* U.S. entering an era of high weather volatility
* Corn yield trends seen sliding next 5 years
* High nighttime temperatures biggest threat to crops
By Christine Stebbins
CHICAGO, Dec 5 (Reuters) - A top Iowa State University agricultural scientist predicted on Wednesday that the United States would see below-trend yields for a fourth straight year in 2013 and spotlighted long-term climate risks for farmers in the years ahead.
"It's looking likely that we will have a fourth year of below-trend U.S. corn yields. But not as bad as in 2012, but still below trend," Elwynn Taylor, Iowa State University climatologist, told Reuters Ag Forum, an on-line chatroom.
Taylor is using a corn trend yield of 160 bushels an acre, based on the U.S. Agriculture Department's past 30 years of crop data. He sees the 2013 U.S. corn yield at 147 bpa, compared with the USDA's drought-hit estimate of 122.3 for 2012.
"Our short-term trend is decreasing, and I do not see a change in the trend within the next five years," said Taylor, adding that the effects of this year's historic "drought will probably ease, but not be erased."
U.S. farmers recently saw six straight years, 2004-2009, of increasing corn yields. But volatile weather patterns from floods to droughts have hurt crop production, including this year's drought that cut yields and affected two-thirds of the U.S. land mass.
The drought, the worst in 50 years, continues to stress winter-seeded crops such as hard red winter wheat and causing nightmares for grain shippers on Midwest rivers as low water slows barges moving grain to Gulf export terminals. The U.S. remains the largest single exporter of corn, soybeans and wheat.
Key will be moisture, a wild card, as well as trends such as El Nino and La Nina - weather anomalies that occur irregularly with changes in sea-surface temperatures in the South Pacific and that in turn affect weather patterns around the world.
"At present the trend is toward a La Nina pattern, but neutral, trending toward La Nina," Taylor said. "On average during a La Nina year, there is a 70 percent chance of below-trend yield for the Corn Belt. For 2013, the chances are somewhat higher because of the existing subsoil dryness."
Subsoil moisture in big crop states such as Iowa and Nebraska is already rated more than 90 percent short to very short, according th e US DA's latest crop condition reports.
Iowa soil moisture map: link.reuters.com/dab54t
Taylor does not expect subsoil moisture to be fully recharged by the 2013 spring planting in Minnesota, Iowa, the Dakotas and Illinois, but he said Indiana and Ohio should be in better shape. The western Corn Belt is suffering from below-normal rains, but last summer's root development also further depleted soil moisture, he said.
"Rooting conditions in 2012 were near ideal and corn and soybean roots of more than 8 feet were reported in numerous locations," Taylor said in an Iowa State newsletter released this week. (link.reuters.com/pyz44t)
"Normally it takes 12 inches of precipitation between October and May to fully recharge soil moisture. This year it's going to take 16 inches," Taylor said.
He also expects shipping conditions on the Mississippi River to worsen.
"The long-term drought does not begin to correct until the subsoil moisture is fully replenished," Taylor said. "History would say that the long term drought will not fully correct the rivers in 2013. At least the period before June will have disrupted shipping. We really have no idea of how much correction will happen in June, normally the wettest month of the year for the Midwest."
Taylor, who has studied agricultural climates for decades, says farmers and river shippers will have to get used to more volatile weather for years to come.
"We have periods of low weather volatility for perhaps 19 years followed by a period of about 25 years of high volatility, which we are just entering," Taylor said.
Commenting on whether the United States is facing a return of the Dust Bowl of the 1930s, when Great Plains suffered from drought for almost a decade, Taylor said Dust Bowl-like conditions occur on a roughly 90-year cycle.
"The harshest years were 1847 and 1936. If we are going to have Dust Bowl-like conditions in this century it will be near 2025, which does fall within the expected 25 years of high weather volatility that we are entering at this time," said Taylor.
He added that "increasing night-time temperature is the principle climate change threat to crops. Research has shown for annual grain crops, rice, wheat, corn night-time temperatures of 4 F higher than normal results in a 15 to 20 percent reduction in yield."
Asked if he worries about climate changes, Taylor said: "I don't worry about them, I study them. If you worry it disrupts your objectivity." (Reporting by Christine Stebbins; Editing by Peter Bohan and Andre Grenon)