The Climate Change Skeptic’s Argument: Natural Solar Cycles or Human Activity?
If there has been no significant net increase in solar irradiance, how can Earth be warming? The answer is the Earth’s energy budget. As solar radiation from the Sun reaches Earth's surface, some of it is reflected back into space and some of it is absorbed. The absorbed energy warms Earth's surface, which in turn radiates the energy back toward space. Molecules in the atmosphere, known as greenhouse gases, absorb some of this outgoing energy and radiate a portion of it back to Earth's surface. Through these two actions, they slow the escape of heat and keep Earth warmer than it would be otherwise. Read “Earth’s Energy Budget” to learn more about what happens to incoming solar radiation when it arrives at the top of the atmosphere.
While there is no question that data clearly indicate a warming trend for Earth’s climate, understanding the relative contributions of solar and anthropogenic factors to the observed rise is a complicated business. How do greenhouse gases work? How do their properties cause the greenhouse effect? Watch the video “The Warming Trend and the Greenhouse Effect” from PBS LearningMedia™, which frames the question of the relative importance of both natural and anthropogenic factors in today’s changing climate and in the build up of greenhouse gases. Read the Background Essay to understand more fully the mechanism of greenhouse gases and how they amplify the greenhouse effect.
Many science textbooks explain the Greenhouse Effect by using the analogy of heat building up inside a car with the windows closed. Although this analogy makes the concept relatable for students, it is important to understand that there is not a uniform layer of greenhouse gases, like the glass in car windows, keeping all the heat in our atmosphere. Greenhouse gases are diffused throughout the atmosphere at all levels in the troposphere, absorbing the solar irradiance coming off the Earth’s surface.
As permanent as geographic features such as deserts may seem, Earth's climate and landscape are dynamic and constantly changing. Geologic evidence has revealed that parts of the Sahara were once lush and filled with animal species more often associated with the savanna far to the south. The video segment, “Natural Climate Change in Djibouti, Africa,” from PBS LearningMedia and adapted from NOVA, explains how Earth's position relative to the Sun might be responsible for the dramatic shift in the climate of what is now the Saharan nation of Djibouti several thousand years ago.
Then, watch NASA’s Striking a Solar Balance to learn how these variations in energy inputs and outputs can affect our planet’s energy budget.
What can upset the balance between incoming solar and outgoing thermal energy on Earth?
Next, play the interactive Solar Activity to see the changes in numbers of sunspots and UV radiation over the past decade. When sunspots appear, what do you notice about the solar activity around those areas? Note: Some of this data will be presented in graphs in the next section.
Finally, watch the “Piecing Together the Temperature Puzzle” video, which provides an overview of global warming. Included is information of the relative role of solar variance as a contributor to contemporary global warming.
Considering the resources you have investigated in this section, what does the data tell you about the relative influence of solar variability on climate change?
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