Scientists in the United States have said that long-term patters suggest rainfall patterns will shift over South-Western states and the North of Mexico in coming decades.

A study by scientists from Columbia University and NASA's Goddard Institute revealed that, over the next few years, a delay will develop in the summer monsoon rains that fall over the area. The North American monsoon delivers as much as 70 percent of the region's annual rainfall, watering crops and rangelands for an estimated 20 million people.

The overall result of shifting rainfall patterns would be that the South-Western USA and North-West Mexico, which have both suffered under drought over the past year, would become even drier. Based on the results of modelling analysis, as climate change takes effect, heavy rains will shift from July and August to September and October.

Richard Seager, a Columbia University climate scientist, said, "There still will be a healthy monsoon which is good news for agriculture in the southern U.S. and northwestern Mexico-the timing is the problem here."

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A delayed monsoon could potentially lower crop yields as rains come later in the growing season, when the days become shorter. By prolonging hot and dry conditions during spring, a late monsoon could also trigger more wildfires and force cities to stretch diminishing water supplies, the researchers said.

They admitted that it is unclear at present how local vegetation, including agricultural crops, will fare under changing conditions, though they pointed out that agricultural lands and rangelands in the region are mostly rain-fed. A late monsoon in 2005 hindered summer grass development to the point that US ranchers had to buy supplemental feed for their cattle

Using the most up-to-date modelling systems, researchers suggested that by the end of the current century (2080-2099), monthly changes in precipitation, combined with rising temperatures, will make it more difficult to form clouds early in the traditional monsoon season when soils are at their driest. This would effectively delay rain and snowfall until enough moisture can be moved in from the oceans.

Earlier evidence suggests that such a shift in rainfall patterns is already beginning to take effect. A 2007 study in the Journal of Climate by University of Colorado scientist Katrina Grantz found July rains had been in decline since the late 1940s, with a corresponding increase in August to September precipitation. However, the Columbia and Colorado climatologists' findings are not universally accepted; Chris Castro, of the University of Arizona pointed out that, although the second-latest monsoon onset ever recorded was in 2005, the 2006-2009 monsoon rains all fell earlier than average or arrived on time.

Though total rainfall is predicted to stay the same, the shift away from the typical recharge periods, alongside warmer summer temperatures, will mean evaporation is likely to become more of a problem. Reservoirs and rain-fed crops are likely to benefit less from the same amount of rainfall under changing conditions, according to the Columbia team.

Study author Benjamin Cook commented, "We hope this information can be used with other studies to build realistic expectations for water resource availability in the future."

Renewed concerns for drought-stricken US Midwest

In the United States' Midwestern and Great Plains, drought last summer had major economic effects (currently estimates place the total cost at around $35 billion) and caused widespread panic, leading maize and wheat prices to rocket upwards in June and July. Higher than average rainfall is currently easing the worst effects of drought in a number of regions, though climatologists have begun to suggest that the damage caused by the country's worst drought in over 50 years will not be undone so easily.

Although conditions in much of the Midwest improved enough to benefit spring planting, above-average temperatures across the recovering drought areas and into the Western states could potentially spread drought conditions this year. Nevertheless, commercial weather forecasters in the US have said long-range predictions suggest conditions will be more favourable this year than last, when soy and maize production were slashed to 1980s levels.

A spokesperson for AccuWeather told news agency Bloomberg "The biggest threat from dry weather this year may be lack of irrigation for the fruit and vegetable crops in California," earlier this week. However, Brad Rippey a US Department of Agriculture meteorologist contended that "very poor" conditions in Colorado, Wyoming and New Mexico, where 'snowpack' feeds the Great Plains' river systems constitutes a cause for concern. These areas have lower than 50 percent average snowpack, though other regions have reported above average levels.

Though snowstorms have deposited a cumulative 60 cm of snow over Kansas, Nebraska and Oklahoma in recent weeks, experts said that this heavy snowfall is equivalent to much lighter rainfall, and may well run off frozen ground rather than contributing to recharge. According to the government Climate Prediction Center, 55 percent of the US land area remains in drought and the most parched areas of the country have not received the amount of moisture needed.

A number of UN organisations, including the Food and Agriculture Organisation, gathered in Geneva this week to encourage national governments across the world to increase provisioning for drought resistance and resilience in the face of more frequent and severe patterns of drought. FAO said in a statement issued last Friday (8th March) "Droughts cause the deaths and displacement of more people than cyclones, floods and earthquakes combined, making them the world's most destructive natural hazard."