North American Monsoons Past and Future

Pam Boedeker

SaddleBrooke Nature Club was fortunate to have Jessica Tierney return to speak with us via Zoom. Jessica is a rare speaker who can translate her scientific knowledge to a language her audience can comprehend.

Jessica earned her doctorate from Brown University where she specialized in Paleoclimatology and Organic Geochemistry. She did postdoctoral research at Columbia University and Woods Hole Oceanographic Institution. Since 2015 she has been in the Department of Geosciences at the University of Arizona. All of her education and research came together in the projects she is doing funded by the National Science Foundation. Her grants total more than one million dollars!

Her research project on monsoons is the topic of her presentation. In general, people think of the Indian seasonal rains when monsoons are mentioned. Other monsoons are in South Africa, Australia, South America, and North America. Very little moisture makes it over the mountains from California to Arizona until the winds shift for the monsoon season from July to September. The system centers itself over The Four Corners, though that may be changing with the climate. Tucson receives 50% of its total rainfall during the monsoon season. We can thank the monsoons for our saguaro. That moisture and lower elevations give our saguaro just the conditions it needs to thrive.

What will happen to the monsoon as our temperatures rise due to global warming? Tucson’s temperatures have risen five degrees since 1950. This is more than the average global warming temperatures. Yet there is no long-term trend in our monsoon. Weather information at the airport shows that the overall pattern hasn’t changed.

Climate models show monsoons might be affected in two different ways. Higher temperatures could cause the monsoons to start later, or monsoons might bring more moisture.

In order to predict the future of monsoons, the past must be examined. Tripti Bhattacharya, who Jessica has worked with, developed the process by which the percent of monsoon rain can be studied as far back as 20,000 years. Scientists look at a wax that coats leaves (Deuterium). These waxes are transported by wind and water to eventually settle on the ocean floor. Core samples are taken to measure the Deuterium/Hydrogen composition. This work is being done in the Guaymas Basin.

Much work is yet to be done. However, it is known that 125,000 years ago the climate was warmer and the monsoons were stronger. The Arizona-Sonora Desert Museum has partnered with this research project. Junior high students are collecting leaf samples to be studied and there is a Paleoclimatology exhibit under construction.

We will be following Jessica and her research findings with great interest. SaddleBrooke Nature Club is making plans for the next Zoom meeting on Dec. 14 at 4 p.m. Jessica Moreno will be discussing wildlife in our area, the Oracle Road bridge and tunnel.

Membership Chair Debbie Grafmiller can be contacted for more information at [email protected]