A new report calls for better water and food management to prevent crises as the global population growth causes stress to natural resources.

The United Nations report issued Monday says more than 1.5 billion people already live in areas where water is scarce. But it says that if the number of humans rises from the current seven billion to at least nine billion as expected by 2050, more people could face water and food shortages.

The report says climate change will increase scarcity by altering patterns and intensity of rainfall. Scientists estimate that in Africa alone, agricultural output could be reduced by 15 to 30 percent by century's end.

They say that in many major food-producing regions, including the plains of northern China, India's Punjab and the western United States, water limits are already being “reached or breached.”

The authors are calling for innovative approaches to farming to yield better crops with less damage to the environment. They also recommend efforts to reduce losses and waste along the food chain, and smart management of natural resources, especially water.

Suggested ideas include better training for farmers, government incentives for environmentally-sound practices, crops that are more suited to scarce or irregular rainfall, water-efficient irrigation techniques and in some countries water reservoirs to help farmers survive dry spells.

The assessment compiled by the U.N. Environment Program and the International Water Management Institute was released at the start of the World Water Week in Stockholm, a global forum on water issues.

The agency U.N. Habitat said in a release Monday that urban residents in developing countries are struggling without adequate water and sanitation. It says up to 50 percent of urban residents in sub-Saharan Africa do not have access to clean water and even more lack adequate sanitation.