Scientists have warned Asian countries that they face chronic food shortages and likely social unrest if they do not improve water management. They say countries in south and east Asia must spend billions of dollars to improve antiquated crop irrigation to cope with rapid population increases. That estimate does not yet take into account the possible impact of global warming on water supplies. Asia's population is forecast to increase by 1.5 billion people over the next 40 years. Asian countries will need to import more than a quarter of their rice and other staples to feed their populations. Asia's food and feed demand is expected to double by 2050. "The best bet for Asia lies in revitalising its vast irrigation systems, which account for 70% of the world's total irrigated land...Without water productivity gains, south Asia would need 57% more water for irrigated agriculture and east Asia 70% more. Given the scarcity of land and water, and growing water needs for cities, such a scenario is untenable." The scenarios forecast do not factor in the impact of global warming, which will likely make rainfall more erratic and less plentiful in some agricultural regions over the coming decades.