Scientist at Work: A Conversation with Hans Christian Steen-Larsen

Climatologist Hans Christian Steen-Larsen joined BIOS in March as an adjunct scientist, to continue and expand his innovative research at the Tudor Hill Marine Atmospheric Observatory. Currently a researcher at the Center for Ice and Climate at the University of Copenhagen, Steen-Larsen has traversed the globe gathering data to reconstruct past climates and to improve current climate models.  At BIOS, he uses clues embedded in water molecules to better understand the climactic processes that drive evaporation from the ocean.

Even though water molecules contain just a single oxygen and two hydrogen atoms, two water molecules can have slightly different atomic weights if the oxygen or hydrogen atoms packed on some extra neutrons. Oxygen atoms, for example, usually contain 8 protons and 8 neutrons in their nucleus, creating an atomic mass of 16. But a small percentage of oxygen atoms carry two extra neutrons, creating an atomic mass of 18. These variants are known as the “light” oxygen-16 and “heavy” oxygen-18 stable isotopes.  Similarly, hydrogen can have one or two extra neutrons, creating “light” and “heavy” hydrogen stable isotopes.

As one might imagine, a water molecule weighed down by heavy isotopes isn’t quite as quick to evaporate as it’s light counterpart. But when it comes time to rain, a heavy water molecule condenses from gas to liquid more readily. Over time, these proclivities rearrange the relative abundance of water isotopes around the globe, creating distribution patterns that reflect the state of the climate. In a conversation conducted via Skype from his office in Denmark, and via email from his field camp on the Greenland ice sheet, Steen-Larsen explained how he is using water vapor isotopes to study climate.

You currently monitor water vapor isotopes around the world, but you started your doctorate research working with ice cores.  Why did water vapor capture your interest?

First of all, water vapor is the most potent greenhouse gas, and every time the atmosphere warms by one degree Celsius it has the potential to hold 7 percent more water vapor.  This is why it’s more humid in the tropics, and a lot drier in the arctic.  That’s also why when you walk outside at night and there’s a clear sky, it’s often colder than when you have a cloudy sky.  The clouds are water vapor acting as a blanket and trapping the heat.  You really notice this effect in Greenland or Antarctica, where I do a lot of my work.

Ice core studies of past climate, and models of future climate, both rely on an empirical relationship established by research conducted during the 1960s.  This study showed that isotope signatures in rainwater collected around the world strongly correlated to the annual temperatures where it was collected. The rain in cold places is depleted of heavy isotopes––the more depleted, the colder it is.  The heavy isotope ratio trapped in ice cores gives us a way to interpret past temperatures. During my doctorate work it became quite clear to me that we really need to start investigating the processes behind the water cycle to better interpret ice core data. What’s really happening during evaporation to create these isotope signatures in the water?  How does the state of the climate––the humidity, the solar insolation––control the isotopic fingerprint? I am drawn towards understanding the process in a quantitative way. And that brought me to Bermuda, because the first step in the hydrologic cycle is evaporation of water from the ocean.

Tudor Hill tower

The BIOS-operated marine atmospheric observatory, known as Tudor Hill.

How do you measure heavy or light water vapor isotopes in your samples?

My field of research was revolutionized when laser spectroscopy became commercialized.  Before, we had to use cryogenic trapping to study water vapor isotopes.  You would suck air through a water trap cooled to -112 Fahrenheit (-80 Celcius), extract it, and take that small vial of water back to the lab to measure on a mass spectrometer.

With laser spectroscopy, if you know how to calibrate the instrument––which I’ve spent a lot of time doing––you can get data points immediately.  The analyzer sucks in air, a laser shoots in a beam and measures absorption, and because the amount of absorption depends on the amount of heavy and light molecules, you get an estimate of the relative abundance and the water vapor isotope concentration. 

I was lucky to get some funding right after my doctorate research to work with this new instrument. In 2011, I established what might be the longest calibrated record of continuous water vapor isotope observations in the world, and that is at the BIOS Tudor Hill Marine Atmospheric Observatory.  It’s running as we speak.

What do you hope to accomplish in your new position at BIOS?

I believe that the backbone of scientific discoveries are long term observations.  With the Tudor Hill data, we are challenging the fundamentals of those old equations that are still being used for the general circulation models of climate.

In many ways we are still in an exploration phase of understanding the powerful tool we have at hand with laser spectroscopy.  We can look at carbonyl sulfide and carbon dioxide in addition to water vapor isotopes and learn how these interact. 

Measurements of isotopes in the ocean can tell us more about long term changes in evaporation and circulation patterns.  The BATS team collects ocean samples for me on every cruise, and collaboration like that is the best way to work.

My next line of research will be hurricanes, and I would like to collect data as tropical cyclones and hurricanes pass through to find out how much water in hurricanes comes from sea spray, versus water vapor.

What else are you looking forward to?

I am organizing a session at the annual European Geophysical Union meeting about using Arduino sensors for educational purposes.   Before, if you needed to measure temperature or wind you had to buy expensive data loggers.  Now you can buy a sensor, put it on an Arduino board, and you’re good to go for less than $100.  To get people excited about science you need to involve young kids and older students, and I think that DIY sensors are really democratizing environmental measurements.

Finally, I enjoy being at BIOS.  When I go there, I work, sleep, eat, hang out with friends, all in the same place.  It’s like being at a summer camp where everyone is open to ideas and collaboration.  It’s really how science should be done.