What's the worst that could happen to our climate? Gavin Foster and colleagues look at carbon dioxide levels over the last 400 million years to find several moments that could model our future, including eras with extremely high quantities of carbon dioxide in the atmosphere. (https://doi.org/10.1038/ncomms14845).
The graph above shows a median line calculated from several distinct proxy measurements compared with computer models of climate variation. The highest levels occur at around 250 and 450 million years ago, when carbon dioxide concentrations soar up above 2000 parts per million, creating an atmosphere that would limit the brain function of contemporary mammals. That date range is also when major deposits of carbon were laid down by plants on land and algae in the sea, forming the oil and coal deposits now mined for energy.
Temperatures at those peaks in the deep past are estimated by Jan Veizer and Andreas Prokoph (https://doi.org/10.1016/j.earscirev.2015.03.008). They draw on a broad database of studies of a proxy for temperature found in the content of an oxygen isotope in fossil shells. They also correct for a slow change in the isotope over long time periods to yield an approximate estimate of temperatures hundreds of millions of years ago. Their numbers show average global temperatures between 30° and 40°C (86°- 104°F) at the peak. Given that current temperatures average 14.9°C (59°F) and the Paris agreement seeks to hold warming to 2°C above pre-industrial levels, about 16°C (60.8°F), those numbers are almost unimaginable. Nevertheless, the worst case scenario projected by the IPCC(Intergovernmental Panel on Climate Change) shows carbon dioxide possibly reaching these levels if nothing is done. In the “business as usual” scenario RPC 8.5, totals could reach 1000 parts per million in this century and perhaps 2000 in the next century if current emissions are maintained. This is unlikely because we are already feeling the effects with a 1°C temperature rise since the mid-nineteenth century and are mustering to address the problem now. The worst case however has been here before.