In this week’s episode, host Daniel Raimi talks with Erika Wise, a professor at the University of North Carolina at Chapel Hill. Wise discusses some of her research, which combines tree ring data with historical climate data drawn from artifacts such as ship logs and diaries, to provide insights about the North American climate of the mid-1800s. Wise enumerates the strengths and weaknesses of different types of paleoclimate records and historical sources. She also highlights the importance of understanding past climate patterns and extreme weather events, which can help inform contemporary adaptations to extreme weather and climate change.
Listen to the Podcast
- Unique records of historical climate and weather data: “Proxy records … are natural archives of climate information—something out there in the environment that’s recording climate information somehow, which can be tree rings, corals, ice cores … There is a field called ‘historical climatology’ that doesn’t quite fit into either paleoclimate or modern climatology: historical climatologists use pre-instrumental or early instrumental human records of climate, and those can be from diaries, ship logs, harvest records—there’s all sorts of sources, because humans have always been interested in climate and weather.” (7:34)
- Tree rings tell us about historical weather events: “1862 (if you’re interested in this time period) is probably the most famous climate year. This is a year when Sacramento and a lot of the Central Valley of California were very heavily flooded, which is recorded by both the tree rings and the historical records quite well. There’s also other regional events in extreme years, like 1864, which was an extreme drought year centered in Southern California. It devastated the cattle industry. Some people think that’s what led to the development trajectory of Southern California, and it’s picked up really well by both tree rings and historical records. Even the spatial pattern of that drought is picked up really well.” (15:53)
- From tree ring data to climate information: “In the western United States in particular, we have very high amounts of data. These data from trees growing at different elevations—they’re different species, so each site has a slightly different part of the climate year that they’re more sensitive to and recording in their rings. By combining and recombining the different sites through statistical analysis, I was able to get climate information. In this case, I was focused on precipitation on a rolling basis throughout the year … I found that, by making these seasonal and monthly records, I was able to get a clear picture of what happened over the entire year—particularly some of those extreme events and precipitation extremes that happened outside of the normal season.” (20:48)
Top of the Stack
- “Sub-Seasonal Tree-Ring Reconstructions for More Comprehensive Climate Records in US West Coast Watersheds” by Erika K. Wise
- “Climate Factors Leading to Asymmetric Extreme Capture in the Tree-Ring Record” by Erika K. Wise and Matthew P. Dannenberg
- Saving Us by Katharine Hayhoe
- Getting to the Heart of Science Communication: A Guide to Effective Engagement by Faith Kearns
The Full Transcript
Daniel Raimi: Hello, and welcome to Resources Radio, a weekly podcast from Resources for the Future. I'm your host, Daniel Raimi. Today, we talk with Dr. Erika Wise, professor in the Department of Geography at the University of North Carolina at Chapel Hill and head of the Climate and Tree-Ring Environmental Science Research Group. Erika's work focuses on understanding the long history of the Earth's climate and what that history can tell us about the effects of human causes of climate change. To do that, she and her colleagues analyze tree rings, ship logs, diaries, and a variety of other data sources to reconstruct annual, seasonal, and even monthly variation in local and regional climates. In today's episode, Erika will give us a fascinating look into how that work gets done and what it can tell us about the future. Stay with us.
All right, Erika Wise, from the wonderful University of North Carolina at Chapel Hill near my hometown of Durham, North Carolina—welcome to Resources Radio.
Erika Wise: Thanks. Glad to be here.
Daniel Raimi: We're thrilled to have you. Erika, we're going to talk about your work on paleoclimate and tree rings, and I'm really looking forward to it, because it's pretty different from a lot of the topics we cover on this show. We always ask our guests how they got interested in working on environmental issues, either as a kid or later in life. So, what steered you into this line of work?
Erika Wise: I have always been interested in the environment, and particularly weather and climate, since pre-high-school age. I don't have a great explanation for why that is.
But as an 18 year old, I went to college with the plan of majoring in Earth sciences. The program I was in at the time was aimed at hard-rock geology, and the normal thing you had to do to graduate was a summer field geology weeks-long class, and I actually had no interest in doing that. So, I found out that I could do a research thesis instead, and I tracked down a paleoclimate modeler, and I ended up doing a project on how the desiccation of the Aral Sea had affected local temperature. So, that not only got me started in climate, but also in doing research—which had not occurred to me that someone could do as a career.
I worked in environmental consulting for a while after that, but I went to graduate school, and my master’s was actually on how climate variability affects air quality. I was at the University of Arizona, which is the global center of tree ring research. I eventually wandered over there and took a class and found out that tree ring research has everything I could want: it has amazing field work, it has really fun lab work, it has nice people. The rest is history.
Daniel Raimi: That's so interesting. It seems like such a cool topic to study. I mean, tree rings—it's like something you hear about as a kid, and then, at least for me, kind of forgot about it for the rest of my life.
Erika Wise: Right. And it's really fun for outreach, too, because kids love tree rings.
Daniel Raimi: That's great. Well, let's talk more about your research. As I mentioned earlier, before we started taping—we talk about climate change a lot on our show, but we mostly talk about it in the context of the recent past and the future. We don't talk much about the long history of the climate. So, can you give us, just in really broad terms, why is it important to understand those historical changes in the climate, and what are some of the ways that information is used to inform the present and the future?
Erika Wise: I'm really glad you asked that question, because most paleoclimate researchers, I think it's safe to say, are doing their work because of concerns about human-caused climate change, but I think that connection is not immediately obvious. I would say there are three main reasons that paleoclimate or pre-instrumental climate are so important for understanding current climate change. I'm sure there are other ones, but I'll just use these three key ones that come to mind.
The first is about context. The most common climate change myth that I hear is that current warming is just part of a natural cycle. The reason we know that's not true is because of our paleoclimate information. So, we can say global temperatures are the hottest in the past 2,000 years. We can say the last time CO2 was as high was three million years ago. And we know those things because of these wonderful paleoclimate proxy archives that we have.
The second reason that I think the paleoclimate is so important has to do with planning for the future. When we think about human-caused climate change, of course we aren't just concerned about it being warmer, right? We're concerned about droughts, and floods, and changes in timing of rain and all these other secondary impacts. One way we can use our paleoclimate information is to ask, What other impacts might we expect in a warmer world based on what we've seen in the past?
The third reason has to do with climate models. Of course we rely on our models for projections of what's to come in the future. The way that we can have confidence in our models is, How well do they replicate the past climate? If we only had our instrumental data, we would only be able to test against this kind of small window of Earth climate. But when we have our knowledge of climate over thousands of years, we can model past environments with those same climate models and make sure they can replicate those past changes that we've seen. Then that gives us confidence in our future projections.
Daniel Raimi: That's a great explanation; thank you so much for that. I should have asked at the outset: The term “paleoclimate”—is that synonymous with pre-instrumental research, or are they different in some ways?
Erika Wise: Yeah, they're synonymous. I think that there's different categories, then, of paleoclimate, because it's kind of interesting, because with the tree ring records, we're interested in something that we call “the Common Era,” which is really just the last 2,000 years or so. But then, of course, with some of our other paleoclimate records, like ice cores, they're going back much, much further. So, there's also different subcategories of paleoclimate once you get into it.
Daniel Raimi: Well, let's talk now about some of your current research. So, you're in the midst of a project that's funded by the National Science Foundation that examines paleoclimate across western North America, and I think focused on the nineteenth century, but please correct me if I'm wrong about that. Can you give us a little bit of background on this particular project, and then introduce us to some of the data sources that you're using to try to reconstruct the climate over this time period?
Erika Wise: Right. This goes into a little bit of what you just asked me about paleoclimate. Mostly, we think about it in terms of what we call these “proxy records.” So, there are these natural archives of climate information—something out there in the environment that's recording climate information somehow. And that can be our tree rings, our corals, ice cores—all these wonderful things out there. But there is a field called “historical climatology” that doesn't quite fit into either paleoclimate or modern climatology. Historical climatologists use pre-instrumental or early instrumental human records of climate, and those can be from diaries, ship logs, harvest records—there's all sorts of sources, because humans have always been interested in climate and weather, of course.
I have this colleague I've known for probably 10 or 15 years, Dr. Cary Mock, who's at the University of South Carolina. He's a historical climatologist who studies western North American climate. Many years ago, we had a conversation about some particular year—I think it was sometime in the 1840s in Utah. He said, "The tree ring record said that was a dry year, but I have this set of diaries from people traveling through or living in that area that showed it was actually kind of a wet year, because it was a very snowy spring." That got me thinking about the ways that those two data sets—the historical records and the tree rings—could be used in a really complementary way.
In this project, we're applying that combination to the West Coast of North America in the mid-1800s, and that's because of previous work by myself but also others that have shown that that time period, which was at the end of what we call the Little Ice Age and also the beginning of the ramping up into our industrial warming period—that it was a really wild period for West Coast climate. There were floods, and droughts, and extreme snow, and rain, and flipping between the two. We think that there's really important lessons to learn from that period that can hopefully inform us about extreme events in the future.
Daniel Raimi: That sounds so fascinating. I want to ask you about what you're finding with some of this research. But first, one more kind of methodological question, which is: I'm hoping you can help us understand in broad terms again, what are some of the relative strengths and weaknesses of these different data sources? Are tree rings good in some ways that diaries are not, or are diaries good in some ways that tree rings are not? Can you just help us understand some of the relative merits of different types of data material?
Erika Wise: Yeah, absolutely. Really, the whole reason we started this study was because of those relative strengths and weaknesses. So, trees are amazing. The United States has this wealth of some of the most incredible trees in the world. The huge strengths of tree rings are that they're annual and can be precisely dated to a calendar year, which is really unique in paleoclimate records. They're also really sensitive to climate, so they record climate really well, but of course they do have limitations.
First, we're talking about one ring a year, so there's not a lot of room for nuance there. Second, each individual site is more sensitive to a certain part of climate—maybe it's cold temperature, maybe it's drought. And it also tends to be more sensitive to a particular time of year. So, at any given site, it's going to be somewhat selective in the climate information that it contains.
Historical records are almost the opposite in some ways. Their strength is that they are often daily climate records, and sometimes they're even hourly in the case of something like ship logs. Another thing is, it's hard to argue with them, because if someone wrote in their diary, “It rained all day,” it probably rained all day, right? We have no reason to doubt that. On the other hand, they do tend to be very limited in duration and also spatial coverage. So, a ship might have overwintered in a port for an entire winter, but that same ship probably won't be back the next year in the same place. A person might keep a really careful weather diary for a couple of years, but probably not for decades. Although, I mean, there are exceptions. Some people kept them for decades. But in general, they're in these little, discreet blocks of time. And then, over space, there will be a record maybe at the coast, or maybe a record from a town, but they're not always quite where you want them to be. So there's going to be these spatial gaps where you're finding them.
These two sources of data information tend to fit together well and have these complementary strengths and weaknesses.
Daniel Raimi: That's so interesting. A follow up question about the ship logs, which, again, I know zero about: Were ship logs kept consistently during this period? Would every ship have a logger, and did they have common methods that they used across space and time? Can you just talk a little bit more about that process?
Erika Wise: Yeah, these are some of the most useful records, because they tend to record a couple of things—one of them is temperature, but another one is barometric pressure. They were recording atmospheric pressure measurements, which are really important for understanding all sorts of weather and climate events.
The reason these are historical records and not instrumental records is that they were different types of instruments, and we can't say that they're equally calibrated amongst ships. But when they did record this data, they wrote it down in these log books, and they tended to be very consistent over time. So again, this is an opportunity to have not just daily data, but usually multiple times a day, they were recording this climate information. It's a really interesting source of information that's also been pretty well-preserved, so that's also really useful.
Daniel Raimi: If we have more time, I'd love to ask you about that preservation process, and then how one goes about finding that information. It just sounds really fascinating.
But let's turn now to some results of the work that you're doing. Can you give us an instance or an example or two of some times when these data sets differed considerably, and why those differences may have arisen, and what it tells us about the climate in a certain place at a certain time?
Erika Wise: One of the exciting parts of this project is that, even though we did launch it with the intention of finding those differences and then figuring out why they occur, what we've found so far is that those two data sources don't differ considerably, for the most part—they tend to agree quite well.
Now, that's not true every single year—there are some years that are different. For instance, our historical records suggest that the winter of 1848 to 1849 was very wet in California, but it appears kind of average in tree ring records. So one way that can happen is if the rain comes mainly in the form of extreme rain events—a lot of rain in a short time, because that can result in runoff across the land, and then that water is not usable by the tree, so the tree could be missing that water.
There are other really wet years in the mid 1800s, though, that are captured by both data sources. 1862 (if you're interested in this time period) is probably the most famous climate year. This is a year when Sacramento and a lot of the Central Valley of California were very heavily flooded. That's recorded by both the tree rings and the historical records quite well. There's also other just kind of regional events in extreme years, like 1864, which was an extreme drought year centered in Southern California. It devastated the cattle industry; there's people who think that's what led to the development trajectory of Southern California eventually. That's another one where it's picked up really well by both tree rings and historical records, and even the spatial pattern of that drought is picked up really well by both.
Now, we did pull in a third data source as part of this ongoing story, which is something called reanalysis. And reanalysis is kind of a model-data hybrid. So, there's this background model running, but it brings in actual data whenever possible to kind of self-correct. When you're at recent times, it's mostly data with just a little bit of model, and the further back you go in time, it becomes more model and less data. It now goes back to the 1830s, but of course, in that early period, it's mostly model. These data are really valuable, even up to the present time—I use them all the time in my own work. But what we found was that, in that early part of the record, they didn't really match up well with either the tree ring or the historical data, and that was particularly when it came to rain and snow.
The people who developed this product are aware that they don't have anything to test against during that period, because there aren't other data products that go back that far. So, one of the goals of this research is to provide these other climate data sources, so that we can test the model and hopefully eventually improve it in the places where it's not getting things right.
Daniel Raimi: One question that just popped into my head is sort of a pedantic one, but it's about when to use the word “weather” and when to use the word “climate.” So, when I asked you about what the “climate” was like in a certain year in a certain place, is that the right word to use, or should I have said “weather”?
Erika Wise: We generally say “climate” because, especially with these—like, in the case of tree rings again, often we're talking about just a yearly record, so that we would consider more of a climate record. Even though the historical records have these daily data, and they are kind of a weather journal, when we are doing this research, we are scaling it up to a matter of months, or even seasons or years. So we would normally say “climate.”
Daniel Raimi: Okay, great. So I will keep saying “climate,” then, too.
This next question goes to what you were just talking about different time scales. One of the outputs of this project so far, that I was able to take a quick look at, was a paper that estimated precipitation patterns using tree rings—but instead of doing it at an annual level, you try to do it at a quarterly and I think even a monthly time scale. Can you help us understand (again, for the non-expert) how you do that? And what are some of the benefits that you can derive by looking at those smaller time scales? And also what are the challenges of doing it?
Erika Wise: I would say a big goal, overall, of my research is to try to take these really great tree ring records and see if we can make them even better. One specific goal of this paper that you mentioned had to do with developing sub-annual records. We talked about trees having these annual growth bands, so traditionally, we are getting one climate record per year, but I and other people as well have been exploring ways to get additional information out of the tree rings. That includes measuring something called earlywood and latewood separately—those are just the light and dark part of each tree ring, if you can picture the inside of a tree. Also, analyzing other parts of the wood, like cell size and stable isotopes and density of the wood.
All these approaches try to dig out additional climate information. And what I did in the study you mentioned was to use the power of many different tree ring sites. This is data that's been shared by many different researchers, and in the western United States in particular, we have very high amounts of data—so, high data density. These data from trees growing at different elevations—they're different species, so each site has a slightly different part of the climate year that they're more sensitive to and recording in their rings. By combining and recombining the different sites through statistical analysis, I was able to get climate information. In this case, I was focused on precipitation on a rolling basis throughout the year. The reason that that's important goes back to some of the things we were talking about earlier with the limitation of the tree ring record.
I found that, by making these seasonal and monthly records, I was able to get a clear picture of what happened over the entire year, and particularly the ability to capture some of those extreme events and also precipitation extremes that happened outside of the normal season. The ultimate goal is to get the most accurate picture of past climate that we can. And what I found is that, by using this approach, I was really able to get just that extra little bit of information out—especially for those times of the year that trees traditionally haven't been that strong.
Daniel Raimi: That's so fascinating. When you're doing that work with tree rings, are you mostly going into archives and using tree rings that have been collected by others, or are you harvesting wood? Are you going out there and measuring data from living trees? How do you physically do that?
Erika Wise: Yeah, I'm glad you asked that, too, because I always—whenever I talk to people and especially classes about tree ring work, usually somebody asks before I can remember to say that we're not cutting down any trees. I always forget to mention that, but we're not cutting down living trees. We have these just small-increment bores—it's something that foresters use to test growth in their managed forest, but you can just drill this straw-sized thing into a tree and take out a sample where you can see all their rings, but it's not going to kill the tree. That's what we use for when we're getting our own data and for a big project like the one we were just talking about, that's really depending on other people sharing their data.
I do collect my own data, and some of my own data went in there, but tree ring field and lab work takes a very long time. To be able to do a really large spatial study like that, we have to be able to rely on each other to share our data, so we can get those points from other people and combine them in these useful ways.
Daniel Raimi: Yeah, that's very interesting. It reminds me of oil and gas, which is a world that I know a little better—when companies are trying to assess whether they want to drill in a place, they'll take a core sample of a rock that's deep underground, and it sounds like a somewhat similar process, although of course happening at smaller scale and above ground.
So, one more question before we go to our final Top of the Stack segment, which is, again, just about how you do this work. I'm really fascinated to hear more about these ship logs and about how you get access to them, how you examine them, where physically are they found, how well preserved are they? Can you just talk a little bit more about working with that information?
Erika Wise: Yes, absolutely. But I do have to say that this is mostly the work of my collaborator, Cary Mock, but he does focus mainly on the big archive centers. So, there's the US National Archives in Washington, DC; there's the British National Archives in London. Those British archives in particular tend to have many of those ship logs because of British Naval and colonial history, so there's a lot of those ship logs there. He does also go to small places. Sometimes these little historical museums that are in the actual locations we're studying, like California—they often have some of those original diaries and things like that that locals have donated when they were cleaning out their attic or something along those lines. I do have a tiny bit of real experience, because I went to the special collections here at UNC [the University of Carolina at Chapel Hill] just last week to look at something that Cary was interested in.
We have a really good collection here that's really focused on Southern studies. I was looking at a diary from a man who was from Alabama, and he had been a 49er and traveled across the country, and he kept this neat little ledger of his expenses every day and also a little summary of the daily weather. So when I went in there, I was able to actually hold these physical objects—they were these small, leather-bound journals—and I took photos of the contents and sent them to Cary. Turns out it's a really special skill to be able to read 1800s cursive handwriting. I can make a lot of it out, but it's pretty slow for me, and Cary's shown me other things where I wouldn't be able to read a word. He has developed a very special skill to quickly read through and record what he needs from all those archives.
Daniel Raimi: Wow. Such fascinating work. Since you're working with someone from South Carolina, I have to ask you the most important question of the day: Which is your preferred barbecue style? Do you argue about barbecue style with mustard-based versus vinegar-based barbecue?
Erika Wise: I've been a vegetarian since I was 19, so I am not the person to ask.
Daniel Raimi: Oh, that's great. Thank you Erika, so much, for coming on the show. This has been just totally fascinating and hopefully for our listeners, too, just learning about something very different from what we normally talk about on the show.
Now I'd invite you to share something with our audience that you've read or watched or heard that's related to the environment—or, not really, whatever is on your mind, but just something that you think is great and you think our listeners might enjoy. Just asking you to recommend what's at the top of your literal or your metaphorical reading stack.
Erika Wise: I have two books that I'm going to mention that are aimed at different audiences, but I feel like they're connected by a common theme. They're both books that I've read recently, and they were both published in 2021.
The first is aimed at a more general audience, and it's a book called Saving Us, by Katharine Hayhoe. The other one is aimed more at practitioners, and it is called Getting to the Heart of Science Communication, and it's by Faith Kearns.
A main theme that I think connects them is the topic of listening, and listening as a part of having effective communication with other people. Saving Us is a book that's specifically about climate change and a lot about how difficult communication about this topic can be. And then Getting to the Heart of Science Communication is about communicating difficult science topics, including climate change (but not just climate change), but both books really emphasize the idea of listening and being empathetic to whoever you're trying to communicate with when you're sharing something that's really important to you.
I feel like these both are really timely books and really makes some food for thought about how we communicate about environmental issues, and they're both fun to read, too. I think they’re really useful books for different groups of people.
Daniel Raimi: Those are great recommendations. We will have links to both of those in our show notes, as well of course as links to your research and the papers we've talked about today. I would encourage listeners to check all that out and just want to thank you one more time, Erika Wise, from the University of North Carolina. Thank you so much for coming on to Resources Radio and telling us about your work.
Erika Wise: Oh, it's been my pleasure. Thanks so much for talking to me.
Daniel Raimi: You've been listening to Resources Radio. Learn how to support Resources for the Future at rff.org/support. If you have a minute, we'd really appreciate you leaving us a rating or a comment on your podcast platform of choice. Also, feel free to send us your suggestions for future episodes.
Resources Radio is a podcast from Resources for the Future (RFF). RFF is an independent nonprofit research institution in Washington, DC. Our mission is to improve environmental, energy, and natural resource decisions through impartial economic research and policy engagement.
The views expressed on this podcast are solely those of the podcast guests and may differ from those of RFF experts, its officers, or its directors. RFF does not take positions on specific legislative proposals. Resources Radio is produced by Elizabeth Wason, with music by me, Daniel Raimi. Join us next week for another episode.