One of my favorite aspects of writing articles for this website is I am constantly learning new things about the Earth and the climate. There are so many aspects to understand and there is always more to learn. Being open to new information is the true nature of the scientific method. It is also one that has broken down in the global warming debate because so few people are open to contrary findings.
The unexpected new piece of the puzzle that I stumbled across deals with the temperature of the land in different seasons. It focuses on the United States, but the behaviors described would apply to all regions of the Earth to some degree. Specifically it deals with how the temperature of the Earth itself varies at different depth over the course of the seasons. It is also a perfect example of how time lags show up in the climate cycles.
This chart provides the average temperature of the ground at different depths over the course of a year. The surface of the Earth responds identically to the atmosphere above it. The peak temperature takes place 46 days after the summer solstice. This would indicate that the surface is receiving more energy than it is losing up until August 6th. After that the surface of the Earth starts to lose energy at a faster rate than it is gaining energy.
The surface loses energy in two directions. It loses it to the atmosphere above it and to the cooler ground beneath it. The rate that it loses energy to the atmosphere is greater than the the ground beneath because there is only thermal conduction available to the ground beneath it.
The 2 foot (0.61 m) depth is only influenced by the temperature of the ground above it and the temperature of the ground below. It gains energy from whichever is warming and loses to whichever is cooler. As the surface stops warming and starts to cool, the amount of energy that the 2nd layer gets from the surface decreases. It reaches the maximum temperature 14 days after the surface does. At that point it is losing energy to the cooler ground beneath it at the same rate it is gaining from the still warmer ground above it.
The 5ft (1.5m) depth lags even further behind the 2ft mark and reaches the maximum temperature more than a month after the surface reached it’s maximum temperature. Each lower layer has less of a temperature change though and as a result the 5ft mark has about half the temperature change that the surface has. At that point it is gaining and losing energy at the same rate, but after that it will lose more than it gains so it will start cooling.
The last depth on the graph is 12 ft (3.6m). This depth has half the temperature change that the 5ft depth does and the maximum temperature change takes place almost two months after the surface does. It takes that long for the the soil to respond to the changes in the surface temperature.
This is also why spring temperatures are colder than fall temperatures. In the spring the Earth is still cold from the winter and the ground is warming up as a result. In the fall the Earth is still warm and has energy to transfer to the atmosphere. So the same energy from the Sun results in warmer fall temperatures than spring temperatures.
Due to the lag the deeper into the Earth the less influence it has on the surface temperatures. Deeper than 5ft the response time is more than a month. The full depth of the surface influence is about 25 ft (7.6m), but the time lag could be longer than a year at those depths.
Here is a chart I created long ago to describe the impact of insolation on the annual temperature cycle. See how the insolation leads the atmospheric temperature. This atmosphere and the surface stay in equilibrium, but the depths lag the surface. This is a clear indication that solar insolation CAUSES the other changes.
It could also be noted that the way that the Earth orbits the sun is what causes the differences in the insolation at different times of the year. So the true cause of the seasons reverts back to what you learned in school. The tilt of the Earth causes the seasons. Of course that might imply that the orbit is also involved in the glacial/interglacial cycle and not CO2 levels.