The following is an excerpt from National Geographic.
In his book Storms of my Grandchildren, noted climate scientist James Hansen issued the following warning: "[I]f we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."
Venus, the hottest planet in the solar system, may have experienced runaway greenhouse effect early in its history. Illustration by Detlev van Ravenswaay, Science Source
Venus has a thick atmosphere that is 96.5 percent carbon dioxide, which keeps its surface at nearly 900°F (482°C). The planet's water boiled off to space long ago. Could that really happen on Earth, which is farther from the sun, and where the CO2 level is just now rising past 400 parts per million?