Thanks to a centuries-long love affair with fossil fuels, we’ve released billions of tons of heat-trapping carbon dioxide into our atmosphere.

And that’s shaking up all of Earth’s systems – including the living ones. Our planet is heating up at a truly alarming rate.

And as droughts and wildfires become more frequent, forests become extra vulnerable to damage from insects whose populations are  now all out of whack. Streams are heating up— disrupting the trout that live there. And rising temperatures push animals into unfamiliar territory— like thousands of walruses meeting up on Alaskan beaches, or armadillos strolling the Indiana streets.

Because so much of life is interconnected, a change in one organism's environment, like a rock dropped in a pond, can cause a ripple effect that reaches hundreds, if not thousands of others – including us. And trust me, climate change is the biggest rock you have ever seen. Hi!

I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Now, climate change may be the biggest  metaphorical rock you’ve ever seen but this is literally the rockin-est theme music you will hear all day. [THEME MUSIC] Ever since we started burning fossil fuels  to power our homes, cars, and smart toilets, we’ve been altering Earth’s natural climate.

And a changing climate disrupts the entire chain of life, from tiny cells to whole ecosystems. Some changes may seem very small, but even microscopic changes can have a huge impact. Like, there are these tiny cells in a pine tree that make a sticky ooze called resin which traps and kills mountain pine beetles that want to eat the tree.

But, when the tree is stressed by heatwaves and droughts, its cells don’t have the water that they  need to make enough of that precious resin. That means entire forests transform into  all-you-can-eat buffets for these beetles. Or, consider the American pika— a little puffball that you might mistake for a living Pokemon.

They’re high in cuteness, but not so much in heat tolerance. As the summers get hotter, pika struggle to regulate their body temperatures. They are forced to move higher and higher up the mountain slopes where they live, and eventually run out of room at the top.

Now, I know what you’re thinking, [Cartoonishly Evil Sammy] “I dare say, Dr. Sammy, if climate change’s effects end there, with the pika and the pine tree, maybe that’s manageable. Sad…—one moment,  I must blot my tears— sad but manageable, I say.” [record scratch] [Dr.

Sammy] I hear you, cartoonishly evil alternate universe me, but the problem is, they don’t end there. As one population changes, so does another, and another. And even though the ripples may start with one species, climate change’s effects  can turn them into waves that disrupt an entire ecosystem.

Let’s head over to the Thought Bubble… A kelp forest is much more than a bunch of seaweed. It’s also a carbon storage container, an ocean oxygen-booster, and a defender against coastal erosion. And for a bustling community of hundreds of species, it’s a place to call home.

Basically, kelp forests  are the hot neighborhood. But as the oceans warm, that becomes true…in a more literal way. See, many bacteria love the heat provided by a warming climate, so a lot of them are doing great.

But as their population grows, they make sea stars sick, taking up their oxygen supply. With less oxygen, their muscles waste away, and eventually, they die. Sea stars are a natural predator of the sea urchin— the spiky purple stress balls of the kelp forest.

So a decrease in sea stars means an increase in sea urchins. As kelp falls to the ocean floor, sea urchins gobble it up. Which is an important job— but without enough  sea stars to rein them in, urchins overdo it.

They chomp the kelp forest to smithereens. And that affects the whole neighborhood— the whales who use the kelp forest like a daycare, the fish that sleep there, the birds that treat its canopy like a charcuterie board. These creatures have few choices: pack it up and move on —or simply die out.

And long after everything else is gone, the urchins remain— lying in wait, devouring new kelp sprouts before they even have the chance to grow. So by affecting just one population, climate change can have a profound effect on a whole community —and even turn it into a zombie town. So much for “a beautiful day in the neighborhood.” Thanks, Thought Bubble!

There’s no single story that captures how Earth’s organisms and ecosystems will respond as the climate changes. Plants and animals alike will face shifts in water availability, temperatures, extreme weather,  exposure to disease, and more. Depending on how well a species tolerates those changes, their story might be a tale of resilience—or tragedy.

Three big climate-caused stories are already playing out in many ecosystems. First, climate can cause populations to vary wildly depending on the species. Some species do better in a hotter, drier climate – and they multiply, like the mountain pine beetles we met earlier, living large on stressed-out trees.

But other populations, like the pika, struggle in a warmer climate and are dwindling. Second, many species pack up and move as the climate changes. That can mean whole communities  splitting up or reorganizing.

For example, herring, one of the puffin’s favorite fish, are moving north as oceans get warmer. Puffin parents are substituting a larger  species of fish as dinner for their chicks, but they're often too big for the chicks to eat. Third, climate change is shifting many species’ phenology, the seasonal timing of their life cycle.

For a lot of organisms, their reproduction, hibernation, or feeding is dictated by the seasons. Which is way more complicated than the ferocious hankering for sweet potato pie that I get every fall. When summers lengthen, the whole dance changes.

Species that depend on each other end up with different rhythms—or they miss their dance partners entirely. Like, an early spring cues Arctic plants to sprout sooner. But the caribou who count on those plants are still migrating by the old rhythms.

So when they arrive, expecting their favorite food, dinner is already over. And there aren’t enough leftovers to go around. Dr.

Tanisha Williams, a plant ecologist and botanist, found a similar thing was happening with a plant called Pelargonium— which you might know as the common geranium in your Grandma’s flower pot. Over the past century, South Africa’s temperature has increased by almost 3 degrees Celsius —more than twice the global average. By comparing today’s plants to historical records, Williams found that South African Pelargoni are now blooming prematurely compared to their early 1900s counterparts —by almost 2 weeks.

That might not sound like a lot. But if the bees can’t sync up their pollination schedules with the flowers, that could threaten both species. And it’s not just climate change  itself that is making waves —but also the billions of tons of carbon dioxide in our atmosphere.

Plants use carbon dioxide to fuel growth. So you might imagine that  they’re filthy-rich with the stuff. But that doesn’t easily translate into riches for plants.

Plants are complicated systems. So turning up the carbon dioxide dial doesn’t necessarily improve access to any of the other stuff they need, like nutrients and water. Many plants also thicken their leaves at higher carbon dioxide levels —as if the atmosphere is muscle milk and every day is leaf day.

But thicker leaves are actually less efficient at pulling carbon dioxide out of the atmosphere —so more of it stays trapped there, where it contributes to global climate change. Nonliving systems like oceans get messed up, too. High carbon dioxide levels change its  chemistry, causing it to be more acidic.

That acidity makes it harder for organisms like coral and clams to build their calcium-rich shells and structures. It even messes with the sharks’ ability to hunt by scent. And while carbon dioxide can sometimes increase plant growth, it won’t necessarily be the plants we want.

Weeds will grow even faster, competing with the crops people depend on for food. Those crops will also be facing other stresses, like less water and drier soil, as temperatures rise. And many crops grown at high carbon dioxide  levels are actually less nutritious, losing crucial vitamins, minerals, and  proteins that make them good eating.

So these ripples will affect any creatures that eat plants —including us. And this nutrient loss could cause millions of people to face protein, iron, and zinc deficiencies by 2050. So, when we talk about the effects of increased CO2 and climate change on living things —and even non-living things— we’re not just talking about communities of urchins and kelp.

We’re talking about communities of people—like you and me. But also, like, your Grandma, and all your Grandma’s bingo buddies, and the cashier at the store who asks you about your day. These aren’t far off, hypothetical situations— the impacts are being felt right now.

Even when you don’t see the original stone drop, by tipping the balance of Earth’s living and nonliving systems, climate change sends out waves that affect all of those people— risking their food supply, their housing, and their health. And while climate change does involve everybody, its worst effects aren’t shared by everyone evenly. When heatwaves, droughts, wildfires, or floods strike, people with the fewest resources also have fewer options for how to respond.

Low-income communities and people who consistently face inequities will be most affected by climate change. They’re more vulnerable when their homes flood, their crops fail, or temperatures soar. A lot of these people are concentrated in communities where the effects of climate change are especially intense.

Each community has different needs, so there’s no one-size-fits-all approach for the challenges ahead. But reducing poverty and investing in infrastructure can help the most vulnerable communities adapt. That includes supporting and  conserving ecosystems— like coastal mangroves that defend against floods— to protect those communities and pull carbon out of the atmosphere.

But really, there’s only one surefire way to limit climate change’s effects: reducing emissions from carbon dioxide and other greenhouse gases. There are some cool ways that people are working to do just that, you can learn more about them in  Crash Course's Climate & Energy series. But it’s not just a matter of gases way up there; it’s also about justice down here.

The effects experienced in any community won’t happen in isolation. That’s true for all living things— whether they’re a person, a puffin, or a pine tree, or, yes, a platypus. Sometimes it can be hard to see the connections we all share— you, me, and our billions of neighbors, on this little blue-and-green rock.

But climate change makes those connections impossible to ignore. In the span of a few human lifetimes, we and our fossil fuels have altered Earth’s climate at a rate that has never happened before. What began as a ripple is now a tidal wave, rushing outward to affect virtually every species and every ecosystem on Earth, including us.

There is a lot happening, and no simple, single story that can capture it all. But, by working together, we can shape what happens next. We’ll see more of that in our next episode, as we learn about how life’s dazzling diversity is under threat, and what we can do to change course.

This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course. [Cartoonishly Evil Sammy] Thanks for watching this episode of Crash Course Biology, which was filmed at our  studio in Indianapolis, Indiana, and was made with the help of all these dapper people. If you want to help keep Crash Course free for everyone everywhere you can give us your money on the Patreon.

Yes yes, good show old bean, good show.