This surprises me, because I can hardly recall reading any statements by climate scientists online that say they are unsure of whether humans are influencing the climate or contributing to global warming. Even the few scientists (generally not climate scientists) who think "global warming is not much of a problem" still think that humans are the primary cause of recent warming.
CO2 was shown long ago to be a greenhouse gas in laboratory conditions, such as this:
Experiment - The Greenhouse Effect
It is undeniable 1) that higher levels of CO2 trap more solar radiation on Earth, 2) that CO2 levels have been rising since the dawn of the industrial age, and 3) that human activity is the cause of most of the rise in CO2. Therefore, a warmer Earth is to be expected. This is confirmed by the paleoclimate record, which clearly associates higher CO2 levels with higher global temperatures.
CO2 is just one of several greenhouse gases, but it is responsible for significantly more of the greenhouse effect than the others.
Likewise, greenhouse gases are just one of the factors influencing long-term climate change, and it is incorrect to frame the AGW climate change debate in terms of "are humans the sole cause of climate change or not?" or "how is it possible that climate changed in the past without human interference?"
It is generally recognized that in previous times greenhouse gas levels have acted not as an initiator but as an amplifier of trends triggered by orbital cycles. This article explains why there is a lag between temperature increase and CO2 increase in the paleoclimate record:
CO2 lags temperature - what does it mean?
Here is a really superb article on the history of climate science and how each important discovery was made:
The Carbon Dioxide Greenhouse Effect