As the world's population soars past 7 billion, farmland and freshwater are becoming increasingly valuable resources.

And, in response, a growing number of companies and investors — Wall Street traders, Chinese state corporations, Gulf sheiks — have been buying up farmland abroad. The trade has been booming since 2007, when a spike in grain prices got everyone fretting about shortages. The purchases help countries like China and Saudi Arabia secure food supplies and conserve water domestically. But critics worry that the trade has also spurred a rise in "land grabs" — when sellers in countries like Ethiopia or Cambodia forcibly acquire the farmland from locals in the first place.

So how big is the trade? A new study in Environmental Research Letters finds that at least 126 countries are now involved in purchasing or selling global farmland. The most active buyers are investors in the United States, China, Britain, Germany, India, and the Netherlands. They're typically seeking out land in South America, Africa, and Asia — particularly Brazil, Ethiopia, Philippines, Sudan, Madagascar, Mozambique, and Tanzania.

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