There is a general agreement of all scientists about the this issue :
« Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. »
(IPCC Summary report, D.3)
Each of the assertions in the IPCC report is given with a mathematicial probability, as in any scientific text. For instance « extremely likely » means between 95 and 100% probability. The statments made by the climatologists are thus very cautious.
If I tell you there's a 20% chance that your house will catch fire, what do you do? You evacuate just in case, or you stay because after all it's quite possible it won't burn down?
Here we're 95% sure the house is burning, but we can't evacuate. Of course there's a 5% chance that's it's not correct, and there's of course some uncertainty about the detailled forecasts, but is that sufficient to ignore the problem?
A lack of critical opinions ?
On the 2nd of September 2014, the newspaper Le Temps quoted an internet user's opinion: « This book starts from the principle that climate change is an established fact, without giving the opposite opinion » - see the full article « Les Jeunes UDC s’attaquent aux professeurs taxés de gauchisme ».
There seems to be some confusion between political opinion and scientific conclusions.
If I say « The Swiss economy cannot sustain a minimum salary of 23 francs an hour », it's a political opinion - I don't know what the real consequences for the economy of such a salary will be, and no economist can assert it with certainty. Therefore the subject is open to debate, and it's interesting to listen to all different opinions.
Here we are talking about a scientific conclusion, which is sumarized in the IPCC report. Of course, scientific reports can also be incorrect, but in that case only another scientist can study the exact conditions of the experiment, possible biases, etc. - or redo the same calculations independantly.
An analogy with tobacco
When doctors state that « Tabagism is the cause of lung cancer in 90% of cases », it's the result of statistical studies on multiple cases of the disease, confirmed by a scientific study of the relationship tabacco => genetic mutation => cancer cells. It's not a public opinion, but a scientific conclusion. The political debate starts when we discuss the actions to take: forbidding tobacco in public places, prevention in schools, publicity for cigarettes, etc.