Settled by the Celts, invaded by the Romans and the Normans, and ruled by the English, Wales' population has regularly changed over the ages. Wales, particularly South Wales, became a melting pot (熔炉) of European nationalities and cultures. At first workers from England, Ireland and the rural areas of Wales rushed into the coal mines of South Wales. By 1911, workers from Ireland, Italy and Spain had joined the industrial workforce (劳动大军) there. Many of these newcomers were prepared to work for less pay.

    But during this period, workers, dissatisfied with regular pay cuts, poor safety, returned to the land. In 1891, the rural population of Wales was about 616,000 and by 1911 just over 649,000 people lived and worked in these areas.

       The rural population continues to grow slowly even today. People from richer parts of the UK, particularly southern England, are buying holiday homes in Wales. In some areas this has led to a lack of housing for local people. This, combined with the lack of employment in rural Welsh communities, has started a new movement of people in search of jobs and homes into the towns and cities of Wales and to other parts of the UK and Europe.

    As so many Welsh people have moved to other countries over the last 200 years, it's really no surprise that many visitors come to see friends and relatives. These visitors strengthen their links to their land of origin and continue to support the Welsh's development. During the hot summer days, about 28% of the visitors visit the countryside and nearly 70% visit the seaside.

    Even if trees cannot walk, they are still on the move.

    In parts of the Arctic, entire forests are moving northward. Across the Arctic, temperatures are rising faster than anywhere else in the world. As that happens, the tree line that marks where forests stop and the treeless tundra (冻原) starts has been shifting northward. Trees growing along the tree line must protect themselves from the cold wind. To dothis, plants tend to grow horizontal (水平的) branches low to the ground. The energy it takes for trees to grow this way means they don't have enough energy to make seeds.

    But as Earth's climate has been warming, trees no longer have to just grow horizontally. Many can instead grow up toward the sky. This takes less energy. And with all that leftover energy, these trees have started producing more seeds. This happens especially in places where white spruce (白云杉) grows.

    White spruce, which is a North American tree, is quite able to produce a lot of seeds, which can move long distances in the wind. When wind­blown seeds end up on the tundra beyond the tree line, they eventually can sprout(发芽) new trees. This explains how a forest can move. Of course, the process would work only if the tundra were warm enough. But in recent years, the whole planet has been warming.

    New trees will provide shelters for some snow. Keeping the sun's rays from making the white surface disappear. Instead, the trees absorb the sun's heat. This warms the surrounding air. The extra warmth encourages even more trees to produce seeds. That further boosts a forest's ability to expand. In addition, more trees will trap more snow, preventing much of it from being blown away. Snow can trap heat in the soil below, which encourages trees to grow. The recent rise of temperatures has helped more trees grow past the tree line. People worry about impacts on the animals that depend on frozen conditions for food and shelter.

    The organic food has gained a lot of [popularity as they are being considered as healthier and tastier.  A fair number of people advocate a large-scale shift to organic farming from conventional agriculture. But this might not seem well-founded.

    Since the mid-19^th century, conventional industrial agriculture has become incredibly efficient on a simple land to food basis. Conventional farming gets more and more crop per square foot of land, which can mean less wilderness needs to be transformed to farmland.

    To make farming more efficient, conventional agriculture uses a significant amount of synthetic fertilizer（合成肥料）each year, and all that nitrogen（氮）enables much faster plant growth. However, the cost is paid in vast polluted dead zones at the months of many of the world's rivers, because much of the nitrogen ends up running off the soil and into the oceans. This also makes conventional farming one of the major threats to the environment.

To weaken the environmental impact of agriculture, improve soil quality as well as produce healthier foods, some farmers have turned to organic farming. Environmentalists have also welcomed organic food as better for the planet than the food produced by agricultural corporations. Organic practices — refusing artificial fertilizers and chemical pesticides –are considered far more sustainable. Sales of organic food rose 7.7% in 2010, up to $26.7 billion—and people are making those purchases for their moral senses as much as their tongues.

    Yet a new meta-analysis in Nature does the math and comes to a hard conclusion: organic farming produces 25% fewer crops on average than conventional agriculture.

    In the Nature analysis, scientists performed an analysis of 66 studies comparing conventional and organic methods across 34 different crop species, from fruits to grains. They found that organic farming delivered a lower output for every crop type, though the difference varied widely. For crops like fruit trees, organic trailed（落后于）conventional agriculture by just 5%. Yet for major grain crops and vegetables – all of which provide the world's main calories – conventional agriculture outperformed organics by more than 25%.

    What that means is that while organic farming may be more sustainable than conventional agriculture, there are trade-offs（此消彼长）with each. So an ideal global agriculture system may borrow the best from both systems rather than upholding merely organic or conventional practices.

    In this Pennsylvania city, Pittsburgh is shrinking but getting wealthier. Since 2000, its population has declined by 95,000 while its income per capita (人均) has shot up 24 percent. The trend is taking hold in many other cities, like Buffalo in New York, Providence in Rhode Island and New Orleans.

    Some of these areas have created more high-paying jobs in energy, health care or education. Others have managed to reshape their producing industry for a new economy. Higher-paying jobs have a greater effect because they create demand for additional services. "The story in Pittsburgh is very positive, and other areas are looking at it as an example of the transformation that might be possible," said Guhan Venkatu, who wrote an economic history of the area called “Rust and Renewal" for the Federal Reserve Bank of Cleveland.

    Carnegie Mellon University and the University of Pittsburgh have helped bring tech jobs and innovation(革新) to the area by sponsoring tech centers that help graduates start companies without moving to Silicon Valley or San Francisco. This has helped keep Pittsburgh's educated young population growing even as the entire population in the city has dropped.

    Pittsburgh has more STEM (science, technology, engineering and math) jobs than other shrinking cities, about 80, 000 or 7% of all jobs. STEM jobs add productivity and income growth to the area. Manufacturers of high-tech medical equipment in the Pittsburgh area also have doubled employment in the last 10 years.

    However, some experts question whether growing income per capita can really make up for a declining population. According to Patrick Adler, a researcher at the University of Toronto, population loss does matter if it means lower-skilled workers have fled because of a lack of opportunity. What's more, high-paying jobs in education and health care can disappear if the population declines too greatly. So it'd be wise to find ways to increase the population.

    Beneath the water in the China's Qiandao Lake lies a hidden treasure. The ancient city of Shi Cheng (also known as Lion City), located in the province of Zhejiang, has been submerged underwater for 53 years. Shi Cheng was founded about 1,300 years ago in a valley surrounded by the Five Lion Mountains. When the Chinese government decided they needed a new hydroelectric power station (水力发电站)， a dam and man-made lake were constructed and the city was left at the bottom of this new body of water. Now, it sits at depths ranging from 85 feet to about 103 feet.

    Much like the Machu Picchu ruins in Peru, this underwater city remained forgotten until some renewed interests lead to its rediscovery. Qiu Feng, a local official in charge of tourism thought about utilizing Shi Cheng for entertainment on the lake and as a destination for diving clubs. On September 18, 2001, scuba divers made their first attempt underwater. "We were lucky. As soon as we dived into the lake, we found the outside wall of the town and even picked up a brick, "Qiu told Guangzhou Daily in an interview. More research was conducted, and it was discovered that the entire town was intact (1881), including wooden beams and stairs. After the initial findings, international archaeologists and a film crew then recorded the preservation of the lost ruins.

    Looking at surface of the lake, dotted with over 1,000 islands, you'd never know that an entire city was below.

    It's incredible to think that after all this time, the structure still remains intact and we could one day visit it. Until then, Shi Cheng will continue to remain inconspicuously submerged.

    If you're encouraged by the tiny house movement and think 160 square feet is just enough for your needs, you may want to contact the Academy of Construction and Design (ACAD) at IDEA Public Charter School in northeast Washington.

    Students in the program built a micro house with a kitchen, a bathroom, a sleeping loft with space for a queen-size bed and a storage loft, all set on a trailer for mobility. The exterior (外部) of the house was part of a continuous living exhibit in 2015and was moved to the IDEA campus so students could work with builders to complete the interior earlier this year.

    McMahon said the D.C. government's approval to push companies to hire District residents (居民) was at odds with the school system not preparing students for careers in construction, exploring or electrical work. McMahon gathered industry and community leaders to establish ACAD in 2005 and he said 100 percent of the companies he contacted responded positively to the idea, including major firms such as JBG, Clark Construction, Hines Construction and Boston Properties.

    "When students make the connection between what they are learning and a potential career, their academics improve dramatically," said Carol Randolph, chief operating officer of the D.C. Students Construction Trades Foundation. "Some of them who didn't think college was an choice now have a better chance because their classes have become relevant to a job."

    "We teach them life skills and explain that even if you start as a laborer, there are opportunities to move up quickly," he said. "We rewrite the story for them and explain that they can work for a few years, make good money, get promoted, and start their own business or go to school with less debts."

    "Parents and school advisors can be the biggest obstacles because of the negative idea of construction as a blue-collar career," Karriem said. "I get middle-school advisors on board to talk about the opportunities this education provides. These kids are learning lifelong skills that can help them in other fields, provide them with income and allow them to take care of their homes."