3 . Burning coal for energy adds planet-warning carbon dioxide (CO₂) to Earth’s atmosphere. As the planet heats up, experts warn that simply cutting greenhouse gas emissions (排放) will not be enough to avoid global warming. CO₂ must also be removed from the atmosphere.

Existing experimental machines that pull CO₂ directly from the air are too expensive to be widely used. But a new effective technology to remove CO₂ already exists. It is not expensive and easy. It is forests. Planting trees and watching forests are effective ways to clean the air.

Forests used to cover large areas of the Appalachian Mountains in the eastern United States. In the state of West Virginia, coal mining let the land there bare, without trees. Over the years, coal mining and cutting forests took over 90 percent of the red spruce (云杉) forests.

Chris Barton works for the University of Kentucky. He started a group called Green Forests Work, aiming to put trees back on the roughly 400,000 hectares of land.

However, Barton explains the land has problems. “If you planted trees on these places, they just didn’t grow. The ground was too hard. Water didn’t infiltrate (渗透). The trees can’t root. Oxygen can’t circulate in those environments.” Using heavy equipment, workers tear the ground. In this way, the trees put down roots.

Barton says not everyone believes the solution is a good idea. “We’ve had a lot of doubtful look at us twice from people. But after we do it, there’s no question that it was the right thing to do.” And it has worked. Forests are coming back to the grounds.

Scientists say that, in West Virginia alone, restoring red spruce forests to the area could send what is equal to 56 million barrels (桶) of oil into the ground. But it will take time — a long time. Around the world, experts say, nature offers powerful tools to fight climate change. But patience is needed. Nature works, but slowly, in its own time.

4 . About 97% of the world’s water is salty and is found in our oceans and seas. But, as we can’t drink sea water, how can it be important?

Every part of our seas and oceans contains an amazing number of animals and fish that live at different ocean depths. Most of the different species of animals and fish depend on simple plants for their food. These simple plants called algae (海藻) drift near the surface of the ocean and use sunlight to turn carbon dioxide and water into food and oxygen. In fact, algae produce over half of the oxygen people breathe. How important sea water is!

Each plant or animal in our seas and oceans is an important link in a food chain. The algae are eaten in large amounts by microscopic animals, which are in tur consumed by larger animals. These food chains are delicately (微妙地) balanced.

The bad news about the food chains in the oceans is that they are under threat because of man. People once thought that the oceans were so big that it didn’t matter if we dumped rubbish into them or caught huge quantities of fish and whales for food. But we now know this is not true and fish stocks in the oceans have started to drop.

Thankfully, the world is taking steps to protect the future of our oceans by introducing international agreements to protect marine habitats. Most counties have introduced fishing restrictions (限制) to protect fish stocks in the oceans and new techniques are being pioneered to cope with pollution. Finally, the importance of protecting oceans is being made known to more people. This is just the beginning of a long process to protect the oceans for our future. We depend on the oceans for fish which are an important part of the human diet. How important sea water is!

5 . Sometimes we may find that every week there are a lot of new stories about how climate change is affecting the planet, or new plans to battle its effects. But the concept itself isn't new at all --- in fact, scientists have been exploring questions about climate change for almost 200 years.

The idea of “greenhouse gases” goes back to 1824, when Joseph Fourier wondered what was regulating the earth's temperature. Fourier concluded that the atmosphere must be responsible for containing the heat absorbed from the sun and described it as a box with a glass lid: As light shines through the glass, the inside gets warmer as the lid traps the heat. As Fourier's ideas spread, it came to be called “the greenhouse effect”.

Scientists continued to study the greenhouse effect. Not until a Swedish chemist named Svante Arrhenius came along, did scientists understand how global warming actually works. After years of work, Arrhenius determined that the level of carbon dioxide in the atmosphere did in fact have a direct effect on global temperatures.

Arrhenius found that CO₂, and other gases trap radiation, which warms the atmosphere. Arrhenius was the first to suspect that burning coal could contribute to the greenhouse effect. But Arrhenius welcomed the warming effect on the planet. At a lecture later that year, Arrhenius noted that creatures of a warmer earth “might live under a milder sky and in less barren surroundings”.

While Arrhenius' findings won him the 1903 Nobel Prize in chemistry, scientists kept debating whether the greenhouse effect was increasing until 1950, when researchers finally began to find strong data supporting it. By the end of the 1950s, American scientists had been sounding the alarm on the long-term consequences of climate change. Climate change research has come a long way since Fourier first described the greenhouse effect – still, maybe Arrhenius should have been more careful of what he wished for.