Over the last few weeks, the world has been following the floods in China's Yangtze basin, the worst in 44 years. Official figures indicate that more than 2,000 people have drowned and 13.8 million have been driven from their homes by the floodwaters.

Damage is extensive. Crops have been totally destroyed on 11 million acres or 3 percent of the national cropland total. Industrial output has suffered as the floods have forced factories to close. The transportation of both goods and people has been disrupted by closing of roads, rail lines, and docks. The overall effect on the Chinese economy will be felt for many months.

The Chinese government is treating this disaster as an act of nature, and indeed it is. Floods during the monsoon season from June through September in southern China are a regular occurrence.

But there is also a human hand in this year's floods in the form of deforestation and intensive land development. The Yangtze basin is home to 400 million people, making it one of the most densely populated river basins on earth. To understand the population pressure in eastern and southern China where most of the country's 1.2 billion people live, imagine squeezing the entire U.S. population into the area east of the Mississippi River and then multiplying it by five.

With such a density of population, the human pressure on the land is everywhere. To begin with, the Yangtze river basin, which originates on the Tibetan Plateau, has lost 85 percent of its original forest cover. The forests that once absorbed and held huge quantities of monsoon rainfall, which could then percolate slowly into the ground, are now largely gone. The result is much greater runoff into the river.

The construction of buildings and roads in the basin is increasing at a staggering pace. With the average household in China consisting of fewer than four people, a population of 400 million means 100 million housing units. And land hunger is forcing more and more homes to be built on the river floodplain.

The extent of factory construction also defies the imagination. Although there are no precise data on the size of the industrial workforce in the basin, a rough estimate would be at least 50 million. With the average factory in the private sector employing fewer than 100 people, this means half a million factories. Each factory needs a warehouse for storage of the raw materials coming into the factory and for the finished products while they await shipment. Each factory must have a road connecting it with the rest of the country. Collectively these homes and factories cover a vast area, further reducing the capacity of the land to absorb rainfall.

At the global level, the human influence on the floods is less direct but no less real. The global temperature during the first seven months of this year was the highest of any comparable period on record. Furthermore, the margin of increase over the previous record is itself a record. The month of July is the warmest month since recordkeeping began in 1866.

Higher temperatures mean more evaporation, more intense storms, and more rapid snow melt. All three could be contributing to the floods. While there is no way of conclusively linking global warming with specific weather events, the likelihood that they are linked has grown with each passing year in which higher temperatures are accompanied by more extreme weather events.

That higher temperatures mean more evaporation is certain. And when more moisture goes up, more comes down. Where it comes down is less predictable. But the Yangtze basin may well be one of the areas getting some of the additional rainfall.

Another likely effect is more intense monsoons-the results of seasonal warming over the continents as summer unfolds. As temperatures climb over the land, the air rises, pulling moisture-laden air from the oceans inland. The higher the temperature over land, the stronger the monsoon.

Higher global temperatures are also leading to increased snowmelt. We don't know with certainty whether the temperature has risen this year in the snow-covered regions that feed the upper Yangtze but, given the dramatic rise in the global average in recent months, it likely has.

While this flood may be the worst in 44 years, we can expect even worse floods in the years ahead. If the basin adds another 100 million people as projected over the next few decades, China will need to build another 25 million homes. As industrialization continues at a rapid pace, factory and road construction will also continue, further reducing the area of land that can absorb water and increasing the amount that will ultimately flow into the Yangtze.

With the international community unable to agree on a meaningful effort to reduce CO₂ emissions, rising atmospheric levels of this greenhouse gas promise even higher temperatures in the future with the potential for more evaporation, more rainfall, and even stronger monsoons.

Over centuries, the Chinese have developed a remarkable capacity for shoring up dikes and protecting themselves from flooding. For example, 1.6 million troops of the People's Liberation Army have been mobilized in recent weeks to help protect the dikes and to move people out of areas being flooded. Literally millions of civilians are involved in this enormous human effort to contain the Yangtze.

Despite this effort, however, the Chinese have had to make difficult decisions. To save major industrial cities, such as Wuhan, they have had to open floodgates upstream, flooding local areas once they were evacuated. When open floodgates do not release enough water, workers are dynamiting holes in the dikes. More than half a million people have been evacuated from areas that either have been flooded or may be flooded in the effort to save major cities.

The Chinese deserve a lot of credit for the capacity that they have developed to deal with flooding. But even this is likely to be overwhelmed in the future as human activities, both local and global, increase the sheer volume of water flowing into the river. With the region so densely populated, there is simply no place to put additional water, except where people already live. Future evacuations are likely to dwarf those up until now.

The 400 million Chinese living in the Yangtze basin are beginning to feel directly the effects of altering the environment. Other countries with rapid population growth can now see the consequences of waiting too long to stabilize population. And it is not too late for the international community to begin working together to lower CO₂ emissions before climate change affects even more people.