The single most important issue is whether climate change is natural or human-caused.
In principle, both options are plausible.
But we know that climate changes naturally, on time scales of decades and centuries. So which is the more important cause: the human release of gases or natural cycles? One cannot go by the majority view — science does not work like politics. We cannot use the fact that there is a rough correlation between temperature increase and increase in CO2. During much of the 20th century, from 1940 to 1975, the climate cooled while CO2 levels rose. And during the past decade, climate has again been cooling in spite of rising CO2.
The answer is to look at the pattern of warming trends and see if it agrees with what models predict. The models all predict the existence of a “hot spot” — a maximum warming trend in the tropical region. The observations from weather balloons show the opposite result, a slight cooling trend. This disagreement between calculated and observed fingerprints of temperature trends is the strongest argument against any appreciable human contribution to climate.