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Mitigation cuts the emissions that cause warming; adaptation reduces harm from the changes that come; both are measured and chosen by what they achieve for their cost.
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By the end of this lesson you will be able to tell mitigation from adaptation, and measure each with tons avoided, percent cuts, costs and design heights.
You know what causes warming, how it is measured, and why its impacts differ from region to region. You can compare risk-reduction options with numbers. This lesson sorts the responses to climate change into two kinds and shows how each is measured and chosen.
| Term | What it means |
|---|---|
| Mitigation | Reducing the greenhouse gas emissions that cause warming. |
| Adaptation | Reducing the harm from climate changes that are already happening or coming. |
| Maladaptation | A response that raises risk, for the one who acts or for others. |
| Offset | Paying for an emissions cut or removal somewhere else instead of cutting at the source. |
| Renewable energy | Energy from sources that are not used up, such as wind, sunlight and water. |
| Cost per ton | What a measure costs for each metric ton of carbon dioxide it avoids. |
Every climate response acts on the cause, the consequences, or both:
Measures are compared by what they achieve for their cost:
$$\text{cost per ton} = \dfrac{\text{cost}}{\text{tons avoided}}.$$
Another way: picture
Picture a bathtub overflowing onto the floor. Turning down the tap is mitigation; mopping the floor and moving the rug is adaptation. Mopping alone never ends; turning down the tap alone leaves today's puddle. Sensible people do both.
Another way: steps
A sustainability claim needs a time horizon, affected groups and a system boundary. A floodwall can reduce frequent flooding locally while encouraging more building behind it. More development increases the value exposed to a rare overtopping event, which can create pressure for an even higher wall. That is a reinforcing feedback if protection encourages exposure that in turn encourages further protection. Positive feedback means amplification, not a beneficial outcome.
A balancing response counters a change. If falling groundwater levels trigger enforceable limits on pumping and levels recover, the response opposes depletion. A new efficient pump is not automatically balancing: lower costs might encourage farmers to irrigate more land. Compare total withdrawals with recharge, not only water used per hectare. Track impacts outside the project boundary and across seasons.
For an invented aquifer, annual recharge is 40 units and extraction is 55. Storage falls by 15 if other flows balance. Cutting extraction to 35 permits a 5-unit gain under those assumptions. A plan also needs an allocation rule: a sustainable total does not prove equitable access. Ask whether households, farms and ecosystems retain water, how enforcement works and what happens in a dry year. Separate reducing greenhouse emissions from adapting to an impact, while checking whether one intervention helps or undermines the other.
Because greenhouse gases already in the air will keep warming the planet for decades, some change is coming whatever we do. Adaptation prepares for it. Because further emissions would add more warming, mitigation reduces them.
The two answer different questions. Mitigation asks how much warming there will be; adaptation asks how well places will cope with the warming that comes. Every region needs a plan for both.
Mitigation follows the sources of emissions. In electricity, it means replacing coal and gas with wind, solar, hydropower and nuclear. In transportation, it means efficient and electric vehicles, transit and less driving. In buildings, better insulation and heat pumps.
Industry and agriculture are harder: making steel and cement releases carbon dioxide from the process itself, and cattle and fertilizers release methane and nitrous oxide. Protecting and growing forests adds a land sink.
In 2005 coal made about half of the electricity in the United States. By 2023 it made about a sixth, according to the Energy Information Administration, replaced mostly by natural gas and by wind and solar, which together made about a seventh.
That shift is the main reason United States emissions from electricity have fallen since 2005. It shows that mitigation can move quickly when a cheaper clean option arrives, and that the same kilowatt-hour can come with very different emissions.
A cut is measured in tons avoided: the emissions before minus the emissions after. As a percent, it is divided by the emissions before. A household cutting from $5$ to $2$ tons a year avoids $3$ tons, a $60$ percent cut.
Dividing by the new emissions instead would give $150$ percent, which cannot be a cut. A percent cut always compares with the starting amount.
Many plans promise to cut emissions by a fixed percent each year. Each cut applies to what is left, so emissions of $100$ cut by $4$ percent a year become $100 \times 0.96^t$ after $t$ years.
This path falls fast at first and slowly later, and never reaches zero. That is why plans aiming at net zero need deeper changes over time, not only a steady yearly rate.
Comparing mitigation options by their cost per ton avoided shows where money cuts the most. A program costing $500$ thousand dollars that avoids $10000$ tons costs $50$ dollars per ton.
Some measures, such as insulation and efficient lighting, save money over time, so their cost per ton is below zero. Others, such as capturing carbon from factories, cost far more. Planners usually buy the cheapest tons first.
An offset pays for a cut or a removal somewhere else, such as planting trees, instead of cutting emissions at the source. Offsets can help, but they raise hard questions.
Would the trees have been planted anyway? Will they burn in a fire and release their carbon? A reduction at the source is certain; an offset is only as good as its answers to those questions, which is why careful plans count them separately.
Adaptation reduces the harm from changes that come. Coastal cities raise streets, build seawalls and restore marshes. Farms switch crops and planting dates. Cities open cooling centers and plant trees for shade. Water agencies store more water and reuse it.
Adaptation is local: it depends on each region's impacts, which the last lesson showed differ greatly. Its measure is the harm avoided, much like the risk reduction studied earlier in this course.
An adaptation built today must serve for decades, so it is designed for the climate expected over its life. A seawall's height adds the projected sea-level rise, the storm surge and a safety margin.
The projected rise depends on how much mitigation happens. More warming means a higher wall, at greater cost, which is one way mitigation and adaptation are linked.
Checking an answer. A percent cut lies between zero and one hundred. Emissions under a steady percent cut fall each year but never reach zero.
Dividing a cut by the starting emissions is allowed because a percent cut is defined as the share of the original removed. Using the new emissions would measure something else.
Writing a steady percent cut as a power is allowed because each year multiplies the remaining emissions by the same factor, and repeated multiplication by one number is exactly what a power means.
Some responses backfire. A seawall that protects one property can push erosion onto the neighboring beach. Air conditioners protect people from heat but, powered by fossil fuels, add emissions that worsen warming.
Building behind a levee can raise exposure, the levee effect studied earlier. Naming maladaptation as a category keeps plans honest about who bears the risk they move.
Some of the best responses help in two ways. Street trees shade sidewalks and store carbon. Restored marshes absorb storm surge and hold carbon in their soils. Efficient buildings stay cooler in heat waves and use less energy.
Planners look for these double wins because they count twice for the same money. They also tend to bring other benefits, such as cleaner air and cooler streets.
Adaptation has limits. A seawall can be raised a few feet, but not indefinitely; crops can be changed, but not every region can keep growing food as heat rises. The more warming, the harder and costlier adaptation becomes.
That is why the two are needed together. Mitigation keeps the changes within the range that adaptation can handle, and adaptation protects people from the changes mitigation can no longer prevent.
The most common slip is to call any climate action mitigation, when only actions that cut emissions are. Another is dividing a cut by the new emissions instead of the old.
A third is treating an offset as the same as a cut at the source. A fourth is thinking a steady percent cut reaches zero, or that adaptation makes mitigation unnecessary.
Miami Beach sits on a low barrier island, much of it only a few feet above high tide, and its porous limestone lets seawater rise up through the ground. As the sea has risen, some streets flooded on sunny days at the highest tides of the year.
Beginning in the 2010s the city spent hundreds of millions of dollars on adaptation: raising roads, installing pumps that push floodwater back into the bay, and requiring new buildings to sit higher. Flooding on some of the worst-hit streets has become much rarer.
The program also raises the questions this lesson names. Raised roads can drain water onto lower private property next to them, a risk of maladaptation, and the pumps discharge polluted street water into the bay. And the height the city builds to depends on how much the sea will rise, which depends on how much the world cuts emissions.
Georgetown, a city north of Austin, owns its electric utility. In the 2010s it signed long-term contracts to buy the electricity it needed from wind and solar farms, and became one of the first American cities to match its yearly electricity use with renewable power.
City leaders said the choice was mostly about price and certainty: wind and solar contracts fixed the cost for decades, while the price of gas could swing. The result was also mitigation, since the city's electricity came with far lower emissions than from the regional average.
The story has limits as well as lessons. Wind and sun do not always blow and shine when power is needed, so the city still relies on the regional grid at times, and in some years its contracts cost more than expected. Georgetown shows that mitigation decisions are made with numbers like cost per kilowatt-hour and tons avoided, weighed against risk, just like any other choice.
It is natural to think that if we can build seawalls, plant new crops and air-condition our homes, we can simply adapt to whatever warming comes. But adaptation has limits: a wall can be raised only so far, and some regions will become too hot or dry for the crops and ways of life they support now.
The more the climate warms, the harder and costlier every adaptation becomes. Mitigation keeps the change within the range adaptation can handle, and adaptation protects against the change that is already coming. They are partners, not alternatives.
A city buys electric buses. Classify the purchase.
$\text{mitigation}$
It cuts emissions.
It opens cooling centers for heat waves. Classify them.
$\text{adaptation}$
They reduce harm from heat.
It restores a marsh that absorbs surge and stores carbon. Classify it.
$\text{both}$
Two effects.
A developer walls off a beach, eroding the next one. Classify it.
$\text{maladaptation}$
Risk moved onto others.
A home's energy caused $9$ metric tons a year; after upgrades it causes $6$. Find the tons avoided.
$9 - 6 = 3$
Each year.
Find the percent cut.
$\dfrac{3}{9} \times 100 \approx 33\%$
Compared with the start.
Find the tons avoided in twenty years.
$20 \times 3 = 60$
Metric tons.
The upgrades cost $6000$ dollars. Find the cost per ton.
$\dfrac{6000}{60} = 100$
Dollars per ton.
Say what lowers the true cost.
$\text{lower energy bills}$
The upgrade also saves money.
Planners expect $2$ feet of sea-level rise. Record it.
$2$
Feet.
A major storm adds a $6$-foot surge. Find the still-water level.
$2 + 6 = 8$
Feet above today's high tide.
Add a $1.5$-foot safety margin.
$8 + 1.5 = 9.5$
The design height.
The present wall is $7$ feet. Find the extra height.
$9.5 - 7 = 2.5$
Feet to add.
Suppose stronger mitigation holds the rise to $1$ foot. Find the new design height.
$1 + 6 + 1.5 = 8.5$
A foot lower.
State the link between the two responses.
$\text{less warming, cheaper adaptation}$
Mitigation lowers the wall.
Convert the cost to dollars.
$300 \times 1000 = 300000$
Dollars.
Divide by the tons avoided.
$\dfrac{300000}{5000}$
Dollars per ton.
Evaluate the cost per ton.
Suppose a resort builds a seawall that pushes erosion onto the neighboring public beach. What kind of climate response is this?
Complete the worked solution: a city's seawall stands $8.5$ feet above today's high tide. Planners expect the sea to rise $1$ feet over the wall's life, a major storm surge of $7$ feet, and want a safety margin of $2$ feet. Find the still-water level in a storm, the design height, and how much higher the wall must be built.
Find the still-water level in a storm.
$\text{sea rise} + \text{surge} =$ a
Feet above today's high tide.
Add the safety margin.
$\text{still water} + \text{margin} =$ d
The design height.
Subtract the present wall.
$\text{design} - \text{present wall} =$ e
Extra height needed.
Name the kind of response.
$\text{adaptation, sized by how much warming comes}$
More warming means a higher wall.
Match each term to its meaning.
| reducing the emissions that cause warming | reducing harm from changes that are already coming | paying for an emissions cut somewhere else instead of at the source | a response that raises risk, for the actor or for others | |
|---|---|---|---|---|
| mitigation | ||||
| adaptation | ||||
| offset | ||||
| maladaptation |
A household's home energy caused $6$ metric tons of carbon dioxide a year. After insulating and installing a heat pump, it causes $1.5$ metric tons a year. Fill in the tons avoided each year, the percent cut, and the tons avoided over ten years.
| value | |
|---|---|
| tons avoided each year | |
| percent cut | |
| tons avoided over ten years |
A state emits $80$ million metric tons of greenhouse gases a year and plans to cut its emissions by $5$ percent every year. Write its yearly emissions, in millions of metric tons, as a function of the years $t$ from now.
Answer:
A program costs $90$ thousand dollars and avoids $3000$ metric tons of carbon dioxide. What does it cost per metric ton avoided, in dollars?
Answer: dollars per ton
Electricity from the U.S. grid causes about $0.4$ kilograms of carbon dioxide per kilowatt-hour. Suppose solar panels on a garage roof in Colorado make $9000$ kilowatt-hours a year that would otherwise come from the grid. About how many metric tons of carbon dioxide do they avoid each year?
Answer: metric tons
A fictional levee lowers annual flood likelihood, but 7 new homes are built behind it each year. The town claims permanent sustainability because smaller floods no longer enter. Mark all sentences that provide a supported review of the feedback, climate response and limits of that claim.
This task has no paper form; do it on a device.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A state emits $120$ million metric tons of greenhouse gases a year and plans to cut its emissions by $3$ percent every year. Write its yearly emissions, in millions of metric tons, as a function of the years $t$ from now.
Answer:
You can distinguish climate responses. Explain why a city that builds a seawall still has a reason to cut its emissions.
27. Your turn: a program costs $300$ thousand dollars and avoids $5000$ metric tons of carbon dioxide. What does it cost per ton?, step 3
$60$
Dollars.