Right now, the amount of carbon dioxide in our environment is hovering at 400 parts per million, a number that is already large enough to start affecting our environment. Carbon dioxide is a greenhouse gas, meaning that it traps heat within the Earth’s atmosphere. That is why governments around the world are trying to reduce the amount of carbon dioxide put into the atmosphere annually, in the hope that the planet will not get so warm that parts ofthe planet become uninhabitable, But what if those reductions do not happen, and instead, everything goes horribly wrong?

    Looking at a computer model of a world completely covered in water (a simple analog of the Earth, which is 71 percent covered by water) Popp and colleagues looked at what would happen if the carbon dioxide levels rose to staggering levels. They found that when the level of carbon dioxide in the atmosphere reached 1,520 parts per million,temperatures at the surface of the world would reach nearly 135 degrees Fahrenheit, evaporating vast amounts of water, and sending them high up into the atmosphere, near space. In this scenario, called the "moist greenhouse" in the paper, Popp estimates that water could easily escape from the world’ atmosphere into space. Worse, they found that once moist greenhouse conditions were reached, they could not be reversed, even by removing the excess carbon dioxide.

    A similar situation could happen in a few billion years as the sun brightens in the natural course of its evolution, sending out so much more heat and light that the temperature of the Earth's surface rises, creating a similar moist greenhouse effect.

    But there is no need to worry right now about whether the world will end in fire or gas. Given the lengthy time scale to reach either situation (milions if not billions of years); these are more geological doomsday scenarios than human ones. Is not that comforting?

(Adapted from http:/www-popsci.com/how-worlds-oceans-might-eventually-vanish-into-space)

In paragraph 2, the author presents ....