It is an actual, physical thing discovered indirectly but definitively based on multiple independent lines of evidence. (discovermagazine.com)
However, we also have strong evidence from multiple independent lines of data that CO2 sensitivity is between 2 and 4.5 and at some compelling evidence that net feedback due to clouds is positive. (realclimate.org)
Why it is that scientists are convinced by the evidence by multiple independent lines of evidence that human caused climate change is real and it represents a risk. (sciencefriday.com)