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California during the last Glacial Maximum

Updated on August 19, 2009

Most in the science community are united in the belief that California was a cool and wet region during the last glacial maximum (18 kya BP). While it is relatively easy to understand why California was cool during the last ice age, discerning why California was predominantly wet is a more challenging exercise.

This hub will examine California’s climate during the last glacial maximum, and in the process verify the cool and wet interpretation given by the scientific community. If the experts are correct, we should find lines of evidence to support their interpretation.

This hub does not endeavor to provide an exhaustive analysis of California’s ice age conditions, and indeed is bounded by the rather limited quantities of the region’s ice age data sets. Moreover, past climatic conditions will be depicted in a non-quantitative manner, which is more appropriate for the short confines of this hub.


In order to realize California’s ice age conditions, this hub will investigate several terrestrial aspects of the region, that, when taken collectively, may paint a convincing picture of the climatic conditions that persisted 18 kya.

Pollen and plant evidence in general will be examined under the assumption that the relationships between the plants that generated the pollen and its environment are similar to relationships we observe today. Using this approach will enable us to utilize “observations on present-day relationships between pollen deposition and environmental conditions to make detailed reconstructions of past vegetation and environments.” (MacDonald 1996)

Plant macrofossil collections from packrat middens will also be examined. The information gathered from analyzing packrat middens is invaluable because packrats are mostly specific herbivores whose range of operations are estimated at one hectare centered at the midden (Spaulding, Betancourt, Croft, Cole 1990). Because packrats contribute to middens over spans of time, examination of the fossil middens enables us to reconstruct past vegetation, and in so doing, provide us with an idea of what the past environmental conditions were like. As in pollen analysis, we rely on the assumption that plant-environment relationships then are similar to what they are today. An effective moisture index will be employed to assist in the recreation of past climatic conditions.

Lake levels during the last glacial maximum will also be examined. 18O isotopic values will be utilized in the reconstruction of glacial maximum lake levels in California. This examination is critical to the realization of California’s wetness 18 kya.

Finally, this hub will inspect an atmospheric general circulation model (GCM) based upon the work of COHMAP(1) members. The predictions of the GCM are of great interest because the model takes into account various variables inherent in climatic conditions; these variables are not accounted for in the other methods employed by this study. These variables include orbitally determined insolation, mountain and ice-sheet orography, sea-surface temperatures, sea-ice limits, snow cover, land albedo, and effective soil moisture.

By examining physical data, it is hoped that a glacial maximum climate for California can be described. By augmenting the physical data with insight gained from the GCM, this hub seeks to provide a compelling argument either for or against the claim that California was a cool and moist region during the last glacial maximum.

Today’s Environment

California is interesting in the sense that the entire state is characterized by varying geographical features, and these diverse features are often not far removed from each other. The Pacific Ocean which bounds the state on the West, is in close proximity to the state’s various major mountain systems, such as the Sierra Nevadas in the north, and the Santa Monica mountains in the South. Collectively, these major mountain systems strongly influence the climates we experience in California today. Aridity in the interior of California can be attributed to the rainshadows caused by these mountain systems.

Precipitation in California is influenced by the migratory low pressure cells that follow westerly flow in the autumn, winter and spring (Thompson, Whitlock, Bartlein, Harrison, and Spaulding 1993). During the summer, the westerlies are shifted relatively further north, encouraging the Pacific subtropical high to expand, and thus resulting in a relatively dry West Coast.

The major vegetation groups are forest, woodland, chaparral, and grassland. Forest vegetation increases in direct proportion to elevation, occurring mostly on the upper levels of mountain systems. Orographic precipitation in conjunction with the cooling effect of increased elevation enable forests to thrive where otherwise the effective aridity would not permit. In northern and central California, the spectacular coastal redwood forests occur.

Woodlands generally exist below the forests and are comprised primarily of widely spaced trees. Primary woodland constituents include junipers, piñon pines and scrub oaks, while chaparrals are comprised of primarily of evergreen, scrub oak, sumac, and buckthorn (Thompson et al. 1993). Grasslands endure primarily on the central valleys of California, where relatively more moisture (or more effectively-distributed precipitation) is observed.

The vegetation groups are illustrated as follows: forest is described by figure 1, woodland and chaparral by figure 2, grassland by figure 3. The figures have been adapted from Harris’ 1985 maps; information for neighboring states have been left in to allow for visual continuity that may prove helpful in the discussion of climate and vegetation.

Figure 1: Modern potential forest vegetation. 1 describes spruce-cedar-hemlock; cedar-hemlock-Douglas fir. 2 describes fir-hemlock; western spruce-fir; 3 describes northern floodplain forest. 4 describes Oregon oakwood and cedar-hemlock-Douglas fir. 5 describes mixed conifer; red fir. 6 describes silver fir-Douglas fir; western ponderosa; Douglas fir; cedar-hemlock-pine; grand fir-Douglas fir; eastern ponderosa; Black Hills pine forest; pine-Douglas fir; Arizona pine forest. 7 describes redwood and 8 describes alpine meadows.

Figure 2: Modern potential woodland and chaparral. 1 describes saltbush-greasewood. 2 describes Great Basin sagebrush. 3 describes Californian oakwood. 4 describes chaparral. 5 describes blackbrush. 6 describes creosotebush-bursage; palo verde-cactus shrub. 7 describes pinyon-juniper woodland. 8 describes CA coastal sagebrush, mountain mahogany-oak scrub, and oak-juniper woodland.

Figure 3: Modern grassland vegetation.  1 describes fescue-wheatgrass and wheatgrass-needlegrass in the case of MT and WY.  2 describes grama-needlegrass-wheatgrass in MT & WY, while Californian steppe in CA.  3 describes grama-galleta steppe.  4 describes grama-buffalo grass.  5 describes foothills prairie.  6 describes tule marshes.  7 describes grama-tobosa prairie.  8 describes fescue-wheatgrass and oak-bluestem shinnery in the case of NM.  9 describes wheatgrass-bluegrass and sandsage-bluestem prairie in the case of CO.  Finally, 10 describes alpine meadows.

Lake levels in California today are not to the extent they were during the last glacial maximum. Permanent freshwater lakes are more likely to be found in higher latitudes, or higher altitudes. Indeed, the basins of south-western deserts “contain dry or seasonally wet playas.” (Thompson et al. 1993) In this study, δ18O information gathered at Owens Lake will aid us in reconstructing a picture of what California was like 18 kya. The Owens Lake area was the first in a series of Pleistocene lakes that at times encompassed a vast territory that stretched to Indian Wells and Death Valley. The floors of this once bustling lake are now playa lakes. The number of perennial lakes that developed directly reflect the amount of precipitation that the basins received. Thus, a bustling Owens Lake meant extremely wet conditions.


In this study, changes in the elevation occurrence of plant taxa is interpreted as an indicator of past climatic changes. A simple logic is followed: an elevation increase of the upper range limits of plants may indicate warmer temperatures (longer growing-season temperatures), while an elevation decrease of the lower range limits of plants may indicate just the opposite – colder temperatures.

The geographic location of plants is also indicative of past climatic changes, in a manner similar to elevation changes. If evidence is found to support a northward migration of specific plant species, then such evidence may point to warmer temperatures. Likewise, if evidence is found that indicates a southward migration of plant species, then such evidence may reflect colder temperatures. This geographic line of reasoning is only applicable in the northern hemisphere; one cannot argue the same for South America, for instance.

Thompson and his colleagues indicated that a 1981 study by Adam et al. Of California’s Clear Lake observed that “the modern oak-dominated vegetation was absent at 18 kya and that instead junipers and pine grew at the site.” Based on their pollen analysis, they believed the temperatures at glacial maximum were “7–8˚C cooler than present levels and precipitation was 300-350% greater.”

By sorting fossil matrix at Los Angeles’ Rancho La Brea Tar Pit Museum, I was able to recognize various materials from plants that no longer persist in the region today. Coast redwood remnants and evidence of various pines were in sight, and I believe these materials may have been deposited by moving bodies of water from the nearby Santa Monica mountains. I also discovered juniper and chaparral constituents such as scrub oak and walnut. Sustaining the aforementioned species meant that California 18 kya must have been cooler in temperature and must have exhibited a more pronounced effective moisture relative to today. Harris (1985) interpreted ice-age floral remains in Carpinteria, Santa Barbara, to be closely similar to those found on the Monterey Peninsula, some 330 km to the north. This interpretation supports the general cool and wet interpretation of California’s ice-age conditions.

Litwin, Frederiksen, Adam, Andrle, and Sheehan, in a USGS Open-File Report (93-683 Pollen) called Core OL-92 from Owens Lake, Southeast California, used Pinus relative frequency to test for response to changes in temperature and precipitation associated with glacial events. The group chose Pinus because it was a dominant element in their terrestrial record, because the genus is admittedly a prolific pollen producer, and finally, because its relative frequency had a high rate of change. The team found that indeed, the pollen evidence described a proliferation of Pinus which coincided with interglacial intervals. This finding further supports California’s cool and wet glacial maximum climate interpretation.

Paleoclimatic interpretations are often aided (particularly in the Southwest) by plant macrofossil assemblages recovered from ancient packrat middens. In this study, we briefly examine three sites to see if indeed, midden records will help us establish an ice-age climate for California. But before we proceed, an index needs to be defined. Effective moisture is a term “used to take into account the effect of variations in evaporation rates associated with temperature changes. For example, although portions of the West were cold and received relatively little precipitation during the Late Wisconsin, the reduced temperatures and increased cloud cover apparently lowered evaporation rates, which made these sites ‘effectively’ more moist than they are today.” (Thompson et al. 1993) The following keys will be used to denote an area’s effective moisture: W denotes wetter conditions, or more effective moisture, than today; D indicates drier conditions, or less effective moisture, than today; N denotes no difference from today.

An investigation of a packrat midden assemblage in King’s Canyon reveals plant species that needed a colder than today climate as well as a wetter than today effective moisture in order to flourish. The midden assemblage primarily constituted Western juniper, sugar pine, ponderosa pine, and single-needle piñon pines. The fact that this collection of plants were present in a geographical area where they could no longer exist today (for it is too warm and dry) means that when the midden was assembled (measured at 18-19 kya), California had to have been cooler and moister. We shall classify this site as a “W.”

Wells and Woodcock in 1985 conducted a packrat midden study at California’s Death Valley and arrived at the same conclusion – that from the assemblage emerged evidence of plant life that couldn’t exist in that environment today. The interpretation, as with the King’s Canyon study, is that California was, in fact, cooler and moister during the last glacial maximum. This site will be classified as a “W.”

Finally, we will take a look at Spaulding’s 1985 investigation of midden assemblages from Eleana Range.  Figure 4 below is borrowed from Spaulding’s work:

Figure 4: Eleana Range chronosequences.  Figure from Spaulding, 1985.
Figure 4: Eleana Range chronosequences. Figure from Spaulding, 1985.

As figure 4 indicates, at some point ~17 kya BP, midden debris supplemented by sloth dung provided evidence for the assertion that California’s climate was predominantly cool and effectively moist. Indeed, as we progress from 17 kya to 11 kya, we see a marked decrease in Limber pine and steppe shrubs. At the same time, woodland thermophiles were on the rise as the climate became warmer. We shall therefore mark the Eleana Range site with a “W.”

The three packrat midden sites investigated by this report, namely, King’s Canyon, Death Valley, and Eleana Range were all characterized by “W” indicating wetter conditions, or more effective moisture, than today. This conclusion is in accord with that reached by the pollen/plant analysis conducted earlier in this hub. However, if indeed California was wetter, should this water be evident elsewhere as well, say at the various lakes that existed then?

A quick glance at figure 4 will indicate that Searles Lake was at capacity during the last glacial maximum, although the issue of whether or not it overflowed is subject to debate (see Benson et al. 1990). In this study we shall investigate Owens Lake, seeking compelling evidence that would indicate whether the lake was at capacity, whether it overflowed, or whether it desiccated during the last glacial maximum.

Benson, Burdett, Kashgarian, Lund, Phillips, and Rye in 1996 published a study on Owens Lake. In their study, they found that δ18O values determined between 52.5 ky and 15.5 ky BP were generally low (at less than 22 per mil). This indicated that Owens Lake had been overflowing during that time period, because under a near-equilibrium state, the δ18O content of a large closed lake should be in the neighborhood of 30 per mil. That it was less than 22 per mil indicated a great outflow from the lake. Therefore, the low isotopic values pointed to a climate that was extremely wet.

Thus far, evidence has been in favor of a pluvial, cool California during the last glacial maximum. The physical evidence ranging from pollen to sloth dung has enabled us to interpret what the conditions were like 18 kya. However, as noted in the beginning of this hub, the physical evidence does not take into consideration various other parameters which invariably influence the climate of the Earth. Those factors include, but are not limited to, orbitally determined insolation, mountain and ice-sheet orography, sea-surface temperatures, sea-ice limits, snow cover, land albedo, and effective soil moisture. Sophisticated climate models are able to address these parameters, and we will examine two global circulation models (GCMs) – one that models California’s climate 18 kya, and one that models today’s climate.

Figure 5 describes the major prevailing conditions during the last glacial maximum.  Sans ice sheets, there would have been a single jet stream path in lieu of the two depicted above.  During the last ice age, the 3 km thick Laurentide ice sheet caused the jet stream to split into two distinct paths, with the bottom path displaced to approximately 30 degrees latitude from its former path at approximately 50 degrees latitude.  This displacement effectively weakened the pacific subtropical high, but more importantly, brought along with it cooler winds and increased winter precipitation to California.  In essence, the GCM paints an ice-age California climate just as the scientists claim it is: cool and effectively moist.
This image depicted below (figure 6) is a general circulation model that describes California’s climate as it is today.  Note that in the absence of the ice sheets, the jet stream has retreated back to approximately 50 degrees latitude, where it brings cool air and moist conditions to the Pacific northwest.


Undoubtedly, the quest to understand past climates involves multiple techniques and a lot of patience, as one method alone may not yield an accurate picture. In this hub, I analyzed plant and pollen composition, packrat midden assemblages, lake levels, and atmospheric general circulation models in order to gain an understanding of California’s climate during the last glacial maximum. California’s diverse environment necessarily added challenge to an already formidable task. Only after all the lines of evidence converged was I able to draw any conclusions. In this case, the evidence clearly states that 18,000 years ago BP, California experienced cooler temperatures and increased exposure to effective moisture. Unfortunately, the same cannot be said today.


1. The COHMAP experiments were the first to simulate a sequence of paleoclimatic conditions covering the period from 18 kya to the present. (1988)


1997 Chapter 2. Northern Hemisphere Deglaciation: Processes and Responses of Ice Sheet/Ocean Interactions; in: Martini, P.I. (ed.), Late Glacial and PostGlacial Environmental Changes; Oxford University Press, Oxford, p.9-27.

1996 Climatic and Hydrologic Oscillations in the Owens Lake Basin and Adjacent Sierra Nevada, California; Science, 274:746-749.

1995 Goode’s World Atlas; Rand McNally, Cambridge, 371 p.

1985 Late Pleistocene Vertebrate Paleoecology of the West; University of Texas Press, Austin, 293 p.

1998 Continental-marine correlation of Late Pleistocene climate change: Census of palynomorphs from core OL-92, Owens Lake, California; [ online ]

1996 Chapter 22. Non-aquatic Quaternary; in: Jansonius, J. & McGregor, D.C. (ed.), Palynology: principles and applications; American Association of Stratigraphic Palynologists Foundation, Vol. 2, p.879-910.

1989 Trees and Shrubs for California Landscapes; Land Design Publishing, Claremont, p. 3-15.

1990 Chapter 9. Vegetational and Climatic Development of the Mojave Desert: The Last Glacial Maximum to the Present; in: Betancourt, J.L., Van Devender, T.R., Martin, P.S. (ed.),
Packrat Middens The Last 40,000 Years of Biotic Change; University of Arizona Press, Tucson, p.166-199.

1993 Chapter 18. Climatic Changes in the Western United States since 18,000 YR BP; in: Wright, H.E., Kutzbach, T.W. III, Ruddiman, W.F., Street-Perrott, F.A., Bartlein, P.J. (ed.), Global Climates since the Last Glacial Maximum; University of Minnesota Press, Minneapolis, p.468-513.


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