Exploring ancient caves for modern climate solutions.
How do scientists predict when a tornado may strike? How do economists figure out how the world will be affected by rising temperatures? And how do we figure out what the best thing to do in the face of the growing and serious effects of climate change?
The key to these predictions and decisions is reliable data. We need to know what the climate used to be like in order to figure out how it’s changing. Data about past climate comes in a lot of forms – it ranges from direct information like written records by settlers and explorers to tribal art and folklore hinting at variations in the weather. Other clues lie in nature itself. For example, the concentric circles that tell us how old a tree is can also tell us whether it survived a drought or suffered a cold winter. But for Dr. Kathleen Johnson, the ancient climate data she studies hides inside the cool darkness of caves.
Dr. Kathleen Johnson, a member of the Grand Traverse Band of Ottawa and Chippewa Indians, is a geochemist and paleoclimatologist. She’s fascinated by how we can reconstruct the history of climactic events using cave formations. Dr. Johnson has degrees in geological sciences and geology, and she’s now a geoscience professor at University of California, Irvine. Her team uses archives of climate variability hidden in cave deposits to understand past precipitation, temperature, and atmospheric circulation in diverse regions including Laos, Vietnam, China, Mexico, and California.
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Decoding cave secrets
Caves tell stories about what the climate used to be. But how does Dr Johnson understand the stories caves are trying to tell us? One of the ways that caves guard climate secrets is in speleothems. These are mineral deposits that accumulate inside caves upon years of sedimentation and water seepage. You may recognize stalactites and stalagmites as typical cave formations– both of these are forms of speleothems.
Speleothems are formed as a by-product of a reaction between water (usually rainwater), carbon-dioxide from the atmosphere, and rocks that contain calcium carbonate. The chemical reaction between these three forms calcite deposits, which is the same material that chalk is made from. Speleothem formation is a very slow chemical process that takes tens of thousands of years. They grow almost 1000 times slower than human hair!
The structures of these slow-growing rocks hide clues about the lifetime of the speleothem, kind of like how you can read tree rings to decipher the climate that trees have experienced over their lifetimes. There are two forms of data inside these rocks can give us: (1) how old they are, and (2) the climate they experienced.
To tell how old a speleothem is, scientists use uranium dating. Uranium is commonly found in trace amounts in speleothems. It can be used to tell time because it has an unstable nucleus. If an atom has an unequal number of protons (positively charged particles) and neutrons (chargeless particles), it becomes unstable because it has too much energy. That extra energy is slowly emitted from its nucleus as radiation. As it continues to radiate energy over a long period of time, it ‘decays’. Uranium in speleothems decays by half every 4.5 million years. Measuring how much uranium is left in a speleothem can allow us to calculate how long it’s been around. The amount of uranium in a cross section of the speleothem can also help us figure out how much water content the deposits have been exposed to over their lifetime, which clues us into changes in climate patterns.
What are isotopes?
An element is defined by the number of protons, or positively charged particles, it contains in its nucleus. Oxygen, for example, has 8 protons. But an atom’s nucleus also contains uncharged particles called neutrons. Usually, an element has the same number of protons and neutrons in its nucleus. But that’s not always true! If an atom has a different number of neutrons in its nucleus, it’s called an isotope. An isotope of oxygen, for example, has the same number of protons (8) but a different number of neutrons. Increasing the number of neutrons can make an element ‘heavier.’ ¹⁸O – one of the isotopes in speleothems – has 10 neutrons, making it heavier than the most common form of oxygen. Changing the number of neutrons in an atom doesn’t change the chemical properties of it, but it can change the way the atom behaves physically.
To figure out what the climatic conditions speleothems survived, geoscientists can look at the relative amounts of different isotopes inside the rocks. This technique is called oxygen isotope analysis, and uses a variable called δ¹⁸O (delta-18-O), which measures the ratio of two isotopes of oxygen: ¹⁸O and ¹⁶O. This number is determined by the properties of the ancient water that seeped into the cave to make the rock formation. It depends on altitude, evaporation temperature, source of the water vapor, and distance to source water. For example, seawater that contains ¹⁶O evaporates faster than seawater that contains the heavier isotope ¹⁸O, an effect that is even more dramatic when it’s colder. So the relative ratio of these isotopes can tell climatologists a lot about the planet’s past, including the temperature, humidity, and precipitation.
But δ¹⁸O is a complicated metric that is influenced by tons of factors. To make accurate inferences about past climate, scientists from all around the world collect huge amounts of speleothem data. These large datasets help provide more accurate approximations of what cave isotope abundances truly mean for ancient climate.
Linking caves, climates, and communities
Kathleen Johnson’s work uses this method of analyzing speleothems to understand monsoons in Southeast Asia. Monsoons are a major season for tropical countries in South America, Africa and Southeast Asia caused by periodic shifts in winds. They’re times of large amounts of rainfall that serve as the primary source of water for many communities. Early human settlements track with monsoon patterns, since they allowed for sustained agriculture and food. So past climate changes not only tell us about the environment but also how human settlements rose and fell as a result of environmental shifts.
For example, Dr. Johnson’s team has done some fascinating work on understanding how climatic changes in modern-day China impacted the rise and fall of different dynasties. The atmospheric conditions of China extrapolated from the stalagmites in Wanxiang Cave has given additional insight into the fall of the Tang Dynasty in the 9th century. Dr. Johnson started by analyzing the samples of rock from the cave and comparing it with data collected from other caves in China. She then compared climate patterns with historical notes and documents from the Tang Dynasty to draw parallels between the availability of water, crop growth, and political unrest in the region. The rise and fall of political powers is really complicated and depends on a lot of factors, but Dr. Johnson’s work shows just how important weather like monsoons and solar cycles are to things like agriculture, population growth, trades, war, treaties, and cultural progress. So the climate can have a huge impact on the political and social world of humans! Being able to go back in time and associate a community’s flourishment or decline with the climate conditions is a super powerful tool, one that might help us understand and prepare for the changes that are coming with rising temperatures.
Emphasizing Indigenous inclusion
As a scientist, Dr. Johnson searches for clues about the impacts of the climate on past communities, but she also cares deeply about how climate change is disproportionately affecting communities today. Native American tribes across the country are already experiencing the environmental consequences of climate change, including coastal erosion, lack of drinking water and severe storms. These impacts are made even worse by the systematic neglect of these communities. Even though indigenous people are highly impacted by environmental issues and highly involved in land management in the United States, Native Americans are heavily underrepresented in the geosciences. This is an issue close to Dr. Johnson’s heart, and one she works hard to address.
To combat this lack of inclusivity, Dr. Johnson set up the American Indian Summer Institute for Earth System Science, a summer institute for 130 indigenous youth from all over the United States that introduced them to earth science and educated them on the environmental issues that impact their native lands. The students conducted fieldwork and completed research projects related to tribal environmental issues that range from water quality issues to renewable energy and waste management. To ensure that this program was as effective as possible, Dr. Johnson emphasized the need to incorporate a Native American perspective into teaching earth science. The institute brought in native experts to discuss traditional environmental knowledge while also incorporating other cultural practices like storytelling, singing, and dancing. “Native Americans are the most underrepresented of all ethnic minorities in the earth sciences,” Dr. Johnson has said, “I can truly say that it is my work with Native youth that has provided me with the most joy and hope for the future of our planet.”
Looking to the past for the future
As the debate on tackling climate change intensifies, scientists turn to the past to gain insight into how our ancestors survived harsh and unpredictable environmental conditions. Dr. Johnson and her team are striving towards developing methods and tools to better understand climate systems and predict climate variability. Maybe by stepping back in time, Dr. Johnson will help us stay one step ahead of the climate changes yet to come.
Written by Manasvi Verma and Kas Sivaguru
Edited by Katie Fraser and Caroline Martin
Illustrations by Sal West
Activities by Lindsey Oberhelman
Sources and Further Reading:
Terrestrial Paleoclimate and Geochemistry Lab by Kathleen Johnson
Uranium–Thorium Dating of Speleothems by Kathleen Wendt, Xianglei Li, and R. Lawrence Edwards
A test of climate, sun, and culture relationships from an 1810-year Chinese cave record by Kathleen Johnson et. al.
Dive deeper into the world of caves, geochemistry, and climate science.
Complete (20-30 minutes): How much did you learn about Kathleen's research and climate science? See if you can fill in the correct words to complete the sentences in this activity. If you don't know the answer, do some research on your own to see if you can figure it out!
Grow (3 weeks): Stalactites and stalagmites take thousands and thousands of years to form inside caves. But you can grow your own tiny versions of the rocks at home much quicker! Follow the steps in this activity and see what you can make.
Measure (1-2 hours): Have you ever noticed that tree stumps have concentric rings inside them? You can use those rings to tell how old the tree was! Find a tree stump near your home and follow along with this activity to learn about it.
Experiment (45-60 minutes): Dr. Johnson uses radioactive dating to figure out how old her rock samples are and when they formed. Experiment with a simulation of radioactive dating to see if you can figure out the age of different object. Follow along with this worksheet to record your findings.
Contextualize (1-2 hours): As scientists it is important to be aware of the context of our work. Kathleen Johnson is a member of the Grand Traverse Band of Ottawa and Chippewa Indians. She understands the environmental issues that native communities face. There are many native-led movements working to combat climate change, including the Water is Life Movement, Standing Rock Dakota Pipeline Protest, Indigenous Environmental Network, Land Back Movement, Defend Kumeyaay Land Movement, Coastline Gasoline Pipeline Movement, Honor the Earth, and Indigenous Climate Action.
Pick a group and spend some time researching why the group formed, who their members are, what they do, what their goals are, and their plans to achieve those goals. Then summarize what you learned to a friend or grown-up!