20th century global warming may have been due to decreasing aroma from trees
The finding below are particularly interesting in the aftermath of Munshi's demonstration that the buildup of CO2 in the atmosphere is NOT of anthropogenic origin. So therefore anthropogenic CO2 emissions CANNOT explain the slight degree of global warming seen in C20. So what does explain it? The best explanation so far is Svensmark's theory that variations cosmic rays reaching earth affect cloud formation and that earth was substantially shielded from such rays by enhanced solar activity in C20.
The finding below builds on that and looks at another factor that could affect cloud formation. It finds that aromatic output from trees can encourage clouds. So the extensive deforestation that occurred during C20 could have reduced clouds and caused some warming. Now that deforestation has on a global scale run most of its course, therefore, we should have a C21 temperature stasis -- which is exactly what we do have. We may have seen the complete end of a warming period
What I say above is just an attempt to put in layman's terms what Lubos Motl says below. My apologies to Lubos if he thinks he had already done that
CLOUD, the experiment that measures the birth of clouds at CERN, has released new papers:
CLOUD has done lots of measurements of the processes that are needed to create clouds which, as many kids have noticed, usually cool down the weather.
The experiment has been taking place at CERN because the cosmic rays (emulated by the CERN's sources of beams) are important for the creation of the cloud (condensation) nuclei. Even in the new papers, cosmic rays are found to increase the nucleation rate by 1-2 orders of magnitude.
Recall that the Sun's activity may influence the cosmic ray flux, and therefore its variations may be responsible for "climate change". Svensmark's theory generally argues that a stronger solar activity means a more perfect shielding of the cosmic rays, therefore less cloudiness, and therefore warmer weather.
However, the focus of the new papers is on something else than the cosmic rays: the molecules that should be present for the cloud nuclei to emerge and surpass the critical mass.
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".