The Exchanges

Q And how much of it do we emit? And I mean, you said it's the third most important behind CO two and methane. Just contextualize it. How important is it relative to those two?

A Okay, well, about seven percent of the radiative forcing is, on the hundred-year timescale, is attributed to N to O. Then I should also add, though, in addition to its effect on radiative forcing or on global warming, it's also a ozone-depleting substance. The, the ozone is the protective, the good ozone and up and high in the stratosphere that protects us from Ultraviolet radiation, and N-to-O is one of the reactants that destroys that ozone, and now that we've been successful through the Montreal Protocol in reducing emissions of chlorofluorocarbons and now hydrofluorocarbons, N-to-O remains as the largest currently emitted ozone depleting substance. So it really is kind of a double whammy in terms of its impact on atmospheric processes, both as a heat-trapping gas that contributes to radiative forcing or global warming, and a reactant in destruction of stratospheric ozone.

AI assessment note: “about seven percent of the radiative forcing is, on the hundred-year timescale, is attributed”

Q the trend line been? So it's seven percent of anthropogenic, or of radiative forcing, rather, Today, has that trend line over the course of decades and since the industrial revolution, has that been growing, um, as a share of radio forcing? Is it, is it flat? Is it declining? Just where are we on the trajectory of Of N-to-O emissions relative to the trajectory we're on with CO₂ and CH₄?

A Well, N-to-O emissions are going up at an increasingly rapid rate. So, um, the atmospheric N-to-O prior to the Industrial Revolution was around 270 parts per billion. That's a B as in boy. Um, and it's now up around 335. So it's gone up by about 25%, and it's going up more rapidly in the last decade than the decade before and the decade before that. So it's now increasing at about a rate of one part per billion per year. Uh, whereas if we look at CO₂, uh, yeah, CO₂ is still going up, um, but there's some signs of it starting to, um, the rate of increase Has gone down in certain years. Uh, it blipped back up recently, but, you know, there's, there's some reason to see that the CO two trend could be leveling off and, and we could even, uh, imagine with, with efforts to mitigate CO two that it's going to eventually level off and the rate of increase will start to decrease. Uh, methane is much more complicated because we actually had a period where methane leveled off for a while and we weren't really sure why.

AI assessment note: “N-to-O emissions are going up at an increasingly rapid rate.”

Q move on to what happened in the industrial evolution, cause we're going to talk a lot more about agriculture and agricultural soils as a, as a source of N two emissions. Um, but in that tropical forest soil context, you know, before humans were doing, uh, organized agriculture, what can you just describe the mechanism what's happening in those tropical forest soils that causes N two emissions in the atmosphere?

A Well, the N-IIO is produced by bacteria that live in the soil, and there are two different groups, uh, called nitrifying bacteria and denitrifying bacteria, and actually, if you want to get real technical, there's also a group of organisms we call archaea, nitrifying archaea, but anyway, these are all microorganisms that live in the soil. And the reason that they do so well in tropical soils is, first of all, it's warm and moist, which makes good conditions for these processes of nitrification and denitrification. And the other thing that's kind of unique about most Lowland tropical forests is that nitrogen is, uh, an element that's relatively abundant compared to other limiting factors like phosphorus, and that's because, um, those native tropical forests have a significant amount of this process we called nitrogen fixation, which is another set of, uh, organisms that, uh, Pull nitrogen gas out of the air and turn it into a form that the plants can use. It's usually in a symbiotic association with the microorganism in the plant, and so they take nitrogen out of the air, put it into a form that living things can use, um, and then that cycles through the ecosystem. The, the trees use it to make their leaves, the leaves fall onto the soil, the roots turn over in the soil, and that Adds nitrogen to the soil, and some of these bacteria Nitrifying and denitrifying bacteria can use t…

AI assessment note: “Nitrifying and denitrifying bacteria can use that, and in the process of using it, they convert”

Q here, because you have schooled me offline about nitrous oxide, and I'm excited for you to reschool me online. Let's start with the basics. So tell me about nitrous oxide as a greenhouse gas. What, how potent is it? Uh, how long does it last in the atmosphere? Like what are the things we need to know? As we start to talk about NTO in the context of climate change.

A Okay. Well, first of all, NTO is considered the third most important anthropogenic greenhouse gas, or greenhouse gas produced by humans. CO₂ being the most important, methane second, and NTO third. NTO has a global warming potential around 275, 300 times more than CO₂. That's on a hundred-year time frame. Um, and actually, it's very similar on shorter time frames, because CO₂ and N₂O have similar, um, estimated mean residence times in the atmosphere, although CO₂ is difficult to calculate, because it exchanges with the ocean and the, and the biosphere more. But anyway, um, N₂O, estimated mean residence time in the atmosphere, is around a 110 years So estimates have varied over the years depending on who's doing them and the methodologies, but somewhere a little over a hundred years mean residence time in the atmosphere. So it's, it's a long lived greenhouse gas.

AI assessment note: “NTO has a global warming potential around 275, 300 times more than CO₂.”

Q methane does not. So it's, it's particularly concerning as part of what has me sort of obsessed with it. I guess the final question, just in level setting for you, you've been citing numbers around how much N-toil emissions we have, and so on. How good have we been about measuring N-toil emissions? How much do we know? Um, and how much of a blind spot do we still have?

A Well, we're very good at measuring concentrations. That we know is happening. It's going up, and we have, um, for the last several decades, have had pretty good measurements of it. Uh, going back in time, um, we can, um, Estimate N-to-O concentrations back in time by looking at what's captured in bubbles frozen in ice cores from Greenland and Antarctica. But that's concentration. You were asking about emissions, and I'd have to say that, well... We more or less know the outlines of the N to O budget in terms of where the major sources are, and the minor sources are, and what the major sinks and minor sinks are. But there's a good deal of uncertainty in each one of those individual estimates.

AI assessment note: “We more or less know the outlines... But there's a good deal of uncertainty”

Q specific, but maybe at a, at a high level, do we have a sense of, let's just say, uh, Farmer X implements the sort of optimal version of the four R's How much of an NTOO reduction could we expect? Like you said, are they incremental? Like, this is an aggregate of, I don't know, 10 or 20% impact on NTOO emissions, or could they represent a significant mitigation lever?

A Well, the nice thing is that that Extra little bit, final increment of fertilizer that the, the farmer may put on as an insurance policy is the part that's most likely to be lost, and so if we can pull back just a little bit on the amount of fertilizer the farmer thinks that he needs, then we can actually have a proportionally larger impact on reducing N-to-O emissions. So that's a good thing. Um, as to how low could it go, uh, I've heard, I've seen some Some modeling studies that suggest you could reduce N-to-O emissions maybe by half, but you know, as you said, it's very, uh, site-specific and, um, and, uh, situation-specific, and so I'd be reluctant to push that number forward very strongly, but yeah, there could be some very significant reductions, and in fact, if we look, when I was thinking, when I used the word incremental, I was thinking more on sort of On the national scale, um, in the last few decades, um, in the U.S., for example, nitrous oxide emissions from agriculture have been sort of flat. They've maybe gone up a little bit in the last few years, but they haven't gone up much, and that's because we are making some improvements in efficiencies. We're producing more food with about the same amount of fertilizer and about the But on the other hand, the NTO emissions aren't going down. So, yeah, it's good news that we're being a little bit more efficient, producing …

AI assessment note: “modeling studies that suggest you could reduce N-to-O emissions maybe by half”

Q baseline scenario, of what the proportion of radiative forcing N-to-O could represent in 2030, 2040, 20, 50 might be. If it's seven percent today, could it be, as CO₂ starts to level off and decline, who knows what happens with methane, could N₂O end up being 10%, 15% of radiative forcing? Is that a realistic scenario, or, uh, or is it hard to imagine it representing that big a share?

A Well, it certainly could go up if CO₂ levels off and N₂O does not. I don't really have the estimates, uh, offhand as to, you know, what the upper limit of that would be, but yeah, and of course, of course, the CO₂ in the atmosphere will continue being a strong radiative forcing forcer, um, until we actually bring CO₂ concentrations down, which is probably going to be many decades off, but in any case, um, uh, N₂O is concerning because, um, It has such a long lifetime that what we're emitting now is going to stay in the atmosphere for a hundred years or more and so, you know, it's, it's going to have an impact on several future generations.

AI assessment note: “Well, it certainly could go up if CO₂ levels off and N₂O does not.”