Why is Climate Changing Today?

Why is Climate Changing Today?

How Models Simulate 20th Century Climate With (top) and Without (bottom) the Imp

The red line shows model simulations of climate taking human impacts into account. The blue line shows how model simulations look when humans are left out. The black line shows temperature actually changed over the 20th Century.
Credit: Commonwealth of Australia

Climate is changing. During the 20th Century Earth’s average temperature rose 0.6° Celsius (1.1°F). While climate has changed throughout Earth’s history, this is the first time that humans are the main cause.


There are several causes of recent climate change, but  all have something in common – people. Recent warming is a side effect of many things that people do – like driving cars, heating homes, running factories, and transforming forests into towns. Our landfills and farm animals are to blame for recent climate change too.

Burning fossil fuels like coal, oil, and gas releases greenhouses gases such as carbon dioxide into the atmosphere. These gases trap heat in the atmosphere that was on its way out to space, causing Earth's greenhouse effect to grow more intense, warming the climate.

Changes in the way land is used also have an impact on climate. For example, when trees and other plants are cut down to make way for new buildings or parking lots, climate warms because less carbon dioxide is taken out of the atmosphere without the plants. Plus, the asphalt and buildings tend to hold onto more heat than forests and fields, so when warmed by the Sun, they stay warm for some time.

Agriculture also adds greenhouse gases to the atmosphere. Farm animals release methane, a particularly strong greenhouse gas, as they digest food. Factory-made fertilizers release nitrous oxide, another strong greenhouse gas into the atmosphere.

Just Add Humans.

What would climate be like on Earth today if humans were not affecting the system? We can answer that question without making everyone move to Mars. With models of the Earth system run on supercomputers, scientists have simulated what our world’s temperature would be like over the past century both without human influences and with human influences. Because computers don’t know what actually happened over that time, the model takes into account only the factors that might effect climate and not any data about what actually happened.

The result:

  • Without humans, climate is affected mainly by changes in the Sun and volcanic eruptions. Under those conditions, the models found that Earth would have cooled slightly over the past century, which doesn’t match what actually happened.
  • With humans, climate is affected by the activities of humans as well as the Sun and volcanoes. Once the models included the activities of humans that caused an increase in the amount of greenhouse gases to the atmosphere, they pretty accurately predicted what happened to Earth’s temperature over the past century.  

What does the future hold?

There will be more global warming is in our future according to the results of computer models summarized by the Intergovernmental Panel on Climate Change – the IPCC - a group of hundreds of experts from more than 100 countries organized by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP).

The IPCC predicts we will see about 0.2° Celsius warming for the next two decades. If we continue to emit as many, or more, greenhouse gases, this will cause far more warming during the 21st Century than we saw in the 20th Century. During the 21st Century, various computer models predict that Earth’s average temperature will rise between 1.8° and 4.0° Celsius (3.2° and 7.2° F) depending largely on how humans change the ways they live on the planet.

Although there is a certain amount of global warming that we are going to have because of our activities during the past century, there are many ways to help slow the rate of warming. In fact, the range in possible warming over this century is mainly because we are not sure how humans will affect the planet. If we add less greenhouse gases to the atmosphere, we’ll cause less warming.