Clouds, the sun, volcanoes, the earth's orbit around the sun, ill-defined 'natural cycles' - all have been brought up recently by a range of non-scientists arguing against the current scientific consensus that anthropogenic greenhouse gases are responsible for the global warming observed in the last century. I'm a neurobiologist, so my knowledge about climatology is rather limited, which means I don't really understand too much of the complex mechanisms underlying climate variabilities and trends. However, I find it conspicuous that these arguments are being publicized as if nobody had thought of them before. I find it hard to believe that a bunch of amateurs should come up with serious stumbling blocks for a mature science such as climatology.
But, one never knows. So I went to one of the most easily accessible archives of scientific communication, the journal Nature, and searched for "global warming". I sorted the list of publications from oldest to newest and hand-picked according to my own, subjective judgment what seemed like interesting titles and went through the most easy to follow parts of the papers. The list shows an ongoing debate among climatologists going back as far as the 1950s. As far as I could tell, all the usual suspects have been covered decades before any blogger has brought them up. Well, decades before the internet. Given that now, after basically a whole generation of debate, most climatologists are coming to a consensus (a rare event among scientists!), any amateur who thinks they can come up with something that hasn't been thought of before, should have a good and thorough read of the literature covering the last 60 odd years. This short list (and the references therein) should provide some good starters:

1950s:
The ice ages and past variations of the earth's climate
Twentieth century man against antarctica
1960s:
The new look of climatology
1970s:
CO2 versus aerosols
Man-made carbon dioxide and the "greenhouse" effect
Wither climate now?
Atmospheric carbon dioxide levels as indicated by the stable isotope record in wood
Glacial advance relative to volcanic activity since 1500 AD
Fluctuations in climate
Aerosol and climate: hotter or cooler?
Causes of climatic change
Cause and effect of global cooling
Volcanic triggering of glaciation
Ocean temperatures and large scale atmospheric variations
Man's influence not yet felt by climate
New data on climatic trends
West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster
Volcanic dust and changes in Northern Hemisphere temperatures

Climatololgy supplement 1978, e.g.:
Predicting temperature trend in the Northern Hemisphere to the year 2000
Solar-terrestrial influences on weather and climate

Clouds and the long-term stability of the Earth's atmosphere and climate
Impact of CO2 on cooling of snow and water surfaces
An empirical determination of the heating of the Earth by the carbon dioxide greenhouse effect

1980-1985:
Scenario for a warm, high-CO2 world
Coupled effects of atmospheric N2O and O3 on the Earth's climate
Albedo change by man: test of climatic effects
Global warming?
Detecting CO2-induced cliamtic change
Surface temperature sensitivity to increased atmospheric CO2
Ice core sample measurements give atmospheric CO2 content during the past 40,000 yr
Increasing atmospheric carbon dioxide and its consequences
Changes in the solar constant and climatic effects
Global production of methane by termites
El Nino Southern Oscillation phenomenon
Marine biological controls on atmospheric CO2 and climate
Two views on whether more means doom
Volcanic, CO2, and solar forcing of Northern and Southern Hemisphere surface air temperatures
Modelling the global climate response to orbital forcing and atmospheric carbon dioxide changes
Does the ocean-atmosphere system have more than one stable mode of operation?
Analytical solution for the effect of increasing CO2 on global mean temperature
A 150,000-year climatic record from Antarctic ice
Future global warming from atmospheric trace gases


I stopped collecting after this last review from January 1986, because the number of papers was just getting way too large. This is not my field of expertise, but it seems to me that all the arguments which are currently being brought up are known in the climatology community for at least 30 years, some of them have been debated for as far back as 60 years. Given such a thorough debate for such a long period of time, I trust that the current consensus of my climatologist colleagues is the best possible we can currently do. That doesn't mean it won't change, it most certainly will, that is the hallmark of science. However, given this track record, it seems unlikely that such new development will come from some blogger on the internet. Indeed, as always, there is a lively debate among climatologists, it's just not about such basics any more. Heck, I'm sure, in the next 30-60 years, some blogger will come with an argument currently being hotly debated among climatologists.

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SAWYER, J. (1972). Man-made Carbon Dioxide and the “Greenhouse” Effect Nature, 239 (5366), 23-26 DOI: 10.1038/239023a0