17.3: The Present

A Mediterranean Climate

The most common way to describe California’s climate is “Mediterranean.” This is true but too simplistic.

A Mediterranean climate means warm, dry summers with wetter, temperate winters. It is not unusual for rains to cease in spring and not return in any appreciable amount until fall (see Figure \(\PageIndex{1}\):). While this description matches climate patterns for much of California, this state has much elevation variation, numerous differences between coastal and inland areas arising from the buffering effects of ocean proximity, and spans many degrees of latitude–from latitude 32 near the Mexican border to latitude 42 near Oregon.

Figure \(\PageIndex{1}\): San Francisco's annual precipitation profile, with months compared to millimeters of precipitation, using a NOAA dataset from 1991-2020. "SF monthly precipitation" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

California’s climate experiences significant variations on a year-by-year basis. In 2013, average statewide rainfall totals were only 20 cm, while in 1983 that number was 108 cm. When even a straight-forward question such as “How much rain does California get?” is answered by 400% variations, that makes broad characterizations suspect.

California’s climate variation is highly local. For example, rainfall in Death Valley averages only 5 cm/year, while Eureka typically receives over 100 cm/year. One reason for Death Valley’s peculiar aridity is its position in an exceptional “rain shadow,” meaning that surrounding mountain ranges squeeze moisture from clouds before they can reach Death Valley. There’s precipitation nearby, but it just doesn’t reach Death Valley.

Rain Shadows

The first step in understanding Death Valley’s local rain shadow is to know that the dominant flow of wind is from the west to the east, with moisture laden clouds first rising up over the Sierra Nevada foothills. This rising creates orographic precipitation, which is to say that as moisture-rich clouds are forced to higher altitudes, they cool, condense, and drop their moisture. In Death Valley, this orographic precipitation first occurs as clouds move over the Sierra Nevada from west to east, then descend into Owen’s Valley, then are driven up over the White-Inyo Mountains, then descend into the Panamint Valley, then rise up over the Panamint Range, and only then does any remaining moisture have a chance of reaching Death Valley. That’s why Death Valley is so arid, while the nearby Sierran peaks have frequent snowfall and glaciers.

A profile of the Death Valley rain shadow may be found in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): The rain shadow effect across California from Monterey to Death Valley. As winds trend from west to east, oceanic moisture is blown over the Coast Ranges toward the Sierra Nevada, where it is pushed to higher elevation, triggering precipitation. This pattern repeats eastward in the Owens Valley and Panamint Valley before reaching Death Valley, which experiences very low precipitation due to this phenomenon. "Rain shadow" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Death Valley lies within a region of extremes. The lowest point in the contiguous United States is Badwater in Death Valley, 86 meters below sea level, while the base of the highest point, Mt. Whitney (4221 meters), is just a two-hour drive away. Death Valley is also the site of the hottest recorded temperature on Earth: 56.7°C (134°F), July 1913 (though some dispute this claim). By contrast, the town of Truckee, north of Lake Tahoe, is so cold that it regularly experiences winter days below -30^oC.

Trying to squeeze so many diverse conditions into one label–“Mediterranean”–is a fool’s errand. To mangle Tolstoy, in California all climates are alike, but each is extreme in its own way.

Köppen Climate Classification

The “Mediterranean” term derives from a taxonomy known as the Köppen climate classification. This figure shows the Köppen system applied to California.

Figure \(\PageIndex{3}\): The Köppen climate classifications for California showing the distribution of 11 different climate zones. "Koppen climate classification" by Adam Peterson is licensed under CC BY-NC 4.0. Access a detailed description.

As Figure \(\PageIndex{3}\): above shows, four out of eleven Köppen zones are described as Mediterranean, with their areas accounting for a plurality of the entire state. However, a closer examination will reveal many other zones: the Mojave is described as “hot desert,” while the High Sierra is called “dry-summer subarctic,” and Mt. Shasta and the high White Mountains are a “tundra.” The variation in Köppen zones–tundra to hot desert–reinforces the idea that representing the whole of California by any one category makes little sense, and we should approach describing California’s climate in highly local terms.

Trends

Rising anthropogenic CO₂ causes many climate forcings and responses. Elevated carbon dioxide affects many climate systems in a multitude of ways. A good analogy for the role of carbon dioxide in climate is the role of elevated blood sugar in diabetes. Chronically high blood sugar hurts nearly every organ in the human body: kidneys, eyes, nerves, skin, the heart. Likewise, chronically high carbon dioxide affects nearly every aspect of the climate: temperature, precipitation, hurricanes, oceanic circulation.

Higher temperatures are a major effect, and these temperatures exacerbate long term drought trends. These droughts have led to reductions in a major way California sustains its water, the vital snow pack. Concomitant with drought is wildfire, and hardly a summer now goes by without major wildfires burning through California’s tinder-dry dead vegetation, some of which dies because of invasive insects previously suppressed by colder winters. One study found after the 2012-2015 drought a 30% rise in ponderosa pine (Pinus ponderosa) deaths due to the warmth-induced accelerations of the life cycle of western bark beetles.

But even in a warmer, drier climate, the extremes may present in the opposite way, with atmospheric rivers dousing California with intense storms, leading to flooding, landslides, problems with reservoirs, or even land subsidence.

The bottom line: most of these changes are for the worse. We’ll begin with temperature.

Trends: Higher Temperatures

People could hardly believe the forecasted heat.

It was 29 June 2021, and swaths of the west coast baked under relentlessly sweltering temperatures as a ridge of high pressure trapped air masses. The heat hit worst in Canada, with Lytton, B.C., clocking in at 49.6^oC (121.3^oF), hotter than Abu Dhabi on the same day, and nearly as hot as a medium-rare steak should be cooked. The following day 90% of the town of Lytton was destroyed in a wildfire, with two fatalities. Over 500 fellow Canadians died from heatstroke. In Seattle, portions of interstate highway 5 closed when its pavement itself buckled due to thermal expansion. Portland, OR, reached 47°C (116°F), shutting down its light rail system, and spiking sidewalk temperatures at over 82°C (180°F). Redding, CA, achieved 46°C (114°F). Just as electrical demand surged, three separate power plants in California shut down because of the excessive heat. California Governor Newsom declared a state of emergency.

While these scorching temperatures would not be unknown in Phoenix, AZ, the big difference is that air conditioning is much rarer in these cities than in Phoenix, where A/C is fundamental for life support. Relatively mild California temperatures, both in winter and summer, mean that many older homes were built without insulation, double-paned windows, or central air conditioning. One study found that only 54% of California homes used central air conditioning. In temperate locales such as the Bay Area, that number drops to 47%. California has the highest percentage of any mainland state of homes without air conditioning.

This lack of home temperature control in California is becoming unsustainable. As Figure \(\PageIndex{4}\) details, since 2010 there has been a surge of high temperatures, compared to the previous century.

Figure \(\PageIndex{4}\): Extremes in maximum temperature, 1910-2022, with trend lines shows the degree of variation. "US West Extremes" by NOAA licensed under public domain. Access a detailed description.

Trends: High Pressure Ridges

California’s climate is characterized by long, dry summers, contrasted with brief, mild winters in which most of the annual precipitation falls in a series of storms. The mountain snow pack created by such storms then gradually melts, supplying some flow to rivers throughout the long dry spell.

At least, that’s the way things are supposed to happen.

In recent years, California’s precipitation patterns have been disrupted by the presence of the North Pacific High, an area of high pressure that is parked, semi-permanently, off the coast of California. This high pressure area interacts with the jet stream to deflect incoming Pacific moisture, and is a primary reason why California experiences such prolonged periods without significant precipitation (typically May to September). A related phenomenon appeared between 2011-2017, a ridge of high pressure exacerbated by warm ocean temperatures; because of its longevity and effective blockage of incoming storms, it was termed the Ridiculously Resilient Ridge. The Ridiculously Resilient Ridge is thought to be a prime reason for the dramatic droughts in the 2010s.

When west-to-east wind flow encounters the North Pacific High, the winds, and their moisture, is often deflected to the north. Seattle typically receives 100 cm/year of precipitation, while Los Angeles receives about 36 cm/year.

The typical climate pattern is for the North Pacific High to migrate southward in winter, allowing a series of storms to reach California. The winter period, and in particular the months of December, January, and February, is when the majority of moisture reaches parched California.

Trends: Drought

Concomitant with higher temperatures is drought. Four periods of major drought have afflicted California in the new millennium: 2000-2003, 2007-2009, 2012-2016, and 2020-2022. The overall period from 2000 to now constitutes the driest regime in the last 1,200 years.

These periodic droughts are frequently broken by relatively short periods of torrential rainfall. For example, the 2020-2022 drought ended due to winter 2022 rains, with many areas of the state exceeding 200% of normal precipitation.

Trends: Snow pack

At higher elevations, much of California’s winter precipitation falls as snow. The California snow pack is a crucial component to the state’s water supply because much of the rainfall is not captured and flows directly back toward the sea. (It is somewhat surprising given our periodic droughts that California allows so much of its precipitation to go uncollected, but because of the nature of so much water falling all at once, cities go to great lengths to allow the water to exit, thus avoiding flooding.) The snow pack therefore acts as California’s water reserve, and its gradual melting keeps rivers flowing in warmer months; the snow pack is responsible for about 30% of the state’s water. This flow is important not only for preserving life in streams, but for holding back saltwater encroachment in the Delta region (see the chapter on California’s Water for information about saltwater intrusion).

By custom, snowfall is assessed annually at a station near Donner Summit, and the snow water equivalent (SWE) is determined. One cannot simply measure the inches of existing snow because there is so much variation–loose fluffy snow mixed with compacted snow. SWE is what would result if all the snow in a column melted instantaneously. During the deep drought year of 2015, the Donner snow measurement, performed on 1 April 2015, showed only 5% of the long-term average SWE (see Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Snow Water Equivalent at Donner Summit, CA by Frankson, R., L.E. Stevens, K.E. Kunkel, S.M. Champion, D.R. Easterling, W. "Snow water equivalent" by NOAA is in the public domain. Access a detailed description.

Another issue is where snow falls. With the warmer temperatures of climate change, the snow line–the elevation at which precipitation drops as snow–rises, but it can’t elevate indefinitely. So future increases in the snow line can produce reductions in the snow pack, and hence, in California’s water system.

Trends: Atmospheric Rivers

Each of the four recent drought periods mentioned above (2000-2003, 2007-2009, 2012-2016, and 2020-2022) ended with periods of unusually high precipitation, due to the phenomenon of atmospheric rivers. Atmospheric rivers involve a series of Pacific storms, one after another, moving straight toward California in a shape that from the satellite view resembles a river (see Figure \(\PageIndex{6}\) and Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{6}\): Illustration of an atmospheric river. "Atmospheric river" by NASA is in the public domain. Access a detailed description.

Figure \(\PageIndex{7}\): A 4 January 2023 satellite picture of an atmospheric river. "Atmospheric river" by NASA is in the public domain. Access a detailed description.

In this soundless video below, Video \(\PageIndex{1}\), we see North America enveloped in rotating precipitation patterns as they migrate from west to east across the Pacific Ocean toward California. Note how the circulation patterns seem to take aim directly at California compared to other western states; this is the "atmospheric river" regularly soaking California.

Video \(\PageIndex{1}\): Atmospheric Rivers

A video showing the movement of an atmospheric river

Atmospheric rivers reflect the fact that California not only tends to get most of its precipitation during a few months, but even within those wet months, almost all of that precipitation falls during very brief storms, often lasting only a few days. These atmospheric river events account for roughly half of the water in the western U.S.

One might imagine that drought-stricken California would wish to capture this bounty of water, but drainage is actually a more critical concern, and much of this inundation is encouraged through flood infrastructure to return freely to the ocean; the sudden downpouring of atmospheric rivers create severe issues with flooding and landslides. Nonetheless, there have been recent investigations into capturing more of this precipitation via increased dams to ease California’s persistent droughts.

All such efforts, though, pale in comparison to the comically-absurd audacity of what has become known as the “Reber Plan.” This fantastical proposal set out to capture all the drainage of the Sacramento River, California’s largest river, by installing dams across San Francisco Bay (see Figure \(\PageIndex{8}\)). While this postwar lark would have stored immense quantities of fresh water, its hydrologic plausibility was always suspect; nonetheless, to test this idea, a giant working scale model of the San Francisco Bay was constructed in Sausalito by the U.S. Army Corps of Engineers, and can be seen today at the Bay Model Visitor Center.

Intriguingly, if the Reber Plan had ever been constructed, and a dam from Marin to Richmond controlled the flow of the Sacramento River, this dam would have offered unanticipated protection for inland California from the problem of climate change–induced sea level rise.

Figure \(\PageIndex{8}\): An illustration of the Reber Plan, showing areas in red where dams would have been constructed in the waters of the San Francisco Bay. "Reber plan" by State of California, Department of Public Works is in the public domain. Access a detailed description.

Query \(\PageIndex{1}\)

References

1. U.S. Energy Information Administration (March 2023). Highlights for air conditioning in U.S. Homes by state, 2020. Retrieved December 6, 2023, from https://www.eia.gov/consumption/residential/data/2020/state/pdf/State%20Air%20Conditioning.pdf
2. Bump, P. (2022, September 7). Californians may no longer be able to avoid air conditioning. The Washington Post. https://www.washingtonpost.com/politics/2022/09/07/californians-may-no-longer-be-able-avoid-air-conditioning/
3. Williams, A.P., Cook, B.I., and Smerdon, J.E. (2022). Rapid intensification of the emerging southwestern North American megadrought in 2020-2021. Nature Climate Change, 12(3), 232-234. https://doi.org/10.1038/s41558-022-01290-z
4. National Public Radio (2023, January 9). California is getting drenched. So why can't it save water for the drought? NPR.org. Retrieved November 9, 2023, from https://www.npr.org/2023/01/07/1147494521/california-weather-storm-water
5. Shao, E., Rojanasakul , M., & Popovich, N. (2023, March 17). A Very Wet Winter Has Eased California’s Drought, but Water Woes Remain. The New York Times. https://www.nytimes.com/interactive/2023/climate/california-drought.html
6. Cassidy, E. (January 2023). Atmospheric River Lashes California. NASA Earth Observatory. Retrieved December 8, 2023, from https://earthobservatory.nasa.gov/images/150804/atmospheric-river-lashes-california
7. Vartabedian, R. (2023, January 13). In a Drought, California Is Watching Water Wash Out to Sea. The New York Times. https://www.nytimes.com/2023/01/13/us/california-drought-storms-water-storage.html