“We are all connected through the atmosphere and that is really fascinating” 

Dr Heather Graven is a PI on the LEMONTREE project with her research focused on the use of atmospheric measurements to understand the global carbon cycle and its response to human activities and climate change. Heather uses radiocarbon and stable carbon isotopes to distinguish fossil fuel and biogenic influences on carbon dioxide and methane and to investigate carbon cycling in the ocean and land biosphere.  

She is a Reader in the Department of Physics and the Grantham Institute at Imperial College London. She worked previously at the Scripps Institution of Oceanography, USA and at ETH Zurich, Switzerland.  

We ask Heather 10 Questions to learn more about her research, role in LEMONTREE and her life in California…  


1.   Your research focuses on measurements of atmospheric gases and how climate change will affect the carbon cycle. Can you tell me a bit more about your specific role in LEMONTREE? 

My role in LEMONTREE is to evaluate the models using carbon isotopes and atmospheric carbon dioxide (CO2) to look at patterns at the ecosystem and global scale. We want to understand whether the advances we are making in models are bringing us towards a better representation of what we see in the atmosphere. Earlier models haven’t captured this well. For example, with the seasonal cycle of atmospheric CO2, we’ve seen a big increase in the amplitude and the models so far haven’t really been able to explain that. When we look at the carbon isotopes, we can look at specific processes and those also tend to not be simulated very well in models at present.   


2.   You are based at Imperial College London where you lead the Carbon Cycle Research group. What does your group aim to achieve? 

With this group, we are focused on greenhouse gases, in particular CO2 and methane (CH4). We are trying to understand how these gases are changing in the atmosphere and what’s driving those changes. We do different types of measurements such as atmospheric concentrations and isotopic composition, and then we use existing models and develop new models to try and understand what is happening in the carbon cycle and what humans are doing.   


3.   Most people associate methane emissions arising from agriculture, but there are many other sources of emissions, both natural and anthropogenic. What is it about methane that makes it such an important compound in the atmosphere? 

Methane is really fascinating because it is rising so rapidly now, much more quickly than we’ve expected in the last few years. It has been going through large changes in how rapidly it increases in the atmosphere over time. There was a period where it stopped rising for a few years but now it is accelerating faster than ever. There have been lots of hypotheses as to what is driving these variations but really, we still do not know.   

There are lots of sources of methane and trying to pick apart how much each of those is contributing to the methane in the atmosphere, as well as these variations, is very challenging. We are using isotopes in methane, radiocarbon, 13C and deuterium to try to understand this better.  

At the recent COP meeting in Glasgow, methane was a big focus and many countries have signed up to the Global Methane Pledge, but if we don’t fully understand the sources, then it is going to be hard to reduce emissions effectively.  


4.   You are affiliated with the Grantham Institute at Imperial, which has the vision of creating a sustainable, resilient, and zero-carbon society. What do you think as individuals is the most effective step to becoming zero-carbon? 

I don’t like to emphasize individual actions because climate change is not a problem we can solve individually. We can make changes like switching energy sources in our homes, in our cars and eating less meat. But we need to think about our entire society and our economy. Every decision we make must consider the implications for greenhouse gases. We also need to think about adaptation and how we are going to deal with our changing climate.   


5.   You carried out your education at Caltech and Scripps Institution of Oceanography. Did you grow up in California and what did you love most about living there? 

I was born in Canada, and I lived there until I was ten. Then I lived in Colorado and then in California. I love living in London, but I do miss California, especially the ocean. Because of the pandemic, I was not able to get back for a few years, but I got to spend a month there this summer which was great. We spent lots of time at the beach. Crashing around in the water with my three-year-old daughter was just wonderful.  


6.   What do you think the future holds for California with the severe droughts they are experiencing and the increase in intensity and frequency of wildfires? 

When I left 8 years ago, there was a big drought, and now there is another extreme drought. Now it seems more like a constant state of drought, rather than cycling between drier and wetter conditions.   

The forests in California have grown in an environment that is no stranger to wildfires, fire is part of their ecosystem. However, if the local environment is changing to a regime that is hotter and drier to an extent that they are not adapted to, then that is when we get the risk of losing them, including trees such as the giant sequoias. Nothing is really going to escape climate change.   

In terms of the emissions, the fires are emitting carbon that was taken up by plants originally, so it is not necessarily having a significant impact on the CO2 concentration. But if ecosystems are lost or do not regenerate, then it can affect the CO2 concentrations. With the increase in wildfires, we could see changes to the ecosystem and changes for the people that depend on the forests for their livelihoods.   


7.   LEMONTREE is focused on terrestrial vegetation. But quite a lot of your research is also focused on the oceans, how the oceans regulate the heat associated with climate change. What impact is our warming world having on the chemistry of the oceans? 

The oceans absorb a lot of the heat and the carbon that is in the atmosphere. CO2 emissions themselves are affecting ocean chemistry because the CO2 that is absorbed into the oceans leads to ocean acidification, which can have detrimental impacts on organisms as water becomes more acidic.   

Then of course with climate change and the associated warming, the oceans are absorbing more heat. This is having devastating impacts on corals with the mass bleaching events that we hear about. As the temperature warms, then there is less oxygen dissolved in the ocean which can also harm marine organisms. Temperature can also change the ocean circulation patterns as well causing the oceans to become more stratified which can restrict the nutrients getting to the organisms near the surface.  

We are already seeing changes outside of the realm of variability that we have seen in the past.  


8.   Who has had a substantial influence on your professional life? 

Well, of course, Dave Keeling, of the Keeling Curve and his research in atmospheric carbon dioxide concentrations and climate change. His son, Ralph Keeling, was my supervisor at Scripps. They are both very inspirational in the work that we do. It was a real thrill and pleasure to work with them at the Scripps CO2 program and learn from them.   


9.   Outside of work, what do you like to do, aside from being at the beach in California? 

I have two young daughters, aged one and three, so they take up all my spare time. But it is great, they are such a blessing, and they give you a new perspective. When you look at your children and think that some of these future projections we are making are going to happen in our children’s or their children’s lives, it makes me feel we should do more outreach to help spread the message.    


10.             What made you get into atmospheric science and what advice would you give young women wanting to get into the sciences? 

I really liked maths and chemistry, the sciences in general, when I was in high school. But no one in my family was in academia and I did not even know what a PhD was. I went to Caltech for my undergraduate degree which has a big focus on research, and it was normal for students to do research projects in the summer. I did one research project in chemical engineering, but I really did not like that, and it put me off doing a PhD.   

But the next summer, I did an atmospheric research project, and it was wonderful. Then I got the opportunity to go to Scripps, which is right on the beach, so I thought, ‘Oh yeah, I could spend a few years here learning about something interesting’!  

I love making new kinds of measurements. Getting new data is so exciting to me. We are all connected through the atmosphere, and that is really fascinating.  

There are still challenges as a woman working in the sciences and I have experienced sexism. But I would say to young women, just follow your curiosity and do what interests you.  If you had asked me when I was a teenager, I never would have expected that I would end up as a physics lecturer in London!