Burning coal, oil and natural gas adds CO2 to the air.

How do greenhouse gases help warm Earth's surface?

• They slow the loss of heat to space.
• They stop most sunlight before it reaches the surface.
• They make the Sun send more energy toward Earth.

Which change can result from a warming climate?

• Glaciers grow because warmer air freezes more water.
• More glacier ice melts on land.
• Glaciers stay unchanged because land ice cannot melt.

With other conditions unchanged, what would much less greenhouse warming do to Earth's average temperature?

• It would rise because more heat could escape into space.
• It would stay the same because sunlight is the only factor.
• It would fall because heat could escape faster.

These made-up school plans cut yearly CO2 emissions by 12 kg and 17 kg. How many kg greater is Plan B's cut than Plan A's?

Why can warming seawater raise sea level even without extra meltwater?

• Warmer seawater takes up more space.
• Warming creates new water molecules in the ocean.
• Warm water weighs more and pulls the surface upward.

A mountain town has warmer winters and earlier snowmelt. Heavy spring rain then falls while snow is melting. How can rain and snowmelt together affect river flooding?

• Only rainfall matters; snowmelt adds no water to rivers.
• Only snowmelt matters; rain adds no water to rivers.
• Rain and snowmelt can combine to raise flood risk.

Ravi says cars are the only fossil-fuel CO2 source, so replacing car trips with walking would stop all these emissions. Which fact limits his claim?

• Walking can reduce emissions only when every car stops.
• Factories and power plants burn fuels too.
• Cars release CO2, but burning fuel for electricity does not.