A massive dust storm hit Mongolia, China, South Korea and Japan last week. Last Monday, China was hit by the largest and strongest sandstorm in a decade, with 12 provincial areas in the country's north blanketed in yellow sand and dust.

It began in the Gobi Desert in the south of Mongolia. From there, the dust moved thousands of kilometres east, hitting northern China, the Korean peninsula and parts of Japan. Then, in the middle of the week, the sand storm turned around and hit China again.

Air quality plunged in China. And in Mongolia, where the sand storm originated, at least 10 people were killed.

Sandstorms have become less common in recent years in China, and many people attribute that to decades of efforts to combat desertification.

The Three-North Shelterbelt Forest Program, launched in 1978, is a national ecological effort that involved the planting of millions of trees along the edge of encroaching deserts in northern China. It is also known as the Great Green Wall project.

The project has helped the world to be a greener place today than it was 20 years ago. According to NASA, global leaf area has increased by five percent in the last two decades, an area equivalent to all of the Amazon rainforests. At least 25 percent of that gain was in China.

As the wooded areas expand, the average number of days in spring with dust storms has fallen from 12 in the 1960s to three days now in China. Before this week, the last sandstorm in Beijing was four years ago. So why did it happen again this year?

There are several factors in creating such a monster dust storm – a dust-rich place, strong wind to drive it forward, and an unstable atmosphere that can lift particles higher up into the sky.

And all three ingredients were fully prepared this time. The huge Gobi Desert is located in the continental interior, far from the ocean, offering lots of sand. And this year there was an unusually warm spring. Most of the area saw temperatures of 20 to 25 degrees Celsius in the middle of February, intensifying the dryness of the region. Moisture tends to weigh dirt down and keep it in place, while drought creates more dust storms.

Also, when the desert is hotter than other places, an imbalance of temperature creates a strong air flow, or wind. The storm that ripped through this year had wind speeds strong enough to lift tons of fine dust up to 1,000 meters and more above the ground, making trees unable to stop its progress.

Once dust has reached that height, atmospheric circulation can deliver it thousands of kilometres away. That's why dust storms sometimes cross oceans and continents.

China's efforts to build a Great Green Wall continue. And international cooperation is needed to help Mongolia, where over 70 percent of the land is degraded from overgrazing, deforestation and climate change.

Sand storms are bad for health. But they also have a positive role to play in biology. NASA satellite images reveal that every year over 2,800 tons of dust from the Sahara cross the Atlantic, and the minerals they carry fertilize plants in the Amazon forests.

Sandstorms are a natural phenomenon that cannot be artificially eliminated.