It comes down to the evolutionary history of Earth throughout life's 4 billion year tenure on this planet. The climate has gone through EXTREME changes [1][2], including the Chicxulub impact, which radically impacted the atmosphere's composition and life on this planet. Yet, the biosphere has always adapted and life continued.
Assuming that mankind is destroying the biosphere at an unprecedented rate (we are), and burning hydrocarbons at an even faster rate (we are), are these two factors enough to turn the earth into a Mars-style wasteland, completely devoid of all life?
Based on the data I`ve linked below, it just doesn't seem plausible. We can use linear regression models of the atmosphere and show temperatures rising until life becomes impossible, but that is not how complex biospheres work.
In any case, climate science is definitely a worthy scientific discipline to study, but it is becoming akin to economics in the prediction department. As in, the principles are sound, but the predictions of the experts are laughably wrong on a regular basis, possibly due to the fierce politicization of this branch of science.
1. http://www.biocab.org/Geological_Timescale.jpg
2. http://earthguide.ucsd.edu/virtualmuseum/images/CO2History.h...
I'm all for the walls, but the money that drives governments don't seem to agree.
I think we'd be better off figuring out how to cope with climate change and generally reduce our consumption of natural resources, rather than focusing to tightly on one evil (carbon).
Examples of this include ice core data, which has been gathered on multiple sites in the world, and the concentration of gases contained within is a really good indicator of how things were in the past.
Another thing that can be used is tree ring data and other information from proxies. You can study the fossils of fauna and flora deposited over different locations and periods of time to get a good idea of the climate conditions in each place.
An interesting correlated piece of data is examining human remains from people. Humans have been buried with foods at the time of their death, and the bones can help establish cause of death. If you go to Peru you can see that the food and living conditions of peoples during El Niño match the surrounding geological data very well.
Glaciers are also interesting indicators of climate phenomena, since they form and melt under specific conditions.
[1]"The Devonian extinction saw the oceans choke to death" http://www.bbc.com/earth/story/20150624-the-day-the-oceans-d...
Also the global mean surface temperature has not been well defined during this time period, it has changed as stations close and new ones are added, and as new technology is introduced.