The Habitable Planet: A Systems Approach to Environmental Science
Agriculture Interview with Pam Matson
Interviewer: Tell me about what you do and how you came to be interested in this project.
PAM: I’m a professor in geological and environmental sciences at Stanford University and I’m a biogeochemist by training. I study the chemical interactions between plants and microorganisms and soil and water and atmosphere systems, and I focus, in particular, on nitrogen. A lot of my research is focused on what happens with nitrogen fertilizers in agricultural systems and how they affect the environment.
Interviewer: Would you describe the nitrogen cycle in layman’s terms for us.
PAM: The biogeochemical cycle of nitrogen is one of the grand element cycles of the planet. All organisms need a lot of nitrogen: plants and people and all different kinds of animals and microorganisms use nitrogen in our cells. We use it in proteins and amino acids; plants use nitrogen in chlorophyll, the machinery of photosynthesis. We need a lot of it and, of course, we’re literally bathed in it. The atmosphere is 78% di-nitrogen (two molecules of nitrogen connected together making it very, very hard to break down) so there should be plenty. But the problem is that most organisms don’t have access to that di-nitrogen in the atmosphere. They can’t use it. They can’t get at it. But there is a group of microorganisms called nitrogen-fixing bacteria that have the ability to break apart the di-nitrogen that’s in the atmosphere into parts that they can use in their own biomass. Then when they die and decompose, the rest of the organisms on the planet get access to that nitrogen. Once that nitrogen’s in plants and in soil, we use it in our cells, microorganisms use it to grow, and some of it gets stored in soils in different forms, in organic forms and in inorganic forms that can be easily lost in water or to the atmosphere. When microorganisms use nitrogen, some of them release di-nitrogen back into the atmosphere. And so that completes the cycle. We have di-nitrogen in through the microorganisms, all other organisms use it, it’s stored in soils and water systems, and ultimately bacteria de-nitrify it back out to the atmosphere. That’s the natural nitrogen cycle.
That’s how the world worked until about 50 years ago when people started fixing nitrogen out of the atmosphere using energy provided by fossil fuels, by oil and gas. We use the energy from fossil fuels to break apart di-nitrogen and make it into fertilizer nitrogen like ammonia fertilizer, nitrate, urea fertilizers that we use to grow plants and agricultural crops. We also started purposefully growing a lot of nitrogen-fixing crops like soybeans and peas and beans. Those two activities – making the fertilizer and growing nitrogen-fixing crops – have together added a huge amount of nitrogen into available forms around the planet. We’ve actually more than doubled the amount of nitrogen that now becomes available every year for all these plants and animals to use. And that’s a huge change in the global cycle of nitrogen.
The consequences of all that extra nitrogen are some good and some bad. The good ones are of course that it allows us to grow more food. We need to add fertilizer nitrogen to maximize the yields of a lot of different crop plants. By yields I mean the amount of food that’s produced per unit area of land surface. Fertilizer nitrogen has been very important in the successful increase of food production over the last 30 or 40 years. But one of the problems with all that extra nitrogen going into land in the fertilizer is that it doesn’t all get used by plants. In fact, maybe on average, 50% of the nitrogen doesn’t stay in agricultural systems but rather gets transported out of agricultural systems and goes off to the atmosphere in a number of different forms, including nitrous oxide, which is a greenhouse gas, nitric oxide, which is an air pollutant. Also some of it goes off as di-nitrogen, that di-nitrogen that the atmosphere is filled with and that has no environmental consequences. Some of the nitrogen in agricultural fields just leaches out through the soils into groundwater systems, or runs off the surface into surface-water systems like streams and rivers and lakes. And there it causes a number of different problems.
It can cause acidification of the lakes in some cases, and can cause disease in humans if there’s enough nitrate in the water that we drink. But probably the biggest problem for nitrogen is that it moves off of the land, through the rivers, down into the coastal waters of the continents. And in the coastal marine waters, a lot of extra nitrogen can cause huge blooms of algae, of phytoplankton in the oceans. Those huge blooms ultimately result in the phytoplankton dying off, sinking down, and being decomposed. And when that happens, sometimes the waters are driven to very, very low oxygen levels — they’re anoxic waters — and that kills fish, kills shellfish, drives fish away, and causes lots of harm to the coastal marine environments. It also causes a lot of economic losses to the fisher people who use those resources. So there’s a lot of inadvertent negative consequences of fertilization. One of the big challenges that we’re trying to address is how you can use fertilizer to grow food and meet the needs of people for food, but at the same time do it in a way that has less negative consequence for the environment that we use.
Interviewer: Why is it that in the coastal waters the phytoplankton, single-cell organisms, are always there even when the nitrogen-rich water washes comes off the crop land and washes into the river systems?
PAM: Coastal waters off of the continents tend to be relatively rich waters. There are lots of organisms that live there, lots of phytoplankton, which are single-celled plants basically, that have the ability to fix or to carry out photosynthesis, and lots of animals as well. But the phytoplankton are especially influenced by nutrients coming off of land, nutrients like nitrogen that come off of land into the river systems. Their growth tends to be limited by the amount of nitrogen that’s there. If you add extra nitrogen, they respond by growing very, very well and that leads to huge blooms of the phytoplankton. That’s not necessarily a good thing for those ecosystems because those phytoplankton die eventually and sink down low into the ocean water column. and as it sinks down, the phytoplankton gets decomposed by bacteria and other kinds of microorganisms in the water column. And in decomposing those phytoplankton cells, they take a lot of oxygen out of the water, leaving the water to become very, very low in oxygen. That’s called anoxic. That, of course, kills off animals that are there that can’t move, like the shellfish, and drives away any other animals that can swim away. So you run into areas that are called dead zones where there’s very little life left in those systems, at least life in terms of fisheries. That has tremendous impact on the fishermen and women who make their livelihoods on fishing in the coastal waters, and it also has really negative consequences, of course, on the biodiversity and the health of the coastal ecosystems.
Interviewer: About 50 years ago, the Haber process started creating a more abundant supply of nitrogen. What has happened to all that nitrogen and how does that influence the nitrogen process?
PAM: The Haber process that you mention is a process by which we use energy from gas and oil to break apart the di-nitrogen in the atmosphere and make it into fertilizer forms like ammonia or nitrate or urea. Since we learned how to do that process and started creating and making fertilizer nitrogen, we’ve added as much nitrogen through our human-made process as happens naturally in the global system. Some of that excess nitrogen goes off to the atmosphere in the form of an air pollutant and a greenhouse gas form; some of it goes back to di-nitrogen, which we already have a lot of in the atmosphere. Some of it goes into water systems where it leads to pollution of various sorts. The big issue for us is how to manage that nitrogen fertilizer better, so that we can still increase plant growth, we can still get high yields, but we prevent the loss of that nitrogen into the atmosphere or into the water system. That’s been the focus of a lot of the research that we’ve done, and particularly the focus of the research in the Yaqui Valley. In that environment, in the Yaqui Valley, the farmers apply a lot of nitrogen, way more than normal amounts of nitrogen, 250-260 kg per hectare of nitrogen, to their wheat crops. And they apply most of it about a month before they plant, a month before they put the seeds in the ground. We expected to see a lot of that nitrogen lost to the atmosphere. We expected to see high emissions of ammonia, of nitric oxide, which is a greenhouse gas. But we also expected to see a lot of it leaching out through the soil into water systems. And we were concerned because, first of all, it was a lot of nitrogen being applied, but also it was being applied not well timed to the demands of the crop. In our studies we measured all of these things in the field and showed that, indeed, very large losses of nitrogen were occurring, given the kind of management practices that the farmers were using.
Interviewer: When was the first time you went to the Yaqui Valley and why there?
PAM: Ros Naylor, one of my colleagues at Stanford, and I knew that we wanted to work together on this issue of fertilizer in agriculture, that we wanted to find those win/win situations where farmers could maintain yields and profits but at the same time reduce the environmental cost of fertilization. We had developed a plan of what we would like to do and we were invited to come to the Yaqui Valley by Dr. Ivan Ortiz-Monasterio, the director general of CMMYT, the international maize and wheat research institute, and an agronomist in the Yaqui Valley, whose own research was focused on nutrient use by plants and by agricultural plants. He visited the Yaqui Valley and thought he had just the right place to carry out our plan. When we met Ivan, we knew that we had the perfect match: that the three of us, an economist, an agronomist, and a biogeochemist could work together on this issue, looking at how farmers manage fertilizer, trying to understand why they do what they do, and looking for alternatives that made sense for the farmers as well as the environment. The Yaqui Valley provided the perfect place to do that.
Interviewer: What had been the traditional practice in the Yaqui Valley?
PAM: The farmers in the Yaqui Valley were growing wheat, irrigated wheat, and their typical practice was to apply a lot of nitrogen, around 200 kilograms per hectare of nitrogen, in November or early December, and then follow that with an irrigation of the dry soils, and then three or four weeks later, they would plant their seeds. About a month after that, they would add a little more water and a little more fertilizer. So altogether they were applying about 250 kilograms per hectare of nitrogen. 75% of that typically was applied about a month before the seeds were planted. When we heard about that practice and when we saw that practice, we realized that they were probably losing a lot of nitrogen before they ever even got the seeds into the ground. And so, when we began doing our research there, we asked, first of all, what were the farmers really doing, why were they doing it that way, what were the logical and rational reasons that they decided to manage fertilizer in that way, and what were the consequences of that management practice, and then finally were there alternatives that would make sense for the farmers and for their economic well being, as well as for the environment?
Interviewer: The farmers must have had the attitude, “Well, we’ve been farming for years. We know how to do this. We’ve got the best yields in the world.” What was working about that practice that the farmers would decide to keep going with it even though it doesn’t seem to make sense?
PAM: When wheat agriculture first started in the Yaqui Valley back in the 1950s and 1960s, they had very low yields and farmers typically were not applying very much fertilizer at all. Not even all of the land was being fertilized, and where it was being fertilized, the levels of fertilization were very, very low. That was a problem because they couldn’t optimize the growth of the plant. They weren’t applying enough nitrogen to get the best growth out of their crop. So the Mexican government started subsidizing the application of fertilizer. They made fertilizer cheap on purpose so that farmers could apply more. Over the years more and more farmers applied more and more nitrogen because it was cheap and why not? It actually was very good because it led to an increase in yields in the wheat in the valley. In the 1980s everybody was applying fertilizer and they were applying about enough—the right amount of fertilizer to get the maximum yields. But for some reason, they kept applying more and more and more. This is one of these interesting cases where they overshot the need. They added more than they actually needed. Partly it was because every once in a while they’d have a really great crop-growing year because of the climate and they might get an exceptionally good yield that year. In those years they probably needed that extra nitrogen. Typically they wouldn’t have needed all that nitrogen, but they couldn’t tell what the year was going to be like, they couldn’t tell which year was going to be great and which year was going to be bad climatically, so they just put a lot of nitrogen on, the more the better. I think also they didn’t realize that they were losing a lot of nitrogen. They didn’t have that information. One of the questions that we asked is why do they apply so much of the nitrogen before they plant? It turns out that farmers all over the world do that and they do it for pretty good reasons. The farmers in the Yaqui Valley told us that they apply it a month before planting because it helps them spread their labor. You know, they could take care of the fertilization and then later on they could take care of the planting. They didn’t have as much machinery, they didn’t have to have as many people working. That’s one reason. Another reason is because they thought that the crop, the wheat plants, really needed a lot of nitrogen right away, so they wanted to make sure they had it on at the very beginning. And a third reason, and maybe the most important reason, is that they were worried that if they didn’t get it on early, they might not be able to get it on later when it starts raining, or if it starts raining. They were trying to avoid risk by getting it on very early and then not having to worry about it anymore. Of course what they didn’t realize is that they were losing a lot of it; they were basically pouring money down the drain by doing that.
Interviewer: How did you come up with a better scheme and then how did you convince the farmers that they should try this?
PAM: In the first few years that we worked there as a team, we spent a lot of time trying to understand what farmers were doing, why they were doing it, and what the consequences were. We interviewed farmers, we carried out farm surveys, we did many, many measurements of nitrogen processes in the soil, and we truly learned what was going on in the valley. The next step, then, was to say, “Well, what could they be doing instead?” If we know that they’re losing a lot of nitrogen and that’s costing them money, what are the alternatives they could try?” So we carried out a number of experiments in farmers’ fields and also on the experiment station there, the land on the experiment station. We compared what happens under different kinds of fertilizer management, and we looked at how well the crop grows, how much the wheat yield was, what the quality of the grain was, what happened to the nitrogen in the soil, how much went off to the atmosphere, how much was lost into water. And we did the economics of it, too. We tried to figure out what the costs and benefits of different management practices were. And we identified some very good practices that would reduce the amount of nitrogen lost while still saving farmers a lot of money an maintaining their yield and grain quality. In our best practice ,farmers would have applied less nitrogen, but they would have applied more of it at planting, none of it a month before planting. They would have had to add less nitrogen, but it would have been more carefully timed to crop requirements. It seemed like a great idea. It worked in most farmers’ fields, it worked in the experiment station, and we talked with farmers about it. But they didn’t adopt it. It seemed like a great idea, but they weren’t going to do it.
In phase two of the research we tried to figure out why they wouldn’t, why they couldn’t adopt it. What was the problem for them? In that research we found that there’s actually a lot of variability out there year to year. Some years the climate’s perfect for growing wheat, some years it’s terrible, some years it’s in between. We also realized that not all of the land there is the same. There’s a lot of variation in soils in that valley. It looks all the same, but it’s quite different. And finally, we realized that different farmers do different things. Some farmers are absolutely great managers and some aren’t quite so good in terms of managing fertilizers. Variability was a huge part of their life. Our one best practice just wasn’t going to be good for everybody under every condition and they weren’t adopting it. But the other thing that we discovered in our research is that one of the reasons they weren’t trying the new practice is that their credit unions were telling them not to. The credit unions in the Yaqui Valley are kind of like farmer associations. They do provide credits, that is they loan money to the farmers, but they also give them advice, give them access to markets, get good fertilizer deals for them, and seed deals and so forth. So it’s a very important part of the farmers’ lives. And those credit unions were risk-averse, too. They weren’t ready for the farmers to completely change their fertilizer management practice, even if it seemed like it was a good idea in many places under many environmental conditions. When we realized that, when we understood, first of all, the importance of variability and risk aversion because of variability, and we also understood the importance of the credit unions and the decisions, we changed our research strategies. Then Ivan Ortiz-Monasterio began a new set of research to look for real-time measures of nitrogen availability in the fields so that farmers would know what it’s like today, this year, under these conditions, on this field, and they could decide how much fertilizer to apply based on that knowledge.
Interviewer: Can you talk about the hand-held radiometers and how this great breakthrough came about?
PAM: When we realized the issue of variability and the need to have more information on a real-time basis, Ivan Ortiz-Monasterio began collaborating with others who had been developing a hand-held radiometer, an instrument that would allow measurement of the nitrogen content of the leaves of the plant—something that you can walk out into the field with and learn something about the nutritional level of the plants right then and there. He developed a management strategy that would allow him to compare the nitrogen content of a fertilized strip of land compared to an unfertilized field. When the radiometer’s information indicated that the plants really needed nitrogen or they wouldn’t grow to their maximum, then it was time to apply the nitrogen. It allowed farmers real-time information and on a site-by-site basis. When the radiometer’s data suggested it was time to fertilize, then they’d bring the fertilizer in and apply it as needed. The interesting thing there is that the credit unions became interested. We actually tried to get them to be interested in the process. Ivan invited them to become part of the research. They were interested in that because they’re always looking for ways to save money. Ultimately, when they saw that the hand-held radiometer approach works and that it could save their farmers a lot of fertilizer and thus save them a lot of money, they decided to invest in them for their members. So the credit unions themselves bought the hand-held radiometers for all of their members to use, and they bought it because it was a money savings device.
Interviewer: You’ve mentioned the importance of the work of Mike Beman, a member of your team. Can you tell us a more about that.
PAM: When we were evaluating what happens to nitrogen in the farmers’ fields, we found that quite a bit of it, maybe 20-50 kilograms per hectare of nitrogen, actually leaches out of the farmers’ fields and moves into the groundwater or the surface-water systems. Those systems lead right out to the Sea of Cortez, to one of the world’s biodiversity hot spots. We were very interested in what that meant to the ecosystems of the marine system of the Sea of Cortez. So Mike Beman, a graduate student who worked as part of our team, began a study to evaluate the consequences of all of that nitrogen and other stuff moving from land to the ocean on the processes going on in the Sea of Cortez. Mike used a couple of different approaches, including sea-based measurements of what’s going on in the water itself, as well as remote sensing studies in which he used satellite data to look at chlorophyll blooms, at these phytoplankton blooms, and to try to relate those blooms of algae in the water to what’s going on on the land. Mike’s results were absolutely amazing, surprising. First time ever that anyone was able to use very high temporal resolution, that is, very high time frequency of sampling from satellites to look at phytoplankton blooms in the water. And because he could do that so well, because he could have so much day-to-day information about phytoplankton blooms, he was able to relate those blooms of algae to fertilizer and irrigation events on land. What he was seeing was that fertilizer on land makes its way through the water systems into the ocean and very often leads to very high phytoplankton blooms that then go on to affect the whole Sea of Cortez.
Interviewer: What happened when he showed the data to the farmers?
PAM: The results of Mike’s research made its way back to the farmers. Ivan Ortiz-Monasterio and Mike Beman both spoke with farmers about it and the farmers, I think, were surprised, impressed, and concerned that their actions on land, their management on land, was actually able to have an effect far away, off of their fields. Most people don’t have that kind of information, so they don’t understand what the issues are. Those results, I think, got a lot of people’s attention. The farmers could see their money going down the drain, and moreover, they could see it heading out to the ocean as well.
Interviewer: How quickly did the nitrogen cause the blooms?
PAM: Very quickly, actually. Mike used another one of the satellite sensors to measure sea-surface temperature and he used that information to tell him when there was upwelling of nutrients, because the up welled water is very cold water. He could tell when the cold waters were at the surface of the ocean there; any phytoplankton blooms that were going on when there was cold water was probably because of the nutrients being up welled. So he separated those out and got rid of those from his data set and just looked at the rest of the algal blooms. He could time those within a week to the fertilizer irrigation events that were going on on land. In less than a week after most of the farmers fertilized, he could see the results in the ocean. Of course the key here in the Yaqui Valley is that most farmers do things at exactly the same time, so it’s very synchronous irrigation fertilization. All happens over a week- to two-week period. And Mike Beman was able to see the reflection of that in the blooms of algae in the Sea of Cortez.
Interviewer: Obviously Yaqui is just one small place, but are you somehow using this to apply to other places?
PAM: We selected the Yaqui Valley as a study site in part because it is so representative of so much of the wheat-growing area in the developing world. It’s agro-climatically representative of about 40% of the wheat-growing area so what we learned in that place is relevant to other places. We also selected the valley because it is the home of CIMMYT, the International Maize and Wheat Research Institute. Researchers from all over the world come to CIMMYT to learn new methods, and CIMMYT’s researchers go out to other parts of the world with their new technologies. So we knew that we had an automatic way to influence what was going on in other parts of the world. Another reason that we worked in the Yaqui Valley is that it was small enough to really get our hands around, and yet what we could learn there was relevant in a lot of other places. The tools that we developed, including the mathematical models that we developed, things like the hand-held radiometer or new fertilizer management practices, are all things that can be extended to many other parts of the world. Finally, the Yaqui Valley is also a case study, in a sense, of how interdisciplinary teams can work together on sustainability issues for sustainable agriculture. People around the world now recognize the Yaqui Valley as an example of how ecologists and agronomists and economists and farmers and credit union advisors and others can work together to develop new approaches that make sense for people and for the environment.
Interviewer: What’s the role of the farmer in all of this?
PAM: The farmer, of course, is the ultimate decision maker. If there’s one thing we’ve learned in this work, it’s that we have to be in constant contact and interaction with decision makers in order to make sure that our research is actually useful and usable and, in the end, used by them. It has led me to believe that if university research teams like ours, or any research team, wants to be effective, wants to have what they learn be actually used by someone, it’s probably very smart to partner with the decision makers, in this case with the farmers.
Interviewer: What excites you about this work?
PAM: I love the opportunity to learn new things about how the world works. I’m a researcher and I love that sense of discovery, and we did learn some brand new things in this project. But I think more importantly for me, it’s having the feeling that what we are doing will actually help somebody; it will help people but it will also help the environment. That sense of making sure that while we’re contributing to new knowledge, we’re also contributing to problem solving is very important for me.
Interviewer: In our final program in the series we’re trying to wrap up the entire course by giving a hopeful ending. Do you think human beings have the capacity to be able to share the earth’s bounty and have quality of life for everybody?
PAM: Sometimes when you realize all the unmet needs of people worldwide, the fact that there are people who go to bed hungry every night, millions and billions of people who don’t have access to clean water and to modern energy and to education and employment, you can get very, very depressed. And when you think about all the environmental problems that we’re facing: change in the atmosphere, climate change, land-use changes, loss of biological diversity, you can get even more depressed. But you know, I’m an optimist. I see great progress. I see great movement toward a transition to sustainability. I think if you look at corporations, governments, individuals, so many people are taking actions now to balance the need, to meet the needs of people and the environment. There’s so much good news out there. The key is that we’re going to have to move a lot faster, we’re going to have to push this transition to sustainability ahead much, much more quickly. But the bottom line is we’ve already taken the first couple of steps and we’re moving in the right direction.
Interviewer: Is there an historical precedent for problems of similar size that have been managed somehow?
PAM: I’m thinking about Jarred Diamond’s book Collapse. He talks about communities of people that made it and communities that didn’t. I think one of the messages that comes from his writing is that if people act, if they recognize that there are multiple and interacting challenges that they’re faced with and they act, they can survive. And I think there are examples of that in our ancient history. In modern history, yes, there are still some examples. Fifty years ago, human population was growing incredibly rapidly and we didn’t have the food to feed them. The green revolution, for all of its challenges, was dramatically successful in that we’ve been able to keep food production at pace with human population growth. Yes, there are still people who go hungry, but it’s not because of lack of food. It’s because of lack of access to the food that’s there. It was a tremendous success. And it was a success that’s thanks in part to the global community coming together and saying we need to work hard on this. The irony is that some of the challenges that we’re facing now are a result of that success, and we just need to go forward, identify, and use new approaches for agriculture that continue that success in terms of feeding people but at the same time reduce the environmental consequences that will influence the ability of our children to have their needs met.
7.1 Agriculture Video
Will world population outrun food resources? The "Green Revolution" of the 20th century multiplied crop yields, in part through increasing inputs of pesticides and fertilizers. How can farmers reduce their use of agricultural chemicals and still produce enough food?
Unit 1 Many Planets, One Earth
Astronomers have discovered dozens of planets orbiting other stars, and space probes have explored many parts of our solar system, but so far scientists have only discovered one place in the universe where conditions are suitable for complex life forms: Earth. In this unit, examine the unique characteristics that make our planet habitable and learn how these conditions were created.
unit 2 Atmosphere
The atmosphere is what makes the Earth habitable. Heat-trapping gases allow ecosystems to flourish. While the NOAA Global Monitoring Project documents the fluctuations in greenhouse gases worldwide, MIT's Kerry Emanuel looks at the role of hurricanes in regulating global climate.
Unit 3 Oceans
Oceans cover three-quarters of the Earth's surface, but many parts of the deep oceans have yet to be explored. Learn about the large-scale ocean circulation patterns that help to regulate temperatures and weather patterns on land, and the microscopic marine organisms that form the base of marine food webs.
Unit 4 Ecosystems
Why are there so many living organisms on Earth, and so many different species? How do the characteristics of the nonliving environment, such as soil quality and water salinity, help determine which organisms thrive in particular areas? These questions are central to the study of ecosystems—communities of living organisms in particular places and the chemical and physical factors that influence them. Learn how scientists study ecosystems to predict how they may change over time and respond to human impacts.
Unit 5 Human Population Dynamics
What factors influence human population growth trends most strongly, and how does population growth or decline impact the environment? Does urbanization threaten our quality of life or offer a pathway to better living conditions? What are the social implications of an aging world population? Discover how demographers approach these questions through the study of human population dynamics.
Unit 6 Risk, Exposure, and Health
We are exposed to numerous chemicals every day from environmental sources such as air and water pollution, pesticides, cleaning products, and food additives. Some of these chemicals are threats to human health, but tracing exposures and determining what levels of risk they pose is a painstaking process. How do harmful substances enter the body, and how do they damage cells? Learn how dangers are assessed, what kind of regulations we use to reduce exposures, and how we manage associated human health risks.
Unit 7 Agriculture
Demographers project that Earth's population will peak during the 21st century at approximately ten billion people. But the amount of new cultivable land that can be brought under production is limited. In many nations, the need to feed a growing population is spurring an intensification of agriculture—finding ways to grow higher yields of food, fuel, and fiber from a given amount of land, water, and labor. This unit describes the physical and environmental factors that limit crop growth and discusses ways of minimizing agriculture's extensive environmental impacts.
unit 8 Water Resources
Earth's water resources, including rivers, lakes, oceans, and underground aquifers, are under stress in many regions. Humans need water for drinking, sanitation, agriculture, and industry; and contaminated water can spread illnesses and disease vectors, so clean water is both an environmental and a public health issue. In this unit, learn how water is distributed around the globe; how it cycles among the oceans, atmosphere, and land; and how human activities are affecting our finite supply of usable water.
unit 9 Biodiversity Decline
Living species on Earth may number anywhere from 5 million to 50 million or more. Although we have yet to identify and describe most of these life forms, we know that many are endangered today by development, pollution, over-harvesting, and other threats. Earth has experienced mass extinctions in the past due to natural causes, but the factors reducing biodiversity today increasingly stem from human activities. In this unit we see how scientists measure biodiversity, how it benefits our species, and what trends might cause Earth's next mass extinction.
unit 10 Energy Challenges
Global energy use increases by the day. Polluting the atmosphere with ever more carbon dioxide is not a viable solution for our future energy needs. Can new technologies such as carbon sequestration and ethanol production help provide the energy we need without pushing the concentrations of CO2 to dangerous levels?
Unit 11 Atmospheric Pollution
Many forms of atmospheric pollution affect human health and the environment at levels from local to global. These contaminants are emitted from diverse sources, and some of them react together to form new compounds in the air. Industrialized nations have made important progress toward controlling some pollutants in recent decades, but air quality is much worse in many developing countries, and global circulation patterns can transport some types of pollution rapidly around the world. In this unit, discover the basic chemistry of atmospheric pollution and learn which human activities have the greatest impacts on air quality.
Unit 12 Earth’s Changing Climate
Earth's climate is a sensitive system that is subject to dramatic shifts over varying time scales. Today human activities are altering the climate system by increasing concentrations of heat-trapping greenhouse gases in the atmosphere, which raises global temperatures. In this unit, examine the science behind global climate change and explore its potential impacts on natural ecosystems and human societies.
Unit 13 Looking Forward: Our Global Experiment
Emerging technologies offer potential solutions to environmental problems. Over the long-term, human ingenuity may ensure the survival not only of our own species but of the complex ecosystems that enhance the quality of human life. In this unit, examine the wide range of efforts now underway to mitigate the worst effects of man-made environmental change, looking toward those that will have a positive impact on the future of our habitable planet.