“Hate. It has caused a lot of problems in this world but has not solved one yet.”
– Maya Angelou
Global Warming Minute – Why is the IPCC “95% Certain” that Climate Change is Manmade?
Why Climate Changes?
The Earth’s Thermostat – Keeping the Earth Warm
Throughout its history, the earth has received heat from the sun. The earth reflects or releases a certain fraction of the sun’s energy right back into space.
This results in a thermal balance that has enabled the earth to remain a hospitable planet for most of its inhabitants most of the time. What is truly remarkable is how incredibly steady the average temperature of the earth has been for so long. A change in the average global air temperature of a few degrees is a big deal, and a swing of around 5°C (9°F)-up or down-puts us at the extremes of all-time historical records. We recognize that the earth is currently in a warming mode.
There are many natural factors that influence the earth’s temperature on a time scale that ranges from days to hundreds of thousands of years. We will discuss these natural cycles and the changes that have been set into motion since humans found a way to derive energy by burning fossil fuels. This chapter is about energy-how much the sun sends to us, how much the earth gets to keep, and how and why that balance is changing.
The Earth’s Orbit
The earth goes through cycles of day and night and an annual progression of the seasons. The earth’s axis of rotation at times points toward and then away from the sun. Throughout all this, the temperature of the earth averages out to establish a very stable historical baseline. So, when we measure a statistically significant change that approaches just 1°C, it gets scientists’ attention.
If the earth’s orbit were a perfect circle, its distance from the sun would not change. However, as is the case with all the planets in the solar system, the distance from the sun varies as the earth dutifully follows the precise elliptical path determined by the laws of Newton and Kepler. This means that at some points during that orbit. The earth is slightly closer to the sun than at other times.
Because of the shape of the earth’s path around the Sun, the earth is closer to the sun in the winter months. The earth actually receives more solar energy in the winter than in the summer. Figure 4-2 shows how the solar intensity varies throughout the year.* This may seem surprising at fi rst because it is it is colder in the northern hemisphere during the winter months (December through February). However, the reason for the lower winter temperature is that in winter the sun strikes the northern hemisphere at a greater angle than during other seasons. The incoming sunlight is spread over a greater area, causing the northern hemisphere winter to be cooler despite the fact that the sun is closer to the earth. Since the earth spins like a barbequed chicken on a rotisserie as it goes through its daily day-and-night temperature swings, these variations do not affect the average temperature. The orbital variations repeat year to year. When all is said and done, the average annual temperature of the earth for many centuries has held within a tight range of only a few degrees.
* Solar intensity varies inversely with the square of the distance between the earth and the sun. At its closest approach, the earth is about 147 million km from the sun and at furthest distance it is 152 million km away. Taking the square of the ratio of these distances, we find that solar intensity varies by about 6.9 percent between the least intense and most intense orbital positions.
