Over human history, one of the major ways that humans have adapted to changing environmental and economic fortunes has been to migrate from unproductive or impacted regions to more productive and hospitable regions. But today, the world’s population has grown too large. There is less usable, unpopulated territory to absorb migrants. National borders are less open, so it is difficult for people to move to other countries when droughts, floods, famines, and wars occur. These boundary effects could be particularly severe for small and poor countries, whose populations are often unwelcome in richer countries. In the 1840s, more than 1 million Irish people emigrated because of the potato blight. Can you imagine an equivalent migration of many millions of people today? Keep in mind that there were only about 1 billion people on Earth then. There are 6 billion now.