As a geologist I am very comfortable with the multiple working hypothesis – I would like someone to start examining the other ideas out there. The Danes have been on the case for a long while, studying the sun. Who would have thought the sun would be involved in warming? The first paper to read is Friis-Christensen and Lassen (Science; 1991). If you can find the entire issue in the reference library, you will see the editors comment referred to this paper as hitting the ball into the anthropogenic court. The causation is under scientific review, however, and while the radiation from the sun varies only in the fourth decimal place, the magnetism is awesome. However, we all realize correlation is not causation. The correlation broke down when Pinatubo erupted in 1991; my tomatoes did not ripen that summer either. Is this the exception that proves the rule?
The important correlation between warming and cooling is the sunspot peak frequency, not the actual number of spots. Sunspot peak frequency proxies for the rise and fall of the sun’s magnetic field. Cosmic radiation is currently at its highest ever measured because the sun and earth’s magnetic shields are down; climate is changing. The climate celebrities, however, are linking climate and the carbon economy. Maybe not evil; just wrong.
The third ranking gas is CO2 (0.0383%), and it does not correlate well with global warming or cooling either; in fact, CO2 in the atmosphere trails warming which is clear natural evidence for its well-studied inverse solubility in water: CO2 dissolves rapidly in cold water and bubbles rapidly out of warm water. CO2 has been rising and Earth and her oceans have been warming. However, the correlation trails.
Svensmark of the Danish National Space Center has experiments scheduled for the Hadron collider to test his basement experiment. Elevated solar flux (> 10 protons per cc) appears to cause fog in the Great Lakes and clouds too. The hypothesis of the Danish National Space Center goes as follows: quiet sun allows the geomagnetic shield to drop. Incoming galactic cosmic ray flux creates more low-level clouds, more snow, and more albedo effect as more is heat reflected resulting in a colder climate. An active sun has an enhanced magnetic field that induces Earth’s geomagnetic shield response. Earth has fewer low-level clouds, less rain, snow and ice, and less albedo (less heat reflected) producing a warmer climate.
That is how the bulk of climate change likely works, coupled with (modulated by) sunspot peak frequency there are cycles of global warming and cooling like waves in the ocean. When the waves are closely spaced, all the planets warm; when the waves are spaced farther apart, as they have been for this century, all the planets cool.
Many answers yield many new questions: the change in cloud cover is only a small percentage, and the ultimate cause of the solar magnetic cycle may be cyclicity in the Sun-Jupiter centre of gravity. We await more on that.
Although the post 60s warming period appears to be over, warming and attendant humidity have allowed the principal green house gas, water vapour, to kick in with more clouds, rain and snow depending on where you live to provide the negative feedback that scientists use to explain the existence of complex life on Earth for 550 million years. We can likely kick much of the carbon economy sometime late the twenty-first century, but we must not rush to judgement for the wrong reason. The planet heats and cools naturally and our gasses are the thermostat. Nothing unusual is going on except for the Orwellian politics. In other words, it is not the heat; it is the humidity.