CO2 was higher for the first 4 billion years of Earth’s 4.5 billion year history than it has been in the past 500 million years.
We have been in a cooling period since the Eocene Thermal Maximum 50 million years ago.
The Earth was an average 10 to 20 degrees C. warmer then.
The Arctic and Antarctica were ice-free and covered in forest.
Glaciers began to form in Antarctica 30 million years ago, and in the northern hemisphere 3 million years ago.
Today's climate is one of the coldest climates in the Earth’s 4.5 billion year history.
Antarctic ice cores show that for the past 800,000 years, there have been periods of major glaciation followed by interglacial periods in 100,000 year-cycles.
These "Milankovitch" cycles are linked to the eccentricity of the Earth’s orbit and its axial tilt.
It's likely these cycles are related to solar intensity and the seasonal distribution of solar heat on the Earth’s surface.
CO2 lags temperature by an average of 800 years during the most recent 400,000-year period, indicating that natural global warming is the cause of more CO2 in the air (out-gassing of CO2 from oceans as they gradually warm from natural causes).
20,000 years ago, during the peak of the last major glaciation, 2 miles of ice was on top of what is now Montreal.
Chicago was covered by a half mile of ice.
With all that frozen water on the land, sea level was 400 feet lower than today.
7,000 years ago all the low-altitude, mid-latitude glaciers had melted.
One third of all manmade CO2 emissions were released from 1997 through 2015 -- a period the UK Met Office says there had been no statistically significant global warming.