What is the single most effective way to reduce greenhouse-gas emissions? Go vegetarian (素食)？ Replant the Amazon? Cycle to work? None of the above. The answer is: make air-conditioners better. On one calculation， replacing refrigerants (制冷剂) damaging the atmosphere would reduce total greenhouse gases equaling 90bn tonnes of CO₂ by 2050. Making the units more energy-efficient could double that. By contrast, if half the world’s population gave up meat, it would save 66bn tonnes. Replanting two-thirds of tropical forests would save 61 bn tonnes. A one-third increase in global bicycle journeys, just 2.3bn tonnes.

Air-conditioning is one of the world's great overlooked industries. Automobiles and air-conditioners were invented at roughly the same time, and both have had a huge impact on where people live and Work. Unlike cars, though， air-conditioners have drawn little criticism for their social impact, emissions or energy efficiency. Most hot countries do not have rules to govern their energy use. There is not even a common English word for “coolth” (the opposite of warmth).

Yet air-conditioning has done more than most things to benefit humankind. Lee Kuan Yew, the first prime minister of Singapore, called it “perhaps one of the signal inventions of history”. It has transformed productivity in the tropics(热带地区)and helped turn southern China into the workshop of the World. In Europe, its spread has pushed down heat-related deaths to 10% since 2003, Men 70,000 people than usual, most of them elderly, died in a heatwave. For children, air-conditioned classrooms are associated with better grades at school.

Environmentalists who call air-conditioning“a luxury we cannot afford” have half a point, however. In the next ten years, as many air-conditioners will be installed(安装)around the world as were put in between 1902(invention time)and 2005.Unless energy can be produced without carbon emissions, these extra machines will warm the world. At the moment, therefore, air-conditioners create a vicious cycle. The more the Earth warms, the more people need them. But the more there are, the warmer the world will be.

Cutting the impact of cooling requires three things. First, air-conditioners must become much more efficient. The most energy-efficient models on the market today consume only about one-third as much electricity as average ones, Minimum energy-performance standards need to be raised raised, orintroduced in countries that lack them altogether, to push the average unit's performance closer to the standard of the best.

Next, manufacturers should stop using damaging refrigerants. One type called hydrofluoro-carbons, is over 1,000 times worse than carbon dioxide when it comes to trapping heat in the atmosphere. An international agreement to knock out these pollutants will come into force in 2019.

Last, more could. be done to design offices, malls and even cities so they do not need as many air-conditioners in the first place. More buildings should be built with overhanging roofs orbalconies for shade, or with natural ventilation (自然通风).Simply painting roofs white can help keep temperatures down. Providing indoor air-conditioned comfort need not come at the expense of an overheating world.

Researchers in Australia, who studied 1 , 500 people and their lifestyle, have found that having good friends can help you live longer. An American study of 1 0,000 students, over a period of 35 years, also found that if you make more friends than the average person at school, you’ll receive a higher salary（薪水） in later life. People need good social skills at work to manage people and work in a team successfully. These are the same skills we use to make friends at school.

On average(平均), teenagers aged between 1 5 and 1 7 have 500 “friends”on their favourite social networking site. Adults have 1 30. So if you believe this research, you might live for a long time and be very rich.

However, according to Professor Robin Dunbar from the University of Oxford, it probably won’t make any difference. Dunbar studied the number of messages between users of a popular social networking website, each of whom had between 200 and 2,000 friends. He found that they always communicate with a maximum (最大量) of 1 50 people.

Among these 1 50, Dunbar believes that around five people are close friends. You’ve mostly known them for a long time; they are probably old friends and you share all your good and bad experiences with them. Then there are ten more friends. Although they’re close to you, you may not keep in touch with them every week. Next there are 35 people who you might spend time with because of a shared interest. You aren’t close. And finally, there’s a large group of 1 00. You see or speak to these people at least once a year, but you don’t know them well. Beyond this number, Bunbar says, it’s impossible to make any relationship meaningful.

If you have a lot of online “friends”,try this experiment: First take away anyone you haven’t been in touch with for a year. Then remove people you can’t remember and, finally, take away friends who you wouldn’t mind losing touch with. How many do you have left? How many of these people are actually good friends? According to the research, these are the only people that really matter.

To solve a big environmental problem, chemists have been thinking small. Really small: a new mini robot with the purpose of helping clean up tiny plastic polluting water across the globe.

The new micro-robots, each of which is no bigger than the tip of a pencil, are magnetic (磁性的) and shaped like four-pointed stars. When the sunlight shines on them, they can swim in a specific direction; when the sunlight disappears, they stop moving. Finding a piece of plastic, they hold onto it, produce chemical reactions and start to break it down.

The project is led by chemist Martin Pumera, a researcher who also studies ways to build micro-robots at the Czech University in Prague. About ten years ago, he noticed the micro-plastic was everywhere, from the bottom of the ocean to the ice on the top of mountains. It even turned up in drinking water, both bottled and tap water. Just think about how much plastic you encounter every day. It doesn't easily degrade (降解), which is a big problem. Therefore, Pumera chose to focus on the problem of water pollution caused by micro-plastic.

The researchers tested the micro-robots on four types of plastic in the lab. After a week, all four began degrading, losing between 0.5 and 3 percent of their weight, which indicated they were breaking down. The robots also turned the plastic's smooth surface into rough. Finally, they could be collected for reuse along with the plastic waste without causing new pollution.

In fact, Pumera says they still have a long way to go. These micro-robots are unlikely to succeed in degrading all types of plastic. They'll also need a lot of tests to show that they're safe in open waterways, such as at sea. But he thinks that these challenges can be overcome. Someday, the micro-robots will play a big role in a worldwide cleanup effort.