Our planet’s climate has been warming much faster over the past 100 years than it has over the 10,000 years before that. In this article, we will explore how we know that the climate has changed so quickly in the last century, what carbon dioxide (CO2) has to do with climate change, and why humans are responsible for the recent increase of CO2 in the atmosphere. Understanding the problem is the best way to find a solution!
Global Warming or Climate Change?
It is about 1°C warmer today on Earth than 60 years ago. 1°C can seem like a very small change. After all, the difference in temperature between winter and summer can be 30°C in some regions! So why does global warming of a few degrees worry scientists so much?
Do you remember the last time you had fever? The usual human body temperature is around 37°C. When you have a fever, your body gets warmer by 1 or 2°C but you cannot concentrate well anymore, your body does not work as usual, and you need to rest. In the case of your body, it is a matter of a few degrees between being healthy and being sick. The effect of global warming on the Earth is like that of a fever: when the air at the surface gets warmer, the entire planet does not work as usual. The oceans warm as well. The thick ice melts away in cold and mountainous regions. All over the planet, living beings need to get used to new conditions or move to other regions. The warming of the planet affects everything within it, which is why we prefer to speak of climate change rather than of global warming. It is not only a problem of air temperature—there are changes in all elements of the environment around us.
Climate Change at Present
In the past, the Earth has been warmer and colder than it is today. These temperature differences were due to changes in the Earth’s position compared to the Sun, or to large natural events such as volcanic eruptions. These periods of warming and cooling occurred over several thousands of years. Nature and humans could therefore slowly adapt to the changing climate conditions.
This time it is different: climate change is happening very quickly. In Figure 1, you can see a reconstruction of the Earth’s average surface temperature over the past 11,400 years. You can easily see that the temperature has risen much faster in the last decades compared to the temperatures that the Earth experienced previously. It only took 60 years to warm the Earth by around 1°C. Scientists have calculated that the Earth might be 3°C warmer—or even more—in 80 years, if we continue living like we are at the moment. Keep in mind that ice ages were only about 4°C cooler than today’s temperatures, so this is actually a big deal! But…what is the difference between today’s climate change and past climate changes? And what do humans have to do with it?
To answer these questions, let us go back some years in time. In the 1950s, a young scientist named Charles Keeling was wondering why the Earth had been warming so much over the past 100 years. He decided to measure the amount of carbon dioxide (CO2) in the atmosphere, or what scientists call the atmospheric CO2 concentration. Why CO2? Charles knew that this gas is a greenhouse gas, meaning that it has a role in adjusting the temperature of Earth’s atmosphere. The more CO2 in the atmosphere, the more heat the atmosphere can absorb and send back to the Earth’s surface, leading to global warming.
From that moment on, Charles measured the atmospheric CO2 concentration every day—for years. Every year, he saw more atmospheric CO2 than in the year before (Figure 2). This increase continues today: in 2019, atmospheric CO2 concentrations were higher than they were at any time in at least 2 million years .
Why Is There More and More CO2 in the Atmosphere?
Charles discussed his findings with other scientists. They knew that more CO2 in the atmosphere meant that the Earth was warming. And they knew that such warming could have dramatic consequences on the whole climate. They decided to find out what was causing this increase in CO2. They hoped that tracking down the cause might help prevent more CO2 from getting into the atmosphere.
As a first step, the scientists looked at the amounts of CO2 in the atmosphere in the past. They found that, at least for the last 10,000 years (and probably for the last million years!), the amount of CO2 in the atmosphere had never been higher than the amount Charles measured!
In the past, CO2 was added and removed from the atmosphere through natural processes. Plants and oceans, for example, can store CO2 and regularly exchange it with the atmosphere. Sometimes plants and oceans take up CO2 from the atmosphere, reducing atmospheric CO2 concentration, and sometimes plants and oceans release CO2, increasing atmospheric CO2 again.
These natural processes are very slow—they can take thousands of years to absorb or release large amounts of CO2. This is why natural processes cannot possibly explain the rapid accumulation of CO2 in the atmosphere during the last century. If the CO2 is not coming from nature, then where is it coming from?
The CO2 concentration started to rise very quickly around 150 years ago, when we started using machines to accomplish more work in a shorter time. To run these machines, we needed energy. We got this energy from burning oil, coal, and natural gas, which are called fossil fuels. When we burn fossil fuels, CO2 and other greenhouse gases, including methane and nitrogen dioxide, are released (emitted) into the atmosphere.
Over time, we needed more energy to power our cars, to heat our homes, to charge our smartphones, and so on. So, we burned more fossil fuels, leading to more greenhouse gas emissions. In parallel with industrial development, the human population has grown rapidly. To feed more and more humans, forests had to be cut down to create large fields for agriculture. As a result, less CO2 could be taken up by trees. It is therefore we humans who were behind the fast increase in atmospheric CO2 over the last century. And, since we have not yet stopped burning fossil fuels, we are still responsible for adding more and more CO2 into the atmosphere!
The amount of CO2 we emit is very small compared to that exchanged naturally between plants, oceans, and the atmosphere. So why does this small amount emitted by humans have such a big impact on our planet’s temperature? Cannot plants and oceans just take up more CO2? Let us take a sneak peek into the natural “CO2-control task force” and how the atmosphere, plants, and the oceans divide the work between them.
Humans Introduced An Imbalance in Natural CO2 Exchange
Nature likes a balance between different elements. For example, when you open your window in winter, cold air from outside and warm air from inside mix until they reach a common temperature—the indoor and outdoor temperatures are balanced. Over thousands of years, nature adapted to keep such a balance in the Earth’s CO2. If the amount of CO2 in the atmosphere was higher than usual, the plants and the oceans would take up more CO2 than usual, until CO2 amounts were all in balance again. The atmosphere, plants, and the oceans form what could be considered a natural CO2-control task force, because together they regulate the exchange of CO2.
When humans started burning fossil fuels, more CO2 started accumulating in the atmosphere in a short period of time. The change in atmospheric CO2 was so fast that the CO2-control task force could not keep up with establishing the balance. The fast rise in atmospheric CO2 concentration and the resulting atmospheric warming put the plants and the oceans under pressure. The task force did not have time to adapt to higher amounts of CO2, as it could in the past. As a result, the atmospheric CO2 concentration increased even more, leading to more atmospheric warming, putting plants and oceans under even more pressure, and so on. This is what we call the vicious circle of the carbon cycle (Figure 3). This vicious circle has only a small effect compared to the human CO2 emissions coming from the burning of fossil fuels, but it is expected to have a larger effect in the future! You should know that there are many such vicious circles in nature. For example, different vicious circles are responsible for the very fast warming in polar regions. Other greenhouse gases are also increased by human activity, although CO2 is the biggest cause of climate change. Methane (CH4) and nitrous oxide (N2O) have also increased a lot since pre-industrial times because of the burning of fossil fuels and the increase in agriculture.
The amount of CO2 that humans send into the atmosphere may appear small at first glance. However, it is enough to throw off the balance between the atmosphere, the oceans, and plants.
The good news is that Charles’ measurements started a scientific investigation into the changes in atmospheric CO2. Ever since Charles’ first measurements in the 1950s, the crowd of scientists trying to understand climate change has grown. Today, we know much more than we knew in Charles’ time. We know that it is humans who are behind the rise in atmospheric CO2 concentration. And it is therefore also humans who will be able to stop climate change. By reducing CO2 emissions, we can slow down climate change: every ton of CO2 that does not go into the atmosphere means less warming. Our actions now are essential to avoid future climate change ! We need smart, creative, and hopeful people to think of new ways to live our lives! Maybe you will be one of those people?
Climate change: we hear about it almost every day now. We hear that our planet is warming and that this warming is caused by us. But why is that a problem? And what exactly are the mechanisms driving climate change at the moment? In this article we try to explain to a younger audience (8–11 years old) what are the causes of the recently observed changes in our climate. These changes are occurring faster than past changes. They cannot be caused by natural events only. Understanding the causes of climate change is crucial to figure out what is going on and what can be done to limit its consequences.
Global Warming: ↑ The warming we are experiencing due to the increased greenhouse effect.
Climate Change: ↑ The change of the Earth’s climate due to the increased greenhouse effect causing warming of the Earth’s surface.
Carbon Dioxide: ↑ A gas in the atmosphere that is created when we burn fossil fuels. CO2 is a greenhouse gas and can stay in the air for many years. It is the main cause of climate change.
Atmospheric CO2 Concentration: ↑ When measuring gases like carbon dioxide, the term concentration is used to describe the amount of gas in a given volume of air. It is usually measured in ppm (part per million), which is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, parts per million means out of a million.
Greenhouse Gas: ↑ A gas in the atmosphere which can absorb heat and cause the planet to warm up. These occur naturally, such as carbon dioxide and water vapor, but human activity is putting more greenhouse gases into the air leading to the planet getting warmer.
Fossil Fuel: ↑ Fuels, such as coal, oil, and natural gas that were formed millions of years ago when plants and animals died and became buried. Burning fossil fuels creates CO2, which goes into the atmosphere.
Vicious Circle: ↑ A chain of events in which a change compared to the usual situation has the effect of creating new problems which then cause the original change to change more, creating new problems and more change again and again.
Carbon Cycle: ↑ The movement of carbon through nature—plants absorb CO2 from the air as they grow, and release it again when they die. The water in the oceans dissolves CO2. This cycle is called the carbon cycle.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
 ↑ Marcott, S. A., Shakun, J. D., Clark, P. U., and Mix, A. C. 2013. A reconstruction of regional and global temperature for the past 11,300 years. Science. 339:1198–201. doi: 10.1126/science.1228026
 ↑ Available online at: https://climate.nasa.gov/vital-signs/global-temperature/
 ↑ Keeling, C. D., Bacastow, R. B., Bainbridge, A. E., Ekdahl, C. A., Guenther, P. R., and Waterman, L. S. 1976. Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii. Tellus. 28:538–51. doi: 10.3402/tellusa.v28i6.11322
 ↑ IPCC. 2021. “Summary for policymakers,” in Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, eds V. Masson-Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (Cambridge: Cambridge University Press).