The really large scale use of fossil fuels only started around 1950. Remember that prior to WW2 only the 'first world' nations were industrialized. Now, heavy industry is just about everywhere. Yet, the recent warming trend began in 1910. An effect cannot begin before its cause. If that seems to be the case, then the theory is faulty.
The CO2 level has increased from 270ppm to 400ppm. The amount of warming that should cause, if CO2 were the only greenhouse gas, is easily calculated from Arrhenius' equation of infrared absorbtion. There is no mystery about that, it's a relatively simple equation. Yet, how often do you see a climate propagandist going through the calculation? NEVER! Why not? Go figure.
The actual equation output is about 1.7C. Which is significantly more than the warming which has been observed.
However, CO2 is not the only greenhouse gas. There is more than ten times the amount of water vapour in the atmosphere than CO2. As with resistors in parallel, you cannot just add the effects of greenhouse gases working in parallel. Consider a 10k and a 1k resistor in parallel. If the 10k resistor goes out of tolerance by, say, 10%, how much difference does that make to the overall value? 10%? Nope. More like 1%.
In the atmosphere, if an infrared photon leaving the surface has been deflected (absorbed then re-emitted in a different direction) ten times by water vapour molecules, then it makes no real odds if the same happens once more with a CO2 molecule. Each time there is a 50/50 chance of it being re-emitted upwards or downwards. Increasing the number of molecules beyond a certain point where most photons are deflected a few times enroute through the atmosphere, has little effect on those odds. That point is reached with about 40ppm concentration, for CO2.
You can experiment with the variables here:
http://climatemodels.uchicago.edu/modtran/Notice how tiny a change in the absorbtion band is caused by changing CO2 from 400 to 270ppm. (There is another CO2 absorbtion band not shown here, but it is not relevant to outgoing radiation. Also we call it absorbtion but scattering would be a better term. Absorbtion, because a dark line in a spectrum results when one wavelength in a beam of light is scattered around while the rest travel in a straight line.)
Prof. Nahle of Monterey has calculated that the mean free path of an outbound photon, between encounters with a CO2 molecule, is a few tens of metres at 400ppm. That is a surprisingly short distance for such a low concentration of gas molecules, but the proof is well documented. It is therefore unsurprising that further increases in CO2 concentration have only a small effect on the number of photons which escape the atmosphere to space. Its consequences match the results of Arrhenius' equation and MODTRAN, so the evidence is overwhelming that it is correct.
The IPCC claim that this tiny effect of CO2 will be increased by 'feedbacks' in the climate system to several degrees Celsius. There are two problems with this hypothesis. Firstly, creating
stable gain by positive feedback is extremely hard. Secondly, since there is no known way that the presence of CO2 could itself trigger a feedback, the input to AND the output from any such feedbacks would have to be the air temperature. Therefore, this is like having an opamp with the + input directly connected to the output, or a better illustration might be a guitar amo on which the guitar and speaker are connected by the same pair of wires. (Don't try it, you'll blow it.) Commonsense tells us that such an arrangement cannot work. As soon as you have slightly more than unity gain, the whole thing oscillates uncontrollably.
When I challenged a climate scientist over this, he replied that the feedbacks in climate science are not the same as those in electronics. To which I said that feedback is a concept in the broader sphere of physics, not specifically in electronics. Therefore if he means something other than the physical concept of feedback, then he is simply repurposing the word to have an undefined meaning. In which case he might as well call it whatever he likes, phlogiston, aether, or whatever, because it means nothing.
The alarmists claim that the science regarding greenhouse gas effect is settled, and that no debate is possible. Well, they are right. It was settled over 100 years ago and has not significantly changed since.
Thing they avoid mentioning is, the settled science doesn't support their claims. Without going into too much detail, sea level rise claims are similarly based on a true principle, but the figures are out by factors of thousands.
Ocean acidification claims are worse. They typify propaganda. They are expressed that way to make them sound scary. If it were accurately stated that the alkali in the oceans was being neutralised, leaving something nearer to pure water, would that be scary?
That's even before you consider how small the change in alkalinity is.
Science is about measurements. If there are no measurements, or no proof of where those measurements came from, then it is not science.