Jessica Tierney and other paleoclimatologists have a strong and growing grasp of temperature and hydrology shifts of the past. What can they tell us about our rapidly changing climate now?
by Allen Best
Jessica Tierney is a historian, but with a longer perspective than most. She specializes in the last 100 million years. She’s a paleoclimatologist.
Like all historians, though, she studies the past to help understand and possibly guide the future. With a view of that deep past, she reports that we live in a remarkable time.
Global temperatures have gone up 1.4 degrees Celsius, or about 3 degrees Fahrenheit, since humans began burning fossil fuels at scale about the time the United States came into being in the late 1700s.
It’s a simple proposition. Fossil fuels were created from biological material deposited roughly 300 million years ago, putting underground carbon dioxide that had been in the atmosphere. Now, in burning fossil fuels (coal, oil, natural gas), we’re returning the carbon dioxide into the atmosphere.
We have been doing this in a hurry. The atmospheric concentrations of CO2 were maybe 280 parts per million about when the United States came into being in the early industrial age. Concentrations are about 430 ppm now.
The importance of those numbers can be difficult to accept. Even some who accept that atmospheric accumulations C02 and other greenhouse gas emissions will pose a problem for humanity doubt the immediacy of the problem. U.S. Energy Secretary Chris Wright, for example, contends we have another century to figure this out. Today, he argues, the advantages to exploiting fossil fuels vastly outweigh the costs of warming temperatures and more erratic and destructive hurricanes, floods and, in the West, aridification of the Colorado River Basin.
Others dispute the science altogether, arguing that these occurrences are just natural climatic variability.
Tierney describes this as a short-term view, akin to see seeing a few plays in a football game and trying to understand the rules of that game. To get a strong grasp of the rules you must watch a full game. And that’s true about understanding climate. You must go deep into the past to understand what lies ahead.
Resolving uncertainties
Predicting the future is an ambition rife with uncertainty. We all know that in our personal lives. In some cases, it doesn’t matter all that much if predictions don’t bear fruit. To continue the football analogy, will the Denver Broncos win 11 or 12 games this season? What is the likelihood of winning 15? Or just 7? The outcome matters to diehards, perhaps, but only until the next season.
More is at stake in climate change. If warming temperatures and more erratic weather are a natural phenomenon, there is little we can do about it except try to adapt, no matter how expensive. If we are responsible, the equation shifts. Will near-term investments yield long-term cost savings? Trillions of dollars are at stake.
In 2003, when I began poking around the climate change debate on behalf of a ski industry trade magazine, greenhouse gas emissions stood at 376 parts per million. That was a sharp rise from the 313 ppm that scientist Charles David Keeling recorded atop Hawaii’s Mauna Loa n 1958. And, again, from he 280 when Ben Franklin was flying kites in Philadelphia.
What exactly will the climate be like if today’s 430 ppm doubles to 860 ppm? In the past, computer models have delivered a wide range of answers. Tierney and other paleoclimatologists aspire to narrow the uncertainty.
Tierney is a professor of geosciences at the University of Arizona in Tucson. She spoke in Avon on Aug. 11 at a forum sponsored by the Vail Symposium and Walking Mountains Science Center. She’s young enough to have tattoos cover much of one arm plus portions of her back. She’s also old enough or at least smart enough to have been a lead author on the most recent Intergovernmental Panel on Climate Change. The report, the sixth so far, was released in 2023.
A major outcome of that IPCC report, said Tierney, was the conclusion that it’s indisputable that human activities are making extreme climate events more frequent and severe.
“Now, this is actually a very strong statement from an organization, the IPCC, that I can tell you, having worked for them — for free, by the way, it’s all volunteer based — that is very conservative about these statements. Every statement needs to be verified and by every single country that participates in the IPCC,” she said.
“This is a strong statement, right? This is saying yes, we can link these extreme climate events to human-caused climate change because a warmer climate is a more energetic climate, which creates more extreme events. And so, a large part of the report was dedicated to that finding.”
Still, is an average increase of 3 degrees Fahrenheit all that much?
“Small numbers in the climate system mean big changes, and we’re starting to see that in our own backyards,” said Tierney. She cited the heat dome in 2021 that produced 116-degree heat in Portland, Ore., a place with a climate that previously had need for relatively few air conditioners. People died.
Tierney showed a slide of a wildfire near Lake Tahoe. She grew up skiing at resorts there while living in California’s Marin County, north of San Francisco. Those wildfires in the West, including Colorado, are also at least partly a manifestation of a warming climate that is creating drier summers. “That makes ignition so much more likely,” she said. In southeastern states, the heating has been manifested in record flooding.
Tools used by paleoclimatologists
But always the questions return. Just exactly what do we know and how do we know it? What exactly is the fuss about the increasing concentrations of a few parts per million of greenhouse gases in the atmosphere?
Paleoclimatologists use proxies to figure out climates of the past. Tree rings reflect heat or drought in the most immediate past. In the Colorado River Basin, tree rings testify that the current “hot” drought has produced the leanest flows of the past 2,000 years. Corals are like the tree rings of oceans. (Any studies of corals you can point to here?)
Ice cores from glaciers – mostly in Greenland and Antarctica but in other places, too — provide clear evidence about carbon dioxide content of the atmosphere during the past 800,000 years.
Ocean and lake sediments also provide clues about past climates. Even the geology evident in road cuts can help Tierney and other paleoclimatologists draw conclusions about past climates.
Study of rocks, however, has allowed paleoclimatologists to reconstruct the temperatures of the planet for the last 450 million years. For example, sedimentary structures in sandstones and limestones, fossils, geochemical signatures, and trapped materials (like gases or isotopes) preserve measurable clues about temperature, atmospheric composition, ice volume and sea level, and ocean chemistry through time.
All this evidence points to a simple conclusion. The rate of climate change now underway is extraordinary. Low concentrations of C02 resulted in cooler climates where humans evolved.
Conversely, high concentrations of C02 in Earth’s history have produced extremely warm temperatures. About 50 million years ago crocodiles crawled and swam in the Arctic. They need average temperatures of 68 degrees and above.
So, temperatures have swung wildly in the past. And in these swings, there is a direct correlation between the carbon dioxide in the atmosphere and temperatures.
Remarkable now is the power of human intervention. “The amount of fossil fuels we burn has raised CO2 to a level that is far beyond anything we’ve seen in the last million years. Actually, we have to go back three million years to find examples of CO2 that are as high as 430 ppm.”
Tierney, in her Vail-area presentation, talked about climate sensitivity. Paleoclimatologists, working with climate models, have been trying to get a sharper focus on what lies ahead given our global emissions. With a car, you can test drive it. With climate models, testing them after 30 to 50 years doesn’t work as well.
“This is something the climate community has been really stuck on for decades,” said Tierney.
The trend of the Earth’s climate has been toward cooler. Wide swings, but since the dinosaurs (and crocodiles in the Arctic), generally cooler temperatures. The last hothouse climate was 50 million years ago, during the early Eocene. Oceans then were 400 feet higher with less water locked up in ice. Florida was under water. The mean temperatures on the Earth’s surface were between 25 and 30 degrees Celsius, compared to 15 degrees today. Conversely, the glacial maximums had temperatures about 6 degrees C colder than today’s temperatures.
Again, it’s useful to remember that temperatures have so far advanced 1.4 degrees Celsius warmer since the late 18th century, when Colorado was still a young state.
When the Earth was ice free
A deeper view of history tells a more significant story. In contrast to today’s global average temperature of about 15 degrees C, it was maybe 35 degrees C for much of Earth’s geologic history. That compares with the human body’s temperature of 38.39. For much of the last 500 million years, though, the Earth has been ice free.
This information opens the door for some of those skeptical about human-caused climate change. “Oh, well, you know, the planet’s been so warm. Like, what’s the problem? Then why are people complaining about a couple degrees Celsius change when it’s changed more before?”
“It’s all about speed,” said Tierney. She showed a graph about a past shift in climate. “The modern rate of warming is probably at least 10 times order of magnitude higher than some of the most rapid changes” in the past.
“Humans can change the climate faster than the Earth we’re normally used to, and things can’t adjust, including us. We’re an ice age species.”
She allowed that scientists do not yet have the tools to understand whether rapid change occurred in the past over the course of a century, as is now occurring.
In other words, uncertainty remains. That is the challenge to Tierney and other paleoclimatologists. They have been trying to input the appropriate information into computer models that will more precisely tell us what can be expected.
Water constitutes a key uncertainty. Scientists still struggle to fully understand clouds. Clouds, of course, deliver water. How much water will clouds deliver in Colorado and other southwestern states as the atmosphere rapidly warms?
“You have this weird dichotomy, where the atmosphere is warmer. It’s demanding more moisture out of the soil. It’s drying out the soils, and at the same time, its more energic atmosphere could fuel a stronger monsoon with more severe storms, with more rain.”
About three million years ago, surface temperatures in the Pacific Ocean off the California coast were 8 degrees C warmer than now. “That’s toasty.” It produced enough rain to create lakes in the Southwest in places now almost imaginable.
A glimpse of that was evident in 2022, when monsoonal rains produced flooding in Death Valley, of all places.
To diminish the uncertainty of climate models, Tierney and other paleoclimatologists have examined the wax on plant leaves. “It sounds crazy,” she said. She then proceeded to tell a detailed detective story.
All plants have waxes on their leaves, she explained. These waxes protect the leaves from water loss, among other purposes. Given a good enough microscope, they can be found in soils and sediments.
Why do paleoclimatologists find them interesting? Well, carbon and hydrogen constitute the waxes, and it’s the hydrogen that leaves clues. A hydrogen isotope called deuterium behaves just like hydrogen but is created differently. The plant uses rain to make the wax, so the ratio of deuterium in the hydrogen reflects the rain. As such, it indicates how much rain was evident when the wax was made.
Our emissions have us at a global temperature increase of 1.5 degrees Celsius, but at our rate of emissions temperature increases of 2 to 3 degrees are not far down the road.
What they found is that when the climate is colder, the monsoon is weaker and the rainfall less intense. “That was a bigger change than we were expecting, right? This is showing that the summer rains are really dynamic, so they’re responding to climate change very sensitively,” said Tierney.
Then, what happens to monsoons in a warmer world? The climate historians then had to find an ancient warm climate. For this, they went back a mere 3 million years, to the Pliocene, the last time that the C02 concentrations were similar to those now.
“And in the Pliocene, global temperatures were about 4c warmer, and we lost most of the Greenland ice sheet, and we also lost ice from the West Antarctic ice sheet — not entirely gone but mostly gone. So we lost a lot of ice, and that’s a bit scary in and of itself, right? CO2 levels were actually a little bit lower than where we are today.”
Losing the glacial ice may take 1,000 to 2,000 years. But this could fuel much more severe storms during monsoons. Gentle summer rains? No, think about flooding.
“It starts completely raging, and we get a huge amount of summer rain, not just in Arizona, but into Southern California, and even farther north,” said Tierney.
Once again, she emphasized the speed of C02 now being added to the atmosphere and the corresponding climate responses.
Colorado River water
In response to a question, Tierney offered her view of today’s Colorado River problems. Hers, she stressed, was a personal, not professional, opinion. She said that water administration was not necessarily built on the idea of efficient water use.
“We’re starting to change that, right. But I think we need to completely rebuild how we manage water and use water in the West,” she said. A large part of that story will be in agriculture. In Arizona, 70% of water goes to ag, 30% to cities.
“Cities can be very water efficient; Tucson has seen our water use flatline for the last several decades, despite huge population growth. Other cities can do the same (as Denver has), but agriculture is going to need some examining, right? It’s the conversation that nobody really wants to have.”
In Tierney’s hour-long presentation, she also offered an interesting explanation about how plants that make waxes have been valuable in helping paleoclimatologists understand past climates. You can find out more about this topic and also her basis for optimism if you want to watch the video. It will likely be posted to the Vail Symposium video page in the next month or so.
More immediately, you can find a Washington Post story from September 2024 that describes a study she helped lead.
- A historian with a very long view - August 17, 2026
- Geothermal interest in southern Colorado - July 31, 2026
- Colorado has new leader for carbon management - July 28, 2026
