Lecture 07 - Hands on with Ecosystem Services - Part 1

BEFORE CLASS INSTALLATION NOTE: Please follow these steps before class.

Install InVEST from https://naturalcapitalproject.stanford.edu/software/invest/invest-downloads-data
Open the application to ensure it works.
Download the InVEST sample data. This can be done within the InVEST application on the main page by clicking the Settings (hamburger?) icon in the upper-right. Download the data for all of the services so that it is organized as follows: /Files/base_data/invest_sample_data/

Reading: Polasky and Segerson 2009

Slides as Powerpoint: Download here

Content

Introduction to Lecture 7

This lecture continues our exploration of inclusive wealth before transitioning to one of the central elements of this course: ecosystem services. The discussion of inclusive wealth requires approximately five to ten additional minutes to complete the material that was rushed at the end of the previous lecture due to time spent setting up VS Code, though that investment of time was worthwhile for future work.

Inclusive Wealth and Sustainability

Foundational Concepts and the DICE Model

Ecosystem services and the provision of value over time represent concepts that generate broad agreement across the field. The DICE model serves as one representation of these concepts, though previous discussions have highlighted various limitations and aspects of reality that DICE fails to capture. The DICE model and its variants, including the RICE model, attempt to address not merely what sustainability is, but rather what constitutes a good type of sustainability. Within the DICE framework, this translates to optimizing utility. When defined broadly enough, this approach proves correct. However, the challenge emerges from necessary simplifications, such as assuming a single sector, a single region, or simplified utility functions, none of which represent ideal conditions. This raises the question of what economic theory itself can contribute to defining sustainability.

Defining a Good Metric for Sustainability

The search for a good metric leads directly to inclusive wealth. Though often perceived as an environmental concept, inclusive wealth actually encompasses much more than natural capital alone. The framework includes other undervalued forms of capital, particularly social capital, and maintains the flexibility for further extension. Understanding the mathematics behind this concept provides important intuition, though the emphasis remains on conceptual understanding rather than technical complexity.

Mathematical Framework of Inclusive Wealth

In most economic models, production functions serve as the foundation. The inclusive approach extends this foundation to incorporate all different inputs, especially all types of capital. The mathematical representation begins with production Y at time t equaling CT plus ST, where C represents consumption that generates utility, and S represents savings that convert into capital stock for production in the next period.

The equation of motion describes how savings contribute to the evolution of the system over time. While solving such systems requires substantial mathematical work, the intuitive understanding proves most valuable. Production equals the sum of consumption and savings, but the transition of ST over time requires careful consideration. The equation of motion demonstrates how savings contribute to the capital stock while accounting for depreciation. Through rearrangement, S equals YT minus CT minus beta KT, which constitutes the equation of motion.

Optimization and Utility Considerations

The optimization process focuses on what actually generates value. While consumption contributes to happiness, representing this relationship solely through consumption constitutes a massive simplification. Nevertheless, this simplification serves our analytical purposes. The standard inclusion of utility forms the basis of our analysis.

A critical distinction emerges regarding whether the discussion concerns the utility of an individual or society as a whole. The framework addresses utility for all of society, as defining inclusive wealth only to exclude everyone except one individual would lack usefulness. However, aggregating utility across individuals creates microeconomic problems. The assumptions learned in foundational economics courses, such as separability, often become violated when aggregating utility. This violation prevents maintaining the cherished laws of welfare economics, which explains why some economists express skepticism toward aggregate utility functions.

Despite these concerns, the analysis proceeds with V representing the value of utility based on consumption CT, dependent on an initial stock of capital K0. Working in continuous time proves more convenient for this analysis. The framework extends to the end state T, with indexing according to tau equals t. The vector C represents consumption choices, while the exponential term captures discounting of utility. The discount rate parameter determines discounting according to tau minus t, indicating distance into the future. This approach remains standard when solving problems in continuous time.

The Definition of Inclusive Wealth

The optimization problem involves maximizing the value function by choosing CT, subject to the equation of motion. This yields the definition of inclusive wealth as non-declining human well-being. More specifically, within this framework, the derivative of the value function with respect to the capital asset stock must remain non-declining.

Several ways exist to violate this condition. Within this setup, utility derives from maintaining the capital stock. Within any given time period, utility also comes from not saving, as consuming now produces happiness but erodes the capital stock. This creates an intertemporal optimization problem. Maintaining the derivative above zero requires preserving the capital stock such that the value to the aggregate utility function never falls below zero. In essence, future prospects must always remain non-declining.

Types of Capital and Substitutability

K and C represent vectors of goods and different capital assets. Inclusive wealth moves beyond a single representative capital stock to emphasize different types, particularly natural capital. The representation could expand further to track all individual capital assets, demonstrating how each contributes to present and future value.

A significant debate centers on substitutability between types of capital. Consider K1 as manufactured capital and K2 as natural capital. What happens if society spends down natural capital while increasing manufactured capital? Depending on the coefficients, producing enough manufactured capital might offset the loss of natural capital. This argument suggests that sufficient production of manufactured capital can offset natural capital losses, preventing decline in future generations’ happiness while maintaining sustainability.

This definition may cause discomfort for some, as the idea that all natural capital could be spent down and replaced with manufactured capital assumes perfect substitutability between the two. The question becomes whether all types of nature can truly be substituted by manufactured capital. Many express skepticism about this assumption, which only holds if one accepts substitutability between natural and manufactured capital.

Practical Implementation and Measurement

The graphical representation of these concepts, sometimes humorously referred to as the “elephant on the page” due to its appearance, illustrates the key relationships. Beyond the mathematics, the intuitive understanding centers on how producing welfare over time involves improving capital stocks through savings. Production divides between consumption and savings, with savings augmenting capital stocks for future production. Without savings, capital stocks decline along with the sum of all capital stocks, leading to declining welfare. With adequate savings, capital stocks maintain or increase, keeping welfare constant and satisfying sustainability requirements.

The substitutability issue remains evident even when achieving sustainability by this definition. The composition of capital changes over time, with some forms (potentially natural capital) declining while others increase. The framework continues assuming substitutability, allowing increases in other capital forms to offset natural capital losses.

The unsustainable path offers temptation through higher well-being in the short run, similar to a sugar rush, but creates future problems as utility declines. The sustainable path maintains well-being consistently over time.

When pressed to identify the optimal metric, inclusive wealth that satisfies this criterion emerges as the answer, assuming acceptance of substitutability. This metric proves both conceptually sound and calculable. Efforts to measure inclusive wealth include work by Kenneth Arrow and the UNEP’s inclusive wealth reports, demonstrating that the concept extends beyond theory to practical measurement.

Challenges and Current Efforts

Several caveats accompany these measurements. Early estimates focused on measurable elements like fossil fuels and forests but failed to capture everything. Ongoing efforts aim to develop more complete metrics. The UN Statistics Division now oversees the system of national accounts, including the System of Environmental Accounting (SEA), which attempts to make environmental accounting both precise and understandable.

Accountants demonstrate deep concern for precision and methods, recognizing the importance of establishing rules for international comparisons. The World Bank produces its own estimates through the “Changing Wealth of Nations” series.

Despite measurement challenges, inclusive wealth represents a persuasive and practical definition of what society ought to maximize, superior to simple GDP maximization. Throughout the remainder of the course, when discussing optimization decisions, inclusive wealth serves as the metric being optimized. While practical calculations prove difficult, the principle guides the analysis.

Ecosystem Services

Introduction to the Concept

Ecosystem services represent a departmental strength and a key reason many students choose this program. The concept has already appeared in earth economy modeling discussions, but now receives focused attention. The implementation occurs through the Natural Capital Project and the Invest tool. The provisioning side of ecosystem services forms the current focus, with valuation postponed until the next lecture. Carbon storage, as the most straightforward service to understand and calculate, provides the starting point for hands-on work with the Invest model.

Natural Capital and Its Relationship to Ecosystem Services

Natural capital resembles other forms of capital in economic production functions. Many types of capital exist, and natural capital extends the tradition of emphasizing human and social capital. The ecosystem services and natural capital framework appeals particularly because it employs economic language, increasing persuasiveness with policymakers. Expressing environmental concerns in economic terms enhances the likelihood of influencing decision-makers.

The distinction between natural capital and ecosystem services requires clarification. Natural capital encompasses the assets and biophysical processes that make life possible, including trees, minerals, the water cycle, and the oxygen cycle. Without natural capital, virtually no utility could be generated. These fundamental processes enable life itself.

However, policy analysis typically focuses on the flow of benefits from natural capital to people—ecosystem services—rather than the total value of capital. Most decisions occur at the margin, and ecosystem services represent the flow of value (often monetary) from natural capital to people. In standard economics, services constitute the flow of value from capital. Ecosystem services specifically represent the flow of value from natural capital stocks.

Valuation and Policy Implications

The emphasis often centers on assigning dollar values to ecosystem services, a topic reserved for detailed discussion in the next session. The rationale for valuation stems from the frequent omission of environmental benefits when decisions rely solely on market prices. Cost-benefit analysis typically considers development costs and opportunity costs but often ignores ecosystem services lacking market prices, such as watershed protection. Including these values can fundamentally alter the analysis.

Ecosystem service methods aim to provide scientific, justifiable approaches for pricing ecosystem services and correcting their omission in traditional analysis. This approach gains persuasiveness by using economic language to argue for nature’s consideration.

Controversies and Development of Tools

The concept generates controversy as well. A famous article by Costanza and colleagues estimated the total value of all nature at $33 trillion, a figure so large it created demand for more rigorous methodology. The Natural Capital Project, a partnership led by Stanford and other institutions, developed the INVEST Toolkit—Integrated Valuation of Ecosystem Services and Trade-offs. This free, open-source collection of tools calculates how ecosystem changes affect ecosystem services.

INVEST organizes separate models for individual ecosystem services, including nutrient retention, sediment retention, scenic quality, and recreation. The design facilitates easier estimation of ecosystem service values. INVEST also integrates within a broader stakeholder engagement process, helping define objectives, compile data, generate scenarios, and assess outcomes. The modeling component accepts scenario inputs, particularly land use and land cover data, along with other biophysical and socioeconomic data, most of which are spatial in nature.

The models generate results including biophysical indicators (such as water produced for consumption) and valuation outputs (pricing services). This comprehensive approach bridges the gap between ecological understanding and economic decision-making.

The Invest Carbon Model

Overview and Basic Concepts

The Invest Carbon model provides a practical introduction to ecosystem service modeling. The carbon model estimates carbon stocks and changes (sequestration) as a function of land use and land cover. Carbon storage represents the mass of carbon at a given time, measured by summing mass and calculating carbon content. Sequestration captures the change in carbon storage over time, typically driven by land use change rather than plant growth. The value of carbon storage relates to the social cost of carbon, a concept previously discussed and revisited in Assignment 3.

Scientific Foundation

The scientific approach proves straightforward, essentially consisting of a literature review examining carbon in different pools, including above-ground biomass, below-ground biomass, soil, and dead wood. Total carbon equals the sum of these pools. The model’s simplicity compared to others makes it an ideal starting point for learning ecosystem service modeling.

Practical Implementation

The practical implementation begins with launching Invest and working with sample data. The base data should be saved to a specific directory structure for organization. Each model includes example data, with the carbon model serving as the introductory example. The process involves using QGIS to examine inputs visually.

The Invest sample data carbon folder contains files such as LULC_Current_Willamette.tif, representing land use and land cover for the Willamette Valley. Opening this map in QGIS provides visual context. Color display issues can be resolved by adjusting symbology settings to use a color palette rather than grayscale.

Within Invest, the Carbon Storage and Sequestration model requires several inputs. First, the workspace must be defined by navigating to the appropriate folder and creating an Output folder. This serves as the designated workspace for model outputs.

Next, the baseline land use and land cover raster must be specified. Invest validates the data to ensure compatibility. The carbon pools biophysical table, typically a CSV file, lists land use codes and the amount of carbon in each pool for each land use class. The model performs matrix multiplication, multiplying these values by the area in each class.

When all validation checkboxes show green, the model can run. Completion occurs quickly for this simple model. The output includes files such as C_carbon_storage_bas.tif, representing baseline carbon storage. Adding this to QGIS creates a map showing where the most carbon is stored, highlighting important areas for conservation.

Extensions and Future Applications

Assignment 4 will require running the carbon model with additional options enabled, particularly sequestration calculations. Enabling sequestration requires an alternate land use and land cover map. The difference between two maps provides the sequestration estimate.

Future work will examine sediment retention, an important issue for soil degradation and declining agricultural yields, particularly in areas dependent on subsistence agriculture. The consequences of soil degradation can prove severe, potentially leading to agricultural collapse similar to scenarios depicted in popular culture, where crop failure threatens civilization.

Conclusion

This lecture has established the theoretical foundation of inclusive wealth as a comprehensive metric for sustainability while introducing the practical application of ecosystem services through the Invest toolkit. The connection between these concepts demonstrates how economic theory can inform environmental decision-making and provide tools for quantifying nature’s contributions to human well-being. The hands-on experience with the carbon model provides a foundation for more complex ecosystem service assessments in future sessions.

Transcript

Welcome to Lecture 7!

Yes, we’re first going to finish up a bit on inclusive wealth. I had to rush at the end last time because we spent extra time getting VS Code up and running, but I think that was worth it. We’ll talk about inclusive wealth for about 5-10 minutes, then switch over to one of the key elements of this course: ecosystem services.

Let’s dive right in. As I said before, ecosystem services and thinking about the provision of value over time is something we can all agree on. We’ve seen this with the DICE model, which is one representation. I walked through different limitations and aspects of reality not captured by DICE. The main point is that the DICE model and its variants, like the RICE model, are all trying to answer not just what sustainability is, but what a good type of sustainability is. In the DICE model, that’s optimizing utility. If you define it broadly enough, this is the right approach. The challenge is that simplifications—like a single sector, region, or simplified utility functions—are not ideal. So, what can we do using economic theory itself to define sustainability?

That’s where we get to defining a good metric. I want to spend more time on this today. Last time, I just read it out, but let’s talk about what it really means. In most economic models, we have some sort of production function. Being inclusive, we extend this to all different inputs, especially all types of capital. Inclusive wealth, although often thought of as an environmental concept, is really about including more than just natural capital. It also takes into account other undervalued things, such as social capital, and can be extended further.

I want to walk through the mathematics of this, not with too much emphasis, but to give an important intuition. We’ll write something similar to what we’ve seen before, but a bit different. Our production Y at time t is CT plus ST. C is consumption, which gives us utility, and S is savings, which is converted into capital stock for production in the next period.

We give this an equation of motion. You’d spend more time on how to solve systems like this, but let’s do it intuitively. Production is the sum of consumption and savings, but how does ST transition over time? The equation of motion shows how savings contribute to the capital stock, minus depreciation. Rearranging, S is YT minus CT minus beta KT. That’s the equation of motion.

Last time, I went a little too fast and didn’t talk about what’s actually being optimized. Maybe that’s my bias—I’m always thinking about the production side, where there’s more detail and data. On the utility side, we know consumption contributes to happiness, but it’s a massive simplification to say it’s just consumption. Still, we’ll use it. What does it look like? We’ll have the standard inclusion of utility.

One big thing to note: so far, we haven’t specified if we’re talking about the utility of an individual or society. We’ll talk about utility for all of society. It wouldn’t be useful to define inclusive wealth and then exclude everyone but one individual. But aggregating utility across individuals creates a microeconomic problem. Think back to your 8001 class—there are assumptions you learn, like separability. Aggregating utility often violates separability, so we can’t keep our cherished laws of welfare economics. This is why economists are sometimes skeptical of aggregate utility functions.

However, I’m happy to proceed. Let’s round out the math: we’re going to have V, the value of utility, based on consumption CT, and it depends on an initial stock of capital K0. Unlike before, I’ll do this in continuous time, which is more convenient here. We’ll go to the end state T, indexing things according to tau equals t. We have a vector C, and the last bit is the term with the exponential—this is discounting utility. The parameter here is the discount rate, and it’s discounted according to tau minus t, showing how far into the future we are. This is standard when solving in continuous time.

Now, our choice is to maximize, by choosing CT, this value function, subject to the equation of motion. That gives us our definition of inclusive wealth: non-declining human well-being. More specifically, given this framework, the derivative of the value function, given the capital asset stock, must be non-declining.

Looking at this, in what ways might we violate that? In this setup, utility comes from maintaining our capital stock. Within a given time period, utility also comes from not saving—consuming now produces happiness but erodes the capital stock. This is an intertemporal optimization. To keep the derivative above zero, we need to maintain our capital stock so that the value to our aggregate utility function never falls below zero. In short, future prospects are always non-declining.

K and C are vectors of goods and different capital assets. Inclusive wealth isn’t just a single representative capital stock; it emphasizes different types, including natural capital. We could expand this representation and track all individual capital assets, showing how each contributes to present and future value.

A big debate is about substitutability between types of capital. Suppose K1 is manufactured capital and K2 is natural capital. What if we spend down natural capital and increase manufactured capital? Depending on the coefficients, it might be possible to produce enough manufactured capital to offset the loss of natural capital. This argument suggests that as long as we can produce enough, the loss of natural capital can be offset, and future generations’ happiness won’t decline. That is still sustainable.

Does anyone feel uncomfortable with that definition? The idea that we could spend down all natural capital and replace it with manufactured capital assumes substitutability between the two. But can all types of nature be substituted by manufactured capital? Many are skeptical. This assumption only works if you’re willing to accept substitutability between natural and manufactured capital.

Now, let’s talk about the figure from the key reading. I called it the “elephant on the page”—an inside joke about how the graphic looks like an elephant. Math aside, keep in mind the intuitive understanding of how this works. The basic decision of producing welfare over time is about improving capital stocks through savings. Production can go to consumption or savings, which augments capital stocks for future production. If we save nothing, capital stocks are spent down, and the sum of capital stocks declines, leading to declining welfare. If we save more, capital stocks are maintained or increased, and welfare remains constant, satisfying sustainability.

Notice the substitutability issue: even if we’re sustainable by this definition, the composition of capital changes. Some capital (maybe natural capital) goes down, but others increase. We’re still assuming substitutability, so increases in other capital offset the loss of natural capital.

The unsustainable path is tempting because it offers higher well-being in the short run—like a sugar rush—but leads to problems in the future as utility declines. The sustainable path maintains well-being over time.

If you pushed me on what metric we should optimize, I’d say inclusive wealth that satisfies this criterion, assuming we’re willing to accept substitutability. It’s a good, calculable metric. There have been efforts to measure inclusive wealth, such as by Kenneth Arrow and the UNEP’s inclusive wealth reports. These efforts show it’s not just a concept but something measurable.

However, there are caveats. Early estimates focused on things that could be measured, like fossil fuels and forests, but didn’t capture everything. Ongoing efforts aim to develop more complete metrics. The UN Statistics Division now oversees the system of national accounts, including the System of Environmental Accounting (SEA), which tries to make environmental accounting precise and understandable.

Accountants care deeply about precision and methods, and setting the rules for how countries compare themselves is important. The World Bank also produces its own estimates under the “Changing Wealth of Nations” series.

There are challenges in measurement, but the key point is that inclusive wealth is a persuasive and practical definition of what we ought to maximize—better than just maximizing GDP. Any questions on inclusive wealth? Is it persuasive? Does anyone dislike it? It’s okay if you do. No? Great.

That rounds out inclusive wealth. For the rest of the course, when we talk about optimizing decisions, you can think of inclusive wealth as the metric we’re trying to optimize. In practice, we rarely calculate it because it’s hard, but that’s the principle.

Now, let’s switch to the newest PowerPoint slides on our Google Drive and transition to ecosystem services. Who here had heard about ecosystem services before this class? Was it a key reason for coming to this department? That’s great—it’s a departmental strength, and we’ve decided to lean into it.

From here, I want to introduce the concept. We’ve already seen some of it with earth economy modeling, but now we’ll focus specifically on ecosystem services. We’ll discuss implementation through the Natural Capital Project and the Invest tool. I’ll talk about the provisioning side of ecosystem services, and we’ll hold off on valuation until the next lecture. We’ll start with carbon storage, the easiest to understand and calculate, and actually run the Invest model as a hands-on workshop.

Natural capital is similar to other capital in economic production functions. There are many types of capital, and natural capital extends the tradition of emphasizing human and social capital. I like ecosystem services and natural capital because they use the language of economics, which increases persuasiveness with policymakers. Expressing things in economic terms makes it more likely to persuade decision-makers.

What’s the difference between natural capital and ecosystem services? Natural capital is the assets and biophysical processes that make life possible—like trees, minerals, the water cycle, and the oxygen cycle. Without natural capital, there would be almost no utility generated. These are the fundamental processes that make life possible.

However, policy analysis often focuses on the flow of benefits from natural capital to people—ecosystem services—rather than the total value of capital. Most decisions are made at the margin, and ecosystem services are the flow of value (often monetary) from natural capital to people. In standard economics, services are the flow of value from capital. Ecosystem services are the flow of value from a stock of natural capital.

Early research focused on how humans get this value. A key paper by Heather Tallis and others described the broader social-ecological system, where ecosystem structure leads to a supply, and the ecosystem service production function converts that supply into a valuable service for humans. This function sits at the interface between biophysical and human systems. For example, a wetland that attenuates floods becomes a service when humans benefit from it. If there’s no city to protect, there’s no ecosystem service value.

The emphasis is often on putting a dollar value on ecosystem services, which we’ll discuss next time. The reason for valuing them is that environmental benefits are often ignored when decisions are based only on market prices. Cost-benefit analysis typically considers development costs and opportunity costs, but often ignores ecosystem services without market prices, like watershed protection. Including these values can fundamentally change the analysis.

Ecosystem service methods aim to provide scientific, justifiable ways to price ecosystem services and correct for their omission in traditional analysis. This makes the concept persuasive, as it uses economic language to argue for considering nature.

It’s also controversial. A famous article by Costanza et al. estimated the value of all nature at $33 trillion, a huge number that created demand for more rigorous methodology. The Natural Capital Project, a partnership led by Stanford and others, created the INVEST Toolkit—Integrated Valuation of Ecosystem Services and Trade-offs. It’s a free, open-source set of tools that calculate how changes in ecosystems cause changes in ecosystem services.

INVEST is organized as separate models for individual ecosystem services, like nutrient retention, sediment retention, scenic quality, and recreation. It’s designed to make estimation of ecosystem service value easier. INVEST also fits within a broader stakeholder engagement process, helping to define objectives, compile data, generate scenarios, and assess outcomes. The modeling part takes scenario inputs—especially land use/land cover—and other biophysical and socioeconomic data, most of which are spatial.

The models generate results, including biophysical indicators (like water produced for consumption) and valuation outputs (putting a price on services). Any questions about the overall framing? If not, let’s move to a specific ecosystem service and run it on your computer.

Let’s talk about the Invest Carbon model. Go ahead and launch Invest. The carbon model estimates carbon stocks and changes (sequestration) as a function of land use and land cover. Carbon storage is the mass of carbon at a given time, measured by summing up the mass and calculating the carbon content. Sequestration is the change in carbon storage over time, usually driven by land use change rather than plant growth. The value of carbon storage is the social cost of carbon, which we’ve discussed before and will revisit in Assignment 3.

The science is straightforward: it’s a literature review of carbon in different pools (above-ground, below-ground, soil, dead wood, etc.). Total carbon is the sum of these pools. The model is simple compared to others, making it a good starting point.

Let’s walk through an example. You should have downloaded the base data. Save it to your username/Files/base_data/Invest_Sample_Data. Each model has example data; we’ll use the carbon model. Open QGIS as well to look at the inputs.

In the Invest sample data/carbon folder, you’ll find files like LULC_Current_Willamette.tif (land use/land cover for the Willamette Valley). Open this map in QGIS. If you get grayscale instead of colors, go to Symbology and set it to a color palette.

Now, open Invest. Click on the home button if needed, then select Carbon Storage and Sequestration. Define the workspace by navigating to the Invest sample data/carbon folder and creating an Output folder. Select that as your workspace.

Next, point to the baseline land use/land cover raster (LULC_Current_Willamette.tif). Invest will validate the data. Then, select the carbon pools biophysical table (carbonpoolsWillamette.csv). This file lists land use codes and the amount of carbon in each pool for each land use class. The model multiplies these values by the area in each class—a matrix multiplication.

If all checkboxes are green, hit run. The model should complete quickly. Open the workspace and look at the output, especially C_carbon_storage_bas.tif, which represents baseline carbon storage. Add this to QGIS. If you can’t see it, make sure it’s above other layers. If it’s grayscale, set the symbology to a color ramp.

This map shows where the most carbon is stored, highlighting important areas for conservation. You’re now GIS and ecosystem service experts. The last few minutes of class, I’ll preview where we’re going.

If you want to work ahead, Assignment 4 will ask you to rerun the carbon model with extra options enabled, like sequestration. Enabling sequestration asks for an alternate land use/land cover map. Calculating the difference between two maps gives you sequestration.

We’ll look at sediment retention next, which is important for issues like soil degradation and declining yields, especially in areas reliant on subsistence agriculture. Think of the movie Interstellar, where the corn stops growing—soil degradation can lead to similar problems.

That’s it for today. Any questions? We’ll see you next time.