Off the Grid wrote:
But what is this influence vs general change? The climate will tend to change somewhat regardless, non?
If we look at the Stern Report, and the projected 2deg change it projects, what portion of this change can be attributed to naturally occurring phenomena? I realize it is not that simple, but roughly.
That is more or less the million dollar question. I will say this though: We are currently in an interglacial period which, unlike the ice ages is supposed to be characterized by relatively stable climate. It isn't natural for the temperature to simply change by several degrees in a century (at least during an interglacial period) without there being some kind of major influence.
Increasing solar activity probably contributed to the warming during the late 19th and early 20th century, but it doesn't explain the warming over the past half century. There hasn't been any massive effusive volcanism or large-scale release of methane hydrates that can explain it. The Milankovitch cycles are actually trending towards cooler temperatures as we move towards the next ice age. Also, some of man's activities have actually been acting to suppress emission-caused climate change such as land-use change (increasing reflectivity) and SO2 emissions.
I wouldn't be willing to put a percentage on the portion of warming that is caused by mankind, as I am not an expert, but I would suggest that it's very high, unless there is some major climatic input that we haven't yet considered and factored into the equation (doubtful).
From wikipedia
Svante Arrhenius (based on research done in the 19th century)
"To a certain extent the temperature of the earth's surface, as we shall presently see, is conditioned by the properties of the atmosphere surrounding it, and particularly by the permeability of the latter for the rays of heat." (p46)
"That the atmospheric envelopes limit the heat losses from the planets had been suggested about 1800 by the great French physicist Fourier. His ideas were further developed afterwards by Pouillet and Tyndall. Their theory has been styled the hot-house theory, because they thought that the atmosphere acted after the manner of the glass panes of hot-houses." (p51)
"If the quantity of carbonic acid in the air should sink to one-half its present percentage, the temperature would fall by about 4°; a diminution to one-quarter would reduce the temperature by 8°. On the other hand, any doubling of the percentage of carbon dioxide in the air would raise the temperature of the earth's surface by 4°; and if the carbon dioxide were increased fourfold, the temperature would rise by 8°." (p53)
"Although the sea, by absorbing carbonic acid, acts as a regulator of huge capacity, which takes up about five-sixths of the produced carbonic acid, we yet recognize that the slight percentage of carbonic acid in the atmosphere may by the advances of industry be changed to a noticeable degree in the course of a few centuries." (p54)
"Since, now, warm ages have alternated with glacial periods, even after man appeared on the earth, we have to ask ourselves: Is it probable that we shall in the coming geological ages be visited by a new ice period that will drive us from our temperate countries into the hotter climates of Africa? There does not appear to be much ground for such an apprehension. The enormous combustion of coal by our industrial establishments suffices to increase the percentage of carbon dioxide in the air to a perceptible degree." (p61)
"We often hear lamentations that the coal stored up in the earth is wasted by the present generation without any thought of the future, and we are terrified by the awful destruction of life and property which has followed the volcanic eruptions of our days. We may find a kind of consolation in the consideration that here, as in every other case, there is good mixed with the evil. By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind." (p63)
He was the first person to predict that emissions of carbon dioxide from the burning of fossil fuels and other combustion processes would cause global warming. Arrhenius clearly believed that a warmer world would be a positive change. From that, the hot-house theory gained more attention. Nevertheless, until about 1960, most scientists dismissed the hot-house / greenhouse effect as implausible for the cause of ice ages as Milutin Milankovitch had presented a mechanism using orbital changes of the earth (Milankovitch cycles). Nowadays, the accepted explanation is that orbital forcing sets the timing for ice ages with CO2 acting as an essential amplifying feedback.
Arrhenius estimated that halving of CO2 would decrease temperatures by 4 - 5 °C (Celsius) and a doubling of CO2 would cause a temperature rise of 5 - 6 °C[4]. In his 1906 publication, Arrhenius adjusted the value downwards to 1.6 °C (including water vapour feedback: 2.1 °C). Recent (2007) estimates from IPCC say this value (the Climate sensitivity) is likely to be between 2 and 4.5 °C. Arrhenius expected CO2 levels to rise at a rate given by emissions in his time. Since then, industrial carbon dioxide levels have risen at a much faster rate: Arrhenius expected CO2 doubling to take about 3000 years; it is now estimated in most scenarios to take about a century.