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.