The Weekly Anthropocene Interviews: Dr. Cassaundra Rose

A Scientist Spotlight Interview

Dr. Cassaundra Rose is the Senior Science Analyst & Climate Council Coordinator for the State of Maine.

A lightly edited transcript of an exclusive interview with her follows. This writer’s questions and remarks are in bold, Dr. Rose’s responses are in regular type. Bold italics are clarifications and extra information added after the interview.

Dr. Rose, thank you so much for joining us today. To start off: how did you get into climate science? What was your career journey?

I started out-I’ve been fascinated by science since I was a little kid. I think many scientists start out just naturally liking science as you grow up, and there are various paths into science as a career. I ended up getting an undergraduate degree in geology, with the gateway of dinosaurs and fossils helping draw me in. And I eventually learned about the wider world of earth sciences. And while I was an undergraduate, I began to learn about climate change, both in Earth’s history as well as modern climate change due to the impacts of human activities. I decided that I wanted to continue into graduate studies and study the history of climate change on Earth. I ended up getting a Master’s degree and then a Ph.D. in paleoclimatology, studying the hydrological evolution of Western North America and Northern Africa respectively. That really gave me a deep understanding of how Earth’s climate system works.

As I was working through my Ph.D., I became aware that I was very drawn to trying to help people with science, and getting very concerned about modern climate change and current, ongoing impacts from human activities on our climate. I decided to switch from an academic career track after finishing my Ph.D. into trying to work to help people with science. I was fortunate enough to work for a few years at a nonpartisan, nonprofit organization called the American Geosciences Institute, which is trying to help connect policymakers with good science in a bunch of different earth science topics, which include climate change but also water, mineral resources, and others.

In 2019, I joined the brand new Governor’s Office of Policy Innovation and the Future to help support the work of the Maine Climate Council, which was a brand new body created by the legislature and Governor Mills to tackle Maine’s climate action plan. And so I’ve been here ever since.

You’ve coauthored a fascinating paper published in Nature, titled Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification. From my understanding, you found that changing global CO2 levels led to expansion of grasslands in Africa from 10 to 6 million years ago, in the late Miocene Epoch. That was of course a key moment in humankind’s evolution, right when our ancestors were coming down from trees into those grasslands. Can you walk through your whole research process here, and your conclusions?

Figure 1 of this paper, showing latitude patterns of modern African rainfall, the δ13C isotopic signatures of major plant cover at different latitudes and modern African ecosystems. C3 and C4 photosynthesis plants have different δ13C signatures, which allows scientists to track their spread over time via plant wax in sediments.

Sure! I was really interested during my graduate studies in trying to answer the question of when did the Sahara region become a desert, and how did that intersect with human evolution. The Sahara Desert is this massive barrier to animal movement, but it’s also a region that fluctuates hydrologically on a very regular timescale. Every 20,000 years, what’s currently a massive hyper-arid desert flips to a wetter environment and becomes a widespread grassland. Northern Africa has these really dramatic hydrological changes on a regular timescale, every 20,000 years. But it has also undergone a long-term evolution towards drier conditions. In the early Cenozoic, a lot of North Africa is under an ocean, and then sea levels dropped over time. For example, we know that areas in Egypt today that are now hyper-arid were actually rainforests. There was much more widespread wetter conditions across Northern Africa. But it’s still an open question when the Sahara region first became a desert. It has been a desert at least, probably, 7 million years, but nobody knows when that shift occurred and how quickly and for what reasons.

So that was the fundamental question I was trying to answer during my Ph.D. With this paper, with this chapter of my dissertation, I took the approach of trying to capture the hydrological gradient between what is currently a hyper-arid latitude in Northern Africa and to compare that with the equatorial region of West Africa. And to look at the evolution of the hydrology and vegetation from the equator to the mid-latitudes in Northern Africa over the last 25 million years. Which is a big study, there aren’t a lot of long-term continental-scale studies out there that have taken this approach. My coauthors and I looked at Northwest Africa, East Africa as well and how those regions evolved hydrologically and how their plants changed over the last 25 million years. So it was a really large-scale study, because we were really trying to see if we could pinpoint these large shifts in both rainfall and vegetation, with the rise and presence of C4 grasslands over that time as well. Through this, we identified that both in Northwest Africa and Northeast Africa, right around 10 million years ago, there was a sudden really quick shift, an expansion towards more dry-adapted modern C4 grasslands. And this has been observed in other regions at different times around the world, this massive expansion of C4 grasslands, but it hadn’t been documented quite this systematically in Northern Africa.

In case you’re unfamiliar with these terms, here’s an overview of C3 vs C4 photosynthesis. Broadly, C3 plants (about 85% of all plant species, including all trees), use normal, basic photosynthesis, converting carbon dioxide (CO2) into sugars using energy from light. This normally works great, but has some key inefficiencies, particularly photorespiration: at high temperatures, the key enzyme “rubisco” sometimes grabs oxygen (O2) instead of CO2, wasting time, energy, and resources. C4 plants spread the photosynthesis process across some extra cells that help process CO2 more efficiently, which is more energy-intensive to maintain but makes them more competitive at higher temperatures and in lower-CO2 conditions.

Figure 3 of the paper, basically showing that C4 grassland expansion was due to Miocene global cooling due to lower CO2, not rainfall or aridification changes. Check out the paper for more detail!

So we were really excited by this result, that we saw a huge shift in the environment. We had to try to answer: why did that happen? There were a number of hypotheses that we tested. From the best available evidence, with both rainfall and CO2 and dust reconstructions, it seems that a fundamental barrier in CO2 was crossed around 10 million years ago that allowed for a big switch in the environment from one type of plants, C3 plants, like trees and shrubs, to C4 grasses. And that had some really big impacts on the evolution of different African organisms, for example grazing mammals. And it would have had an impact on the environments that our human ancestors evolved in as well, because there was a large expansion of grasslands, more open habitat as opposed to more shrubby or tree-covered habitat before.

“It seems that a fundamental barrier in CO2 was crossed around 10 million years ago that allowed for a big switch in the environment from one type of plants, C3 plants, like trees and shrubs, to C4 grasses. And that had some really big impacts on the evolution of different African organisms, for example grazing mammals. And it would have had an impact on the environments that our human ancestors evolved in as well.”

-Dr. Cassaundra Rose

Our best hypothesis is that this was driven by CO2, and wasn’t driven by changes in rainfall, which was not what I had expected. But that’s what happens in science, you have to follow where evidence is pointing to. Our best hypothesis right now is that this was driven by CO2 in Africa, 10 million years ago. There’s still an open question about when the Sahara Desert became a desert, I didn’t quite get there in my Ph.D. But I think that’s exciting, somebody will answer that someday and I’ll be very happy for them when they do.

So what caused the CO2 shift that led to this? Was it a natural cycle, the Milankovitch cycles?

The best understanding is that CO2 declines slowly over the late Cenozoic because global climate was cooling over time. Here’s a USGS review of the Cenozoic Era. Over that period of time, we saw the growth of the Antarctic and Greenland ice sheets, and some pretty large changes to ocean circulation as well. So over time, that allowed for more CO2 to come out of the atmosphere and to go into the oceans, and to be sequestered in geological reservoirs as well. What we think happened was that around 10 million years ago, CO2 was probably gradually declining, and it just declined to a certain point that allowed for a sudden shift in ecosystems. C4 plants are evolved to do really well in lower-CO2 environments. And C4 plants actually include corn, corn is one of the biggest C4 crops, it’s a type of grass. Wheat is a type of C3 plant. So, C4 plants do much better in low-CO2 as well as dry environments. We think some threshold was crossed around 10 million years ago where all of a sudden C4 plants could compete much better against C3 plants in Northern African environments, and so they were able to expand very dramatically. There probably were other environmental factors at play. You wonder about wildfires, and their impact on the landscape to open up habitats. And the hydrological conditions were probably dry enough for C4 grasses to compete. But fundamentally, we think it came down to some sort of CO2 threshold was crossed and then C4 grasses were able to muscle their way onto the landscape and expand in a way that they hadn’t been able to previously.

And that has some interesting implications for CO2 going up in the atmosphere today. Because we’re returning to the CO2 environment of earlier in the Cenozoic. And so, over the course of thousands of years, it might be interesting to see how that impacts what plants are able to do well. I’m definitely not a botanist, but that’s going to be something to think about as CO2 keeps going up in our atmosphere today.

Can you tell me about your leading role as Maine Climate Council Coordinator drafting the landmark Maine Won’t Wait climate action plan? What’s it like working in such a linchpin interdisciplinary role, serving as the climate expert integrating plans from across many different government agencies?

Fundamentally, my role is about supporting the work of the Maine Climate Council. And the Maine Climate Council is a diverse group of people, appointed by Governor Mills to come up with Maine’s climate action plan. The Climate Council is comprised of 39 people, including state agency commissioners, representatives from Maine’s tribes, large and small businesses, municipalities, experts on climate change, and many more. It’s quite a diverse group. And under the umbrella of the Maine Climate Council we also have a number of Working Groups. Six Working Groups and two subcommittees, the Scientific and Technical Subcommittee and the Equity Subcommittee. So all told, it’s almost 300 people, all volunteers, who have worked together to put together a fantastic climate action plan which allows us to reduce emissions, increase the resilience of the state of Maine, tackle equity in climate action, and foster a clean energy economy.

My role is to try to make sure that they can all do their work. That involves a lot of different things, including helping them to plan their work long-term, to support their meetings, and that’s a lot of organizational and communication skills right there, for any students reading your newsletter who want to know what skills to be developing. I’ve also been lucky to really listen in and help facilitate when necessary some of the conversations of the different working groups as they were drafting recommendations for the Maine Climate Council to consider in in the final climate plan, including the public health subgroup. I worked really closely with the Scientific and Technical Subcommittee to write a 370-page report on the impacts of climate change in Maine. That was produced as part of the climate planning process. So, a lot of facilitation, a lot of planning, a lot of communication.

I also as a climate scientist am lucky to occasionally get the opportunity to help track new reports on the impacts of climate change, to try to help understand how we need to take those into account for the climate action planning process. Since we finished the climate plan in 2020, I’ve also helped to track how this plan is being put into action, which means talking a lot with different state agencies and outside groups, helping them to plan how they can put the strategies in the climate action plan into action, how they can work to support those goals. And then reporting back to the Maine Climate Council on how all those actions are putting the strategies of Maine Won’t Wait into reality. So I wear a lot of different hats, but I think it’s exciting, because no two days are the same. I’m always learning new things and new skills, and I get to work with a really fantastic, dedicated group of almost 300 people who are really committed to making sure that Maine is taking action on climate change and that we’re preparing adequately for the impacts that are already happening.

Can you tell me more about how climate change is already affecting Maine? And what should Mainers look out for in the coming decades? How will different emissions pathways play into this?

So as I mentioned before, there’s a really great report, the Scientific Assessment of the Impacts of Climate Change in Maine, if anybody is really interested in learning all the nitty-gritty details. But the high level message is about how climate change is already impacting Maine and will impact Maine. Maine is in a part of the United States that is warming faster than average. We’ve already had more than 3 degrees Fahrenheit of average warming since 1895. And particularly in Maine’s fall and winter seasons, Maine has experienced a lot of warming. We’ve also seen rainfall and snowfall, precipitation going up over time, we have six more inches on average now than we did in 1895. And we’re seeing more of that precipitation coming in large, heavy downpours, 1 to 4 inches or more in a short period of time.

Map of the Jackman, Maine August 2022 road diversion, courtesy of the Morning Sentinel

So there are a lot of obvious shifts that we’re already observing. One reason why Maine communities are concerned about that is that that means we’re at risk from damage to our infrastructure. For example, during a sudden downpour, where you get 2 to 4 inches of rain, that tends to lead to flash flooding, washing out culverts and roads. And there are a lot of isolated Maine communities whose only access might be washed out in a storm, and they may have to drive hundreds of miles to get out of their community to where they want to go. There’s an example that happened earlier this year in Jackman, Maine, where a culvert and road washed out after the area received nearly 2 inches of rain in 24 hours, causing a 143-mile road diversion (see map above).

Outdoor heat pump. Photo courtesy of EfficiencyMaine

We also worry about more frequent heat waves and extreme temperature events, both warm and cold. Maine is a pretty cold climate, but in our rapidly warming conditions, more people are finding that they have to rely on cooling their homes to stay safe during a heat wave. And so parts of the state climate plan are around planning for that, but also trying to ensure that people use technology like heat pumps, which also provide cooling, so they can stay safe during both cold and warm temperatures. And reduce emissions at the same time, and save money.

A deteriorating dike in Machias, Maine that is highly vulnerable to sea level rise. Photo from The Maine Monitor’s excellent The Unstoppable Ocean project.

We see all sorts of changes. Of course, rising sea levels is one impact that’s happening quickly, and we’re already seeing increased nuisance flooding. There are changes in tick activity and increasing vector-borne disease, like Lyme disease. And we have big changes in our ecosystems happening, particularly in the Gulf of Maine, which is one of the fastest-warming areas of ocean worldwide. So there are lot of impacts to catalog. And it can sound overwhelming, but that’s exactly why we have a really detailed climate action plan, to help prepare communities and economies and protect our ecosystems from these impacts and try to prepare them to be as resilient as possible into the future.

“It can sound overwhelming, but that’s exactly why we have a really detailed climate action plan, to help prepare communities and economies and protect our ecosystems from these impacts and try to prepare them to be as resilient as possible into the future.” -Dr. Cassaundra Rose

What would you like to add? What question should I have asked that I haven’t asked?

There are two things. There’s one thing about climate change action in Maine, and then there’s one thing about the time we have left to take action.

Maine Governor Janet Mills addressing the UN General Assembly on the topic of Maine’s climate action commitments in September 2019. Photo courtesy of the Portland Press Herald.

I really have loved working on climate change action in Maine. In part because Maine has taken really seriously the best available science, and has been very science-driven in its approach to laying out emissions reduction goals, and taking action to adapt to the impacts of climate change. Maine has used scientific information and data to help really pinpoint what the biggest action areas are to reduce our emissions. For example, transportation and buildings are two of our biggest emissions sources. So Maine has been really deliberate about using information to guide its decision-making and goal setting, and is using information to track our progress over time so we can make sure we’re targeting our efforts effectively and identifying where more work needs to be done. I think that really deliberative approach will help us to be as effective as possible, and to be really inspiring as an example. We’re a small state, we don’t produce a ton of emissions compared to some other states, but we’re being as ambitious as required to reduce the impacts of climate change to safer levels.

“Maine has taken really seriously the best available science, and has been very science-driven in its approach to laying out emissions reduction goals, and taking action to adapt to the impacts of climate change…We’re a small state, we don’t produce a ton of emissions compared to some other states, but we’re being as ambitious as required to reduce the impacts of climate change to safer levels.”

-Dr. Cassaundra Rose

And then my second message is that, although climate change is really concerning, and it can feel overwhelming, like there’s not a lot we can do, the best available climate science and modeling tells us that we do still have time to take action to reduce warming to the safer 1.5 degrees Celsius levels by the end of the century. I know I often talk to people who are really concerned, and understandably can feel very-I don’t know if I want to use the word “depressed.”

People do feel depressed. I’ve talked to many people who feel depressed about climate change.

Yeah, they feel like we’re out of time or we’ve missed our opportunity. But the best available science tells us that there is still time. We have to be very ambitious and we have to do a lot in a short period of time, but we can still take action to ensure that we minimize the amount of climate warming that happens over this century. I like to share that, because for me that tells me there is still a lot of hope, and we have to make sure that we continue our forward momentum and not lose hope, so that we reach the goals that we have set to protect our communities and protect Earth’s environment from the very worst scenarios.

“We have to make sure that we continue our forward momentum and not lose hope, so that we reach the goals that we have set to protect our communities and protect Earth’s environment from the very worst scenarios.”

-Dr. Cassaundra Rose

Wow. That is a really powerful summarization of this, thank you. Thank you very much.

Thanks. I talk about this topic a lot on behalf of the Climate Council, and I think it’s important to end on a message of hope. It’s very concerning, there’s a lot going on, but it’s important to tell people that there is reason for hope. There’s a scientific basis for hope, that we still have time to take action.

Thank you.

Happy to do it.