* 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 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
"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."
CLIMATE ON FRONT BURNER
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
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
(Reporting by Christine Stebbins; Editing by Peter Bohan and