Do Climates Change? Is Climate Changing Now? Causes of Change
Is Current Change a Problem? Future Climates What Can Be Done?
Earth’s climate system is very complex, and each one of its elements is changing constantly. Most of these changes are not immediately noticeable on a global scale, but over time patterns emerge, and we say that the climate is changing. You can learn more about these patterns in Do Climates Change and Is Climate Changing Now?

Causes of climate change can be either internal, a result of changes in the five elements of the climate system (remember the Introduction to this section?), or external, the result of elements outside the climate system, such as the sun or plate tectonics. When climate scientists talk about these changes, they refer to these various causal elements as “forcing” a response in the climate, such as temperature change. So forcing (link to glossary) means the elements that cause change, and the response is the actual change that occurs in the climate.

Many causes of climate change are natural, have been happening for millions of years, and have little or nothing to do with the actions of humans, while other forcing factors are directly affected by us. The difference between purely natural causes of climate change and anthropogenic effects on climate change can be seen in this figure from the IPCC’s Fourth Assessment on Climate Change:

  In this collection of regional and global graphs, the black lines represent the observed average temperatures for the corresponding areas (i.e. what was measured on thermometers). The blue and red shaded areas are the results of simulations; the red areas are the result of 58 simulations run by 14 climate models, and are predictions based on contributions of natural and anthropogenic forcing factors. As you can see, they are pretty accurate; the actual temperatures fit approximately in the middle of the shaded regions. The blue regions are predictions of what temperatures would have been like if natural causes only had been acting. According to these graphs, human activity has already caused temperatures to increase compared to what we would have expected if only natural forcing factors had been affecting temperatures.

Explore the following pages for more information on specific causes of climate change. Natural causes are things that humans have no control over whatsoever, such as the tilt of the Earth’s axis. Anthropogenic forcing is the part of climate change that we contribute actively to. The third section deals with feedbacks; these occur when a climate forcing occurs, and the climate begins to change, and this change contributes to itself and causes further change, in a loop. Humans influence feedbacks indirectly, because our emissions cause changes in the atmosphere that in turn influence things like ice and snow cover, which further affect how the climate changes.
  Volcanoes & Hot Springs - Earth's Axis - Plate Tectonics - Sun - Asteroid Impacts
  Emissions - Forest Clearing
  Ice Cover - Ocean Currents
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