Southwestern Storms and Molecular Legacies

A Life of Science: A Series by New Scientists

Reconstructing the ancient monsoon to help us understand how rising global temperatures are affecting the modern monsoon.

  
Though I didn’t realize it at the time, I first experienced the North American monsoon in the slickrock country of Southern Utah, a landscape and climate as unfamiliar as could be to this native New Yorker. I was an undergrad taking a summer geology field course and it was my first time in the Southwest. As our van descended into Bears Ears National Monument, the orange and red canyon walls glowed in the late afternoon sunlight against a deep, unbroken blue ceiling. Yet, within an hour of arriving at our campsite, the sky dimmed to black.

The wind picked up, echoing between the canyon walls. Barely able to assemble my new tent, I felt tears well up as I struggled to wrestle the rainfly into position. In the end, the tent was a disappointing refuge, as my poorly attached rainfly vibrated furiously. Fat raindrops crashed. It was a cold, miserable night. The next morning, what had been an inconspicuous creek bed running alongside our camp was choked with red water.

At first these memories stayed with me because the time had been so challenging and uncomfortable. I felt utterly out of place. But this experience also foreshadowed my eventual fascination with the deeper, longer changes in the Southwest that I would come to study—changes that, ironically, also involve storms.

 
 
When I returned to school in the fall, I considered that perhaps studying the climate might be the way to continue in geosciences while avoiding any more time in the field. Then in November, Hurricane Sandy devastated much of Long Island, as well as New York City and New Jersey. The way it caught us off-guard and made us feel vulnerable was reminiscent of the loss of innocence following 9/11. Climate change was happening now, and we finally saw that. Pursuing a career in climate science no longer seemed like a wimpy excuse, but somewhere I could make a contribution. Weather was transformed from just a shade of daily experience into a careful study of trends.

When I searched for graduate programs, I wasn’t interested in the climate of any particular place. From the Arctic to the tropics, it was all fascinating. Ultimately I connected with a faculty member at the University of Arizona, who offered me a position studying the North American monsoon. I hadn’t known there was a monsoon in North America. If you live outside of the Southwestern U.S., you might not either.

The North American monsoon is the smallest such system globally, delivering only about six inches of rainfall in Arizona and New Mexico and about 12 inches of rainfall in northwest Mexico, on average. Compare that to the 63 inches dumped each year by the Indian monsoon. Still, every July and August, the North American monsoon produces the most extreme weather in the United States, generating more lightning strikes in two months than most states experience in a year, driving mile-high dust storms that plow over metropolises, and whipping up storms where rain falls so quickly that canyons and dry washes transform to rapids.

 
Yet all this drama makes the monsoon a scientific enigma. Unlike the constant onslaught of rain other monsoon systems deliver, the North American monsoon is characterized by intervals of dry, calm weather, and bursts of heavy rains. Some years as much rain can fall in a single storm as the region typically receives in a month. With this much seasonal and annual variability, it is difficult to decipher how much of that has been ongoing and how much is the result of human-caused climate change.

The extremes and the scales at which they occur all make predicting how this region’s climate will evolve difficult. Predicting the unpredictable is an irresistible challenge for any zealous, young scientist. It is for me.

The monsoon is too flashy and chaotic to understand well with observational data or climate models. So, instead, I search for present-day analogues in the geologic record. This can help us better understand how rising global temperatures are affecting the modern monsoon. For example, 135,000 years ago the planet was about 1° Celsius hotter, and at the end of the last ice age 20,000 years ago, CO₂ levels rose by 100 ppm. Though these changes were slower and smaller than what we’re experiencing today, they’re similar enough for testing how past monsoons actually responded to climate change.

Because the woolly mammoths and giant ground sloths of the Pleistocene failed to keep diligent climate records for us, I use molecular fossils from sediments buried below the Gulf of California to reconstruct the ancient monsoon.
 

By June 2019, my first monsoon season loomed on the horizon. The opportunity to live in my research area every day, to become fully immersed in the nuances of the monsoon by witnessing them first-hand was exhilarating. Sensing the oncoming storms tied me even more to the deep past whose monsoon secrets I hope to decipher.

It was a hot, bright Tucson summer morning, quiet but for the buzz of cicadas that thrive in the dry heat preceding monsoon season. They are harbingers. Around me, as I biked to my lab, was the stillness of the air, the desiccation of the dirt yards of student housing and the yellowed vegetation of those with xeriscaping. All reminders of the precarious water future here.

Walking into the lab, the cacophony of cicadas was replaced by a medley of low-frequency hums emanating from refrigerators, servers, instruments, and fume hoods. I snapped on fresh purple nitrile gloves, selecting a pair one size too small because the tightness heightens my dexterity.

 
Earlier that year, I had meticulously carved samples from a core of Gulf of California sediments, with each depth representative of a point in time. I have 400 to process for my research. Extracting the specific molecules I need involves several steps where I break down the sample into different components.

To start, each sample is some mix of water and sediment—otherwise known as mud. I isolate the sediment by freeze-drying the samples. If you’ve ever wondered how astronaut ice cream or other space foods are made, this is the very same process. The mud is placed is a vacuum-sealed chamber which is cooled to very low temperatures (think Antarctica cold) so that any water will sublimate into gas, and this sublimated water vapor is then drawn from the chamber due to the vacuum. It leaves behind dehydrated, dusty sediment.

The sediment contains a mixture of organic and inorganic materials, but I’m only interested in the organics. To extract the organic matter, I use a machine akin to a Keurig coffee maker. The machine pumps solvent through the sediment at a high temperature and pressure. Green liquid streams down the inside of the collection vial like rain on a window. The extracts range in color from pale yellow to dark brown, reflective of how much organic material is in there.
 

 
The product is called the total lipid extract. I need to separate specific lipids out of this organic soup using a process called chromatography. At the end of this hours-long operation, I’m left with vials and vials containing just a thin white film at their bottoms.

This is the heart of my data, the leaf waxes I seek—organic material whose chemistry tells us secrets about ancient monsoons. These leaf waxes incorporate different types of water molecules within them, and these water molecules can actually be sourced indicating where their rainwater came from and when.

It’s abstracted, this process. It can be frustrating not to be able to touch these ancient clues. But at day’s end, I will leave the lab and step into a hall whose bank of windows face south. I’ll stop and gaze, analyzing the shapes of the clouds rolling in from the south, noting if there is any rain, virga, or lightning, and how strong the winds are blowing.

 
 
Initial results from my research suggest that the monsoon was more intense during past warm periods, but I’m still working out why that may have been. This intensity also likely came in the form of stronger bursts and longer breaks, meaning the American Southwest might be in for longer dry spells punctuated by more violent monsoons than we’re used to. They’ll make my little storm in Utah at the start of my science career seem quaint.

The importance of the monsoon to water resources in the Southwest is ever-growing. Whether these changes in the monsoon will exacerbate or mitigate the drying trend could make or break the future stability of this already water-starved region.

Storms do come. My first Western storm caught me largely unprepared and naïve. Today, with clues from the past, we need not approach the future so blindly.

Dervla Meegan Kumar is a Ph.D. candidate in the Department of Geosciences at the University of Arizona. Her research combines organic geochemistry and climate model simulations to reconstruct past climate variability in the Southwest, with an emphasis on understanding the role of the North American monsoon in driving regional aridity.

Header photo of North American monsoon over Organ Pipe National Monument by Dervla Meegan Kumar.