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

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.

• Comments are not for extended discussion; this conversation has been moved to chat.
– gerrit
Nov 23, 2019 at 12:56
• @polcott: In addition to global trends over longer timespans, human influence on the climate is observable in global weather data like moisture and temperature of a single day. This somewhat blurrs the classical distinction between climate and weather. Source: nature.com/articles/s41558-019-0666-7 (paywalled, sorry ...)
– user18607
Jan 5, 2020 at 16:20

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.

• Comments are not for extended discussion; this conversation has been moved to chat.
– gerrit
Nov 26, 2019 at 19:36
• "We can wait." It is one big experiment. Welcome to the test tube! I think I prefer modelling based on understanding of climate related processes before it happens regardless of imperfection. Nov 27, 2019 at 0:02
• @KenFabian Yes, when somebody talks of that we have only one earth, and therefore must protect is, that sounds much like a rhetoric figure influenced by science fiction, a metaphor to describe a big problem. But in reality, it is a very simple practical problem. We need to do one of two things: spend extreme effort on climate or not. We have an estimate which it is, but a dispute whether it is valid. We could just try, and then have a consent about what to do on the second earth. "Come on, it's not that hard. What did you say, how many test subjects do we have. One? Seriously?" Dec 7, 2019 at 2:28