High School Earth Science—Semester B

Sample Lesson - Introduction

Lesson 14.03: Climate Feedback

Think this feedback is harsh? Wait to see what Earth can do.
(Source)

When we hear the term "feedback," most of us probably think about the feedback we get from teachers when we have to write essays or do other big projects. So in other words, "feedback" means critiques and criticisms, oftentimes harsh criticisms.

When it comes to climate, Earth has its own way of providing feedback. No, it doesn't involve a red pen, and no Earth doesn't shout things like "lack of evidence," "comma splice," and "where's the negative?" That doesn't mean Earth's feedback isn't harsh, though.

In fact, we'll learn in this lesson that positive feedback is a big reason Earth's climate changes as much as it does between glacial and interglacial periods. We could almost say that Earth's feedback makes the climate over react. We can all relate to that, though, right?

After all who hasn't gotten frightened by all those red marks on a paper and overcompensated on the next paper?

Sample Lesson - Reading

Reading 14.14.03: How About Some Positive Feedback

In the previous lesson, we learned about how Earth has gone back and forth between glacial and interglacial periods about every 100,000 years, in what scientists call Milankovitch Cycles.

As we learned, these cycles are triggered by the eccentricity of Earth's orbit, the wobble in Earth's rotation, and the cycle of equinoxes. Notice that we said "triggered" here. It turns out that there are other things happening on Earth that factor into Earth's climate, too.

After all, it's not like Earth is just a blank slate for the Sun's radiation to play with. Earth has a whole slew of systems already in place that help regulate our climate.

In general, our atmosphere, oceans, and even our magnetic field help maintain our climate at a cozy temperature. In particular, the greenhouse effect, or how gasses in our atmosphere trap infrared heat, keeps Earth from getting too cold and undergoing massive temperature fluctuations.

However, there are several of Earth's systems that create positive feedback when it comes to climate change, meaning that they amplify the effect of Milankovitch Cycles. The three main positive feedbacks we'll take a look at are 1) atmospheric water vapor, 2) the ice-albedo effect, and 3) greenhouse gasses.

Without these three feedbacks, all the stuff going on with our orbit around the sun would result in a drastically different climate scenario for all of us here on Earth.

It's Not the Heat, it's the Humidity

About 70% of Earth is covered with water, so it's probably not surprising that the water cycle plays a huge role in our climate.

As Earth goes through a Milkanovitch Cycle and starts to warm up, more water evaporates into the atmosphere. While we normally think of water vapor as pretty harmless, it turns out that it's actually a greenhouse gas.

That means water vapor traps heat in our atmosphere, which in turn causes more water to evaporate from our oceans, which then means more heat is trapped in our atmosphere, which in turn means…well, we can all see where this is going.

This positive feedback loop amplifies the warming effect as Earth enters interglacial periods. The opposite is also true as Earth goes into glacial periods. As the Earth starts to cool as a natural result of the Milkanovitch Cycle, less water is evaporated, meaning less heat is trapped in the atmosphere.

Whichever way we're going—heating or cooling—water vapor in the atmosphere is a positive feedback. In fact, scientists believe that water vapor is the biggest positive feedback affecting Earth's natural climate change.

Grab them Sunglasses

Ever been skiing or snowboarding on a sunny day? It turns out that snow and ice are pretty glary. This doesn't only result in gnarly sunburns for unwary skiers. It also has a profound effect on our climate.

Whereas most surfaces on Earth absorb sunlight and heat the planet up, Earth's polar icecaps and glaciers act like a mirror. They reflect sunlight right back out into space, which prevents Earth from absorbing that energy. This is called the ice-albedo effect.

When Earth has colder winters, the polar ice caps and other glaciers grow larger, which means the Earth reflects more sunlight back out into space, resulting in an even colder climate. That, in turn, leads to larger ice caps, which leads to more sunlight reflected into space.

Once again, we have positive feedback.

On the flip side, ocean waters are dark and absorb a good amount of the energy from sunlight. That means when the climate warms, not only is there less snow and ice to reflect sunlight back into space, there's water there to suck all that sunlight up.

So just like with water vapor in the atmosphere, the ice-albedo effect creates positive feedback both when the climate is cooling and warming.

A Warm Earth is a Gassy Earth

Ready to not be shocked? Greenhouse gasses in the atmosphere are another form of positive feedback, both when the climate is warming and when it is cooling. In that respect, it's exactly the same as water vapor and the ice-albedo effect. The mechanism is a bit more complex, though.

The general trend is that warm climates release greenhouse gasses from carbon sinks, which in turn amplifies the greenhouse effect in the atmosphere. This, of course, leads to increased warming.

On the flipside, cold climates trap greenhouse gasses in carbon sinks, which reduces the greenhouse effect. This leads to increased cooling.

To understand why a warming Earth releases more greenhouse gasses, we need to take a gander at a couple of Earth's carbon sinks.

The first is Earth's oceans. When water is colder, it can dissolve more CO₂. When water is warmer, more CO₂ gets released into the atmosphere. It's the same principle that makes warm sodas more fizzy than cold sodas. The only difference is that Earth's oceans are a soda pop that cover 70% of Earth's surface.

The second carbon sink is the surface of land, particularly land that is permafrost, or "permanently" frozen during cold periods. When permafrost thaws out, all the dead plants and other organic materials that were trapped in the ice begin to decay. This releases both carbon dioxide and methane.

Similarly, glaciers and permafrost can cover other natural pockets of methane that suddenly get released when that ice melts.

Whichever of these carbon sinks we're looking at, the result is the same: greenhouse gasses are released when the climate warms, and trapped when the climate cools.

The Carbon Heartbeat

While Milankovitch Cycles are the driving force behind glacial and interglacial periods, the three positive feedback loops we just discussed causes Earth's climate to shift rather quickly.

(Source)

As the lower part of graph above shows, the changes between glacial and interglacial periods are abrupt. If it wasn't for the climate feedback loops, we'd probably expect to see the fluctuations be gradual and less severe.

The other important fact to take note of in the graph above is how accurately atmospheric carbon (the top curve) correlates with global temperature (the bottom curve). The two are essentially mirror images of each other, going up and down like a heartbeat in a regular pattern.

Understanding this sort of data about Earth's climate history allows us to analyze our current climate and make predictions about the future. To that end, we need to take note of one last important part of the graph above.

Looking at carbon dioxide levels, we can see that it has had a natural fluctuation between 180 and 300 parts per million for the last 800,000 years. This is the natural fluctuation of carbon dioxide caused by Milankovitch Cycles and Earth's positive feedback loops.

According to NASA, the concentration of carbon dioxide in our atmosphere reached 400 parts per million in 2013 and has continued to rise steadily, ever since. This abnormal increase is caused by human activity and is higher than it's been in the entirety of human history. No wonder scientists are alarmed.

Recap

In this reading, we learned that it's not all about the Sun and Earth's orbit when it comes Earth's climate over the last million or so years. Several positive feedback loops come into play, which make sudden climate change…well, much more sudden, and probably more severe, too.

The positive feedback mechanisms include

• Atmospheric water vapor.
• The ice-albedo effect.
• Greenhouse gasses.

We also learned that CO₂ has fluctuated between 180 and 300 parts per million for the last 800,000 years, but is currently above 400 parts per million.

Sample Lesson - Activity

Activity 14.03a: A Comic Tragedy

We read a whole lot about feedback in this lesson. Well, here's some feedback that students have been hollering at teachers for decades: "Diagrams and pictures are worth a thousand words."

Shmoop agrees. That's why we're asking you to draw a comic strip for this activity. Your goal is to create a comic strip that shows the before-and-after result of the three positive feedback mechanism that help climate change happen.

You don't need to show a full warming and cooling cycle; simply showing how the feedback mechanisms either help the world heat up or how they help the world cool down will do the trick. You're welcome to be as creative and artsy as you want to be with your comic strip. Just make sure it includes the following elements:

• At least one panel that shows the initial trigger of climate change: Milankovitch Cycles.
• At least one before-and-after panel for each positive feedback mechanism, although you might find it helpful to have more than two panels for each of them.
• Include captions, dialogue, or labels to help explain what's going on in each panel.
• Give your comic a title.

So to recap, your comic should include a minimum of 7 panels and clearly show how all three feedback mechanisms work. When you're all done, scan or take a pic of your comic and upload it below.

Sample Lesson - Activity