Introduction

There are some obvious direct impacts of climate change. They 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 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 want you to think climate change isn’t happening. For example, some people who work for oil companies say climate change isn’t real. Or, they claim humans aren’t causing it. But here’s the truth: Nearly all scientists believe climate change is happening. And nearly all believe humans are causing it. What made scientists so sure? They studied a huge amount of climate data. This data suggests several things. One, greenhouse gases— such as carbon dioxide (CO₂)—are causing global temperatures to rise more rapidly than at any time in the last hundred years. Two, these greenhouse gases are being released because of human activity. There is also very clear evidence that the rise in average temperatures is causing major changes. The oceans are getting warmer and more acidic, ice sheets and glaciers are shrinking. Sea levels are rising. Major storms and other extreme weather events are becoming more and more common.

Humans have only had the tools to measure CO₂ levels for the last hundred years. However, today’s scientists have new ways to study the past. They are able to closely examine ice cores, tree rings, and more. This allows them to collect data that stretches back hundreds of thousands of years. That data shows our current rate of warming is not part of a natural cycle. The climate is changing quickly and in different ways than before. And these changes are driven by human activity. How can we be so sure? Let’s explore the connections between data on rising temperatures and data on levels 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. Take a look at Chart 1, on the next page. It measures the global monthly temperature anomaly since 1880. Temperature anomaly is a way of measuring the rise or fall in temperature compared to a baseline. That baseline is an average of temperatures over several decades. Chart 1 uses the average monthly temperature from 1951 to 1980 as its baseline. It shows 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 much warmer than it was in the 1800s. A 1-degree Celsius increase might not seem like much. However, even small changes in global average temperatures will cause tremendous problems. The rise in global temperatures is what’s driving all the other signs of climate change. These signs include ice melt, sea-level rise, and extreme weather.

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 common greenhouse gas released by human activity. The majority of it is released through the burning of fossil fuels. Coal, oil, and natural gas are all fossil fuels.

Chart 2 illustrates the change in the amount of CO₂ that has been released into the atmosphere since 1750. The Industrial Revolution began in 1750. After that, human societies started to increasingly burn 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, we used ever more fossil fuels to power our lives. As a result, CO₂ emissions increased enormously.

Earth is very old. It has been around for 4.5 billion years. And Chart 2 only shows the last 275 years. So how do we know this level of carbon in the atmosphere is actually a big deal? After all, average temperatures have gone up before during the 2 million years of human existence. There have been plenty of ice ages and warming periods. Yet, as Chart 3 shows, 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 the concentration of CO₂ in Earth’s atmosphere has reached levels never before seen. And during this period we burned fossil fuels far more than ever before. This strongly suggests that rising temperatures aren’t part of a 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 enormously because of 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. It shows both temperature anomaly and CO₂ levels over the last 800,000 years. What does it 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

From the four charts we’ve just examined, we can conclude a few things:

• Earth’s temperature is rising quickly. It is rising at a faster rate than anything seen over the last 150 years.
• There is more CO₂ in our atmosphere than any time in the last 800,000 years. Also, CO₂ levels have increased enormously over the last century. This suggests that human activity is responsible.
• There is a strong link between the level 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.