures in the atmosphere lead to greater evaporation from the ground surface and a greater capacity for the atmosphere to retain moisture.

2. MORE AND MORE CO₂: Concentration of CO₂ in the atmosphere from year 1000 to year 2000 (ppm means parts per million, or liters CO₂ per million liters of air). Scientists analyze old ice from glaciers to find out how much CO₂ there was in the atmosphere. For the most recent years, they have analyzed air samples taken directly from the atmosphere as well. The levels of concentrations after 2000 are estimates based on different possibilities for how concentrations could develop in the future, depending on how much CO₂ people emit (see next unit).  Source: IPCC (click to enlarge, 33 kB)

Human activities also increase the amount of particles in the atmosphere. Some of these have a cooling effect that counteracts the greenhouse effect. For example, when sulfur dioxide (SO₂) is emitted into the air, it is transformed to particles that reflect sunlight and thus reduce solar radiation. But contrary to most greenhouse gases that stay in the atmosphere for many, sometimes thousands, of years, particles remain airborne only for a few days. Thus, the cooling effect of particles is short-lived and limited to certain areas.

Human influence comes in addition to all the natural factors that have always affected the Earth’s climate. The climate is affected by conditions outside the Earth’s atmosphere (for example, the intensity of the sun and small variations in the shape of the Earth’s orbit around the sun). The climate is also affected by natural processes in the Earth’s atmosphere, the oceans, vegetation, and snow and ice cover. These factors combine with human influence to shape the Earth’s climate.

A warmer climate will change the planet in many different ways. As the Earth has become warmer, the sea level has risen a few centimeters. An even warmer atmosphere will lead to even greater sea-level rise. This is mainly because higher temperatures in the oceans expand the water, which means it takes up more room. This increased need for space pushes the surface of the ocean upward. In addition, higher temperatures in the atmosphere cause many glaciers to melt. Parts of the huge ice caps covering Antarctica and Greenland could also start melting. The runoff flows into the ocean, adding to the rising sea level. Ice floating in the ocean in the artic regions near the North Pole will also melt, but since the ice is already floating in the ocean, this will not have any effect on the sea level.

Higher air temperatures will also cause more water to evaporate. This can increase drought in some places, and increase rainfall in others – if the water vapor creates rain clouds. A warmer atmosphere can also change prevailing wind directions and ocean currents. The warming will not be evenly distributed throughout the planet; some places may become much warmer, while other place may even become colder (read more about some observed changes in physical climate here). In unit 2, you can read about which climate changes we can expect over the next hundred years, and what consequences the climate changes may have for people, animals, and plants. In unit 3, you can read about what we can do to slow down climate change.
 

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Author: Camilla Schreiner - CICERO (Center for International Climate and Environmental Research - Oslo) - Norway. Scientific reviewers: Andreas Tjernshaugen - CICERO (Center for International Climate and Environmental Research - Oslo) - Norway - 2004-01-20 and Knut Alfsen - Statistics Norway - Norway - 2003-09-12. Educational reviewer: Nina Arnesen - Marienlyst school in Oslo - Norway - 2004-03-10. Last update: 2004-03-27.