144 Reconstructing an 8,000-year Climate History on the Baja Peninsula: Research Outline

Faculty Mentor: Jennifer Watt (Environmental & Sustainability Studies, University of Utah)

The Baja Peninsula is located in a geographical region that is particularly subject to the variable climate trends that result from the compounding effects of El Niño Southern Oscillation and the North American Monsoon (Minnich & Dezzani, 2000). Under normal conditions, the western pacific experiences low atmospheric pressure, while the eastern pacific (specifically equatorial South America) experiences high atmospheric pressure. This atmospheric gradient drives the “trade winds” which flow from east to west; the eastern pacific waters remain cool and nutrient-rich – a result of ocean upwelling due in part to the high atmospheric pressure (NCEI).

On a 2–7-year cycle, this pressure gradient mellows: the pressure in the eastern Pacific drops, resulting in heavy precipitation in equatorial South America and southwestern North America and significantly warmer off-shore waters (NCEI). This phenomenon is termed El Niño Southern Oscillation (ENSO). The reversal of these conditions – when this atmospheric pendulum swings back, passing by ‘normal’ in the direction of the other extreme – is termed La Niña. Essentially, this is a reinforcement of the easterly trade winds, and the result is intense drought in the eastern pacific with heavy precipitation in the west.

While ENSO is a major determinant for the amount of winter precipitation on the Baja Peninsula, the North American Monsoon (NAM) is responsible for summer precipitation. NAM is driven by temperature differences between ocean surface and land surface – a product of differences in the specific heat of water vs land – and this results in a pressure gradient which brings monsoonal precipitation to northern Baja during the early summer months (NOAA). The sum of these two atmospheric phenomena results in variable intensity and frequency of seasonal wildfire (Brunelle, 2022).

A 307 cm soil core (PA-19-A) was collected in 2019 by researchers from the University of Utah. It was obtained from the Palo Atravezado Ciénega, located within Sierra de San Pedro Mártir National Park, Baja California, Mexico. Ciénegas are desert wetlands that show evidence, through sedimentary analysis, of long-term ecological stability (Hendrickson & Minckley, 1985). Since these wetlands are found at the convergence of subsurface and surface water, they are characterized by high levels of deposition, particularly of organic material from the surrounding terrestrial ecosystem (Minckley & Brunelle, 2007). As such, ciénegas provide researchers with sedimentary evidence of historic hydrological patterns.

Sierra de San Pedro Mártir National Park does not experience a high level of anthropogenic disturbance, which renders this soil core particularly representative of an unaltered environment. Analysis of PA-19-A in conjunction with previously collected soil cores (and with those which will be collected in the future) will help us to reconstruct northern Baja’s paleoecology and associated paleoclimate. Specifically, charcoal analysis of PA-19-A will serve to reconstruct the fire history of the northern Baja region.

This reconstruction, when related to a temporal model, will be indicative of fluctuations in plant biomass through time. When this model is compared with pollen analysis, a division between winter and summer vegetation becomes apparent. From this data, we can obtain a better understanding of the paleoclimate: ENSO is attributed to warm, wet winters in southwestern North America, which encourages winter plant growth; seasonal variation in the North American Monsoon can result in fluctuation of summer plant growth. The dynamic relationship of these two climate events is strongly correlated with patterns of wildfire.

In reconstructing the paleoclimate surrounding the Baja Peninsula, I hope to gain an understanding of predictable patterns of global climate change. This model will provide insight for the future, and my hope is that it will help guide land management strategies that incorporate and regard fire as a tool, rather than an obstacle. Following completion of the charcoal analysis, a broader anthropological question I’m interested in is the impact of paleoclimate trends upon indigenous communities of the Baja Peninsula – specifically the impact of ENSO, NAM, and wildfire on the human-environmental relationship that existed prior to European contact.