By what objectively correct process could we determine the truth about the actual severity of anthropogenic climate change?
We have reasonably good observations of the present.  We have moderately decent observations of the past (with larger uncertainties the further back we go).  Unfortunately, we have no observations at all of the future.

There are fundamentally only two ways to know about future impacts:


We can wait.  20 years, 50 years, 300 years.  We will know for sure, but it will be far too late to act on the knowledge to prevent the worst consequences.
We can model the impacts.  All models are wrong, but some are useful.  There is no "objectively correct process to determine the truth about the actual severity", because "actual severity" is not an objectively defined quantity.  However, we can model the impacts on biodiversity loss, sea level rise, desertification, food security, economic losses, and other measurable quantities that we care about.  From a strictly mathematical point of view, we can never be 100% sure.  However, if we are 99% sure of net impacts that most people would consider to be very bad indeed unless we act, the precautionary principle suggests acting is the wisest thing to do.



Source: USA Today, but has been published in many newspapers.