Executive summary: it's not a reasonable hypothesis for the current warming. (And certainly not for the larger problem of ecological overshoot.)
It would be helpful for the public to understand that unlike what they are being told by the media and politicians, many scientists regard our knowledge of what factors affect our Earth's climate, their relative importance and how they interact, to be still in a rather primitive state.
With regard to the role of cosmic ray flux in high cloud nucleation it is important to realize that CRF varies continuously. The periodicity of our solar system's passing through the spiral arms of our galaxy (See the beautiful photos of distant galaxies coming to us from the space telescope) is on the order of 100 Ma. That means that the time between our solar system's passing through a point in its cyclical path through our galaxy and returning to that point is on the order of 100 million years. Nevertheless, the climate changes continuously throughout this cycle, but always withing the bounds of the main, controlling variables. These, according to Shaviv, being the solar flux and the CRF which directly affects the amount of high cloud cover. Shaviv has argued, based on considerable evidence I would add, that although CO2 and other trace greenhouse gases, including contributions from our use of fossil fuels, does play a minor role in determining mean temperature, the combined contribution of these gases is significantly smaller than the contribution of the solar flux and the CRF. The latter, according to Shaviv and his colleagues, determines the nucleation of water to form high altitude water aerosols, i.e., high clouds.
Hansen realized that water vapor was the main greenhouse gas in our atmosphere. Nevertheless, he jumped to an early conclusion re the role of CO2 in contributing to temperature based on insufficiently granular correlation of CO2 with Earth's surface temperature. He also used various, questionable proxy records for temperature. Realizing that CO2 alone was present at too low a concentration in our atmosphere to directly cause much atmospheric warming, Hansen and his laboratory then sought a feedback mechanism that could explain how small changes in CO2 concentration roughly the order of 200 molecules increase among a million other molecules (including water molecules and other greenhouse gasses) could be rationalized as the critical element in determining changes in the Earth's surface temperature.
In the early 1980s, Hansen testified before Congress that he believed CO2 could be important in determining temperature. Unfortunately, he promoted his early, unproven hypothesis via not only scientific venues but also via the mainstream media. The more public attention he brought to his hypothesis, the more entrenched he became in his thinking. At first his hypothesis was accepted as at least feasible. With time, Hansen's hypothesis, via the ceaseless self-promotion of his ideas and his dire warnings of impending doom, gained wide attention, and government money followed. Slowly however, inconsistencies between observation and Hansen's hypothesis began to surface. It was at this point that the IPCC Secretariat began to suppress contravening views, by reducing them to parenthetics and footnotes, or relegating them to the back pages in the heavily edited, periodic IPCC reports. Many of those scientists who's views differed from the mainstream stopped participating in the panel discussions. Despite these overt attempts to suppress conflicting views, the continued production of increasingly reliable and conflicting data caused serious questioning of the correctness of the original Hansen hypothesis.
If this episode of James Hansen and his CO2 hypothesis has taught us anything it should be that scientists should be very wary of public disclosure of preliminary findings before those findings have been thoroughly examined and criticized within the scientific community. Among scientists, it has always been true that ideas breaking from mainstream thinking found a difficult path to acceptance. Their proponents will sometimes be vilified and subjected to ad hominem attack; yet there is a time honored procedure that allows correct hypotheses to eventually be accepted. Science struggles terribly when subjected to layman forums and mainstream media reporting before the scientific community has finished having its say. Unfortunately, this latter situation is where we are today with regard to climate science.
I do not know if James Hansen is right or wrong. Are we on a path to runaway temperature excursion? I have no idea, but if we are, I strongly doubt it has anything to do with CO2 emissions from fossil fuels. I believe the hard evidence is telling us that our Earth is heating and cooling despite our fossil fuel CO2 emissions. Mainly for thermodynamic reasons, I believe we are doing the correct thing to reduce our dependence on fossil fuels. I also believe it is quite possible that direct thermal pollution from man's activities and our wanton destruction of plants, will prove to be more damaging than the past increase in CO2 emissions due to industrialization.
Regardless, I maintain that anyone who thinks the Hansen hypothesis is "settled science" has simply not been paying attention.