 My basic research question is, how long does it take the carbon that gets fixed from atmospheric CO2 by plants? How long does it take that carbon to travel through the plant's soil system until it ends up back in the atmosphere as CO2? So why should we care about that first? We have to understand something about the global carbon cycle and carbon dioxide. We know that carbon dioxide is increasing in the atmosphere and that's warming our planet. What we also need to know is what happens to the carbon dioxide we put in the atmosphere. Right now, only about half of it accumulates there, about 25% dissolves in the oceans, and the other 25% is taken up on land, mostly by processes we don't fully understand right now. So what I want to do is understand what those processes are. And so it's really important to know the CO2 that plants take out of the atmosphere. It's removed and is not warming the planet. So how long can we count on terrestrial ecosystems to store it? To understand how carbon moves through systems and how long it takes, we need something that acts as a clock. In our case, the clock that we use is carbon-14, an isotope of carbon, that was made by atmospheric weapons testing in the 1960s. The weapons testing approximately doubled the natural levels of radiocarbon in the environment. It's dissolved in the ocean, it has been taken up by plants, passed on into soils, and ultimately returns back to the atmosphere. What we do is we would take samples of plant material. So for example, every year since 1963, if we take a leaf sample, we can see that the carbon that's being fits from the atmosphere has a different value every year. It's decreasing a little bit over time as this excess carbon-14 has taken up through the carbon cycle. When we go to tree rings, we can also identify every year going back and look at how old the tree is. Things like roots and soil organic matter, they're derived from the plant material, and so we can look at how carbon moves from one compartment to another as it's moving through the vegetation soil system. The other thing we can do is we can put a chamber over everything and measure the radiocarbon content of what's being respired. That will tell us the average time since the carbon was fixed from the atmosphere to when it came back out. And that integrates across many of these processes that we're talking about. I'm going to illustrate for you three representative findings. We use the bomb radiocarbon to tell a lot of different things about ecosystems. Probably the simplest one to understand is that we determine the ages and growth rates of tropical trees. Normally we use tree rings to get growth rates and calculate the age of a tree, but trees in the tropics have a lot of missing rings or they may not have easily identifiable growth rings at all because the climate doesn't vary over the season like it does in temperate latitudes. So what we do is we use the carbon-14, which differs in every year since the bomb spike, and we can use that as a time signature going backwards to say how fast these trees are growing, and most of them are growing very slowly. So the ages of tropical trees, you might have the kind that you hit with your machete to just clear it out of the way, and that tree can be more than 50 years old. A second example of using bomb carbon-14 is that we can look at faster processes, like how old the sugars are that trees use as their storage reserves. And the most probably interesting example of this is that we used the carbon-14 in maple syrup to determine that the carbon used in that sugar used by those trees was fixed on average three to five years ago. The third example is looking at soils which are much more complex and take a lot longer for carbon to move through. There we have compared the carbon-14 from decades ago to going back to the same place that we are now, and from the increase in bomb-14 globally compared with the decline we see in the atmosphere at the same time, we can understand how much of the carbon goes into soils and how long it stays there. The relevance depends on the question that we were asking. So for example, growth rates and ages of tropical trees, we want to understand how fast these forests might recover from a disturbance and also how long carbon stays in these trees before they die and ultimately decompose. For the maple syrup, knowing that it's made from carbon that was fixed three to five years ago means that any single growing season won't influence maybe so much the yield of maple syrup in the following year. For the soil example, what we really want to understand is how long carbon can be stored in soils because carbon in soils is not in the atmosphere warming the climate. Particularly now, there are a lot of ideas about how to manage soils to retain carbon for longer time periods. If we understand the mechanisms that are storing it for long time periods, now we can think about how to accelerate or augment those methods for the future. We've been looking at a lot of different questions and how bomb radiocarbon is moving through different parts of the system and right now what we're trying to do is to put all these things together, which is a very difficult thing to do and requires more mathematical modeling and mapping of single points to the globe. What we try to do now is, for example, with soils to make a global database so we can find out where all the measurements that have been made are and then try to relate how much bomb radiocarbon is taken up by the soils with either the kind of soil it is or its chemistry, what kinds of minerals it has, what kind of clay minerals, what kinds of stabilization mechanisms exist in those soils. Once we know those rules, we can map that globally and we can start to compare our results, our upscaled data to predictions made by models. What we've found so far is that the models are not very good at predicting what we've observed and so there's lots of room for improvement and understanding of the processes that retain carbon and ecosystems for longer periods of time.