Over the past several million years the geography of the Northern Hemisphere (NH) developed into the driving force in the Earth’s climate. This can be seen in the annual temperature variation (Jones 99).
Notice that the global temperature tracks with the seasons of the NH. The reason for this is simple. The NH has twice as much land as the Southern Hemisphere (SH). Oceans do not change temperatures in the same manner that land does. So every year when there is almost a 4 °C swing in the temperature of the Earth, it follows the seasons of the NH. This is especially interesting because the Earth gets the most energy from the sun in January right now. That is because of the orbit of the Earth. The perihelion is when the Earth is closest to the sun and that currently takes place in January.
The small annual variation in the energy from the sun makes little difference right now to the Earth’s climate. That is because the SH is tilting to the sun at that time of year instead of the NH. This is a good example of local energy changes mattering more than the total global energy changes. The NH receives less energy in the winter than it does in the summer. Again, that is because of the Earth’s tilt.
So even though the Earth is closest to the sun while it is receiving the most energy from the sun, the Earth is the coldest at that time of year because the NH is away from the sun. Looking at local amounts of energy can be critical to understanding how climate changes over time. Here is the difference in energy that a mid-latitude location gets on a daily basis for two different times of year.
The tilt causes the magnitude and duration of energy to vary in the NH by the time of year. It is this variation in the daily energy that causes the seasons. It is very important to understand that even though the Earth is getting more total energy from the sun on Dec 21 than it is on June 21, the Earth is colder on Dec 21 because the NH is tilted away from the sun. Land warms and cools more than the oceans do. Because the NH has twice as much land as the SH it sees more warming and cooling based on the amount of energy it gets from the sun.
Why does this matter so much? In the debate about global warming, people often try to focus on the total energy while completely ignoring the local energy. When the Earth warmed from the last glacial (ice age) to the current climate, it did so because of a change in the local energy that the NH gets in the summer. Even though the total energy the Earth received from the sun had very little change, the amount of energy the NH recieved during the summer months changed by a large amount.
Right now the NH has an annual temperature range of 8-21 °C. The SH has an annual temperature range of 11-17 °C. In the coldest periods of the last glacial, the average temperature of the Earth was about 5 °C colder than it is today. That does not mean that every place on Earth was 5 °C colder. What it means is that the NH was much colder than it is today. The tropics were still the tropics during the last glacial. The difference in the global temperature was caused by a large part to a much colder NH. When the NH is much colder it causes the average temperature of the Earth to drop.
Estimate from: Northern Hemisphere Ice-Sheet Influences on
Global Climate Change
Peter U. Clark, Richard B. Alley, David Pollard
The other factor is the sea levels. They were more than 100 meters lower than they are today. That caused other changes to the Earth’s climate. That water that wasn’t in the ocean was locked up in the ice sheets that covered much of North America and Europe. The oceans near both poles were colder while the sea levels were lower. One of the changes caused by the colder oceans was the reduction in atmospheric CO2 as the colder water absorbed more CO2 from the atmosphere. That is a partially why CO2 levels change with the temperature of the oceans. Colder water absorbs CO2 from the atmosphere and warmer water CO2 releases into the atmosphere.
Many methods in determining the climate sensitivity depend on using the change in overall energy and the overall change in temperature to determine the climate sensitivity. Since there is a small overall change in energy to the Earth, but the average temperature of the Earth changes, the estimated climate sensitivity is higher than it actually is. The strong solar forcing in the NH causes a strong response in the NH that then causes a change to the rest of the Earth’s climate. In the case of climate sensitivity there is a significant difference between what is happening on a global scale and directly to the Northern Hemisphere. Just like the annual seasons are caused by local energy levels to each hemisphere, the current glacial cycles are also determined by local energy levels. This is why the climate sensitivity is overstated when the glacial/interglacial transitions are analyzed.