If you need the ultimate example of a runaway greenhouse planet, look no farther than Venus.
It has an extremely thick atmosphere which is 96 percent carbon dioxide. That concentration of CO2 is the reason Venus’ surface temperature averages around 860-864 degrees Fahrenheit, which is hot enough to melt lead. This is despite the constant cloud cover which blocks solar input to quite an extent. Despite the dense clouds much of the heat which does get to the lower atmosphere and surface of Venus is trapped by the CO2.
Here on Earth, CO2 is up about 40 percent in our atmosphere since the dawn of the Industrial Revolution. While it is something akin to a trace gas, that large an increase is a major factor in our warming (on average) climate. Water vapor is the dominant greenhouse gas which allows life-supporting temperatures on our planet, but water vapor volume is closer to being a near constant. That 40 percent increase in CO2 is another matter entirely, since CO2 volume is not a constant.
Venus is hotter than Mercury. Mercury is much closer to the sun than Venus, but it lacks the runaway greenhouse atmosphere so its heat cannot be trapped like that of Venus. Mercury is broiling in the daytime but frigid at night. Venus is hot day and night.
New NASA research and climate modeling from its Goddard Institute suggest Venus once may have had a shallow liquid water ocean and habitable surface temperatures during its first 2 billion years. Even the 1980s Pioneer probe to Venus showed evidence an ocean may have once existed there.
But there were many factors working against a habitable environment lasting beyond the planet’s earlier years. A Venusian day lasts 117 Earth days. That allowed tremendous daily heating even before the atmosphere developed its runaway greenhouse characteristics. With that heating - even with a sun estimated to be 30 percent dimmer than it is now - the ocean water evaporated over time. The water vapor molecules were broken apart by the sun’s ultraviolet radiation, and the H—hydrogen—in the H20 escaped into space.
The climate modeling shows the removal of hydrogen and destruction of water vapor allowed CO2 to become more and more abundant, setting off the runaway greenhouse effect. So, before the era of staggering amounts of CO2 and evaporated oceans, Venus received about 40 percent more solar input than Earth. But after the thick cloud cover (which also includes sulfuric acid mist) formed, the solar input dropped off sharply.
I have occasionally run into people who don’t even believe CO2 is a greenhouse gas, due either to a lack of understanding of this very basic and simple atmospheric chemistry, or denial. This is not a hypothesis: The CO2 greenhouse effect and its longevity in the atmosphere are irrefutable facts.
What’s NOT irrefutable is how much of an impact it has on warming and at what rate the warming will progress. The ocean has absorbed some of the excess heat and the carbon (the latter leading to harmful acidification), and that ability to absorb heat has sometimes slowed the rate of atmospheric warming on Earth.
There is no danger our planet’s atmosphere ever will develop such astronomical levels of CO2 as exist in the atmosphere on Venus. But an increase in CO2 levels from about 280 parts per million in the late 1700s to a little over 400 parts per million today cannot be brushed aside just because CO2 is a trace gas in the atmosphere.
At least on Venus, no one can blame alien creature activity for the vast increase in CO2 leading to extinction-level changes in the environment. Our comparatively much smaller though significant changes here, however, can be placed at our human doorstep.