To what extent is climate change caused by human factors?

Answer - “None that has been proven”.
Take the “CO2 causes warming” hypothesis. CO2 is a greenhouse gas. CO2 exists in the atmosphere at 400 ppm (parts per million). That is 0.04%. 99.96% of the atmosphere is not CO2. Scientists supporting the idea that increased CO2 causes global warming agree that an increase will not of itself cause much warming, but any increase will trigger an increase in the main greenhouse gas - water vapour (1% to 4% of the atmosphere depending on location). Proving this cannot be done by measurement - timescales are too long, historical records too short. Mercury thermometers were invented in the 18th century and temperature records are very sparse until recent times, especially over oceans and uninhabited areas (most of the planet). Attempts to deduce past temperatures from proxies - ice cores, tree rings, lake sediments etc. - have been made. These all have problems but they can provide estimates over long time scales. The only way to prove the hypothesis is by computer modelling. This is very inexact and complex. Over 50 institutions run models. Results disagree. Projections from past model runs are hotter than the temperatures that happened. Furthermore, the temperature of the earth has scarcely changed while the concentration of CO2 in the atmosphere has risen even more than expected. Therefore, there is zero PROOF that human-produced CO2 causes global warming.