Yesterday I wrote about trees being able to seed clouds. Lubos of the Reference Frame takes it further.
It’s been generally thought that the sulfuric acid was almost necessary. Chimneys (or volcano eruptions etc.) should increase cloudiness. However, there have been inconclusive hints in some papers that some organic molecules are enough. You may have worried: How could have the clouds existed in the past, before the chimneys were built? 😉
Jasper Kirkby and collaborators have found out that the molecules known as “aroma of the trees” may indeed do the same job and that is decisive in the pristine environments without chimneys.
More precisely, the molecules that can do the job are the “highly oxygenated molecules” (HOMs) which are produced by ozonolysis of α-pinene.
The lesson for “global warming” seems clear: deforestation may decrease the amount of aroma from the trees, and therefore the amount of clouds, and it may therefore lead to global warming.
This may be the explanation of the changes in the 20th century and because the deforestation is over, so may be “global warming”.
So … what other periods of global warming took place?
“The Roman Warm Period or the Roman climatic optimum has been proposed as a period of unusually warm weather in Europe and the North Atlantic that ran from approximately 250 BC to 400 AD”
What is Rome famous for (other than killing and conquering and so on)? Roman baths. What did the heat the baths with? Firewood.
“The baths were BIG; the Baths of Caracalla, completed in 235, could handle 8,000 visitors a day. Its 50 furnaces burned ten tons of wood every day to keep the water warm.
Deforestation was huge in Roman times.
Wood was a primary source of heating and used extensively in industry. Wood fuel constituted about 90 percent of the consumption overall,[citation needed] and was a major factor in Roman deforestation. Wood was essential fuel in industries like mining, smelting, and the making of ceramics.[4] Wood and charcoal were the primary ancient fuels in public facilities, households, public baths and industries that produced light and heat.
Forest areas around mining centers were deforested first, consuming all natural resources around the area of work. Once all the natural resources around the area of production were consumed, wood was then shipped and carried in to supply the furnaces and smelters for the mining centers. Eventually, these centers would shut down and move to areas within Roman territory to repeat the same cycle of deforestation, supplying an ever-growing population and consumption demand.
Deforestation and the fall of Rome :
In the ancient world, fossil fuels were unusual enough to be a curiosity, and certainly did not provide any major heating source. Almost all heating was done by wood and wood productsand while it may not seem like such a major factor it becomes a different story if you think about the Roman baths. The public baths were kept constantly at a minimum of 130 degrees Fahrenheit (54 degrees Celsius), and even a very small bath required 228,000lb (103,421kg) of wood per year. The Emperors recognised the importance of the baths in keeping the populace happy, and made keeping them running a primary goal. A whole guild, equipped with 60 ships, was created specifically for the purpose of obtaining bath-heating wood. Large palaces and villas also often had personal central heating systems; one such system has been evaluated and determined to require 2,506,000lb (1,136,722kg) of wood per year in order to properly heat the villa.
sunshine hours 
