Source
In my previous post, I discussed the "hundred-thousand-year" cycle where the orbit of Earth around the sun goes from being circular (where we are now) to more elliptical.

When Earth’s orbit is at its most elliptic, about 23 percent more incoming solar radiation reaches Earth at our planet’s closest approach to the Sun each year than does at its farthest departure from the Sun. Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing, in a cycle that spans about 100,000 years.
Source

I want you to remember the 23 percent. While I won't be addressing the fact that a relatively small change in the eccentricity of our orbit (0.0034 to 0.058) means the difference between being covered in ice and not covered in ice in this article I will refer to it in later posts.

Obliquity

Another cycle that we experience is the change in the "Obliquity" of the Earth. This cycle has a roughly 40,000 year period.

It was last at its maximum tilt about 10,700 years ago and will reach its minimum tilt about 9,800 years from now.

It is good to know that we are talking about a range between 22.1 and 24.5 degrees and we are currently at 23.4 degrees - close to the halfway point. What happened roughly 10,700 years ago? We left the last glaciation. There was sufficient energy pointed at the poles to cause the ice to melt. What can we anticipate in 9800 years? The next glaciation? It is also important to consider what part of the year the north is pointed toward the sun. Although our orbit is almost circular, there is enough difference to give us seasons. The countries in the north experience winter in the latter and early months of the year. The countries in the south experience winter in the middle of the year.

Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere. This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations.

The orientation makes our winters warmer (melting more Arctic ice) and Australia's summers hotter and drier (causing more bush fires). Due to the speed of Earth's orbit, this condition will change and will reverse in about 13,000 years and then about 6500 years from now Spring/Winter will the times receiving more and less solar energy due to the tilt.

Have you ever wondered why they use Antarctic Ice Core data (Vostok) to represent the past levels of temperature and CO2 levels? The Greenland samples tell a different story:

The Vostok data gives the idea the Earth's temperature has been rising over the last few thousand years while the Greenland data points out that the temperatures have been falling.

For the first time, we can state the Roman period was the warmest period of time of the last 2,000 years, and these conditions lasted for 500 years,’ said Professor Isabel Cacho at the Department of Earth and Ocean Dynamics, University of Barcelona.
Roman Warm Period Was 2°C Warmer Than Today, New Study Shows

It is easy to say that temperatures are getting warmer especially when you factor values from the south. However, they are not comparing it to warm periods. When the Vikings came to this area they called it Vinland because of the number of grapevines. This is what it looks like today:

Part 3