Unit 26 · Environmental Science

Fill a bathtub with the drain half open. Water runs in from the faucet, water runs out through the drain, and the level in the tub goes up or down depending on which flow is bigger. That tub is a system: a set of parts that work together and affect each other. The water in the tub is a stock, an amount that can be stored. The faucet and the drain are flows, the rates at which the stock changes. Almost every problem in environmental science can be drawn this way.

A lake is a stock of water with rivers flowing in and evaporation and an outlet flowing out. The carbon in the atmosphere is a stock, with volcanoes, breathing, fires and burning fuel adding to it and plants and oceans taking it away. A forest is a stock of wood; growth adds, logging and fire subtract. If inputs equal outputs, the stock holds steady. Scientists call that a dynamic equilibrium: the stock looks still, but water, carbon or wood is moving through it the whole time.

The trick is that a stock changes slowly even when a flow changes fast. Open the faucet wider and the tub does not fill instantly. That lag is why environmental problems can sneak up on people. The Gulf of Mexico’s oxygen, the level of an aquifer or the carbon in the air respond to years of small changes in flow, not to a single event. By the time a stock is clearly out of balance, the flows that caused it may have been running for decades.

Systems thinking is a habit, not a formula. When you meet a new problem, ask: what is the stock? What adds to it? What removes? Which of those flows have people changed? A good answer to those four questions is often most of the way to understanding the problem, and to seeing which flow could be turned to fix it.

A thermostat and a furnace make a simple loop. The room cools, the thermostat senses it, the furnace turns on, the room warms, the thermostat shuts the furnace off. The result of a change (cooling) triggers a response that reverses it (heating). This is a negative feedback loop. The word negative does not mean bad; it means the loop pushes back against change and keeps the system near a set point. Your body temperature, the sugar in your blood and the population of deer in a forest with wolves are all held in check by negative feedback.

Now picture a microphone too close to its own speaker. A small sound goes into the mic, comes out louder, goes back into the mic, and in a second the room is screaming. That is a positive feedback loop: the result of a change makes the change bigger. Positive feedback is what makes a system run away. Snow is bright and reflects sunlight, so a snowy region stays cold; if warming melts some snow, the darker ground beneath absorbs more sunlight, warms more, and melts more snow. Scientists call this the ice-albedo feedback, and it is one reason the Arctic has warmed faster than the rest of the planet.

Real systems mix both kinds. A prairie in a dry year loses grass; less grass means less shade and more evaporation, so the soil dries faster, a positive loop. But the surviving grasses send roots deeper and the drop in grazing lets seedlings recover, negative loops that bring the prairie back when rain returns. Whether a system settles down or tips over depends on which loops are stronger and how hard it is pushed.

A tipping point is a threshold past which positive feedback takes over and the system does not return to its old state on its own. A lake can absorb some fertilizer runoff and stay clear. Past a certain load, algae shade out the rooted plants that held the mud in place. The water clouds, and the lake flips to a murky state. That state persists even if the runoff is reduced. Knowing where those thresholds lie, and staying on the safe side of them, is one of the hardest and most useful jobs in environmental science.

Drive from Chicago straight west to the Rockies and then to the California coast, and you cross several worlds. Tallgrass prairie gives way to shortgrass, then to sagebrush desert, mountain forest and, near the ocean, a dry scrubland of tough-leaved shrubs. No one planted these borders. Two numbers, average temperature and average yearly precipitation, do most of the sorting. Each large region with its own climate and its own community of plants and animals is a biome. Deserts, grasslands, several kinds of forest, tundra and the great ocean zones are the main ones.

Illinois sits in the tallgrass prairie biome, or it did. Before 1830, big bluestem grass grew taller than a person across about 22 million acres of the state. Roots went 3 meters deep. Fires set by lightning and by Native people swept through most years, killing tree seedlings and returning nutrients to the soil. Then the steel plow cut the sod, and within a few decades almost all of it was corn and wheat. Today only scattered remnants of the original prairie survive, in cemeteries, along old railroad lines and in restored preserves. Illinois still calls itself the Prairie State, but the biome it is named for is nearly gone here.

Biodiversity means the variety of life in a place: the number of species, the genetic variety within each species, and the variety of habitats. It is not just a count for its own sake. A prairie with 200 plant species has some that thrive in drought, some in flood, some that fix nitrogen and some that bloom early or late. Whatever a given year throws at it, some part of the community does well. Experiments at the Cedar Creek research station in Minnesota, running since the 1990s, found that plots planted with many species produced more plant growth and held up better through drought than plots planted with one or two.

That is why ecosystem services, the benefits people get from nature for free, depend on diversity. Pollination, clean water filtering through wetlands, flood control, soil building and pest control all come from communities of many organisms. When a biome is stripped to a single crop, those services must be replaced by machinery, chemicals and money, or they are simply lost.

A cornfield in July is a solar collector. Leaves catch sunlight and use it to build sugar from carbon dioxide and water; this is photosynthesis, and the plants doing it are the producers of the ecosystem. A grasshopper eats a leaf. A meadowlark eats the grasshopper. A hawk eats the meadowlark. Each link in that chain is a trophic level, a feeding level, and energy moves up the chain one bite at a time.

Most of it does not make it. At each level, the organism spends most of the energy it eats just staying alive: moving, keeping warm, growing, repairing. That energy leaves as heat. Only roughly a tenth of the energy at one level is stored as body tissue that the next level can eat. Ecologists call this the ten percent rule. It is a rough average, not a law, but it explains a lot. It is why a field can feed thousands of grasshoppers, hundreds of meadowlarks and only a few hawks, and why there are no predators that eat hawks for a living. The pyramid gets too thin at the top.

Matter is different. A carbon atom in that leaf becomes part of a grasshopper, then a bird, then, when the bird dies, part of the fungi and bacteria that break it down. Those decomposers release the carbon back to the air as carbon dioxide and the nitrogen back to the soil, where a plant takes it up again. Nothing is used up. The same atoms cycle through the ecosystem for millions of years. Energy, by contrast, arrives fresh from the sun every day and leaves as heat that radiates into space. An ecosystem is an open system for energy and, very nearly, a closed loop for matter.

This difference has a practical edge. Because energy is lost at every step, eating lower on the food chain feeds more people from the same land. A field of grain feeds far more people directly than it does if the grain is fed to cattle first. Because matter cycles, the nutrients that leave a field in a truck of corn must come back from somewhere. Where they come from, and where they end up, is the story of the rest of this chapter.

Carbon is the backbone of every living molecule, and it is constantly on the move. Plants pull carbon dioxide out of the air. Animals and decomposers breathe it back out. The ocean absorbs it at the surface and releases it elsewhere. Over millions of years, some carbon gets buried: dead plankton on the sea floor, peat in swamps, all slowly turned into limestone, coal, oil and natural gas. That buried carbon is a stock that was, until recently, almost sealed off from the fast loop. Volcanoes leaked a little back to the air each year.

Then people began digging up and burning fossil fuels. Burning coal, oil and gas moves carbon from the slow, buried stock into the atmosphere in a single afternoon that took millions of years to store. Today that human flow is much larger than all the world’s volcanoes combined. Plants and the ocean take up roughly half of what we add each year. The rest builds up in the air. That is why carbon dioxide has risen from about 280 parts per million before the industrial era to more than 420 parts per million today.

Nitrogen makes up about 78 percent of the air, but plants cannot use it in that form. The two nitrogen atoms in a nitrogen molecule are locked together by a triple bond that is very hard to break. Only a few kinds of bacteria, and lightning, can do it. Those bacteria, some living in nodules on the roots of beans, clover and other legumes, convert nitrogen gas into ammonia, a process called nitrogen fixation. From there, other bacteria turn it into the nitrate that most plants absorb. When plants and animals die, decomposers return the nitrogen to the soil, and yet other bacteria eventually convert it back to nitrogen gas. For most of history, the supply of usable nitrogen was the main thing that limited how much a field could grow.

In 1909 a German chemist, Fritz Haber, found a way to make ammonia from nitrogen and hydrogen with high pressure, heat and an iron catalyst. Carl Bosch scaled it up to a factory. The Haber-Bosch process let people fix nitrogen on an industrial scale, and synthetic fertilizer followed. It is one reason the world can feed eight billion people. It is also why the nitrogen in the Gulf of Mexico’s dead zone traces back to a factory, then a bag, then a field.

Dig a pit in an Illinois cornfield and you see layers. On top, a dark band of topsoil a foot or more thick, rich in humus, the black, crumbly remains of dead plants that decomposers have partly broken down. Below it, lighter subsoil with less organic matter. Below that, the parent material: in most of Illinois, a fine yellowish silt called loess that wind blew off glacial outwash plains more than ten thousand years ago. Prairie roots and ten thousand years of decay turned that silt into some of the deepest, darkest topsoil on Earth. It is the reason central Illinois grows so much corn.

Topsoil is not dirt; it is a community. A single gram can hold hundreds of millions of bacteria along with fungi, tiny worms, mites and insects. Fungal threads glue mineral grains into crumbs, and the spaces between the crumbs hold air and water. Roots, earthworms and burrowing animals keep it open. This structure is what lets rain soak in instead of running off, and it is easily wrecked by heavy machinery, by leaving the surface bare or by plowing until the crumbs collapse into dust.

In the 1930s the United States learned how fast soil can go. Farmers on the southern Great Plains had plowed drought-prone grassland to plant wheat. When rain failed for several years, the bare, powdered soil blew away in storms that darkened the sky as far as Chicago and Washington, D.C. The Dust Bowl drove hundreds of thousands of people off the land. In response Congress created the Soil Conservation Service in 1935, led by Hugh Hammond Bennett, who had spent years warning that erosion was quietly stripping the country’s farms.

The methods that grew out of that disaster are still the core of soil conservation. Farmers plow along the contour of a slope instead of up and down it. They build terraces, plant windbreaks of trees and rotate crops. More recently, no-till farming leaves last year’s stalks on the surface, and cover crops planted after harvest hold the ground through winter. Each of these methods does the same thing the prairie once did for free: keep roots in the soil and cover over it.

Earth is a water planet, but almost all of that water is salty or frozen. About 96.5 percent is in the oceans. Most of the rest is locked in ice caps and glaciers. The fresh water that people, crops and land ecosystems actually use, in lakes, rivers, soil and the ground, is well under one percent of the total. Of that, by far the largest share is groundwater, water that fills the tiny spaces in sand, gravel and porous rock beneath the surface. A body of rock or sediment that holds and yields useful amounts of water is an aquifer.

The largest aquifer in the United States lies under the Great Plains. The Ogallala Aquifer stretches beneath parts of eight states, from South Dakota to Texas. Its water is mostly ancient, left over from wetter times after the last ice age, and in the dry southern Plains very little new rain reaches it. It is, in the language of this chapter, a stock with a tiny input.

Starting in the 1940s and 1950s, cheap pumps and center-pivot sprinklers let farmers pull that water up to irrigate corn, cotton and wheat in a region that gets too little rain to grow them reliably. The green circles you see from an airplane over Kansas and the Texas Panhandle are the result. So is the drawdown. In parts of Texas and western Kansas the water table has fallen by more than 100 feet since pumping began, and some wells have gone dry. The U.S. Geological Survey monitors thousands of wells across the aquifer and reports the change each year. Where the output far exceeds the input, the water is being mined, used once and gone for generations.

Illinois has water troubles of a different kind. Rain is plentiful, but much of central Illinois draws from the Mahomet Aquifer, a buried ancient river valley, and communities and industries argue over how much it can safely supply. Chicago and its suburbs draw from Lake Michigan under a limit set by a Supreme Court decree, while far suburbs that pump deep sandstone aquifers have watched levels drop. Even where water seems endless, the stock-and-flow question is the same: how much comes in, how much goes out, and who decides.

A cell phone holds dozens of elements: copper in the wiring, lithium and cobalt in the battery, silicon in the chips, tiny amounts of gold on the contacts, and rare earth metals in the magnets that make it vibrate. None of them are made in a factory. Each was once ore, rock that contains enough of a useful mineral to be worth digging up, and each had to be mined, crushed, chemically separated and refined. Minerals are a nonrenewable resource: Earth does not make more on a human time scale.

The word worth is doing a lot of work in that definition. A rock that is one percent copper was ore a century ago. Today, many copper mines profitably dig rock that is less than half a percent copper, because bigger machines and better chemistry make it pay. This means the amount of a mineral that counts as a reserve, the part known and economical to extract, changes with prices and technology, not just with geology. When people say the world will run out of a metal, they usually mean that the cheap, rich deposits are gone and the rest costs more to get.

That cost is the environmental part. Lower-grade ore means moving more rock per ton of metal, which takes more diesel and electricity. Crushed waste rock, called tailings, is stored behind dams that have sometimes failed. Sulfur-bearing minerals exposed to air and water make acid that carries metals into streams, a problem called acid mine drainage that can last for centuries at abandoned mines. In Illinois, the main mined resources are coal, crushed limestone for roads and concrete, sand and gravel, and fluorspar, which was once mined in the far south of the state and is Illinois’s official state mineral.

Recycling changes the arithmetic. Metal in a discarded phone is far richer than any ore, and recycling aluminum takes only a small fraction of the energy of smelting it from bauxite. Yet most electronic waste is not recovered, partly because devices are built to be hard to take apart. Designing products so their materials can cycle, the way carbon and nitrogen cycle in an ecosystem, is one of the central ideas in sustainability.

Coal, oil and natural gas supply most of the energy the world uses, and for good reasons. They are dense: a kilogram of gasoline holds about as much energy as a person uses in a full day of hard labor. They can be stored in a tank or a pile and burned when needed, day or night, windy or calm. A century of engineering has made them cheap to find, move and use. Their costs are just as real. Burning them releases carbon dioxide, the main driver of climate change, along with sulfur dioxide, nitrogen oxides, mercury and fine particles that damage lungs. Mining and drilling scar land and, at times, foul water. And the supply is finite.

Illinois sits on one of the largest coal deposits in the country, the Illinois Basin. Its coal is high in sulfur, which is why many Illinois plants installed scrubbers or switched fuels after air rules tightened. Illinois also generates more electricity from nuclear reactors than any other state, roughly half of what it produces. Nuclear plants release almost no carbon dioxide while running, but they are expensive to build and leave radioactive waste that must be stored for thousands of years.

Renewable sources, mainly wind, solar, hydroelectric and biomass, are replenished by nature. Wind and solar have become dramatically cheaper since 2010, and in many places new wind or solar power now costs less per kilowatt-hour than new coal. Illinois’s flat, windy farmland has made it a top wind state; the turbines you pass on Interstate 55 and Interstate 39 are part of that. But the wind does not always blow and the sun sets every day. This intermittency means a grid built on wind and solar needs storage, backup or long transmission lines, all of which add cost. Solar farms take land; hydroelectric dams flood valleys and block fish; growing corn for ethanol competes with growing corn for food.

There is no free source. The honest comparison is a table with many columns: cost per unit of energy, reliability, land used, water used, air pollution, carbon emitted, waste created, and risk. Different communities weigh those columns differently, and the weights shift as technology changes. What science can do is fill in the numbers accurately, so the choice is made with open eyes.

In 1935 an acre of Illinois farmland yielded roughly 25 bushels of corn. Today the state average is often over 200. That change did not come from better soil; the soil was better in 1935. It came from hybrid seed, then from synthetic nitrogen fertilizer, then from pesticides, larger machines, drainage and, most recently, genetically engineered crops and satellite-guided planters. Each is an input: something added to the field from outside. Together they are the reason Illinois ranks first or second in the nation in both corn and soybeans nearly every year.

Look at a field as a system and the inputs and outputs line up. Energy in: sunlight, plus diesel for tractors and natural gas to make fertilizer. Water in: rain, and on much of Illinois’s flat, formerly swampy land, water out through buried drainage tiles, perforated pipes that carry excess water away so roots do not drown. Nutrients in: fertilizer. Nutrients out: the harvest, plus whatever the drain water carries. That last output is easy to forget because it leaves underground.

Growing one crop over a huge area, called monoculture, makes machines and markets efficient. It also builds a perfect home for whatever eats that crop. Corn rootworm and soybean aphid thrive because their food stretches to the horizon. Pesticides hold them down, but over time the pests become resistant, so new chemicals or new seed traits are needed. Nearly all Illinois cropland alternates corn and soybeans, a simple rotation that breaks some pest cycles and lets the soybeans, a legume, fix some nitrogen for the next year’s corn.

Alternatives exist along a spectrum. Sustainable agriculture means farming in ways that keep the soil, water and surrounding ecosystems able to produce indefinitely: cover crops, longer rotations, no-till, precise fertilizer timing, buffer strips along streams. Organic farming rules out synthetic fertilizer and most pesticides; it usually yields less per acre but with fewer chemical inputs. Every option trades something. The question is never whether to have inputs and outputs, but which ones, how much, and where the outputs end up.

Follow a single nitrate ion from a field near Bloomington, Illinois. Spring rain dissolves it from fertilizer and carries it down through the soil into a drainage tile. The tile empties into a ditch, the ditch into a creek, the creek into the Sangamon River, the Sangamon into the Illinois River, and the Illinois into the Mississippi at Grafton, just above St. Louis. From there it is about a thousand miles by river to the Gulf of Mexico. Along the way, some nitrate is taken up by plants or turned back to gas by bacteria in wetlands. But wetlands are scarce now; Illinois drained most of its original wetlands for farming. Much of the nitrate makes the whole trip.

Government scientists have measured nutrient loads in the Mississippi for decades. Their studies point to farm fertilizer and manure as the largest sources of the nitrogen reaching the Gulf, and to a handful of intensively farmed states in the upper basin, Illinois among the biggest, as major contributors. Cities and sewage treatment plants add some; so does the nitrogen that falls from the air. But the shape of the problem is clear: the flat, tile-drained, heavily fertilized corn belt is the main faucet.

In the Gulf, nitrogen and phosphorus act like fertilizer on the sea. Floating algae called phytoplankton bloom in the nutrient-rich river plume. When they die they sink, and bacteria decompose them, consuming oxygen. In summer, warm fresh river water spreads over the colder, saltier Gulf water and does not mix with it. This stratification means the bottom layer cannot draw oxygen from the air. The bottom water goes hypoxic, and animals that cannot flee die. The whole process, from nutrient overload to algae to oxygen loss, is called eutrophication, and it happens in lakes and ponds across Illinois for the same reasons.

In 2001, federal and state agencies, including Illinois, set a goal of shrinking the average dead zone to about 1,900 square miles. It has not come close. The five-year average has stayed several times that size. The stock-and-flow logic explains why: the nitrogen flow has not dropped much, and a great deal of nitrogen from past decades is still stored in soil and groundwater, working its way slowly downstream. Even a sharp cut in fertilizer today would take years to show up in the Gulf.

If the problem is a flow, the solutions are ways to shrink it. The simplest is precision: apply fertilizer at the rate the crop can actually use, in spring when it is growing rather than in fall when it can only leach away. Soil tests and yield maps let farmers vary the rate across a field. This often saves money as well as nitrogen, which is why it has spread faster than most other practices.

The second tool is cover. A cornfield is bare from October to May, exactly when rain and melting snow push water through the soil. A cover crop of cereal rye planted after harvest keeps living roots in the ground all winter, taking up leftover nitrate and holding soil in place. In spring it is killed and the cash crop planted into it. Studies in Illinois and Iowa have found cover crops can cut nitrate loss from tile drains substantially, though the amount varies with weather and management. The catch is cost and timing: seed, an extra pass with equipment, and a narrow window in a wet fall.

The third tool is to rebuild what the prairie had. A strip of grass or a constructed wetland at the outlet of a drainage tile slows the water and lets bacteria convert nitrate back to harmless nitrogen gas, a process called denitrification. A bioreactor, a trench filled with wood chips that tile water flows through, does the same thing in a small space. Restored wetlands along the Illinois River also store floodwater and bring back ducks, herons and fish. Each acre of wetland is an acre not in corn, which is the trade-off farmers and taxpayers weigh.

Illinois adopted a Nutrient Loss Reduction Strategy in 2015 with a long-term goal of cutting the nitrogen and phosphorus it sends downstream by 45 percent. Progress so far has been slow, and most practices remain voluntary. That is the last lesson of the dead zone: the science of the fix is largely known. The harder part is designing incentives and rules so that millions of separate decisions, on farms that are each doing something reasonable, add up to a river that no longer starves the Gulf.

In late October 1948, a temperature inversion, a layer of warm air sitting on top of cold air, trapped the smoke from steel and zinc works over the town of Donora, Pennsylvania. For five days the fog was so thick that people could not see across the street. About twenty people died and thousands fell ill. Four years later, in December 1952, a similar inversion held coal smoke over London, and the death toll ran into the thousands. Air pollution stopped being a nuisance and became, in the public mind, something that could kill.

The pollutants have names and sources. Particulate matter, tiny solid and liquid specks, comes from combustion, dust and smoke and lodges deep in the lungs. Sulfur dioxide comes mostly from burning coal and makes acid rain. Nitrogen oxides from car engines and power plants react in sunlight with vapors from fuel and solvents to form ground-level ozone, the main ingredient of smog, which irritates lungs and damages crops. Carbon monoxide from incomplete burning blocks blood from carrying oxygen. Lead, once added to gasoline, damages children’s brains. Each of these was once simply released into the air.

The Clean Air Act of 1970 changed that. It directed the new Environmental Protection Agency to set national standards for these common pollutants based on health, and required states, including Illinois, to write plans to meet them. Later amendments removed lead from gasoline, required catalytic converters on cars and, in 1990, created a market for sulfur dioxide permits to cut acid rain. The results are measurable. The EPA reports that combined emissions of the main regulated pollutants fell by roughly three-quarters between 1970 and the 2020s, while the economy, population and miles driven all grew. Blood lead levels in American children dropped by more than 90 percent.

Chicago’s air is far cleaner than it was in the 1960s, when steel mills lined the Calumet region and soot blackened buildings. It still fails the ozone standard on hot summer days, and neighborhoods near highways, rail yards and remaining industry breathe more particulate matter than others. Clean air is not a finished job. It is an ongoing measurement, with a standard to compare against and a law that requires the gap to close.

On June 22, 1969, oil and debris floating on the Cuyahoga River in Cleveland caught fire. The river had burned before, more than a dozen times, but this time the story spread nationwide and became a symbol. Lake Erie, which the Cuyahoga feeds, was choked with algae and its fish were dying. In Chicago, the river had been so foul from stockyards and sewage that in 1900 engineers reversed its flow. A new canal sent the city’s waste toward the Illinois River and the Mississippi instead of into Lake Michigan, the drinking water supply.

Congress passed the Clean Water Act in 1972. Its most powerful tool was simple: no one may discharge pollution from a pipe into a river or lake without a permit that limits what and how much. Pollution that comes out of an identifiable pipe or ditch is called point source pollution. Factories and sewage plants are point sources. The law also paid for thousands of modern sewage treatment plants. The results were dramatic. Fish returned to rivers that had been open sewers, and the Cuyahoga now hosts kayakers and restaurants along its banks.

The other kind of pollution was left largely alone. Nonpoint source pollution comes from everywhere at once: fertilizer and manure washing off fields, oil and salt off streets, sediment from construction sites, bacteria from failing septic tanks. There is no pipe to put a permit on. Today, nonpoint runoff, mostly from agriculture, is the leading reason U.S. rivers and lakes fail water quality standards, and it is the source of the nitrogen that builds the Gulf dead zone. Illinois rivers such as the Illinois and the Kaskaskia carry it in every spring rain.

Drinking water is regulated separately, under the Safe Drinking Water Act of 1974, which sets limits for contaminants at the tap. Nitrate is one of them, because at high levels it can interfere with oxygen in infants’ blood. Some Illinois and Iowa towns that draw from rivers or shallow wells in farm country must treat their water or blend it to stay under the nitrate limit. The pollutant that leaves a field as a farm problem arrives in town as a public health bill.

The ozone story has a beginning, a middle and, unusually, an ending in sight. The beginning was chemistry. Molina and Rowland worked out on paper in 1974 that CFCs, which are so stable that nothing in the lower atmosphere breaks them down, would eventually reach the stratosphere, 15 to 50 kilometers up, where hard ultraviolet light could split them and free chlorine. A chlorine atom takes an oxygen atom from ozone, then gives it up again and repeats. It is a catalyst, unchanged by the reaction, so one atom can destroy thousands of ozone molecules over its stay.

The middle was measurement and argument. Halley Bay’s readings and the satellite data showed the hole. The 1987 aircraft flights into it found chlorine monoxide, the smoking gun of the chlorine reaction, rising exactly where ozone fell. Why Antarctica? Its winter is so cold that clouds of ice form in the stratosphere, and on their surfaces chlorine is converted into its most destructive forms, ready to attack when spring sunlight returns. The chemistry explained not just that ozone was falling but where and when, and a prediction that fits the details is far more convincing than one that fits only the headline.

The end was policy. The Montreal Protocol of 1987, strengthened several times since, set a schedule to phase out CFCs and related chemicals. Three things made it work. Substitutes were available or soon developed, so the cost was manageable. Wealthier nations helped pay for poorer nations to switch. And the agreement had a mechanism to tighten controls as the science firmed up. Chlorine in the stratosphere peaked in the late 1990s and has been falling since. Scientific assessments project the Antarctic ozone layer will return to its 1980 condition around the 2060s.

The fix had a side effect worth knowing: CFCs are also powerful greenhouse gases, so the protocol did more to slow warming than any climate treaty of its era. The main replacements, HFCs, do not harm ozone but do trap heat, and a 2016 amendment now phases those down too. Ozone was in some ways an easy case: a few chemicals, a few hundred companies, clear substitutes. Climate change involves the fuels that run everything. But the shape of the solution is the same, and the ozone layer is proof the shape can work.

Earth is warmer than it should be. A bare rock at our distance from the sun would average about minus 18 degrees Celsius, well below freezing. The actual average is about 15 degrees. The difference is the greenhouse effect. Sunlight, mostly visible, passes through the air and warms the ground. The warm ground gives off infrared radiation, heat you cannot see. Nitrogen and oxygen, which make up 99 percent of the air, let infrared pass straight through. But water vapor, carbon dioxide, methane and a few other gases absorb it and radiate part of it back down. They act like a blanket, not a mirror.

This is not new science. In 1859 the Irish physicist John Tyndall built an apparatus that passed heat radiation through tubes of different gases and measured how much got through. Nitrogen and oxygen absorbed almost nothing; water vapor and carbon dioxide absorbed strongly. In 1896 the Swedish chemist Svante Arrhenius, working with pencil and paper for a year, calculated that doubling the carbon dioxide in the air would warm the planet by several degrees. His number was rough, but his reasoning stands, and the effect he described has since been confirmed by laboratory spectroscopy, by satellites that measure infrared leaving Earth, and by instruments on the ground that measure infrared coming back down.

The gas that matters most in the long run is carbon dioxide, because it lasts centuries in the air and because we add so much of it. In 1958 Charles David Keeling began measuring it precisely at an observatory on Mauna Loa in Hawaii, high above local pollution. His record, now called the Keeling curve, shows a sawtooth line. It dips each northern summer as plants grow and rises each winter, a planet breathing. Underneath the sawtooth, the line climbs relentlessly, from about 315 parts per million in 1958 to over 420 today. Air bubbles trapped in Antarctic ice show that before the industrial era it was about 280. In the past 800,000 years it never rose above about 300.

How do we know the extra carbon dioxide comes from fossil fuels and not from volcanoes or the ocean? Fossil carbon has a chemical signature: it contains almost no carbon-14, a radioactive form that decays away over thousands of years, and it is slightly depleted in carbon-13 compared with air. As fossil fuel carbon has flooded the atmosphere, both signatures have shifted exactly as expected. The ocean, meanwhile, is gaining carbon dioxide and turning slightly more acidic, so it cannot be the source. The bookkeeping closes.

A claim this large should not rest on one measurement, and it does not. Thermometer records from thousands of weather stations, ships and buoys, kept since the 1880s and analyzed separately by NASA, NOAA, the UK Met Office and others, show that Earth’s average surface temperature has risen a little over 1 degree Celsius since the late 1800s, with most of the rise since 1970. The groups use different methods and reach the same curve. The warmest years on record have nearly all come in the last decade.

Nature keeps its own records. Ice cores drilled from Greenland and Antarctica hold layers of snow going back hundreds of thousands of years, with air bubbles that preserve ancient atmosphere and chemical signatures that reveal past temperature. They show carbon dioxide and temperature rising and falling together through the ice ages. Tree rings, corals and lake sediments fill in the last few thousand years. Glaciers photographed a century ago have retreated up their valleys. Arctic sea ice at the end of summer, measured by satellite since 1979, has shrunk by roughly 40 percent in area. Spring arrives earlier; birds and plants have shifted their ranges north.

The ocean is the biggest thermometer of all. It stores more than 90 percent of the extra heat, and a fleet of thousands of floating robots called Argo floats has measured its temperature at depth since the early 2000s. Warm water expands, and melting glaciers add more, so sea level rises. Tide gauges and satellites agree that global sea level has risen about 20 centimeters since 1900, and the rate has roughly doubled in recent decades. The ocean has also absorbed about a quarter of our carbon dioxide, which has made it measurably more acidic, a change that stresses corals and shellfish.

Scientists distinguish between what is very certain and what is less so. That Earth has warmed, that human emissions are the main cause, and that more emissions mean more warming is settled beyond reasonable dispute. Exactly how much warming a given amount of carbon dioxide produces, how fast ice sheets will respond, and how rainfall will change in a particular region are areas of active research with real ranges of uncertainty. Good science states both the confidence and the range.

A climate model is a set of equations for how air and water move, how heat and moisture are exchanged, and how sunlight is absorbed and reflected, solved over a grid of boxes covering the planet. Weather forecasters use nearly the same equations to predict next week. Climate modelers run them for centuries. The difference is the question asked. A forecast asks what the sky will do on Tuesday. A climate projection asks what the average of many Tuesdays will be in 2070 if emissions follow a certain path. You cannot predict the next coin flip, but you can predict very well how many heads a thousand flips will give.

Models are tested against the past. Start one in 1850 with the observed changes in sunlight, volcanoes and greenhouse gases and it reproduces the measured warming, the cooling after big eruptions, and the faster warming in the Arctic. Run it without the human-added greenhouse gases and it fails to reproduce the warming since 1970. That is the main way scientists attribute recent warming to human activity: nothing else in the models can make the curve.

Models also made predictions that came true. Projections published in the 1970s and 1980s, from models far cruder than today’s, forecast warming rates close to what was later measured. They predicted that the stratosphere would cool while the surface warmed, which is what greenhouse gases should do and what satellites later found. They predicted nights would warm faster than days and that the Arctic would warm fastest. Each confirmed prediction is a test the theory could have failed.

What models do worst is the local and the sudden. A global model’s grid boxes are tens of kilometers wide, too coarse to capture a thunderstorm or a single city. Clouds are the biggest source of uncertainty, because whether warming makes more low clouds (cooling) or fewer (warming) is hard to simulate. That is the main reason estimates of climate sensitivity, the warming from doubling carbon dioxide, span a range of roughly 2.5 to 4 degrees Celsius rather than a single number. A range is not a failure. It is an honest statement of what is known, and the entire range means substantial warming.

Climate change is often pictured as melting glaciers and drowning islands, far from Illinois. But the Midwest has its own signature, and it is already measurable. Warmer air holds more water vapor, roughly 7 percent more for each degree Celsius of warming. When that air finally rises and cools, it rains harder. Records from Illinois weather stations show that the heaviest downpours, the kind that once came every few years, now come more often, and that total spring rainfall has risen. Chicago’s combined sewer system, which carries rain and sewage in the same pipes, overflows into the river and, on the worst days, into basements and Lake Michigan.

Summers are getting hotter, though not evenly. Nights have warmed more than days, and humidity has risen, which matters for people and for livestock because humid heat is harder for bodies to shed. The 1995 Chicago heat wave killed more than 700 people in a week, most of them elderly residents in apartments without air conditioning. Heat is already the deadliest kind of weather in the United States, and models project many more days above 35 degrees Celsius in Illinois by late in the century.

For farms the picture is mixed. A longer frost-free season and more carbon dioxide can boost some crops, and Illinois growers now plant corn earlier than their grandparents did. But wetter springs delay planting and drown seedlings, hotter July nights reduce corn yields by making the plant burn energy instead of filling kernels, and heavy rains wash away soil and nitrogen. Pests and crop diseases that once died in winter now survive. On balance, most agricultural studies project that the gains fade and the losses grow as warming continues.

Lake Michigan tells a subtle story. Warmer winters mean less ice cover and more evaporation, yet heavier rains push levels up; the lake has swung between record lows in 2013 and near-record highs in 2020, eroding Chicago beaches and flooding lakefront paths. Rivers run higher in spring and lower in late summer. None of these changes ends life in Illinois. They add cost, risk and uncertainty to a place built around a climate that is no longer quite the one it was built for.

Mitigation means reducing the cause: cutting the greenhouse gases we add to the air. The first step is counting. Governments, companies and cities keep greenhouse gas inventories, ledgers that tally emissions by source. For the United States, the largest slices are transportation and electricity generation, each roughly a quarter to a third, then industry, then buildings and agriculture. Agriculture’s share comes mostly from methane, which cattle produce as they digest, and nitrous oxide, which soil bacteria release from fertilizer. Both are far stronger heat-trappers per molecule than carbon dioxide.

Different gases are compared using carbon dioxide equivalent: the amount of carbon dioxide that would trap the same heat over a hundred years. Methane counts about 28 times its weight; nitrous oxide about 265. This lets a farm, a factory or a country add up everything into one number and track it over time. Illinois’s inventory shows the state’s emissions have fallen since the mid-2000s, mainly because coal plants closed and were replaced by natural gas, wind and existing nuclear plants.

A carbon footprint is the same accounting applied to a person or a product. The average American’s footprint is among the highest in the world, roughly 15 metric tons of carbon dioxide equivalent per year, several times the global average. Where it comes from matters more than the total: for most households, the big items are driving, flying, home heating and electricity, and beef. Recycling and reusable bags are good habits, but they move the number very little compared with those four.

Mitigation options are ranked by cost per ton avoided. Some, such as efficiency improvements in buildings and switching from coal to wind, pay for themselves. Others, such as capturing carbon from the air, remain expensive. The world’s emissions are still rising, though more slowly, and the gap between what countries have pledged and what the physics requires is large. Carbon accounting does not close that gap. It shows exactly where it is.

Even if the world stopped emitting tomorrow, the carbon dioxide already in the air would keep the planet warm for centuries. Adaptation means adjusting to changes that mitigation cannot prevent. It is not giving up; it is the other half of the job. A city that cuts its emissions and also builds a bigger storm sewer is doing both.

Chicago’s Deep Tunnel is an adaptation project that began before anyone used the word. Since the 1970s the Metropolitan Water Reclamation District has bored more than 100 miles of tunnels hundreds of feet beneath the city, some 10 meters across. They hold stormwater until treatment plants can catch up. Giant reservoirs in old quarries hold billions of gallons more. The system was designed for the storms of the past century, and heavier rains now push it to its limits. So the city is adding green roofs, permeable pavement and rain gardens that soak water in where it falls.

Heat is the deadliest threat, and its adaptations are cheaper: trees. A shaded street can be 5 to 10 degrees Celsius cooler than a bare one on a hot afternoon. Mapping after the 1995 heat wave showed that deaths clustered in neighborhoods with few trees, more pavement and more isolated elderly people. Planting trees, painting roofs white, opening cooling centers and checking on neighbors are adaptation, and they save lives every summer. Farmers adapt too, with earlier planting, drought-tolerant seed, tile drains sized for bigger storms and cover crops that hold soil through a downpour.

Adaptation has limits. Sea walls protect a coast only up to the height they were built. A crop can tolerate only so much heat. And adaptation is unequal: wealthy places can afford tunnels and air conditioning; poor ones and poor countries often cannot, though they did least to cause the problem. Deciding who pays for adaptation, and how much warming to plan for, is a question science can inform but not answer alone.

Hazel Johnson lived in Altgeld Gardens, a public housing development on Chicago’s far South Side. She came to call the area a toxic doughnut: landfills, a sewage treatment plant, chemical factories and abandoned industrial sites on every side. Her husband died of lung cancer in 1969. She noticed how many neighbors had cancer, asthma and other illnesses, and she began going door to door with a survey. In 1979 she founded People for Community Recovery to fight for cleanup and for information about what was in the air, water and soil. She is often called the mother of the environmental justice movement.

Environmental justice is the principle that no group of people should bear a disproportionate share of environmental harms, and that everyone should have a voice in decisions that affect their environment. Study after study has found the same pattern Johnson saw: hazardous waste sites, highways, refineries and power plants are more likely to be located in low-income neighborhoods and in Black, Latino and Native communities. Sometimes the polluter came first and only cheap housing was left nearby. Often the neighborhood had the least political power to say no. Either way, residents breathe more particulate matter, live with more lead, and have higher asthma rates.

Flint, Michigan, showed how the pattern extends to water. In 2014 the city, under state-appointed emergency managers, switched its water source to the Flint River to save money and did not add the chemical that prevents pipes from corroding. Lead from old pipes leached into the tap water of a majority-Black city where much of the population was poor. Residents complained for more than a year before officials accepted the findings of outside scientists and a local pediatrician who showed that children’s blood lead levels had risen. Lead causes permanent harm to developing brains. Chicago, which has more lead water service lines than any other American city, is replacing them slowly.

Climate change adds a layer. Heat waves kill in the neighborhoods with the fewest trees. Floods hit the low-lying areas where housing is cheapest. In 1994 the federal government directed agencies to consider environmental justice in their decisions, and Illinois now requires extra review and community notice for polluting facilities in already burdened areas. Science can map the burden. Deciding that the map is unacceptable is a choice a society makes.

Suppose a state wants less sulfur dioxide from its power plants. The oldest tool is a regulation: a rule that says each smokestack may emit no more than a set amount, or must install a scrubber. Rules are clear and enforceable, and for something like lead in gasoline, where the goal is zero, they work best. Their weakness is that they treat every plant the same. A plant that could cut emissions cheaply has no reason to cut more than the rule demands, and a plant where cuts are very expensive must pay anyway.

A second tool is a tax: charge each ton of pollution. Now every plant has a reason to cut as much as it cheaply can, and the cheapest cuts happen first. Economists generally favor this because it gets the most reduction per dollar. Its weakness is that the total reduction is uncertain; you set the price and see what happens. Politically, taxes are hard to pass. A few places tax carbon; many more subsidize the alternatives instead, which is the tool the United States has mostly used for wind, solar and electric vehicles.

The third tool combines the two. In cap and trade, the government sets a total limit, the cap, and issues that many permits. Plants that cut cheaply can sell spare permits to plants that cannot. The total is guaranteed and the cheapest cuts still happen first. The 1990 Clean Air Act amendments used this for sulfur dioxide, and acid rain emissions fell faster and far more cheaply than industry had predicted. California and a group of northeastern states now run cap-and-trade markets for carbon dioxide. The risk is a cap set too loose, which makes permits nearly free and changes little.

Choosing a tool depends on the problem. Is the harm so severe that a hard limit is needed? Is the pollution easy to measure at the source? Are there many sources with very different costs? Are the public and lawmakers willing to see a price attached? Environmental science provides the first three answers; the fourth belongs to citizens, which is one reason to understand the tools well enough to argue about them.

When the EPA proposes a new limit on soot, it must estimate what the rule will cost: scrubbers, fuel switches and higher electric bills. It must also estimate what the rule will gain: fewer hospital visits, fewer missed work days and fewer early deaths. This is cost-benefit analysis. Its retrospective studies of the Clean Air Act have found the health benefits outweighing the costs many times over. But the method requires assigning dollar values to things like a year of life or a clear view across the Grand Canyon, and reasonable people disagree about those numbers.

The heart of the problem is the externality: a cost that falls on someone who was not part of the deal. A coal plant sells electricity to its customers; the asthma downwind falls on people who bought nothing. Because the polluter does not pay that cost, it produces more pollution than it would if it did. Most environmental policy is an attempt to make the external cost internal, whether by rule, tax or permit, so the price of a thing reflects what it truly costs.

Trade-offs run in every direction. Cheaper electricity from coal means more mercury in fish. A wind farm displaces coal but kills some birds and bats and changes a landscape people love. Banning a pesticide protects streams but may raise food prices or push farmers to a worse chemical. Requiring corrosion control in Flint would have cost the city a few hundred dollars a day; skipping it cost hundreds of millions and a generation’s health. Not every trade-off is close.

Two ideas help. Risk assessment asks how likely a harm is and how bad, so that a small chance of catastrophe is not weighed the same as a large chance of inconvenience. The precautionary principle says that when a harm could be severe and irreversible, lack of full certainty should not be a reason to delay action. Critics note that waiting for certainty is itself a choice with costs. The ozone case is the model: action came before the science was complete, the science firmed up, and the action was strengthened. Waiting would have been far more expensive.

Sustainability means meeting the needs of the present without using up the ability of future generations to meet theirs. That definition, from a 1987 United Nations report, sounds abstract until you apply it to a town. Where does its electricity come from, and will that source still be there in 50 years? Where does its water come from, and is the aquifer level falling? Where does its food come from, and is the soil holding? Where does its waste go? How do people get to work? A sustainable community is one where the honest answers to those questions do not point at a cliff.

The design tools are the ones this unit has built. Systems thinking maps the stocks and flows. Feedback loops show what will stabilize and what will run away. Life-cycle carbon accounting compares energy choices. Green infrastructure handles stormwater and heat. Walkable neighborhoods with transit cut the largest slice of a household’s carbon footprint without asking anyone to give up going places. Buildings that are insulated and oriented to the sun use a fraction of the energy of ones that are not. Composting and recycling close material loops the way decomposers close them in a prairie.

Examples exist at every scale. Chicago’s City Hall has grown a garden on its roof since 2000 to cool the building and soak up rain. A number of Illinois towns run on electricity from nearby wind farms. Communities from Copenhagen to Curitiba have built transit and bicycle networks that move most trips without cars. None of them is perfect, and each faced trade-offs: cost, space, the disruption of change, and the fact that people like the way things already are.

The last design principle is about who decides. Communities that planned with their residents, especially those in the most burdened neighborhoods, have generally built things that lasted. Environmental science supplies the evidence: the measurements, the mechanisms, the projections, the ranges. It cannot say how much a community should value a clean river against a lower tax bill, or the present against the future. Those are questions for citizens who understand the evidence well enough to argue from it. That, in the end, is why you learned this.