For those that believe that CO2 is the driving force of climate. I would like to show what happened to the Earth 120,000 years ago. The Earth was very warm during the Eemian Interglacial. So warm that many of the dreaded dangers of global warming were actively present. The sea level was higher than today by at least 4-6 m, but possibly even more. They were high enough that Scandinavia was an island separate from the rest of Europe. The reason it is called the Eemian is because fossils of Mediterranean type sea life that were found in boreholes from the Eem River in the Netherlands. Of course that meant that much of the Netherlands were underwater during the Eemian Interglacial.
During the Eemian period forests grew where now there is tundra and glaciers. Temperate trees grew much farther north than they do today. By almost all accounts the Northern Hemisphere (NH) was warmer then than it is today. Even in the United States there is evidence that it was warmer. Instead of prairies there were forests in Texas.
So most of the NH was more pleasant than it is today. The sea levels were higher and things were good. So when people talk about the dangers of global warming, consider the Eemian. Much of what they describe is what the Earth was like then. Skiing was limited in the NH, but it was a pretty nice place to live. Even the Arctic was warmer. It is very likely that it was ice free many summers over the course of thousands of years.
It didn’t last though. The Earth was warm, CO2 levels were up in the 270-280 ppm range. They had been stable for more than 10,000 years in that range. According to the CO2 forcing model, there was plenty of the Greenhouse Effect going on to keep the Earth warm, but it didn’t happen. Instead, the summer energy in the NH started to drop rapidly 120,000 years ago. The Earth was warm and had been warm for about 10,000 years. All the GHG’s were up after being released by the oceans, but once the NH summer energy from the sun dropped, so did the temperatures.
In the period from 120,000 – 114,000 years ago the amount of summer energy to the NH dropped about 50 W/m2. In that same period of time the temperatures from the ice cores dropped 4-5 °C. By 111,000 years ago the temperature anomaly was -6 °C in the ice cores. For most of that drop in temperature, CO2 levels stayed elevated. It was only after the oceans cooled enough about 112,000 years ago that the oceans started to absorb enough CO2 to cause the global levels of CO2 to drop. So there was an 8,000 year period where CO2 levels were high, but the global temperatures dropped.
Here is the sequence of changes that ended the warm Eemian Interglacial which led to the last glacial (ice age).
It is one thing to argue that CO2 forcing at the beginning of an interglacial helps cause the warming after the the Mikankovitch cycle triggered the warming. It is entirely different to argue that the Milankovitch cycle didn’t cause the glacial to initiate from a warm period with elevated CO2. If the CO2 forcing is so strong, then why is it so easily overwhelmed by a drop in local energy in the NH?
The drop in energy was local to the NH. While the NH was losing energy, the SH was gaining energy. It didn’t matter though. The Earth cooled anyway. With almost no change to overall global energy and large amounts of CO2 in the atmosphere, the Earth still cooled. The reason why is very simple. The energy that the NH was receiving from the sun during the summer dropped. That prevented the snow and ice in the NH from melting as much during the summer and glaciers started to expand. The sea levels started dropping and the oceans cooled. During all of that, CO2 levels remained high.
This simple example shows the limitations of CO2 as a greenhouse gas. 8,000 years of elevated CO2 levels were not able to prevent a local change in NH energy from triggering a glacial period that lasted nearly 100,000 years. That the behavior at the beginning and the end of interglacial periods is a strong indicator of what matters most to the Earth’s climate. That driving force is the amount of energy that the Northern Hemisphere gets from the sun during the summer months.
Vostok Ice Core: Temperature and CO2.
Northern Hemisphere: Summer Insolation