Sometimes, perhaps even often, “effects” are discovered before a plausible mechanism for them is identified. Planck devised the basis of the law of black-body radiation long after black-bodies could be approximated in the lab, in the years when Einstein won a Nobel Prize for the photoelectic effect, that until then had no plausible explanation at all.
On the opposite side, more or less at the same time physics modelled the universe as filled of “luminiferous aether”: a plausible mechanism (devised to explain light propagation, among other things), argued favorably by the top scientists and institutions of the time. And something we now know it was just a fantastic self-deception, more and more convoluted as the years went by.
“Contemporary scientists were aware of the problems, but aether theory was so entrenched in physical law by this point that it was simply assumed to exist. In 1908 Oliver Lodge gave a speech in behalf of Lord Rayleigh to the Royal Institution on this topic, in which he outlined its physical properties, and then attempted to offer reasons why they were not impossible.“
It must be noted than regarding black-body radiation and the photoelectric effect, a major advancement of scientific knowledge came about by investigating those “effects” without preconceptions, whilst concerning the “aether” brilliant minds wasted decades of their lives trying to prove the existence of something everybody “knew” it was there. And even when evidence started accumulating against the “aether” still they came up with even more fanciful properties for it: to the point that any observation was meant to confirm the presence of the “aether”.
Fast forward to 2008 when warm spells are ascribed to anthropogenic global warming. Cold spells too. And violent hurricane seasons. And weak hurricane seasons. Increasing global temperatures, and decreasing global temperatures. Explanations are always at hand, and if it is not the aerosols then it’s soot from coal-fueled power stations.
What happened, is called “reification”. There are all indications that the same phenomenon is surfacing also in cosmology, with dark matter, dark energy, WIMPs, gravitational waves, etc. The problem is when an observed effect without a known cause is ascribed to “something unknown” and this “something unknown” is given a name.
As it happens, sometimes scientists are lucky and do “reificate” something that shows up in reality: take P.A.M. Dirac and the neutrino…but in the case of AGW: the “something unknown” is called “radiative forcing“. Guess what? It CANNOT be measured in the real world. It’s no “neutrino” waiting to be found: it can ONLY be “estimated“: because, as it is well known, it’s a useful computational tool and NOT a “real thing”.
Yet, since it does have a name, this subtle but important distinction is getting progressively lost…”radiative forcings” are taken as physical quantities so that an increase in CO2 concentration ipso facto brings an increase in “CO2 radiative forcing“.
Despite the absence of experiments and measures, the mere fact it has a certain name is taken as “evidence”.
A reality-check is in order. The greenhouse effect has been observed only in laboratory conditions, and we still refer to experiments done decades ago by Arrhenius; there is no actual satisfactory theory on how the effect would work in an actual atmosphere, so we are restricted to numerical models with far too many parameters to fiddle with; and even the best models cannot even compute clouds, of which there is aplenty in the actual atmosphere.
That is what is known.
Everything else is a conjecture that could collapse tomorrow, when a new Michelson and/or a new Morley will attempt, for once, to verify in the field that greenhouse gases actually warm up a planetary atmosphere.