What is the evidence for an anthropogenic effect?
The first question we wish to address is whether there is actual evidence indicating that greenhouse gases (GHGs) are responsible for most of the warming. Basically, we observe a temperature rise over the 20th century, and we measure a rise in the global concentration of CO2 and other anthropogenic greenhouse gases. What is the evidence proving that the increase in the GHGs is the cause for the temperature increase?
The truth is that there is no real evidence for this link. Most of the "evidence" often mentioned in the media, is evidence for global warming (e.g., melting of arctic ice-sheets). But who said that this warming (which indeed took place over the 20th century) is because of GHGs? In fact, there is no substantial evidence which proves that CO2 and other GHGs are the primary cause for the warming, and not some other mechanism. ...
An alternative explanation for global warming, or at least part of it
Solar activity appears to affect climate. This can be seen from many different correlations between solar activity on one hand, and climate on the other. These correlations exist on time scales ranging from the 11-year solar cycle to many millennia (for the two most beautiful correlations, see Neff et al, and Bond et al. in the refs below). Such a link is potentially important for global warming because over the 20th century, solar activity has been increasing. ...
The activity of the sun manifests its self in many ways. One of them is through a variable solar wind. This flux of energetic particles and entangled magnetic field flows outwards from the sun, and impedes on a flux of more energetic particles, the cosmic rays, which come from outside the solar system. Namely, a more active sun with a stronger solar wind will attenuate the flux of cosmic rays reaching Earth. The key point in this picture is that the cosmic rays are the main physical mechanism controlling the amount of ionization in the troposphere (the bottom 10 kms or so). Thus, a more active sun will reduce the flux of cosmic rays, and with it, the amount of tropospheric ionization. As it turns out, this amount of ionization affects the formation of condensation nuclei required for the formation of clouds in clean marine environment. A more active sun will therefore inhibit the formation of cloud condensation nuclei, and the resulting low altitude marine clouds will have larger drops, which are less white and live shorter, thereby warming Earth.