By what objectively correct process could we determine the truth about the actual severity of anthropogenic climate change?

Question

I began to do the best that I could to boil the key facts down to the most basic essence of proving significant anthropogenic climate change:

(1) It is a verified fact that since 1959 $\small\mathsf{CO_2}$ increased 98 PPM which is 126 times faster than the next fastest increase of at least 90 PPM in the prior 800,000 years.

(2) It is a verified fact that $\small\mathsf{CO_2}$ is currently 37% higher that its prior 800,000 year high value of 298 PPM.

(3) It is a verified fact that global temperatures have increased about 0.75C since 1970.

(4) It is a verified fact that the only reason the Earth is much warmer than the Moon is the greenhouse effect of greenhouse gases.

(5) It is a verified fact that a significant increases in greenhouses gases cause a corresponding significant increases in global temperatures.

It does seem to be the case that within the premise that all of the above facts are true that significant anthropogenic climate change is proven.

The only pushback that I have gotten from global heating deniers is that they simply do not believe the temperature record.

By what objectively correct process could we determine the truth about the actual severity of anthropogenic climate change?

There are two key goals of the above question:
(1) To make sure that the above five points sufficiently prove AGH.
(2) To find very succinct and compelling ways to validate these five points.

Answer 1

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.