Drought's cause more natural, less human-induced, NOAA finds

The current drought that has gripped California for the past three years is mainly the result of naturally occurring phenomena, according to a study sponsored by the National Oceanic and Atmospheric Administration.

“The preponderance of evidence is that the events of the last three winters was primarily due to the natural variability of the atmosphere ocean system,” said Richard Seager, professor with Columbia University's Lamont Doherty Earth Observatory and lead author of the report released on Monday.

Seager said that though ocean temperatures are rising, global warming did not play a significant role in the lack of rain plaguing the state. In fact, he said that as a result of warming, Californians can expect an increase in mid-winter rainfall.

“The models are actually projecting, not overall drier conditions as a result of rising greenhouse gases, but a shorter, sharper rainy season,” Seager said.

Scientists with NOAA pointed out, though, that the resulting increased precipitation could be less beneficial, because warmer temperatures would lessen the amount that falls as snow.

Winter snow accumulation in the Sierra Nevada Mountains is important for California, because it stores water for distribution as runoff later in the year.

Juliet Christian-Smith, a climate scientist with the Union of Concerned Scientists, said that global warming’s impact on the snowpack will have consequences for Californians.

“Warmer temperatures are widening the gap between water supply and demand by contributing to earlier spring snowmelt meaning less water is available when demands are the highest in mid-to-late summer,” said Christian-Smith. “As global warming continues, decreasing snowpack and earlier snowmelt — coupled with increasing demand for water — will likely exacerbate water shortages.”

The Union of Concerned Scientists released a statement on Monday afternoon, saying the NOAA report didn’t account for temperature’s effect on water shortages.

“The NOAA study points to natural variability, but it is important to note that these random variations are happening in the context of higher overall temperatures caused by climate change and warmer temperatures spell trouble for water in California,” said UCS senior climate scientist Brenda Ekwurzel.

As for the cause of the current drought, scientists said that a ridge of high pressure over the Western Pacific Ocean has kept most storms from California during the past three years. Its longevity led many to dub it the “ridiculously resilient ridge.”

Marty Hoerling, a researcher with NOAA's Earth System Research Laboratory, said that the ridge of high pressure has dissipated and been replaced with a low-pressure system. Hoerling attributed that with recent rainstorms and ones forecast to hit the state later this week.

That new system appears to have gained its own alliterative name.

“Now we seem to be getting into what seems to be a lingering lousy low,” Hoerling said. 

Causes and Predictability of the 2011-14 California Drought

This story has been updated.