Climate change refers to long-term shifts in global temperatures and weather patterns, typically measured over decades or longer. It's important to distinguish between weather the short-term conditions outside today, like rain, sunshine, or a cold snap and climate, which is the average of weather patterns over 30 years or more. Just because a particular day is cold or a region experiences a severe snowstorm does not mean climate change isn't happening or that the Earth isn't warming overall; climate is defined by long-term trends, not single events
Natural vs. Human-Caused Change
- Climate can change naturally (volcanic eruptions, solar activity)
- Since the 1800s, human activities have been the MAIN driver of recent climate change
- The primary cause? Burning fossil fuels: coal, oil, and natural gas
Humans are responsible for global warming
Climate scientists have showed that humans are responsible for virtually all global warming over the last 200 years. Human activities like the ones mentioned above are causing greenhouse gases that are warming the world faster than at any time in at least the last two thousand years.
The average temperature of the Earth’s surface is now about 1.42°C warmer than it was in the late 1800s-prior to the industrial revolution-and warmer than at any time in the last 100,000 years. Scientists can gather information on the environment going back hundreds of thousands of years through ice cores. Ice core samples trap small amounts of air bubbles which allow scientists to gather information on the environment at that time, including the amount of carbon dioxide in the atmosphere.
The last decade (2015-2024) was the warmest on record, and each of the last four decades has been warmer than any previous decade since 1850.
How Ancient Ice Proves Climate Change Is Real (12:25)
Transcript: How Ancient Ice Proves Climate Change Is Real (12:25)
[Dr. Jeffrey Severinghaus] You can see the tiny air bubbles in there? Those are what we study. This is a piece of ice – about 20,000 years old – from Antarctica. And bubbles trap air from 20,000 years ago, so we can find out what air was like back then. Can figure out if carbon dioxide has gone up or down. And what we’ve learned from that is carbon dioxide is higher now than it’s been for at least the last million years, probably the last 20 million years, but that’s less certain. So it’s really quite a dramatic thing that we humans have done to the carbon dioxide.
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[Dr. Joe Hanson] Hey smart people. Joe here. Earth’s atmosphere and climate have changed in a big way, and they are continuing to change. There’s no doubt about that, and we’ve known it for decades. But Earth’s climate has always changed throughout its history. So how do we know this time is different? We know because at places like the Scripps Institution of Oceanography in southern California, we have freezers full of ancient ice that let us look into the past, thousands–even millions of years, and measure exactly what Earth’s atmosphere, and its climate, were like throughout deep history.
I recently stopped by to visit Dr. Jeffrey Severinghaus, who studies ice cores. He’s part of a team working to find the oldest ice on Earth. Each of these little blocks of frozen water can tell us something about our planet’s past, long before we existed – and where it’s heading, now that we do.
And inside these tiny bubbles in this ice, is old bubbles of air that existed on this planet as old as that ice is.
[Severinghaus] Yeah.
[Hanson] That’s the atmosphere of the planet, trapped in those little bubbles.
[Severinghaus] What happens in the polar regions is it’s too cold to melt. So when snow falls it doesn’t melt, it just piles up and piles up, and eventually turns into ice under its own weight. But if you think about what snow is like, if you have a snowflake you have air in between the snowflake. As snow becomes more and more dense, it tends to squeeze out the air between snowflakes, but it turns out it doesn’t squeeze out all the air.
[Hanson] As more layers of snow fall and condense, those tiny voids are literally frozen in time, layer upon layer. And, there are a lot of layers.
[Severinghaus] Some ice cores have annual layers just like trees do, you know how you can count tree rings? So some graduate student sits there and counts 50,000 annual layers. Of course it has to be a graduate student! What a lot of work.
[Hanson] But to study ancient ice, first you have to find ancient ice. Where are you doing this research? Where are you collecting these ice cores?
[Severinghaus] This is from a place called Taylor Glacier in Antarctica.
[Hanson] Taylor Glacier is a 54 kilometer stretch of ice and rock. People like Dr. Severinghaus can read it like a book–full of stories about our ancient climate. Taylor Glacier is special because it’s one of the few places on Earth where the ancient ice has risen to the surface.
[Severinghaus] So, you only have to drill 5-10 meters to get the ice. Which is much easier than drilling a deep ice core which is 3,000 meters and costs 50 million dollars.
[Hanson] It’s basically a cylinder that has little tiny teeth on the bottom. And when you rotate the barrel it carves out the ice, but only a little bit in a ring, and it leaves behind an ice core in the middle. Once the core is pulled up, it’s packed up and sent off, carrying a slice of history inside it.
[Severinghaus] It’s a slow process, it takes like a month for the ship to get here.
[Hanson] Whether you’re standing in the middle of the Amazon rainforest or at the North Pole, you’re breathing roughly the same air. Our atmosphere is pretty much the same everywhere. Which means that a tiny air bubble from that one spot is enough to paint a picture of what the entire planet’s atmosphere looked like so many years ago.
[Severinghaus] This is the freezer. We won’t be in there long, so don’t worry about the cold. So this is what a typical ice core sample looks like. Now you’ll notice that there’s no bubbles. That’s because when you get down below 600-700 meters, the pressure is so high that the air turns into something called a clathrate which is an ice-like substance.
[Hanson] Clathrates are crystals, where instead of bubbles, the molecules are trapped in a cage made by the bonds between frozen water molecules. There’s still gas in there.
[Severinghaus] There’s still gas molecules but they’re not in a gas phase.
[Hanson] Man the patterns are so cool, you must randomly see such cool ice phenomena. It’s cold in here! This cold!
[Severinghaus] Funny how that works.
[Hanson] Okay, but how do you get the ancient air out of the ice to measure it? I mean, without contaminating it with… all this air around us?
[Severinghaus] So this is how we actually extract the ancient air, if you will. We take a piece of ice and put it in a vacuum flask, and pump out all of the modern air, the air we’re breathing right now, using a vacuum line. This is a vacuum pump here. So we make a seal, and close this valve, and then you only have an ice cube and a little bit of water vapor, but no air. Then we melt the ice, and the melting of the ice releases those little air bubbles of ancient air.
[Hanson] So because you already let out the “now air,” the only gasses that are coming out are the ones that are trapped inside the ice.
[Severinghaus] Right, and then we can purify the gas a little bit by freezing the water.
[Hanson] So they pump out all the modern air, melt the ice to let the ancient atmosphere vaporize, re-freeze the water, and pump that ancient atmosphere out so it can be measured. This is a liquid helium tank?
[Severinghaus] It’s cold enough - it’s at 4 kelvin, 4 degrees about absolute zero. It’s cold enough that all the air actually condenses and turns into ice - air ice.
[Hanson] Every gas, will freeze.
[Severinghaus] Every gas except helium.
So then we take it over here. This is the analysis part of it. This tube is actually a bottle, a long skinny bottle that’s capable of dipping itself into the liquid helium.
[Hanson] You wouldn’t want to be getting your own hands too close to 4 kelvin.
[Severinghaus] No.
[Hanson] The frozen air gets put into this, a mass spectrometer, which basically measures the masses of really tiny things.
We measure the chemical composition of the atmosphere using isotopes: they’re like different flavors of atomic elements. Isotopes, those flavors of elements, have unique masses, and the mixture of them in the air bubbles can tell us all kinds of things about ancient earth.
[Severinghaus] We use the isotopes of nitrogen to tell ancient temperature at the time the snow was falling. Ordinary nitrogen has a mass of 14, but the rare isotope nitrogen 15 has a mass of 15. It turns out that relative proportions of N15 and N14 are sensitive to temperature.
[Hanson] So, whatever the temperature is at a particular time, it’s creating different mixes of different flavors of gasses in the atmosphere, like a fingerprint for temperature.
[Severinghaus] That’s right, and that’s trapped in air bubbles for posterity. So the sample here starts out waiting its turn and when its turn comes the sample opens and goes into this little tiny tube, which leads into the mass spectrometer, here, and it gets accelerated by a 3,000 volt electrical gradient, which makes the ions go really fast. And then they hit this magnet and they’re forced to make a 90-degree right turn, and in doing so, heavy things like N15 try to go straight, and lighter things like N14 get bent more.
[Hanson] It’s like being in a car. You can’t turn as fast in a big heavy car. So they swing out, and then the detector is seeing what swung out farther.
So, you’re getting resolution of things that differ by a single neutron when they’re flying through that curve? That’s pretty cool.
The same idea can be used to find out more than just temperature. Labs all over the world use elements trapped in air, trapped in ice cores, to paint a map from our distant past to today. Oxygen isotopes can tell us how oceans changed, mineral dust tells us about how the atmosphere moved around, there are chemical clues about early volcanoes. But maybe most importantly, we can trace changing levels of carbon dioxide.
So the climate has changed before, how do we know that this time it’s us?
[Severinghaus] The way we know, is just like we talked about with nitrogen, the carbon in carbon dioxide also has two flavors. There’s carbon 12, which is ordinary carbon, and then a very rare form of carbon, carbon 13. So, that’s how we know it’s human caused. The atmosphere, as it goes up in CO2 concentration, the carbon 13 of the atmosphere is taking a nosedive. And that’s not what would happen if it was natural CO2. Because fossil fuel CO2 is very depleted in carbon 13.
[Hanson] This comes from the fact that plants prefer to eat CO2 made of carbon-12, and when we burn fossil fuels made from those ancient plants, the fraction of carbon-12 in the atmosphere goes up while carbon-13 goes down. We’ve only been measuring carbon dioxide in the atmosphere since 1957, but using the data from ice cores, we can trace levels back way farther. And this is what we see:
[Severinghaus] CO2 was pretty flat for most of the past 1,000 years. All around 280 ppm. Now we’re going to add in the carbon 13 abundance, this gold line. And you can see that was also pretty constant for most of the last thousand years.
But then around 1850, right when carbon dioxide concentration started to rise, the carbon 13 abundance started taking a nosedive. And this kind of unambiguously tells you that humans did it. That’s why I call it the smoking gun of human causation. There are lots of other ways we know, but this is the simplest.
[Hanson] We’re moving into uncharted territory. The last time something like this shows up in the ice record is around 55 million years ago, when a volcano popped up under an oil field and cooked basically everything.
[Severinghaus] It sent all the carbon dioxide into the atmosphere. So, the carbon dioxide shot up, we think it nearly quadrupled, and the climate warmed by 6 degrees.
The most important thing is right away to solve this global warming problem. We don’t have much time left. We have to put aside all of our political differences, The health and wellbeing of the planet is so much more important than everything else. We can do this, I know we can.
[Hanson] We can. But will we? I hope so. Stay curious.
Many people think that climate change means warmer temperatures. But increasing temperatures are only the beginning of the story. Because the Earth is a system where everything is connected, changes in one area can influence changes in all others.
The consequences of climate change include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.
People are experiencing climate change in diverse ways
Climate change can affect our health, ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where entire communities have had to relocate, while protracted droughts are putting people at risk of famine. In the future, the number of people displaced by weather-related events is expected to rise.
Read more about climate change on the United Nations What is Climate Change page.
Crash Course What is Climate Change
What is Climate Change? (13:57)
Transcript: What is Climate Change? (13:57)
Our planet has been draped in ice sheets, filled with boiling-hot oceans, and dimmed by volcanic ash – all before anything more complicated than a single-celled organism showed up. So climate change is nothing new around here… when it happens gradually, over millions of years.
But something new has happened in the last few centuries. People like us…except, wearing hats like this… began burning fossil fuels like coal, oil, and natural gas to make energy. Most of us aren’t wearing hats like those anymore. But we’re still powering our daily lives and industries with those fuels, releasing billions of tons of carbon dioxide every year. And that’s caused Earth’s climate to change in the span of just a few human lifetimes, the geological blink of an eye.
Hi! I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology! Hey, do you guys smell that? …smells like... theme music?
[THEME MUSIC]
Now, I know what you’re thinking. “Wait, isn’t this Crash Course Biology?” And yeah, this episode is heavy on the gases, low on the golgi bodies. And that might leave you wondering, what does climate change have to do with the science of life?
Here’s the thing: life and climate are tied together, like that tangled pair of headphones at the bottom of your backpack. You know, like you tug one end and then a knot tightens, which is looped around a paperclip, somehow snagged on that tiny notebook where you drew hearts around your crush’s name, which after several tugs is now lying open on the floor and…I’m breaking into a cold sweat just thinking about it.
But back to the point, we can’t talk about life without talking about climate —which doesn’t mean last week’s thunderstorm or a one-day temperature swing – that’s weather. Climate is long-term weather conditions averaged over many years. To understand the difference between them, just remember that knowing the weather will help you decide if you should grab an umbrella before you head out, but knowing the climate will help you decide if you should invest in a good air conditioner.
While the weather might impact your choice of clothing for the day, the climate directly impacts where and when different kinds of life can survive.
The Greenhouse Effect
We owe today’s climate to the fact that our little green-and-blue marble of a home isn’t just floating in space unprotected. It’s wrapped up in an atmosphere; a big, invisible, gassy jacket. Which, granted, sounds pretty weird when you put it that way. But without it, we wouldn’t exist.
This jacket is made of different kinds of gases. And a small fraction of them, known as greenhouse gases, absorb solar energy like, super well. We’re talking about gases like methane, water vapor, and most importantly carbon dioxide – also known as CO2. They account for less than half of one percent of our atmospheric jacket. But they’re a part of what makes it so good at trapping heat, sort of like all the little white feathers in your puffy coat.
When sunlight beams down from space, most of it travels through those gases with no problem. The energy from that sunlight gets absorbed as it strikes the
Earth, warming the surface. That type of warming is normal and seasonal. The Earth naturally bounces some of that solar energy back toward space. Where some of it exits the atmosphere and heads right back out into the inky ether.
But the rest of that energy gets trapped by the Earth’s gassy jacket. Specifically by those super absorbent greenhouse gases, which suck up heat and bounce itback down to us again. This warming process is called the greenhouse effect. Unsurprisingly, It works the same way as a greenhouse – using layers of glass to trap heat inside. And it keeps Earth at a nice, cozy, insulated average of 14 degrees Celsius.
Without it, our Earth would be a chilly average of -18 degrees Celsius. Great for storing ice cream! Not so good for rainforests, swimsuits, or us for that matter. So, the greenhouse effect is a natural, helpful process that makes Earth habitable for all of life!
But you can have too much of a good thing. When our atmospheric jacket contains more carbon dioxide, for example, it gets really good at trapping heat. Like, too good. And the hotter things get, the more water evaporates and joins the atmosphere—and remember, water vapor itself is a greenhouse gas, so that in turn absorbs even more heat, creating a looping system of cause-and-effect that just keeps reinforcing itself.
Measuring the Greenhouse Effect
And while dressing in layers is great if you’re hiking in the Alps, it’s really hard for our planet to shed its extra coats. So all of that heat gets stuck going from the ground to the atmosphere like the worst game of hot potato ever played.
When I learned about this, I was like, "Wow, what a revelation! I can’t believe we’ve only recently figured this out!" But it turns out that this isn’t new knowledge; we’ve known how, and why, this could happen for nearly 170 years.
Let’s pay a visit to the Theater of Life… [Inquisitive music] Back in 1856, Eunice Foote, an American scientist and suffragette, was thinking about how the Sun’s warmth affected different gases. In those days, the scientific community was a bit like a fort with a handmade “no girls allowed” sign out front. But Foote wasn’t deterred, and ran her experiments anyway.
She filled tubes with different combinations of gases, including carbon dioxide. After putting some tubes in the Sun and some in the shade, she compared their temperatures, trying to find the hottest gas. All the tubes in direct sunlight warmed up. But none as intensely as the tube that contained carbon dioxide. The temperature had soared to 51.7 degrees Celsius, hot enough to burn your fingers.
From that insight, Foote theorized that if our atmosphere ever contained more carbon dioxide, the whole planet would warm up as a result, which would mean a lot more than a few scalded fingers. And sure enough, today, that’s exactly the situation we’re in.
Foote was one of the first scientists to recognize carbon dioxide’s potential to affect Earth’s climate. But she wasn’t the only one to connect the dots. For example, just a few decades later, another scientist named Dr. Svante Arrhenius observed that burning coal releases carbon dioxide. As a fossil fuel, coal forms from the decomposed, carbon-based bodies of plants and animals that lived and died a long time ago. And I’m talking before the dinosaurs. So no, I’m afraid that means you aren’t gassing up your car with the remains of a T-Rex.
When we burn those fossil fuels—whether it’s in the form of coal, oil, or natural gas—carbon dioxide gets released into the atmosphere. When Arrhenius ran the numbers, he predicted that carbon dioxide released by burning fossil fuels could warm our climate within a few thousand years if we kept burning them at their current rate. But we’ve far outpaced his estimates.
Our emissions of carbon dioxide have grown and grown and grown — and so has the mountain of evidence that those emissions are warming our planet. Some oil and gas companies have worked to promote uncertainty around the existence of climate change and what’s causing it. While they have only very recently acknowledged its existence, as of 2023, they're still trying to deflect from what's causing it. But the scientific consensus on this is overwhelming. You can learn much more about that in our Climate & Energy series.
We’ve only been reliably taking direct measurements of Earth’s global temperature since the 19th century. But that measly slice of time shows a steady rise in temperature of about 1.1 degree Celsius since 1880. And we know that that’s unusual because we’ve learned to read Earth’s much longer climate diary, in the form of ice cores.
See, when snow hardens into ice, tiny bubbles of air remain trapped in the gaps between snowflakes. These gases and water molecules stay frozen, like entries in a frosty journal. So by drilling deep down into polar ice, we can snoop on what the atmosphere was like hundreds of thousands of years ago.
Carbon Sinks
Ice cores show us that carbon dioxide levels have fluctuated over the past 800,000 years. And temperatures have fallen and risen alongside them, too. But when people started burning fossil fuels, carbon dioxide levels began to spike quickly, like really, really quickly. And they haven’t stopped rising.
Before the Industrial Revolution, for every million molecules of air in our atmosphere, around 280 were carbon dioxide molecules. But by 2022, that number had increased to 422 —the highest concentration of CO2 our planet has seen in 4 million years.
And that surge in carbon dioxide affects more than just the temperature. As the amount of carbon dioxide in our atmosphere rises, it impacts all of Earth’s systems. Just like yanking on the end of that tangled mess of headphones impacted all of the other items in my bag (and eventually, everybody around us when stuff started falling out). It’s all connected, is what I’m saying.
For example, the ocean is our planet’s largest carbon sink. It’s sort of like a big storage container for carbon. In fact, the ocean holds 50 times more carbon than the air or soil do. But a chemical reaction happens when carbon dioxide meets water, it creates an acid. So that influx of carbon has already turned the ocean 30% more acidic since the 19th century.
As carbon dioxide and other greenhouse gases trap more heat, there’s more energy pouring into our planet than going out. That means more energy is pumped into the ocean, fueling hurricanes and typhoons to become more frequent and more intense.
Environmental Justice
As the whole planet gets hotter, that triggers all kinds of changes. Spring arrives earlier, leading to shorter winters and longer summers. Rising temperatures lead to double-whammy droughts and heatwaves. And that leaves forest floors full of dried-up plants that serve as fuel for wildfires to ignite—spreading faster, farther, and more often.
Plus, Earth’s polar ice caps are melting, transforming solid ice to slush and seawater. And that’s causing ocean levels to rise and encroach on land. So while some communities are already facing a problem of not enough water, others are facing a problem of too much.
And because these adverse changes layer on top of existing social inequalities, they disproportionately affect lower-income communities and people of color, making climate change not only a scientific issue, but a matter of environmental justice that has spurred some researchers to political action.
For example, climate scientist Nicole Hernandez Hammer witnessed first-hand, through her field research, the sea-level rise alongside Miami Beach’s Latino communities. But these communities were not included in conversations about climate change or the dangers of rising sea levels. So, Hammer took action, moving into environmental outreach and education.
And, while the threat is still there, these communities are now in conversation about climate change, and able to plan and advocate for the future of their environment. And the good news more broadly is we know exactly why these tangled, complex effects are happening.
The more we burn those fossil fuels, the more greenhouse gases we release —and that’s driving sweeping changes all over our planet. And these changes are impacting life at all levels from the tiniest bacterium, to the elephants of Botswana, to you and me.
We can slow these impacts by breaking up with fossil fuels. But we also have to invest in nature’s carbon sinks —such as soils and forests— which pull carbon out of the atmosphere and back into the land, where it can’t keep heating things up.
Review & Credits
It won’t be the easiest breakup. We’ve designed whole societies and global systems around fossil fuels. We should fully expect to be listening to Jazmine Sullivan on repeat with a gallon of Rocky Road. But this is one relationship we can't afford to stay in.
So, in order to halt emissions, we have to both invent new systems and work more efficiently within old ones, and to do that, we’ve got to get a whole planet’s worth of people on board. And if you’ve ever been part of a group project, you know that last part isn’t easy.
Thankfully, when faced with global crises, humans have one great thing going for us: we are creative. I mean, we’ve been to space, we’ve got electric cars, we made furbies for some reason. So yes, It’s going to take all of our creativity and cooperation to tackle. But the only way out is through, and the only way through is together.
In our next episode, we’ll tune back into the world of living things— and see how our rapidly changing climate involves much more than the atmosphere. It affects every living, breathing, organism on this planet— including you and me.
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.
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 nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.
Climate Change Solutions
Since climate change is a worldwide problem that requires international cooperation. It will affect everyone, although the impacts of climate change are more readily seen in island populations as sea levels rise.
Paris Agreement is an agreement signed in 2015 at the UN Climate Change Conference (COP 21) in Paris. This agreement set long term goals to guide all nations to
- Reduce Global Greenhouse Emissions to hold global temperatures to well below 2°C above pre-industrial levels.
- Periodically assess the collective progress towards meeting long term goals
- Provide financing to developing countries to mitigate climate change, strengthen resiliency and adapt to climate impacts
What is the Paris Agreement (1:39)
Transcript: What is the Paris Agreement (1:39)
[Presenter] What is the Paris Agreement?
The Paris Agreement is a legally binding international treaty on climate change, to limit global warming to well below 2, preferably to 1.5 degrees Celsius compared to pre-industrial levels. This requires economic and social transformation to face the climate challenges now and moving into the future, based on the best available science.
The Paris Agreement works on a 5 year cycle of increasingly ambitious climate action. By 2020, countries communicate their plans, known as "nationally determined contributions". Countries communicate actions they will take to reduce the greenhouse gas emissions in order to reach the goals of the Paris Agreement. Countries also communicate actions they will take to build resilience to adapt to the impact of rising temperatures. This may include information on adaptation and finance flows.
The Paris Agreement also provides a framework for financial, technical, and capacity-building support to those countries who need it. Starting in 2024, Countries report transparently on actions taken. Collective progress under the Paris Agreement will be assessed through a global stocktake. This will lead to recommendations for countries to set more ambitious plans in the next round.