Tropical Revelations: Unearthing the Impacts of Hydrological Sensitivity on Global Rainfall

Jie He

Jie He, assistant professor in the School of Earth and Atmospheric Sciences, wants to predict how rainfall will change in the presence of continuing climate change. — Photo by Jerry Grillo


Georgia Tech researcher Jie He set out to predict how rainfall will change as Earth’s atmosphere continues to heat up. In the process,  he made some unexpected discoveries that might explain how greenhouse gas emissions will impact tropical oceans, affecting climate on a global scale.

“This is not a story with just one punch line,” said He, assistant professor in Georgia Tech’s School of Earth and Atmospheric Sciences, whose most recent work appeared in the journal Nature Climate Change. “I didn’t really expect to find anything this interesting—there were a few surprises.”

He is principal investigator of the Climate Modeling and Dynamics Group, which combines expertise in physics, mathematics, and computer science to study climate change. The team’s latest study, a collaboration with Mississippi State University and Princeton University, examines hydrological sensitivity in the planet’s three tropical basins: the central portions of both the Pacific and Atlantic oceans and most of the Indian Ocean, an equatorial belt girding the Earth between the Tropic of Cancer (north) and Tropic of Capricorn (south).

Hydrological sensitivity (HS) refers to the precipitation change per degree of surface warming. Hydrological sensitivity is a key metric researchers use in evaluating or predicting how rainfall will respond to future climate change. Positive HS indicates a wetter climate, while negative HS indicates a drier climate.

“The projection of hydrological sensitivity and future precipitation has been widely investigated, but most studies look at global averages — nobody had yet looked closely at each individual basin,” He said. “And the real impact on global climate change will come from the regional scale.”

In other words, what happens in tropical waters has far-reaching effects.

Long Reach of the Tropics

He wanted to specifically examine the tropical basins because they already have a well-known influence on remote locations: El Niños and La Niñas. These weather patterns that shift every couple of years are examples of tropical oceanic precipitation changes that have a global impact.

“These precipitation changes create heating and cooling in the atmosphere that set off atmospheric waves affecting remote climates across the globe,” He said. During El Niño winters, for example, the southeastern U.S. typically gets more precipitation than usual.

But El Niños and La Niñas are naturally occurring, whereas the tropical precipitation changes He identified are projected as outcomes of human-induced global warming — a simulation, part of a climate model.

Climate models are an essential tool for He and other researchers, who use them to simulate possible future scenarios. These are computer programs that rely on complex math equations to project the atmospheric interactions of energy and matter likely to occur across the planet.

What surprised He was the substantial difference in HS between tropical basins. Essentially, in He’s model the Pacific tropical basin has an HS more than twice as large as the Indian basin, with the Atlantic basin projected as a negative value.

“It was surprising because these differences can’t be explained by the mainstream theories on tropical precipitation changes,” He said. “In other words, none of the theories we knew would have predicted it.”

Modeling the Sensitive Future

The effects of such diverging hydrological sensitivity would be widespread, according to He. For example, his experiments suggest that the continental U.S. will get wetter, and the Amazon will become drier.

“If these model projections are true, these effects will materialize as the climate continues to warm,” said He, who can’t predict exactly how long it will be before these effects can be detected in actual observations of our three-dimensional world.

That’s because they only have reliable observations of oceanic tropical precipitation since 1979. Precipitation changes over decades are strongly affected by internal climate variability — that is, climate change that isn’t caused by humans. When human-induced precipitation changes are significantly greater than internal climate variability, we should be able to detect the wide-ranging effects of diverging hydrological sensitivity.

But the challenges of continuing climate change do not allow the luxury of waiting until every aspect of climate projection becomes a reality, He noted, adding, “We are relying on climate projections to some extent to guide our adaptation and mitigation plans. Therefore, it is important to study and understand the climate projections.”

Based on the scenario projected by climate models used in He’s research, the effects of El Niños and La Niñas on remote climates will become stronger.

“What we can imply is that this strengthening would be partly due to the diverging HS among tropical basins,” He concluded.

While the future effects of HS on El Niños and La Niñas weren’t discussed in this study, He believes it would make a very interesting research subject going forward.


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2024 Frontiers in Science: Climate Action

Susan Lozier, Julia Kubanek, L. Beril Toktay, and Tim Lieuwen

Susan Lozier, Julia Kubanek, L. Beril Toktay, and Tim Lieuwen

This Earth Month more than 100 campus and community stakeholders gathered near the Georgia Tech EcoCommons for the 2024 Frontiers in Science: Climate Action Conference and Symposium.

On April 18, the College of Sciences hosted more than 20 speakers and panelists from across the Institute and Atlanta community presenting groundbreaking research and discussing innovations and ideas in climate change, challenges, and solutions. 

Georgia Tech President Ángel Cabrera (M.S. PSY 1993, Ph.D. PSY 1995) kicked off the morning sessions by highlighting the Institute’s new Climate Action Plan, which outlines the pathway to achieve net-zero emissions by 2050. Cabrera’s remarks focused on Georgia Tech’s role on the frontlines of research and education informing how we respond to climate challenges — and noted that the Institute’s work must extend beyond our laboratories and classrooms.

“It is essential that we not only do the science, but that we also tell that science to the world,” Cabrera says.

Interdisciplinary inquiry

This year, Frontiers in Science featured an array of climate research and initiatives led by the College of Sciences, fellow colleges across Georgia Tech, and the wider Atlanta community.

Following a three-year hiatus of the Frontiers series, the 2024 edition re-envisioned the signature annual event as a research conference and symposium to convene campus experts — and to incubate seed grant proposals to support the work of early career faculty.

Frontiers previously hosted Nobel laureates and invited thought leaders for individual talks across the College’s six schools, and celebrated milestones like the International Year of the Periodic Table of the Chemical Elements.

“This year, we wanted to showcase what we are doing right here in the College of Sciences and throughout the Institute,” says Susan Lozier, dean of the College of Sciences, Betsy Middleton and John Clark Sutherland Chair and professor in the School of Earth and Atmospheric Sciences. “Our faculty are at the forefront of broadening our knowledgebase and uncovering solutions in areas critical to the planet and our well-being. We wanted to uplift that work and see what sort of connections could be made.”

Connections and collaboration were key themes of the day as faculty, staff, students, and alumni participants representing all six Georgia Tech colleges shared research results and ongoing work and discussed collaborative ideas for horizons ahead.

“Scientists alone cannot [create accurate models],” noted Annalisa Bracco, professor in the School of Earth and Atmospheric Sciences and associate chair for Research, who shared her own research alongside Lozier, who presented a version of her 2024 TED Talk on ocean overturning. “Engineers alone cannot do it. We need social scientists, policy makers, communicators.”

The importance of an interdisciplinary approach was reinforced by the Strategic Energy Institute at Georgia Tech (SEI) and Brook Byers Institute for Sustainable Systems (BBISS), which announced an interdisciplinary seed grant funding opportunity for assistant professors with ideas for new climate solutions.

Frontiers in focus

Across three themed sessions, faculty and leadership from the Colleges of Sciences, Engineering, and Design spearheaded talks on the ocean and cryosphere, biodiversity, carbon cycling, coastal wetlands, biofuels production, and beyond.

Panels on climate challenges across community, technological, and policy initiatives were hosted by Georgia Tech Vice President for Interdisciplinary Research and Professor in the School of Biological Sciences and the School of Chemistry and Biochemistry Julia Kubanek.

Following a networking lunch with climate table topics, Georgia Tech Executive Vice President for Research and Professor in the School of Electrical and Computer Engineering Chaouki T. Abdallah (M.S. ECE 1982, Ph.D. ECE 1988) kicked off the afternoon sessions — which also announced the scholarship recipients of a student video competition and featured videos with a pair of alumnae working in meteorology, climate research, and policy.

Afternoon highlights also included discussions on the Georgia Tech Climate Action Plan and Sustainability Next initiative, led by Jennifer Chirico (B.S. MGMT 1997, Ph.D. PUBP 2011), associate vice president of Sustainability for Georgia Tech Infrastructure and Sustainability, and Jennifer Leavey (B.S. CHEM 1995), assistant dean for Faculty Mentoring in the College of Sciences and interim assistant director for Interdisciplinary Education in the Brook Byers Institute for Sustainable Systems.

Although many of the presentations provided a stern outlook of the state of our ecosystems, the conference concluded with a sense of hope. This optimism was grounded in the range of opportunities that exist to address climate challenges — thanks, in part, to the body of knowledge and solutions being tested and explored by Georgia Tech researchers.

At the end of the day, Katie Griffin, a first year undergraduate student in Environmental Science, read Amanda Gorman’s poem Earthrise and provided this reminder:

All of us bring light to exciting solutions never tried before
For it is our hope that implores us, at our uncompromising core,
To keep rising up for an earth more than worth fighting for.


Experience the event in pictures with the College of Sciences’ Flickr account, and discover the highlights through the day’s live tweets on College of Sciences’ X account.

Frontiers in Science Banner Outside at Sunrise

Frontiers in Science Banner Outside at Sunrise

Jenny McGuire

Jenny McGuire

Frontiers in Science Policy Discussion Panelists: Michelle Midanier, Valerie Thomas and Joe F. Bozeman III

Frontiers in Science Policy Discussion Panelists: Michelle Midanier, Valerie Thomas and Joe F. Bozeman III

Frontiers in Science Participants

Frontiers in Science Participants

President Ángel Cabrera

President Ángel Cabrera

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By: Lindsay Vidal

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College of Sciences at Georgia Tech

Everlasting African Wildfires Fueled by Aerosol Feedback

Fires have been burning in Africa for centuries. The fires are fueled by feedback loop as aerosols interact with the climate. It’s a process that plays a critical role in the regulation of African ecosystems.

Fires have been burning in Africa for centuries. The fires are fueled by a feedback loop as aerosols interact with the climate. It’s a process that plays a critical role in the regulation of African ecosystems.

Africa is on fire. It has been for thousands of years. The continent contains more than 50% of the total area on Earth that is burning, on average, and there is no sign of it stopping — indeed, the migrating, hemisphere-hopping African wildfire season is steadily increasing.

The fire is essentially feeding itself in a kind of feedback loop as aerosols, induced by the perpetual conflagration, interact with the climate. It’s a process that plays a critical role in the regulation of African ecosystems, reinforcing wildfires and paving the way for elevated fire seasons in subsequent years.

Aerosols are tiny particles that have a large impact on the Earth’s climate. They comprise a wide range of materials. Besides the human-induced air pollution that we can see (that brown smog is the interaction of light with aerosols), there are a lot of natural aerosols: salty sea spray, mineral dust, volcanic ash, and wildfire smoke.

Suspended in the atmosphere, the role of aerosols in our climate is complex. But a new study by Georgia Tech researchers demonstrates the role they play in the African wildfire life cycle. The research, published in the journal iScience, could have significant implications for understanding the impacts of fires and climate change in Africa and other regions of the planet prone to wildfire.

“We used to think that aerosols had a short-term, localized climate impact and can be effectively removed by precipitation within a week. But in this study, we’re showing that isn’t necessarily correct,” said Yuhang Wang, professor in the School of Earth and Atmospheric Sciences and corresponding author of “Positive Feedback to Regional Climate Enhances African Wildfires.”

The Wang lab works at solving mysteries of atmospheric pollution, and the team is onto something with its latest research, revealing new clues in its study of wildfires in Africa, where the unique alternation between dry and wet seasons along the equator extends the lifespan of aerosols.

“Basically, with the combination of wildfires and fire-induced aerosols, the impact of aerosols can be longer term, extending over seasons,” said Wang, whose team invented the tool it needed to complete its investigation.

Building a Better Model

Several years ago, Wang’s lab developed the Region-Specific Ecosystem Feedback Fire (RESFire) Model to augment the existing, publicly accessible Community Earth System Model (CESM). Managed by the National Center for Atmospheric Research, CESM is an open-source global climate model that provides computer simulations of the Earth’s climate system.

RESFire improves CESM’s fire simulation capability, helping researchers develop a better grasp of complex fire-climate-ecosystem interactions, “which are still not very well understood,” said Wang, whose team used its CESM-RESFire model to study aerosol feedback in Africa for the latest research.

“We found that the extension of the aerosols’ lifespan in Africa occurs through a positive feedback mechanism,” said Wang.

Aerosols can essentially give clouds a bad case of constipation, absorbing vapor from the atmosphere, making it difficult for clouds to produce large droplets.

“Fire aerosols are transported from burning or dry regions to wet regions,” Wang explained. “That leads to reduced precipitation and drying of fuel loads.”

The Feedback Mechanism

Identifying the fire-aerosol positive feedback mechanism in Africa sheds light on wildfire-related climate feedback globally. Other studies have shown that in some coastal areas, such as the western United States, fire smoke alters local fire weather, resulting in positive feedback. These coastal regions have distinct fire seasons, and the escalation caused by aerosol feedback doesn’t persist into the next fire season.

Africa is different. With its shifting fire regions and prevailing winds, the positive feedback affects the current season and amplifies burning in the subsequent season. And fire weather season has increased by up to 40% in Africa over the past four decades, which means there may be shifts in distribution and variability of burned areas.

“The good news is that this mechanism is self-sustaining. It even has some resilience built in,” Wang said. “The question is what happens in the presence of persistent global climate change. What we know is, the mechanism underlying this natural system of wildfires depends on the current state of the atmosphere.”

The positive feedback mechanism implies that a warmer, drier climate will likely lead to more persistent burning in Africa in the future, the researchers write, concluding, “The systematic fire-climate feedback may also be present in other fire-prone tropical regions and has significant ramifications for understanding the impacts of fires and climate change on humans and plant life.”

Citation: Aoxing Zhang, Yuhang Wang, Yufei, Zou. “Positive feedback to regional climate enhances African wildfires.” iScience.

Funding: This work was supported by the National Science Foundation (NSF) (grant 1743401). 



Yuhang Wang, School of Earth and Atmospheric Sciences

Yuhang Wang

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Team iManhole Wins Fall 2023 EGHI/GT Global Health Hackathon

Winners of the EGHI/GT Hackathon stand together at Tech Square ATL Social

Students tackled climate change in the Fall 2023 Emory Global Health Institute (EGHI) /Georgia Institute of Technology (GT) Global Health Hackathon, Nov. 11, at Tech Square ATL Social. Competing for cash prizes and a spot in GT Startup Launch, first place went to Team iManhole. The team created an integrated system that gathers real-time data from manholes and uses machine learning algorithms to predict flooding to manage traffic and evacuation routes.

“The effects of climate change are felt in every country with the brunt and burden of an unmanaged climate crises threatening to set back global health progress by eroding decades of poverty eradication and health equity efforts worldwide,” said Dr. Rebecca Martin, EGHI director of Emory Global Health Institute.  “Students are an important partner in our work as a global community to mitigate the impacts of climate change on health, safety, and security.”

The EGHI/GT Global Health Hackathon is a partner event between EGHI and CREATE-X. It provides multidisciplinary student teams from Emory University and the Georgia Institute of Technology an opportunity to create technology-based product solutions for global health problems. The target for this fall’s event was creating solutions that address urban flooding, urban heat, or global sea level rise in densely populated, low-resource urban settings. Prizes included $4,000 and a golden ticket into CREATE-X Startup Launch for first place winners, $3,000 for second place winners, $2,000 for third place winners, and $500 each for two honorable mention winners.

“This hackathon continues to be a wonderful partnership between our two institutions that gives these talented students the platform and support to put forward solutions to the most pressing issues we face today,” Rahul Saxena, director of CREATE-X, said. “Each hackathon, I’m increasingly impressed with their ingenuity and their dedication to build something of impact.”

Check out the event program on the EGHI website and see photos from the event on the CREATE-X Flickr account. The full list of the winners of this year’s event includes:

1st Place: iManhole

An integrated system that gathers real-time data from manholes and uses machine learning algorithms to predict flooding to manage traffic and evacuation routes

Team Members: Imran Shah, Leonardo Molinari, and Jiaqi Yang 

2nd Place: Canopy

A climate-tech software platform for democratizing climate analytics using machine learning for urban development planning.

Team Members: Deesha Panchal, Kruthik Ravikanti, Vaibhav Mishra, Nicholas Swanson, Jennifer Samuel, and Vaishnavi Sanjeev

3rd Place: Floodwise

A package of effective simulations and an informed chatbot that help facilitate wise decisions during floods.

Team Members: Ansh Gupta, Dimi Deju, Mukund Chidambaram, and Sahit Mamidipaka 

Honorable Mention

Conquering Heat Islands

Process and hardware that uses excess solar power to mine crypto

Team Members: Rida Akbar, DJ Louis, Edward Zheng, Dmitri Kalinin, and Jade Bondy         

Real-Time Computational Modeling of Urban Flooding and Evacuation in Local Atlanta Communities

Integrated system to gather real-time data from manholes and use machine learning algorithms to predict flooding and optimize traffic/evacuation.

Team Members: Imran Shah, Leonardo Molinari, and Jiaqi Yang

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Balancing Act of Hurricane Season Sways With Climate Change

Hurricane Radar.

Hurricane season is underway and runs through Nov. 30. While the National Oceanic and Atmospheric Administration is forecasting a “near-normal” 2023, experts say that climate change paints a more unpredictable picture for the future.

Behind the 2023 projections is a balancing act of rising oceanic temperatures and the onset of the climate phenomenon El Niño, explains Susan Lozier, dean and Betsy Middleton and John Clark Sutherland Chair in the College of Sciences. The waters of the tropical Atlantic Ocean are currently 1 – 3°C above average, which would typically signify the potential for more intense activity, but the wind shear associated with El Niño acts as a deterrent for hurricane formation.

Increasing Intensity

But what could happen when the shield of El Niño isn't present to counteract the rising temperatures in the tropical Atlantic?

"Climate change is leading to warmer surface temperatures. We know that will lead to more intense hurricanes, but we don't know if it will necessarily lead to more hurricanes. As climate change progresses, we are interested in understanding how weather patterns will be disrupted, including those related to hurricane formation and pathways," said Lozier, who recently served as president of the American Geophysical Union.

She further explained that the increased intensity is a result of the warm waters releasing additional energy into the storm as it forms. This consequence of climate change could present problems for the Tech campus and the city of Atlanta due to the risk of torrential rainfall. According to the National Weather Service, flooding has proven to be the deadliest hazard associated with hurricanes over the past decade.

"When people think about hurricanes, they generally think about damaging winds. Winds are damaging, but increasingly, the most damaging part of a hurricane is the immense amount of moisture they carry," Lozier said, reflecting on the 2017 landfall of Hurricane Harvey. "An area like Atlanta could be affected by heavy rainfall associated with the path of a hurricane. The winds will have mostly died down by the time a storm reaches Atlanta, but as the climate warms, warmer air holds more moisture, and because of that, the expectation is that there will be more rainfall associated with hurricanes and tropical storms.”

Beyond Reducing Carbon Emissions

Fueling the rising temperatures in the world's oceans is an increase in carbon emissions, and simply curtailing them may not be a solution.

"The private and public sectors are increasingly looking at actively removing carbon from the atmosphere because we are unlikely to limit global warming simply by curtailing emissions. Active carbon drawdown from the atmosphere and the ocean are active areas of research right now,” Lozier said.

Tech researchers are at the forefront of this effort, highlighted by a partnership between the Institute, the Georgia Aquarium, and Ocean Visions­­ — the Center for Ocean-Climate Solutions. Lozier represents the Institute as a partnership lead at the center, where the primary focus is the design and delivery of scalable and equitable ocean-based solutions to reduce the effects of climate change and build climate-resilient marine ecosystems and coastal communities.

Associate Professor Chris Reinhard is exploring how coastal ecosystem restoration can permanently capture carbon dioxide from the atmosphere as it becomes buried in sediments on the seafloor. The overall process of removing carbon from the air can be costly. To combat that, a team of researchers in the School of Chemical and Biomolecular Engineering is developing a traditional direct air capture system that is cheaper to operate and more efficient. Helping to craft policy and research climate solutions, Marilyn Brown, Regents’ Professor and the Brook Byers Professor of Sustainable Systems in the School of Public Policy, serves on the leadership council of Drawdown Georgia.

A certain level of unpredictability will always exist when dealing with natural disasters, but understanding humans’ role in controlling climate change could be a key factor in our ability to accurately assess the threat of developing storms. 

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