Temperature of the Earth

To begin the global warming debate it seems reasonable to start with what we know about the Earthly temperature. The most important fact is that the global temperature is continuously changing and has been for over 4.5 billion years. The new Earth was VERY HOT (about 80 deg. C or 176 deg. F) and it stayed that way for ~ 0.7 billion years due to the continuous, violent meteor bombardment that heated the Earth. When the bombardment slowed about 3.8 billion years ago the Earth cooled sufficiently for cellular life to be born.

However, unlike a roast beef that you take from the oven and let cool on the counter, the Earth experienced repeated reheatings when the oceans would boil and repeated freezings when the entire planet was covered with ice. The “Snowball Earth” episodes happened about 2.3 billion years ago and again around 700, 400 and 300 million years ago. In between those severe glaciations the Earth was warm and sometimes quite hot. Around 100 million years ago the whole Earth was a tropical paradise and the dinosours roamed.

Over the last 2 million years the Earth has settled into a temperature oscillation between Ice Ages and warm Interglacial periods on roughly a hundred-thousand-year time scale. These cycles are due to the Earth's orbit around the Sun that changes from nearly circular (warmer) to a pronounced elipse (colder) with that period. The Ice Ages tend to be long (~90-100 thousand years) while the Interglacials are shorter (~10-20 thousand years). The energy received by the Earth can vary by as much as 30% over this entire cycle. Presently the Earth orbit is close to circular and the energy received varies by only 6% from January to June.

The temperature swings are extreme and the change can occur over a relatively short time. For example, the temperature increase during the transition from an Ice Age to a warm Interglacial period ranges from 5 to 15 degrees C, and half of the increase can occur in only 100 years. The last few Interglacials occurred about 420,000 years ago, then again 330,000 and 240,000 and 120,000 (the “Eemian”) and the one we are living in (the “Holocene”) starting around 11,500 years ago. These cycles are just what we expect from the Earth's orbital motion.

In addition to the 100 thousand year temperature oscillations, there are also more rapid, less severe, oscillations caused by variation of the tilt of the Earth's axis on a 41,000 year cycle and the Earth's precession about its axis on a 23,000 year cycle. Each dynamic factor influences how much sunlight strikes the Earth and changes the global temperature. Thus the last Ice Age that began about 110,000 years ago was interrupted by a warming about 55,000 years ago, then reached it’s coldest point around 20,000 years ago. The sea levels at that time were 400 feet lower than today.

The last eight Interglacial periods have each lasted for about half of the 23,000 year precessional cycle, or 11,500 years. On that basis our current warm period is about due to end. I’ll look at the implications of the looming Ice Age in a future post.

So for about 11,500 years we have been in a warming period. The “Climate Optimum” occurred between 9,000 and 5,000 years ago when the Earth was both warmer and wetter than it is today. Biodiversity bloomed, life was good.

But the story is still more complicated and more interesting. The last Ice Age actually ended about 14,000 years ago, when global temperatures rose to nearly the present level (~ 15 C). Then about 1500 years later the Earth was plunged back into a mini Ice Age that lasted for 1000 years before the present Holocene warm period began (with half the warming happening in tens of years). About 750 years later it got cold again and about 750 years later it was warm again, and on and on.

These temperature oscillations of 1500 (+/- 500) years are called Dansgaard-Oeschger cycles after their discoverers. They have been happening like clockwork over at least a million years, through Ice Ages and Interglacials. Their mean temperature change has been 4 C, peak to trough. The recent D-O periods have included the following:

Pre-Christ cooling from 600 to 200 BC
Roman Warming from 200 BC to 600 AD
Dark Ages cold period from 600 to 900 AD
Medieval Warming from 900 to 1300 AD
Little Ice Age from 1300 to 1850 AD
Modern Warming from 1850 to present

Based on the record the Modern Warming temperature should increase another 1-2 C, and the next little icy period should begin in 100-300 years.

In the next post I’ll look more closely at the causes of the temperature changes (the forcings) including the greenhouse effect.

Reference

W. Dansgaard et al, “North Atlantis Climatic Oscillations Revealed by Deep Greenland Ice Cores,” in Climate Processes and Climate Sensitivity (1984).