The scientific consensus is in: human-produced carbon dioxide is causing a rise in temperatures across the planet. There are still those who reject the evidence that humans have an impact on global temperatures, and instead maintain that natural processes are at the root.
One of these natural causes, they say, could be from cosmic rays.
According to one paper, published in 2000 to Physics Review Letters, the Hunacayo neutron monitor detected a heightened number of cosmic rays from regions that had low clouds, less than 3.2 km in altitude. The quantity of these cosmic rays depends on the intensity of the solar wind, since the Earth's magnetosphere grows and shrinks depending on the strength of particles streaming from the Sun. Periods of warming appear to correlate with a decreases in cosmic rays over the 20th century.
When the cosmic rays interact with the Earth's atmosphere, especially the low level clouds, they create ions of varying strength and charge. These ions would then contribute to the formation of dense clouds, blocking the Sun's rays and reducing the effect of heating.
This connection between the Sun's 11-year cycle of sunspot and solar wind activity and the Earth's deflection of cosmic rays was offered up as a possible natural explanation for global warming.
But T. Sloan from the University of Lancaster and A.W. Wolfendale from
Durham University have looked carefully at the evidence and found it unconvincing. They published their results in a new paper called Cosmic Rays and Global Warming. Their research will be presented at the 30th International Cosmic Ray Conference, held in Merida Mexico from July 3 - July 11, 2007.
According to Sloan and Wolfendale, the 2000 paper highlighting the connection between cosmic rays and low-level clouds completely avoids clouds at other altitudes. This is surprising because cosmic ray ionization should increase with altitude. Cosmic rays should be intercepted earlier by the atmosphere and turned into clouds, not down at the lowest altitudes. If cosmic rays were to blame, you would expect the exact opposite, with more high-altitude clouds.
It can't be ruled out, but it's pretty unlikely.
The next piece of skeptical evidence is the likelihood that cosmic rays will create ions that turn into water droplets. The researchers estimated the density of cloud droplets that could be produced by cosmic rays at the lowest altitudes. They found that the rate of ion production was too low generate the number of water droplets required to create clouds.
Global warming skeptics explain the cosmic ray/cloud cover/global warming natural cycle as the interaction between the Sun's 11-year cycle of solar activity and the magnitude of cosmic rays that reach the
Earth's atmosphere. As the solar wind increases, it buffets away cosmic rays that would reach the Earth's magnetosphere.
Ionized particles are channeled towards the Earth's poles, which is why we see the beautiful auroras at the highest latitudes. If cosmic rays were causing additional cloud cover, you would expect the greatest variations around the poles. This isn't the case; in fact, the opposite is true.
Furthermore, there's known to be a 6-14 month delay between the decrease of cosmic ray activity, and the increase in the number of sun spots. Based on these cycles, the researchers found almost no correlation between the rise and fall of sun spots, and levels of cloud cover. They estimated that less than 15% of the 11-year cycle warming variations are due to cosmic rays and less than 2% of the warming over the last 35 years is due to this cause.
If scientists wanted to study the interaction between radiation and cloud cover they could always perform a highly unethical experiment:
release a tremendous amount of radiation into the atmosphere and see what it does to clouds in the environment.
Unfortunately, that experiment has already been performed... accidentally: the Chernobyl disaster.
On April 26th, 1986 the reactor released a huge cloud of radioactive particles into the atmosphere. If radiation increases cloud cover, there should have been clouds surrounding the facility for weeks. There was no evidence of unusual cloud coverage surrounding the facility after the disaster.
Sloan and Wolfendale reviewed the cosmic ray connection to global warming, and found several different ways that discount the explanation. Of course, no matter how good their evidence, for some people this is a political matter now - no amount evidence will ever be enough.
