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Evidence of Human-Induced Climate Change

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

What is climate change, and why is it happening? Learn about the science behind global warming, and the evidence that humans are the primary cause of climate change.

What is Climate Change?

Climate is average weather over a long period of time and a large area. This includes temperature, precipitation (rain and snow), wind patterns, storms, and severe weather. Climate change, therefore, refers to a change in the Earth's temperature and weather patterns over time. While you might hear people deny it, the evidence for climate change is airtight. The world is getting warmer. This process has been traditionally been called global warming. And this global warming has impacts on the climate as a whole.

As the world gets warmer, there is greater evaporation, which means more water leaves dry areas and follows wind currents to eventually rain upon wet areas. Dry places get drier, which causes drought. And wet places get wetter, which can sometimes cause flooding. Global warming can even cause more snow. This might seem odd -- surely a warmer planet would mean less snow? But it's actually pretty simple. If you live in an area that gets wetter, but the temperature doesn't rise enough to go above freezing, you will get warmer weather and more snow at the same time.

The evidence for climate change is incredibly strong. The evidence also tells us that humans are the primary cause. However, due to political and economic forces, many people have been persuaded that global warming isn't really happening, or that it's not as serious as some say it is. In this lesson, we're going to take a look at the evidence that shows that humans are primarily responsible for the current climate change trend that we're seeing.

Greenhouse Gases

You've probably heard about something called greenhouse gases. Global warming is all about greenhouse gases. These include carbon dioxide, methane, and water vapor, and they act like a blanket on the planet. This happens because they don't absorb incoming radiation (light waves) from the Sun, but do absorb outgoing radiation from the Earth's surface.

According to the laws of physics, every gas that exists will absorb radiation of certain, particular wavelengths. Different gases absorb different wavelengths. The wavelengths of light we get from the Sun are not absorbed by greenhouses gases, so they make it all the way to the Earth's surface.

The greenhouse effect
The greenhouse effect

However, all objects produce infrared radiation (including humans and the Earth's surface itself). This radiation heads out towards space, but gets absorbed by the greenhouses gases before it can escape. This keeps the Earth warmer.

The greenhouse effect is wonderful. Without it the earth would be so cold that we simply couldn't exist - it would be 33° C colder (about 58° F colder). Some greenhouse gases are better at absorbing heat than others, and it turns out that even though there is hardly any carbon dioxide in our atmosphere, that single gas is responsible for between 9% and 26% of the greenhouse effect. So it's pretty important.

Overall effect of various greenhouse gases, not including water vapor
Overall effect of various greenhouse gases, not including water vapor

Humans and Carbon Dioxide

Now here's the problem: we humans produce a lot of carbon dioxide. It comes out of our cars, it comes out of our factories, and even out of our power stations. It's also produced when we burn forests - trees store carbon dioxide inside them. Since the Industrial Revolution, we have been producing an unbelievable amount of carbon dioxide. The current level of carbon dioxide in the atmosphere is just under 400 ppm (parts per million). Compare that to its value of 314 ppm in 1960, and you'll see the problem: with more carbon dioxide in the atmosphere, more heat is getting trapped, causing the Earth to get warmer.

Human greenhouse gas emissions by sector
Human greenhouse gas emissions by sector

It's therefore not surprising that we are seeing temperatures rise globally. This isn't something you can see by looking out the window. The rise so far has been less than 1°C (though a 2° C rise would still happen even if we stopped producing greenhouse gases today).

Carbon dioxide increases plotted alongside temperature
Carbon dioxide increases plotted alongside temperature

But while that might not seem like a large increase, even a 3-5 degree C increase can put low-lying countries under water due to melting ice, cause the collapse of the rain forest ecosystem, and cause millions to die because of migrating mosquitoes and failed crops.

Climate change is a real problem for animals like polar bears, who rely on sea ice
Climate change polar bears

Where is the Evidence?

As we will see, we know that climate change is happening, and the evidence in favor of it being human-caused is staggeringly strong. However it's also pretty complicated. So here is a brief summary of the main kinds of evidence that we have.

Evidence that Climate Change is Happening

  • We measure temperature on land, under the sea, in every continent, and almost every country. Wherever we measure, the average temperature is increasing.
  • When we measure heat from satellites in space and stations on earth, we find that more heat is entering Earth than is radiating back out into space.
  • We can measure the heat in the oceans, and we find that most of the heat the earth is gaining is absorbed by the oceans. This is why the effects have been relatively modest so far.

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