Runaway Greenhouse Effect

Global climate change could spin out of control, that is, lead to the "runaway greenhouse effect." It's called "runaway" because it runs away from human control to do anything about it.

There are a variety of positive feedback mechanisms that could lead to this outcome. For example, as the earth warms, and permafrost melts. The organic matter in the frozen muck decays and methane gas is released. As more of this greenhouse gas is released, more solar radiation is trapped leading to more warming. This leads to further melting and decay of the frozen tundra in a positive feedback process.[1]

This is what some NASA scientists had to say about the phenomenon:

The phenomenon, called the ‘runaway greenhouse’ effect, occurs when a planet absorbs more energy from the sun than it can radiate back to space. Under these circumstances, the hotter the surface temperature gets, the faster it warms up [that is, it accelerates]... scientists believe Venus did experience a global runaway greenhouse effect about 3 billion to 4 billion years ago.

So, we hear a lot about sea level rise, disease, and other likely impacts of global climate change, but very little is said about the runaway greenhouse effect. Perhaps the popular media should stir people's imagination with a description of Venus, which is where the Earth could end up if we don't take action soon:

Water and water vapor are extremely rare on Venus due to its high surface temperature that can approach 758 K (900 °F). This extreme temperature is caused by the greenhouse effect. As sunlight heats Venus' surface, the surface radiates infrared energy that is kept from escaping the planet by dense carbon dioxide atmosphere.

The Earth is not Venus. The Earth is in a different orbit around the Sun, the planet sizes are different, the atmosphere of Venus is far more dense; however, the severity of outcome of a potential runaway greenhouse event on Earth subordinates all other considerations relative to taking action; even an economic depression like that of 1930s would be a cake walk compared to the alternative.

Notes and Sources:

[1] Another example, Tropical Runaway Greenhouse, is described in the paper below.

NASA, Ames Research Center, TROPICAL ‘RUNAWAY GREENHOUSE’ PROVIDES INSIGHT TO VENUS,
Kendall Powell, John Bluck, Release: 02-60AR, May 15, 2002