Amy Hessl compares rings on trees to lines of music. She says each ring is like a note. But in this case, the music doesn’t repeat itself, because climate tends to vary in such a way that it doesn’t repeat.
Hessl spoke to Managing Editor Jennifer Gardner about how she uses the rings of trees to understand what past climate was like.
Q. How do you measure climate change based on the rings of trees?
A. Trees, depending on where they are, grow faster when it’s wetter and slower when it’s dryer. If they are limited by temperatures, like if they grow at very high elevations, then they might grow faster when it’s warmer and slower when it’s colder. So their ring widths tell us something about what past climate was like. They have lots of other properties we can look at as well, including the chemical makeup of the wood and some of the physical properties of the cells.
Q. Do you use any special tools or methods to convert the growth of trees to climate information?
A. We use something called calibration. You know when you go to the gym or the doctor’s office and they’re like “hold on we have to re-calibrate the scale”? They’re fixing it so when nothing is on it, it is zero. We do a similar thing with the tree rings, which means we essentially use simple models that relate the current ring widths to current climate conditions and then we forecast that backwards in time. So it’s very similar to what you do when you calibrate a scale.
Q. Where do you study the trees?
A. I have worked most recently in Mongolia. They have really long-lived trees there and the trees are very responsive to past moisture conditions, so they have really accurate representations of moisture change. That’s the ideal situation. There is deadwood that we can use to extend our records further back in time.
Q. What interests you about studying trees in relation to climate change?
A. I’ve always had an interest in the past and past societies and how things might have been different or the the same for them. Some of the things I have observed looking at past climate variability and past societies is that it appears there are similar patterns. There’s an empire I’m looking at right now called the Uyghur Empire. They had a massive empire in central Asia during the 8th and 9th century and they experienced a really severe drought that lasted a long time—around 60 years—and they don’t appear to have realized they were in a drought. I think they got used to it, and I think that’s an interesting aspect of human behavior. You make these gradual adaptations to something and the new normal becomes acceptable and eventually you find yourself in a situation you can’t sustain anymore. I think that’s what happened with the Uyghurs, and I kind of wonder if that’s what is happening with us now.
Q. How is this affecting us now?
A. Climate is changing and we are sort of adapting to this climate change and it’s becoming normal for us to accept these changes in climate. Will we just sort of keep making these incremental adaptations until we can’t adapt to it anymore and it becomes too hot? We’ve lived in an altered climate for at least 20 years, possibly 40, so that’s pretty much my lifetime. I think we have sort of just made incremental adaptations to that and change is normal to us now. The fact that it is hotter this year than it was last year is normalized.
Q. Has there been any “Aha” moment for you in your research?
A. I have a new research project where I am looking at past climate in Tasmania and they have all of these endemic conifers that don’t grow anywhere else in the world and they’re really long-lived. So they can live for a thousand years and they do this thing where the trees die and the wood just hangs out and doesn’t decay. We can create 2,000 years of climate record from these trees. We had kind of a moment of discovery where we were sampling trees that were mostly alive, and we saw these piles of logs at high elevation on top of each other and we thought maybe we should sample those and see how old those are. We were able to connect the growth of the living tree and overlap it with the dead trees.
Q. How are you able to connect the growth of different trees?
A. So you find periods where the dead trees were alive at the same time as the living trees, and then the pattern of responses that they have to climate overlaps, and that’s the way you can date these pieces of deadwood and figure out when they were alive. We’ve done that and we now have this 2,000 year record for Tasmania and it actually tells us some really interesting things about how temperature, and potentially moisture, has changed in the Southern Hemisphere, which is a super useful tool because we have lots of records for what has happened in the Northern Hemisphere over the last 2,000 years, but we have very few for the Southern Hemisphere. The Southern Hemisphere doesn’t behave in tandem with the Northern Hemisphere, so it’s just a really interesting record and it was kind of accidental.