Earth energy budget diagram, with incoming and outgoing radiation (values are shown in W/m^2). Satellite instruments (CERES) measure the reflected solar, and emitted infrared radiation fluxes. The energy balance determines Earth’s climate.

Let’s play pretend for a moment. Pretend, if you can, that you’ve never heard about the idea of global warming before. Pretend you’ve never heard anyone else’s opinions on the matter, including from politicians, scientists, friends or relatives. Pretend that there are no related concerns, like the economy, our energy needs, or the environment.

If you were going to make a genuine inquiry, there would instead be only two questions to ask and answer:

1. is the Earth warming or not,
2. and if so, what’s the main cause?

This is a question that was tailor-made for the enterprise of science to answer. Here’s how we can figure it out for ourselves.

There are really only two things that determine the Earth’s temperature, or the temperature of any object that’s heated by an external source. The first is the energy that goes into it, which is primarily energy produced by the Sun and absorbed by the Earth. The second is the energy that leaves the Earth, which is primarily due to the Earth radiating it away.

During the day, we absorb energy from the Sun; this is the power inputted into the Earth. During both the day and the night, we radiate energy back into space; that’s the power outputted by the Earth. This is why temperatures heat up during the day and cool off during the night, something that’s pretty much true for every planet that has both a day side and a night side.

The Earth and Moon, to scale, in terms of both size and albedo/reflectivity. Note how much fainter the Moon appears, as it absorbs light much better than Earth does.NASA / APOLLO 17

To know what the temperature of Earth ought to be, we need to first understand the energy that comes into our world. The source of this energy is the Sun, which radiates with a very well-measured power: 3.846 × 10²⁶watts. The closer you are to the Sun, the more of this energy you absorb, while the farther away you are, the less you absorb. Over the timespan that we’ve measured the Sun’s power output, it’s varied by only about ±0.1%.

The anatomy of the Sun, including the inner core, which is the only place where fusion occurs. Even at the incredible temperatures of 15 million K, the maximum achieved in the Sun, the Sun produces less energy-per-unit-volume than a typical human body. The Sun’s volume, however, is large enough to contain over 10^28 full-grown humans, which is why even a low rate of energy production can lead to such an astronomical total energy output.NASA/JENNY MOTTAR

Sunlight spreads out in a sphere the farther away you are from it, meaning that if you’re twice as far away from the Sun, you only absorb one-quarter the radiation. At Earth’s distance from the Sun, we encounter a power of around 1,361 watts-per-square-meter; that’s how much hits the top of our atmosphere.

The Earth also orbits in an ellipse around the Sun, meaning that at some points it’s closer to the Sun, absorbing more radiation, while at other times it’s more distant, absorbing less. The variation from this effect is more like ±1.7%, with the largest amount of energy absorbed occurring in early January, and the least amount occurring in early July.

The way that sunlight spreads out as a function of distance means that the farther away from a power source you are, the energy that you intercept drops off as one over the distance squared.WIKIMEDIA COMMONS USER BORB

But that’s not the full story. The sunlight that hits us comes in a variety of wavelengths: ultraviolet, visible, and infrared, all of which carry energy. The atmosphere has many layers, some of which absorb that light, some of which allow it to transmit all the way down to the ground, and some of which reflect it back into space.

All told, about 77% of the energy from the Sun makes it down to Earth’s surface when the Sun is directly overhead, with that number dropping significantly when the Sun is lower on the horizon.

The atmosphere of the Earth, although only 5.15 x 10^18 kilograms in mass (just under 0.0001% of the Earth’s mass), plays a tremendous role in defining the properties of our surface.COSMONAUT FYODOR YURCHIKHIN / RUSSIAN SPACE AGENCY PRESS SERVICES

Some of that energy gets absorbed by Earth’s surface, while some of it gets reflected. Clouds reflect sunlight better than average, as do dry sand and icecaps. Other ground conditions are better at absorbing sunlight, including oceans, forests, wet soil, and savannahs. Depending on seasonal conditions on Earth, the individual locations on Earth vary tremendously in how much light they reflect or absorb.

On average, however, the Earth is very consistent: 31% of the incident radiation gets reflected, while 69% gets absorbed. As far as global effects go, this average has changed remarkably little over time, even as human civilization has transformed the landscape of our planet.

Although various components of the Earth’s surface display huge variable ranges in the amount of light they absorb or reflect, the global average reflectance/absorption of Earth, known as albedo, has remained constant at ~31%.KEN GOULD, NEW YORK STATE REGENTS EARTH SCIENCE

When we put in all the factors we know of:

• the Sun’s power output,
• the Earth’s physical size and distance from the Sun,
• the amount of sunlight that Earth absorbs vs. reflects,
• and the intrinsic variability in the Sun over time,

we can arrive at a way to calculate the average temperature of the Earth.

The result?

We calculate that Earth should be at 255 Kelvin (-18 °C / 0 °F), or well below freezing. And that’s absurd, and completely not reflective of reality.

The Earth as viewed from a composite of NASA satellite images from space in the early 2000s. Note the abundant presence of liquid water on the surface: an indicator of a temperate climate.NASA / BLUE MARBLE PROJECT

Instead, our planet has an average temperature of 288 Kelvin (15 °C / 59 °F), which is much warmer than the naive predictions we just painstakingly calculated. Our world is temperate, not frozen, and there’s one big reason for these predictions and observations to be so thoroughly off from one another: we’ve been ignoring the insulating effects of Earth’s atmosphere.

Sure, the Earth radiates the energy it absorbs back into space. But it doesn’t all go into space straightaway; the same atmosphere that wasn’t 100% transparent to sunlight also isn’t 100% transparent to the infrared light that Earth radiates. The atmosphere is made up of molecules that absorb radiation of varying wavelengths, depending on what the atmosphere is made out of.

The interplay between the atmosphere, clouds, moisture, land processes and the oceans all governs the evolution of Earth’s equilibrium temperature.NASA / SMITHSONIAN AIR & SPACE MUSEUM

For infrared radiation, nitrogen and oxygen — the majority of our atmosphere — act as though they’re virtually transparent. But there are three gases that are part of our atmosphere which aren’t transparent at all to the radiation Earth produces:

• water vapor (H2O),
• carbon dioxide (CO2),
• and methane (CH4).

All three of these gases, when they’re present in any planet’s atmosphere, act the same way a blanket does when you place it over a warm-blooded animal’s body: they prevent the heat from escaping.

An emaciated orphaned elephant calf was rescued from the wild after tourists spotted him struggling. Kenya Wildlife Service and David Sheldrick Wildlife Trust responded to reports of the wandering calf on March 18 and dispatched a rescue team to pick up the calf. Here, a blanket was placed over the elephant calf to help it retain its body heat: an extremely effective technique that humans take for granted in our daily lives.THE DSWT / BARCROFT IMAGES / BARCROFT MEDIA VIA GETTY IMAGES

In the case of an animal, they need to generate less of their own heat to maintain a constant temperature when there’s a blanket on them. And if the blanket is thicker, or if there are a greater number of thin blankets, they need to generate even less. This analogy extends to layers of clothing in any conditions; the more insulation you have around you, the less heat escapes, allowing you to maintain higher temperatures.

For a planet like ours, these gases prevent the infrared radiation from escaping, instead absorbing it and re-radiating it back to Earth. The more of these gases that are present, the longer and more efficiently Earth holds onto the Sun’s heat. We can’t change the energy input, so instead, as we add additional amounts of these gases, the temperature of our world simply goes up.

The concentration of carbon dioxide in Earth’s atmosphere can be determined from both ice core measurements, which easily go back hundreds of thousands of years, and by atmospheric monitoring stations, like those atop Mauna Loa. The increase in atmospheric CO2 since the mid-1700s is staggering, and continues unabated.NASA / NOAA

The water vapor content is something that’s determined by Earth’s oceans, the local temperature, humidity and dew point. When we add more water vapor to the atmosphere or take water vapor out of it, the overall water vapor content doesn’t change at all. As far as human activity goes, nothing we do has any impact on the net amount of H2O in the atmosphere.

The concentrations of the other two gases (CO2 and CH4), though, are primarily determined by human influence. It’s well-documented, for example, that CO2 has risen by more than 50% of its 1700s-era value due to the burning of fossil fuels coinciding with the start of the industrial revolution. According to NASA scientist Chris Colose:

50% of the 33 K greenhouse effect is due to water vapor, about 25% to clouds, 20% to CO2, and the remaining 5% to the other non-condensable greenhouse gases such as ozone, methane, nitrous oxide, and so forth.

At an average warming rate of 0.07º C per decade for as long as temperature records exist, the Earth’s temperature has not only increased, but continues to increase without any relief in sight.NOAA NATIONAL CENTERS FOR ENVIRONMENTAL INFORMATION, CLIMATE AT A GLANCE: GLOBAL TIME SERIES

All of this leads to a very straightforward conclusion: if we increase the concentrations of infrared-absorbing gases in our atmosphere, like CO2 and CH4, the Earth’s temperature will rise. Given that the temperature record unequivocally shows that the Earth is warming, and we have put these additional proverbial blankets onto our atmosphere, it seems like a slam dunk that this is cause-and-effect at work.

It cannot be proven that human activity is the cause of global warming, of course. That conclusion we drew is still a scientific inference. But based on what we know about planetary science, Earth’s atmosphere, human activity and the warming we’re observing, it seems like a very good one. When we quantify the other effects, it’s unlikely that anything else could be the cause. Not the Sun, not volcanoes, not any natural phenomenon that we know of.

The Earth is warming, and humans are the cause. The next steps — of what to do about it — are 100% up to us.