Without carbon dioxide in the atmosphere, the Earth would be a frozen orb.
It is known with a very high degree of certainty that carbon dioxide keeps the Earth warm and more of it will make the Earth warmer.
It is also known with a very high degree of confidence that humans activities have increased atmospheric CO2 content by about 40% since the dawn of the industrial revolution.
There is close to universal consensus among well-informed climate scientists that most of the global warming over the past 50 years was associated with our greenhouse gas emissions.
There is substantial uncertainty regarding how sensitive our climate system is to added CO2, where something like 3 C per CO2 doubling (about 5 F) might be somewhere near the central expectations but with semi-reasonable people arguing for half this or double this.
There is very little consensus regarding how adaptable humans will be to these changes. Humans already live from the equator to the Arctic circle. Houston used to be a malarial hell-hole and now it is a modern air-conditioned city.
At the one end of the spectrum there are people thinking climate change will be an existential threat to human existence. At the other end, there are people who think most people will barely notice the effects of climate change. Neither end of this spectrum represents a tenable position.
My own view is that climate change will impose a substantial cost on society but that climate change is unlikely to be the biggest problem that most people will face in their lives. This is less true for sensitive ecosystems such as coral reef systems.
Humans are like weeds. We are the invasive generalists par excellence. We spread rapidly, grow quickly, and successfully inhabit almost any environment.
Climate change will impact the delicate flowers tuned to a narrow range of environmental conditions; climate change will benefit many weeds, which can take advantage of disruption.
Carbon dioxide also acts as a fertilizer for plants, so there is potential for crop yields to increase under a high-CO2 atmosphere.
When the dinosaurs were around, the atmosphere was rich with CO2 and life flourished. We are not followers of Leibniz and do not think we are living in the best of all possible worlds. There is nothing particularly special about the climate of the pre-industrial era, although it does seem to have been a particularly stable climatic period.
The problem is not that greenhouse gases are pushing us from a better climate to a worse climate so much as the problem is one of rates of change. Will climate change occur so rapidly that the transition imposes costs that were not anticipated, costs that are larger than we would like to deal with?
[Just in case it is not clear, my answer to the final question is ‘yes’. Not only that, even anticipated changes are sufficient to motivate eliminating fossil-fuel CO2 emissions as soon as is practicable.]