Implications of the Changing Climate for Future Water Use

By Branwen Williams

What does the latest science show about future water availability in Southern California?

Precipitation in southern California

Low precipitation in southern California during the winter of 2013-2014 has led to the development of persistent aridity1 and the declaration of a drought emergency in Los Angeles.2 Southern California in general is characterized by dramatic variations in precipitation from year to year ranging from 12 to 90 cm per year since 1895.1 With humans increasing the greenhouse gases in our atmosphere and warming our planet, it is critical to understand how the resulting climate change will impact precipitation patterns in the southern California region. Thus, the goal here is to explain the natural causes of precipitation in the Los Angeles region, and start to explain how changing climate may impact future precipitation levels.  

Atmospheric circulation patterns are a major cause of regional precipitation patterns. The sun’s energy is concentrated at the tropics over the course of a year, causing surface water to evaporate and the air to warm and rise. As the air rises, it releases its moisture causing high precipitation in tropical regions. As the air reaches the top of the lowest layer of the atmosphere, the troposphere, it splits with half of the air moving north and half moving south. This northward-moving dry air at the top of the troposphere will start to sink at 30° N and S as the air cools and converges with other air masses. Although the sinking air will warm as it approaches the earth’s surface, it stays dry.  Offshore of Southern California, the surface waters of the ocean are cold because the dominant direction of the large scale ocean circulation brings cold water from further north to the south and deep waters are brought up to the surface waters via upwelling. This cold surface ocean water does not release heat or water to the atmosphere, maintaining the dry conditions on land. This complicated process, combined with the atmospheric circulation patterns, drives the cool wet winters and hot dry summers of the region that characterize Los Angeles as a Mediterranean climate.

Within a year, the band of warm temperatures at the equator in the tropics follows the sun and moves a little north during the northern hemisphere summer and a little south during the northern hemisphere winter. Considering this, and the fact that the land absorbs and releases energy much quicker than the ocean and because our planet spins, it seasonally changes the patterns of the dry high pressure systems and the wet low pressure systems. Slight changes in the winter atmospheric circulation patterns in the northern hemisphere can lead to winter storms that bring most of the region’s precipitation.

From year to year, changes in the Pacific Ocean related to the El Niño-Southern Oscillation (ENSO) are the primary drivers of wetter and drier years in Southern California.  ENSO is a shift in the atmospheric wind patterns and location of warm water in the tropical Pacific Ocean. Every three to seven years, an El Niño conditions occurs in which the winds at the equator pushing the water to the west relax and warm water come sloshing back eastward across the ocean. When these conditions move all the way to the eastern tropical Pacific, with it comes precipitation because the warm water heats the air causing it to rise and release its water vapor as rain.  With that low pressure system, the Pacific Jet Stream and amplified storm track bring substantial precipitation eastward to the southern US, including California.  This contrasts with a La Niña condition, in which strengthened trade winds push the warm surface water of the tropical Pacific Ocean west, resulting in a persistent high pressure off the California southwest coast that forces the Pacific Jet stream and thus precipitation north, leaving Southern California dry.

Changing Climate

Humans are increasing the greenhouse composition of the atmosphere through land-use change and burning of fossil fuels. These greenhouse gases in the atmosphere interact with the long wave radiation leaving our planet, trapping that radiation in the atmosphere, keeping the energy in our earth system, and warming our planet.  Ice core measurements of the greenhouse gases carbon dioxide and methane in conjunction with temperature provide evidence between the tight coupling of greenhouse gases and temperature.3 Recent increases in greenhouse gases are the major contributor to the approximately 1°C increase in global temperature since the industrial revolution. Global climate models projecting future change estimate that temperatures will continue to rise. The extent of the future warming varies depending on the degree with which we continue to burn fossil fuels.  

Global climate model projections of future precipitation patterns indicate that, broadly, precipitation will increase in already wet areas and may decrease in already dry areas4. Regionally, temperatures are projected to increase in southern California by several degrees Celsius by mid-century5 and precipitation is expected to decrease by 10-40% by the end of the century4, although precipitation may increase for the entire state of California.6

Changes in precipitation in Southern California are expected to occur as a result of the influence of warming and the resulting climate changes on the occurrences and severity of both ENSO and winter storms.  The interaction between the ocean and the atmosphere in the Pacific Ocean is chaotic, even without a human influence, and currently there is no scientific consensus on the influence of warming on ENSO. However, evidence is starting to accumulate that a warming climate has changed ENSO patterns7 and may generate more El Nino events in the future.8 Warm seawater temperatures moving eastward in the tropical Pacific during an El Nino event will bring more precipitation to the west coast of the Americas, including Southern California. Therefore, if the future climate is characterized by more frequent or extreme El Nino events, this would result in more precipitation overall in southern California albeit during periodic events occurring every 3-7 (or fewer) years during El Nino conditions.

Recent trends in atmospheric convection combined with model output may provide a clearer picture of future changes in the southern California precipitation due to a shifting winter storm track. With increased long wave radiation staying in the system warming the planet, the tropics will expand further north and south of the equator, pushing the high pressure systems at 30° north and south toward the poles. This will increase precipitation at the expanded tropics and the higher latitudes that will receive the increased precipitation from winter storms, but decrease the precipitation in the mid-latitudes, including the southern California region. Essentially, the storm track will go north of Southern California, reducing the amount of winter storm-driven precipitation impacting the region. In fact, the northern hemisphere storm track has already moved slightly poleward over the past century9, driven by a variety of factors.10

Tree ring records

One way to explore how precipitation patterns may change in the future is to investigate variability in precipitation in the past. The rate of growth in some species of trees including Douglas Fir respond to water availability such that the tree will grow faster during periods of abundant water and grow less in periods of less water. This variability in growth rate is evident in the width of growth bands in the tree trunk, and measurements of growth band width over the course of a tree’s life can reconstruct ambient water levels. In this way, by measuring growth in numerous trees throughout a region that are stressed by water levels, scientists can create records of past water availability. By comparing these records of past water availability with overlapping instrumental precipitation records, the measurements of the relative widths of tree rings are converted into absolute records of precipitation that can extend far into the past prior to instrumental measurements. Drought records generated in this way for the southwest United States demonstrate that periods of naturally warm northern hemisphere climate in the late 900s to 1200s AD correspond to periods of sustained severe droughts in the southwest US.11 Scientists termed these extended droughts that surpass current dry conditions in duration and intensity as “mega” droughts. Thus, tree ring drought records provide evidence that natural variability in precipitation levels may far exceed present day dryness. This natural variability combined with greenhouse-gas induced drying may lead us into uncharted territory in reduced Southern California water availability.

References Cited

1. National Integrated Drought Information System U.S. Drought Portal. [Online] Available: [2014, April 8]

2. National Oceanic and Atmospheric Administration’s National Climatic Data Center. [Online] Available: [2014, April 8]

3. Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.-M. Barnola, I. Basile, M. Benders, J. Chappellaz, M. Davis, G. Delayque, M. Delmotte, V.M. Kotlyakov, M. Legrand, V.Y. Lipenkov, C. Lorius, L. Pépin, C. Ritz, E. Saltzman, and M. Stievenard. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436, doi: 10.1038/20859.

4. IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Stocker, T.F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P.M. (eds.)] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.

5. Hall, A., Sun, F., Walton, D., Capps, S., Qu, X., Huang, H-Y., Berg, N., Jousse, A., Schwartz, N., Nakamura, M., and Cerezo-Mota, R. 2013. Mid-century warming in the Los Angeles region. Part I of the “Climate Change in the Los Angeles Region” project. [Online] Available: [2014, April 8]

6. Neelin, J.D., Langenbrunner, B., Meyerson, J.E., Hall, A., and Berg, N. 2013. California Winter Precipitation Change under Global Warming in the Coupled Model Intercomparison Project Phase 5 Ensemble. Journal of Climate, 26:6238–6256, doi: 10.1175/JCLI-D-12-00514.1.

7. McGregor, S., Timmermann, A., England, M. H., Elison Timm, O., and Wittenberg, A. T. 2013. Inferred changes in El Niño–Southern Oscillation variance over the past six centuries, Climate of the Past. 9:2269-2284, doi:10.5194/cp-9-2269-2013.

8. Cai, W., Borlace, S., Lengaigne, M., van Rensch, P., Collins, M., Vecchi, G., Timmermann, A., Santoso, A., McPhaden, M.J., Wu, L., England, M.H., Wang, G., Guilyardi, E., and Jin, F.-F. 2014. Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change. 4:11-116, doi: 10.1038/nclimate2100.

9. Bender, F.A.M., Ramanathan, V., and Tselioudis, G. 2012. Changes in extratropical storm track cloudiness 1983–2008: observational support for a poleward shift. Climate Dynamics. 38:2037-2053, doi: 10.1007/s00382-011-1065-6.

10. Allen, R.J., Norris, J.R., and Kovilakam, M. 2014. Influence of anthropogenic aerosols and the Pacific Decadal Oscillation on tropical belt width. Nature Geoscience 7: 270-274, doi:10.1038/ngeo2091

11. Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M., and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science. 306: 1015-1018, doi: 10.1126/science.1102586

© Branwen Williams, 2014