I'm an environmental chemist by trade, I, essentially, make my living by applying what I know about statistics to what I know about aquatic chemistry to figure out if the Council Poo-plant is having a significant effect down stream (by comparing downstream water chemistry to upstream water chemistry, and the chemistry of the effluent being discharged), I also monitor them to ensure that they stay within the conditions specified in their discharge permits - essentially, I enforce the
New Zealand Resource Management Act (1991). I'm a chemist, a statistician, and a law enforcement officer all rolled into one.
As for my personal opinion as to how much of climate change man is
actually responsible for? I don't know - and have never claimed otherwise.
Here's what I do know:
Simple Harmonic Motion (weights on springs) predicts that Carbon Dioxide absorbs infrared radiation over a range of wavelengths, and that that infra red radiation is turned into energy of motion (symetric and asymetric bending, stretching and, I believe, rotational modes).
Quantum Mechanics reinforces this prediction.
Emperical Observation also reinforces this prediction, we can measure it in the laboratory, and we can measure it 'in the field'.
We are burning fossil fuels.
We are burning fossil fuels at an increasing rate - to get an emperical indication of this compare how many cars there were per household 30 years ago to today, or how many coal fired powerstations there were around the world 30 years ago compared to today.
Combustion of Carbon produces carbondioxide.
Therefore, logically, if the rate at which we are burning fossils fuels is increasing, then the rate at which we're producing CO[sub]2[/sub] must also be increasing, and if the amount of CO[sub]2[/sub] in the atmosphere is increasing (and there are other sources of CO[sub]2[/sub] besides us) then the amount of infra red radiation being absorbed by the atmosphere and converted into heat energy must also be increasing.
However, this is where things get less certain.
Now, we know that CO[sub]2[/sub] is absorbed from the atmosphere primarily through three process. Plant growth, Sillicate Weathering, and oceanic absorption.
Plant Growth:
Yes, it's true plants use CO[sub]2[/sub] in photosynthesis, however, plants also respire at night, absorbing oxygen and producing carbondioxide, and only some plants actually have the net effect of removing CO[sub]2[/sub] from the atmosphere and sequiestering it. This Carbon sink represents the smallest carbon sink available, but also the one that responds most rapidly to change (plants, to some extent, respond better to a higher ppCO[sub]2[/sub], however I believe as with all nutrients there is a peak effect).
The next biggest carbon sink is sillicate weathering. This also tends to increase as CO[sub]2[/sub] increases, because as CO[sub]2[/sub] increases, and temperature increases, glaciers start melting faster and retreating, exposing more bedrock to the atmosphere. A greater area of exposed bedrock results in increased silicate weathering, which results in increased removal of CO[sub]2[/sub] from the atmosphere.
So for both of these, it is fairly straight forward in the sense that any natural rise in CO[sub]2[/sub] will result in a small adjustment with some lag time, and the sources and sinks will find a new equilibrium.
The third sink is a little more complicated. The Ocean absorbs CO[sub]2[/sub directly from the atmosphere. It's a straightforward diffusion process that follows well known physical laws (Henry's Law). Increasing the partial pressure of CO[sub]2[/sub] in the atmosphere increases the solubility of CO[sub]2[/sub] in the oceans, it's the same basic principle behind a can or bottle of coke or pepsi. So that's all good, however, if we accept the principles of simple harmonic motion, and the theory that carbon dioxide absorbs infrared radiation, and stores it as heat energy, then as ppCO[sub]2[/sub] increases, increasing temperatures also come into play, because the solubility of a gas in a liquid is inversey proportional to temperature. The hotter it gets, the less soluble a gas is in a liquid, and eventually you reach a point where if the liquid gets hot enough, it doesn't matter how high the pp of the gas is, it's still insoluble, or sparingly soluble in the liquid.
Unfortunately, this isn't the end of it for the oceans, because there's still the Lysocline, and the Carbonate compensation depth, both of which also influence the ability of the oceans to store CO[sub]2[/sub]. essentially, when anyhting with a carbonate shell dies, it's shell starts to sink. If the water is shallow enough, it will sit there, and this tendency increases the amount of CO[sub]2[/sub] the oceans can store through the following chemistry:
CO[sub]2[/sub]+H[sub]2[/sub]O ↔ H[sub]2[/sub]CO[sub]3[/sub] + CaCO[sub]3[/sub] ↔2HCO[sub]3[/sub][sup]-[/sup] + Ca[sup]2+[/sup]
This reaction effectively acts to remove CO[sub]2[/sub] from the water, which is why the oceans are our biggest resivoir of CO[sub]2[/sub].
There's a catch though, because it relies on a diffusion process, the further away you get from the interface in the water (the deeper you go) the less CO[sub]2[/sub] there is in the water, which has the effect of increasing the solubility of the Calcite (common ion effect essentially), eventually you reach a depth where the solubility of calcite increases dramatically, this is the lysocline, below the lysocline the calcite shells become soluble again, at a rate that increases with depth, until you reach a point where the rate at which the calcite is deposited by dying things is equal to the rate at which the calcite dissolves - analgous to the snow line.
The total mass of calcite in the oceans above this depth is what determines the amount of CO[sub]2[/sub] that can be absorbed by the oceans. There's a lot of it, I think the CCD averages something like 1200-2000m in depth. However, because of the volume of water that needs to re-establish equilibrium with the atmosphere. Our largest carbon resivior is also our slowest to respond.
It's these opposing rates of change that make making any predictions difficult, but it gets worse still, because reducing ice cap cover, and replacing it with bedrock changes the average albedo of the planet, which in turn change sthe percentage of radiation absorbed, which changes the effective temperature of the planet, which changes the amount of infra red radiation emitted by the earth. At the same time, water is a better greenhouse gas than CO[sub]2[/sub], however the amount of water in the atmosphere is controled by the temperature of the atmosphere, and the amount of IR absorbed by water is more or less maxed out in most of its absorption windows., however, increasing the amount of water in the atmosphere, I believe has the effect of changing the type and extent of cloud cover which can have conflicting effects on long wave retention.
And that's without even looking at things like increasing aerosol emissions that logically accompany increased fossil fuel consumption (aerosols have, I believe, a net cooling effect).
All of the above (and more, it's time for me to go home now) is why I generally say "Up to a certain point the science is fixed in stone, it's the effects beyond that point which are open to debate".