Penn StateNASA

Module 3: Earth's Climate System

Earth 103 is all about the Earth’s climate. Thus it is essential that you have a solid understanding of how the climate system works. This module is all about the climate system. It is by far the most technical module in the course, and our philosophy is to lay out the science in a comprehensive way, equations and all, so that you can see that Earth's climate is in part fairly simple, governed by physical relationships that describe how heat from the Sun is exchanged on the surface of the Earth and in its atmosphere. Then there are some very complex aspects of the Earth's climate that we will not devote much time to.

Here is an example of why this module is important. Those of us on the East Coast and Midwest of the US and our neighbors in Canada, 187 million people in all, lived through an extremely cold week at the beginning of 2014. Air temperatures without the windchill factored in, reached -35oC in eastern Montana, South Dakota and Minnesota. This cold was a result of the southward expansion of the polar vortex, a whirlwind of cold dense air that is normally restricted to the area around the poles. Understanding the polar vortex, and how it became unstable and swept across the Midwest and eastern parts of Canada and US, is key to interpreting the significance of the extreme cold in early 2014. Without this understanding, you might think that the expansion of cold air is a sign of cooling climate. However, it is likely that the opposite is the case, the recent cold snap is actually a result of warming. This is how it works. As you will learn in this module, the northern high latitudes are warming more rapidly than the rest of the globe as a result of melting sea ice. You will also learn that such warming leads to diminished wind velocities, including the polar vortex. As the vortex weakens, it becomes less stable and begins to wobble and stray from the region around the North Pole. It turns out that the recent cold snap was just one of these wobble events, and the projections are for polar vortices to become more common over North America in the future, just as other extreme events like extratropical hurricanes such as Sandy, heat waves and droughts become more frequent.

Maps show 500-millibar geopotential height on January 5, 2015 and mid November 2013.
Maps show the 500-millibar geopotential height (the altitude where the air pressure is 500 millibars) on January 5, 2014 (left), and in mid-November 2013 (right). The cold air of the polar vortex is purple
Credit: Maps by NOAA, based on NCEP Reanalysis data from NOAA ESRL Physical Sciences Division.

Now right off the bat, we need to make it clear that the "simple" relationships are often portrayed in the module in terms of equations. You do not need to be a Math major to understand these equations, nor do we want you to memorize them. The point of showing the equations is not to cause great anxiety, but to provide an understanding of the relationship between two variables. For example you should be looking to distinguish relationships that are linear (such as a=b*x [where * is multiplied by]) from those that are quadratic (such as a=bx2). This is the level at which we expect you to understand equations. One last word, the lab for this module is designed to strengthen the fundamentals you learn in the reading. By experimenting with climate in the lab, you should come away with a really solid understanding of the climate system.