Effects of Climate Change

California’s coastline will be slammed with more extreme storms with intense rainfall in the coming years. That’s the conclusion of a group of National Oceanic and Atmospheric Administration (NOAA) and US Geological Survey (USGS) scientists at the recent California Climate Change Conference, sponsored by the California Energy Commission and the California Environmental Protection Agency¹.

Climatologists who studied the rainfall and temperature data during the past century have demonstrated that, beginning in the mid-1960s, temperatures began to rise everywhere in California and precipitation to increase in coastal areas (although not in the interior, which is becoming drier). They expect that trend to continue as the oceans warm.

Think of the earth’s atmosphere as a giant machine for redistributing energy and moisture around the globe. That redistribution pattern is changing as warming oceans create more "atmospheric rivers" of wind and water vapor flowing from the equator—called a pineapple express—straight at California, as shown in figure 1.

Figure 1: Atmospheric rivers, "the pineapple express," deliver intense amounts of warm rain. Source: National Oceanic and Atmospheric Administration.

According to NOAA and the USGS, northern California will experience more of the warm pineapple express storms from Hawaii, with very intense periods of rain. Often there is a near balance between a high-pressure area to the south of California and a strong low-pressure area off the Northern California or Oregon coast. In between, the frontal zone can ripple back and forth several hundred kilometers between Hawaii and California picking up more water vapor. When atmospheric rivers strike coastal mountains, the air rises, water vapor condenses and continuous heavy rainfall occurs up to fairly high elevations, as shown in figure 2.

Figure 2: When atmospheric rivers hit coastal mountains, they ascend and water vapor condenses, leading to intense rainfall. Source: NOAA/OAR-Earth System Research Laboratory.

Pineapple Express Floods
These are the types of storms that lead to large amounts of runoff flowing into creeks. "The most feared flooding comes from the slow moving storms with a long southwesterly fetch extending toward Hawaii, the so-called pineapple express," according to the former chief hydrologist of the California Department of Water Resources.² Pineapple express storms have caused:

• All 7 major floods on the Russian River in Sonoma.
• The 9 largest winter floods on the Carson River in Nevada since 1950.
• 44% of rainfall during only 21% of the total time it rained between 2001 and 2006 in San Francisco.
• The New Year floods of 2006 on Corte Madera Creek in Marin County.
• 6 of the highest peak flows on the American, Feather, and San Joaquin Rivers since 1950.

With more pineapple express conditions, the most intense storms, those that currently have a 1% chance of occurring in any given year, will actually occur far more often. Some scientists predict that 1% annual chance rising to perhaps a 10% annual chance in coastal areas.³ Statistics already show double the number of storms in the "danger zone" in the past 40 years versus the prior 40 years⁴.

Increased Wildfires Lead to Floods
The hotter drier summers will, by the most conservative estimates, lead to a 10-35% increase in wildfires by 2070⁵. In addition to loss of vegetation, agriculture and property, wildfires also have a dramatic effect on sediment and water flowing down hillsides. When intense rain hits a recently burned hillside, formerly stable streams and creeks can be transformed into debris channels with hyper-concentrated floods, in what called the fire-flood sequence⁶. Fires further dry soils that were already very dry, changing the molecular structure of the soil, which then forms an almost a waterproof seal. As California gardeners know, water does not easily soak into dry soil. Water that doesn’t soak into the ground instead runs off into creeks, which increases peak flows by several orders of magnitude compared with what they would have been in pre-burn conditions. And when fire removes vegetation, it also removes structural support for a hillside, which further increases sediment flowing into creeks.

Inadequate Infrastructure
Throughout California, levees, dams, flood control channels and bypasses are being forced to manage flows for which they weren’t designed. Even the most forward-thinking 1950s estimates of peak flood flows, such as engineers designed for on San Lorenzo Creek, are now being shown to be at least 50% below what will now actually flow from the hills during each pineapple express storm.

References
"American River Flood Frequencies: A Climate-Society Interaction," Kelly T. Redmond, Western Regional Climate Center, Desert Research Institute, Reno, Nevada, http://meteora.ucsd.edu/cap/kelly_flood.html.
"Analysis of July 2006 Record-Breaking Heat Wave," Dan Kozlowski, California-Nevada River Forecast Center and Laura Edwards, Western Regional Climate Center, http://www.calclim.dri.edu.
"California Flood Risks in a Changing Climate," Dr. Mike Dettinger, USGS and Scripps Institution of Oceanography.
"Climate Change 2007: The Physical Basis," Intergovernmental Panel on Climate Change, www.ipcc.ch.
"Our Changing Climate: Assessing Risks to California," www.climatechange.ca.gov/bienniel_reports/2006report/index.html.
"A Twenty-First Century Observing System for California Weather and Climate," Dr. Marty Ralph, NOAA.

_______________________
¹California Climate Change Conference, September 10-13, 2007.
² "A Half Century of Watching California Floods," Maurice Roos, Chief Hydrologist, California Department of Water Resources.
³ "Our Changing Climate: Assessing the Risks to California," Summary Report from the California Climate Change Center, funded by the California Energy Commission and the California Environmental Protection Agency.
⁴ Dr. Mike Dettinger, USGS and Scripps Institution of Oceanography.
⁵ "Our Changing Climate."
⁶ Thomas Spittler, "California Fires, Floods and Landslides," 2005.

Related Content