3. This increases global air temperature and warms ocean water. The warmer air melts glaciers and snow on mountains, and water flows down to the ocean and causes the sea level to rise.

WHAT'S KNOWN
Greenhouse gases make up only a tiny fraction of the atmosphere, but they can have a big impact, and their proportion is rising rapidly.

In 1850, about 280 out of every million air molecules were carbon dioxide. Now the concentration is 360 parts per million, and if nothing is done by 2100, scientists predict it will nearly double to 700 parts per million.

A similar doubling is projected for methane -- primarily a byproduct of agriculture -- which is scarcer than carbon dioxide but many times more potent than a greenhouse gas.

What Are Greenhouse Gases?