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 societal impacts of those changes, 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, causing 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.

How do we know climate change is happening? It’s a simple question, but there are a lot of conflicting viewpoints and people out there trying to convince you one way or the other. Some, such as people in the fossil fuel industry, might want to convince you that climate change isn’t real, or that human activity isn’t what’s causing it.

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 the huge amount of climate data we now have access to. 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. We also have overwhelming evidence that 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 as a result of these rising temperatures.

Humans have only had the tools to measure CO₂ levels for the last hundred years or less, but thanks to technologies that let us analyze ice cores, ocean sediment, 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 atmospheric concentrations of CO₂.

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, particularly in the last 50 years, global temperatures have increasingly risen above the baseline. 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 and significantly warmer than in the nineteenth century. A 1-degree Celsius increase might not seem like much, but as you’ll learn in the coming lessons, seemingly minor changes in global average temperatures will have disastrous impacts in many parts of the world. The rise in global temperatures is what is driving all the other indicators of climate change—including ice melt, sea-level rise, and extreme weather events.

Greenhouse gas emissions

Scientists believe that the rapid warming illustrated by Chart 1 is a result of emissions of greenhouse gases into Earth’s atmosphere by humans. Greenhouse gases such as carbon dioxide, methane, and nitrous oxide trap heat from the Sun inside Earth’s atmosphere. Carbon dioxide is—by far—the most abundant greenhouse gas that’s released by human activity, particularly the burning of fossil fuels. Chart 2 illustrates the change in the amount of CO₂ that has been emitted globally since 1750. Soon after 1750, with the start of the Industrial Revolution, human societies increasingly burned fossil fuels to power machines. Burning fossil fuels like coal, oil, and natural gas for energy releases large amounts of CO₂ into the atmosphere. Chart 2 indicates that, before the Industrial Revolution, CO₂ emissions were very low. From 1950 to the present, as the population of Earth tripled and we used ever more fossil fuels to power modern life, CO₂ emissions have skyrocketed.

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-ish years. So how do we know that this level of carbon in the atmosphere is actually a big deal? After all, there’s evidence that 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 rapid technological development that saw humanity embrace fossil fuels—the concentration of CO₂ in Earth’s atmosphere has reached unprecedented levels. This is a strong indication 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? One of the most important skills we can develop to become adept at reading data is the ability to compare two different sets of information, and then make correlations between them. By doing this, we gain a deeper understanding than when we look at only one set of data. 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 this chart (in particular, the point labeled “Today”) suggest about the future of climate change?

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 unprecedented in the last 150 years.
• There is more CO₂ in our atmosphere than any time in the last 800,000 years, and the dramatic increase in the last century suggests that human activity is responsible.
• Data from the last 800,000 years shows us that there is a strong correlation between the concentration of CO₂ in our atmosphere and rising global temperatures.

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