Introduction

The direct impacts of climate change include global warming, rise in sea levels, extreme weather, forest fires, drought, and flooding. Later, you’ll learn about the impacts of those changes on society, which we call indirect impacts. But for now, let’s focus on one direct impact: sea-level rise.

Greenhouse gas emissions are trapping heat in Earth’s atmosphere. This causes a general rise in global average temperatures. As global temperatures increase, water that was once stored in polar ice sheets and glaciers melts and enters the oceans. As water gets warmer, it also expands. So, when temperatures in the ocean go up, so do sea levels.

The world is experiencing long-term heating of the climate due to human activities. This is known as climate change.

Some people out there want to convince you that climate change isn’t happening. For example, some people who work for oil companies say climate change isn’t real. Or, they claim human activity isn’t what’s causing it. But here’s the truth: The vast majority of scientists claim that climate change is happening, and that humans are causing it. Scientists have come to this conclusion by examining a huge amount of climate data. This data suggests that human emissions of greenhouse gases—such as carbon dioxide (CO₂)—are causing global temperatures to rise more rapidly than at any point in the last century. There is also very clear evidence that these rising temperatures are causing major changes: The oceans are getting warmer and more acidic, ice sheets and glaciers are shrinking, sea levels are rising, and that extreme weather events are increasing in frequency.

Humans have only had the tools to measure CO₂ levels for the last hundred years. However, thanks to technologies that let us study ice cores, tree rings, and more, scientists have been able to collect data that stretches back hundreds of thousands of years. This data tells us that our current rate of warming is not part of a natural cycle. The climate is changing rapidly and in ways unlike anything that’s happened in hundreds of thousands of years. Climate change is happening, and it’s driven by human activity. How can we say this with such certainty? Let’s explore the connections between data on rising temperatures and data on concentrations of CO₂ in Earth’s atmosphere.

Is it hot in here, or is it just me?

It’s not just you! Earth is heating up at an alarming rate. Chart 1, on the next page, measures the global monthly temperature anomaly since 1880. Temperature anomaly is a way of measuring the increase or decrease in temperature from a baseline average. This chart uses the average monthly temperature from 1951 to 1980 as its baseline. You can see from this chart that global temperatures have increasingly risen above the baseline. This has been particularly true over the last 50 years. In recent years, temperatures have peaked above 1 degree Celsius over the baseline. Looking back to 1880, you can see that temperatures were often below the baseline. The world is warmer than it was just 50 years ago. It is significantly warmer than in the nineteenth century. A 1-degree Celsius increase might not seem like much. However, even apparently minor changes in global average temperatures will cause tremendous problems in many parts of the world. The rise in global temperatures is what is driving all the other signs of climate change—including ice melt, sea-level rise, and extreme weather events.

Greenhouse gas emissions

Scientists believe the rapid warming illustrated by Chart 1 is a result of the release of greenhouse gases into Earth’s atmosphere. And they believe those gases are being released by humans. Greenhouse gases include carbon dioxide, methane, and nitrous oxide. They all trap heat from the Sun inside Earth’s atmosphere. This causes temperatures to rise over time. Carbon dioxide is—by far—the most abundant greenhouse gas that’s released by human activity. The majority of it is released through the burning of fossil fuels like coal, oil, and natural gas. Chart 2 illustrates the change in the amount of CO₂ that has been released into the atmosphere since 1750. Soon after 1750, with the start of the Industrial Revolution, human societies increasingly burned fossil fuels to power machines. Burning fossil fuels releases large amounts of CO₂ into the atmosphere. Chart 2 shows that, before the Industrial Revolution, CO₂ emissions were very low. From 1950 to the present, the population of Earth tripled. During that time, as we used ever more fossil fuels to power modern life, CO₂ emissions increased enormously.

Earth has been around for a long time. A really long time. Four-and-a-half billion years long. And Chart 2 only shows the last 275 or so years. So how do we know that this level of carbon in the atmosphere is actually a big deal? After all, there have been plenty of ice ages and warming periods during the 2 million years of human existence. Yet, as Chart 3 shows us, we have data on CO₂ levels from ice core samples—data that goes back 800,000 years. In this chart, you can see that in the past century—a century of heavy fossil fuel use—the concentration of CO₂ in Earth’s atmosphere has reached levels never before seen. This strongly suggests that humans are transforming our atmosphere far beyond any natural cycle.

Combining data

We know from Chart 1 that global temperatures have been rising in the last century. We also know from Charts 2 and 3 that CO₂ emissions have risen dramatically thanks to human industrialization and the burning of fossil fuels. How are the two trends related, and what does it mean for the future of climate change? Take a look at Chart 4, below. This chart shows both temperature anomaly and CO₂ atmospheric concentration over the last 800,000 years. What does this chart suggest about the relationship between global warming and CO₂ in Earth’s atmosphere? Finally, what does it suggest about the future of climate change? Look, in particular, at the point labeled “Today.”

Conclusion

So, how do we know climate change is real? And how do we know it’s caused by human activity? From the four charts we’ve just examined, there are a few conclusions we can draw:

• Earth’s temperature is rising at a rate that is faster than anything seen over the last 150 years.
• There is more CO₂ in our atmosphere than any time in the last 800,000 years. Furthermore, the dramatic increase over the last century suggests that human activity is responsible.
• There is a strong correlation between the concentration of CO₂ in our atmosphere and rising global temperatures. This is shown by data from the last 800,000 years.

In short, climate change is real. It’s happening. And humanity’s use of fossil fuels is to blame.