Climate Change skeptics? [ot]

This is a relatively new and interesting hypothesis, so it's become popular amongst climate contrarians as an alternative explanation to human-caused global warming. However, it's also been the subject of extensive scientific research over the past few years, and the hypothesis simply has not held up to scrutiny.

First, there's the obvious fact that cosmic rays cannot explain the recent global warming because solar activity and the amount of cosmic rays reaching the Earth's surface have remained flat on average over the past 60 years. The sun and cosmic rays could only be causing global warming if there were a long-term upward trend in solar activity and downward trend in cosmic rays reaching Earth. In fact, the number of cosmic rays reaching Earth has increased since 1990, and reached record levels in 2009 (one of the hottest years on record).

A paper published in the journal Meteorology and Atmospheric Physics this August noted,

"Recent measurements of the cosmic ray intensity show that … even if cosmic rays enhanced cloud production, there would be a small global cooling, not warming."

Two of the authors of that paper (Sloan & Wolfendale) have also just published another cosmic ray research paper in Environmental Research Letters, finding that the contribution of solar activity and galactic cosmic rays (combined) to global warming is "less than 10% of the warming seen in the twentieth century."

Sloan & Wolfendale also examine the influence of cosmic rays on the climate over the past billion years in another new paper published in the journal New Astronomy. They find that changes in the galactic cosmic ray intensity are too small to account for significant climate changes on Earth. This was also the conclusion of a paper published this May in The Astrophysical Journal.

In another paper just recently published in Environmental Research Letters, Rasmus Benestad of The Norwegian Meteorological Institute compares measured changes in the amount of cosmic rays reaching Earth to changes in temperature, precipitation, and barometric pressure measurements. Benestad finds no statistical evidence that cosmic rays can explain the recent global warming.

Finally, a paper published last month in Geophysical Research Letters compared measurements of cosmic rays and cloud cover changes, and found no detectable connection between the two. This study is consistent with many previous papers finding that cosmic rays are not effective at seeding clouds. Likewise, in the CERN CLOUD experiments, Almeida et al. (2013) found

"ionising radiation such as the cosmic radiation that bombards the atmosphere from space has negligible influence on the formation rates of these particular aerosols [that form clouds]"

Thus every step in the galactic cosmic ray-climate hypothesis is fraught with problems. Evidence suggests that cosmic rays are not effective at seeding clouds. Solar activity has been flat, and even slightly downwards over the past few decades. Galactic cosmic ray flux on Earth has been flat, even slightly upwards over the past few decades. 2009, which saw a record number of cosmic rays reaching Earth (meaning it should have been cold), was the 5th-hottest year on record at the time.

provides an example where they derive a simple conceptual model of how the greenhouse effect works from first principles. They show the story behind the expression saying that a doubling in CO2 should increase the forcing by a factor of log|2|.

Some may ask why we keep revisiting the question about cosmic rays and climate, after presenting all the evidence to the contrary.
One reason is that science is never settled, and there are still some lingering academic communities nourishing the idea that changes in the sun or cosmic rays play a role. For this reason, a European project was estaqblished in 2011, COST-action TOSCA (Towards a more complete assessment of the impact of solar variability on the Earth’s climate), whose objective is to provide a better understanding of the “hotly debated role of the Sun in climate change” (not really in the scientific fora, but more in the general public discourse).