Global warming is not complicated. I’ll expand a little below, but here’s how it works, in one paragraph.
The Earth is heated mostly by light from the Sun. Light is energy. Energy is heat. When light is absorbed by something, that thing heats up. Visible light passes right through the air without being absorbed, which is why we can see things around us. The only part of sunlight that is absorbed by the air is infrared light. Most of the atmosphere (nearly all of it) is made up of nitrogen (78%) and oxygen (21%), neither of which absorbs infrared. Some trace gasses do absorb infrared, including carbon dioxide (CO2) and water vapor (H2O). If more of these trace gasses are present, the atmosphere will retain more of the heat energy from sunlight. Humans are adding prodigious amounts of CO2 to the atmosphere. This cannot avoid further heating the air.
It’s really that simple. Anyone who denies that global warming is happening has to explain why physics doesn’t work on Earth.
For more detail, see below, but that’s really about it.
Visible light comes in many wavelengths, from blue to red. But this is only the portion of the light spectrum our eyes can see. There are “colors” of light (wavelengths) both shorter (more energetic) and longer (less energetic). Ultraviolet is beyond blue in one direction, infrared is past red in the other.There are also wavelengths beyond even these, such as X-rays and radar. The Sun produces light in virtually all these wavelengths, some more than others.
Longer wavelengths tend to pass through matter (solids, liquids, and gasses) while shorter wavelengths are more easily absorbed. The less dense that matter is, the easier it is for light to pass through it. That’s why X-rays can pass through your skin and muscles, but can’t pass through your bones, allowing doctors to take X-ray pictures of broken bones. It’s why radar can pass through the air, but is blocked by mountains or airplanes.
Nitrogen and oxygen, which make up most of our atmosphere, aren’t very dense gasses. They weigh less, per unit volume, than more complex gasses like carbon dioxide or water vapor. CO2 and H2O are much denser than nitrogen or oxygen. Infrared can pass right through nitrogen and oxygen, but is absorbed by CO2 and is also absorbed (or sometimes reflected) by H2O.
That’s why sunlight heats the air. The Earth’s atmosphere contains carbon dioxide and water vapor, which absorb infrared light. But there is a further feedback process that captures even more of the sun’s heat.
Light in the visible portion of the spectrum usually passes right through the gasses in our atmosphere. The exception is when some of the denser gasses clump closely together, as when there’s a great deal of water vapor, which makes clouds that can block or reflect even visible light.
I won’t get deeply into reflection vs. absorption here (though we’ll briefly get back to it later). It’s enough to say that when light falls on something too dense for it to pass through, sometimes it is reflected back (like a mirror does), but sometimes it is absorbed, and the energy of the light is transferred to the thing that absorbed it.
The surface of the Earth is made up mostly of solids (rocks, trees, people, dirt, ice, and so on) and liquids (mostly water). These are much denser than air (which is why you can’t walk through rocks). Most of the solids absorb even visible light and so get heated up. That’s why sidewalks get warm on a sunny day.
Even liquids absorb much of the light that falls on them, which is why a glass of beer warms up when left on the porch in summer. (Conduction also plays a part in warming your beer, especially indoors, as the heat from the air outside is transferred through the glass to the beer, but I don’t need to get into that.) That’s also why a swimming pool heats up on a sunny day.
Something that is warm tends to radiate its heat away. That’s why your dinner cools off if you don’t eat it quickly enough. When it radiates heat, it emits infrared light. The graphic at the top of this article is picture of a boy, taken in infrared light. It’s brighter in the places that are emitting more heat. I added the image of the Earth globe he’s holding, also a picture taken in infrared. That’s why infrared goggles let you see things at night—they are sensitive to the heat energy (infrared light) being emitted by the things you’re looking at.
So visible light passes through virtually all the gasses in our atmosphere, but is absorbed by the surface of the Earth, and the surface is thus heated by that light. The Earth’s surface then tries to cool off, by emitting infrared light back upward, into the air. Most of that infrared passes right through the air (remember that most of the atmosphere is nitrogen and oxygen, which lets infrared pass through). That’s why nights are colder than days. The surface isn’t absorbing any sunlight at night, and the heat energy that is emitted is lost out into space.
This process doesn’t happen only on Earth. It happens everywhere. On the daytime side of the Moon, surface temperature can reach 253 degrees F (123 C). The "dark side of the moon", the half facing away from the sun, radiates that heat away, and temperatures can drop to to minus 243 F (minus 153 C).
The Earth (which is the same distance from the Sun that the Moon is) doesn’t experience these extremes of heat for three main reasons.
First, neither nights nor days are as long on the Earth as on the Moon (day and night each average twelve hours on the Earth, while on the Moon day and night are each about 700 hours) so there isn’t as much time for the Earth to either absorb solar heat during the day or to to lose it at night.
Second, the Earth doesn’t get as hot during the day because some of the light that falls on the Earth is reflected back into space immediately by clouds and by reflective surfaces such as glaciers and ice caps. (I told you I’d briefly mention reflection again.)
And third, the Earth doesn’t lose as much heat at night because—well, remember what I said about carbon dioxide and water vapor? Some of the heat that these gasses absorbed during the day doesn’t get lost. It stays in the atmosphere. Additionally, some of the infrared that is emitted by the surface of the Earth never makes it out into space, because it is absorbed by CO2 and H2O.
Without these trace gasses, the surface of the Earth would average about 0 degrees F (-18 C). The oceans would freeze to a considerable depth. (The Earth might retain, as Jupiter’s moon Euopra does, a subsurface ocean, covered in ice, heated by the planet’s interior.) Land masses would be covered in permanent glaciers, like a superbig and forever-lasting ice age. Life on Earth would be impossible.
The presence of carbon dioxide and water vapor in our atmosphere is the only thing that makes Earth habitable. Without these trace gasses, the Earth would be a barren and lifeless snowball.
Now, think back on how much of the Earth’s atmosphere is nitrogen and oxygen. Nitrogen is 78% of the atmosphere. Oxygen is 21%. Together, they account for 99% of our atmosphere. All the other trace gasses amount to about 1%. Clearly, just a little bit of these gasses has an enormous effect.
How much carbon dioxide is there in the atmosphere? For the last 800,000 years, carbon dioxide concentration has been between about 180 and about 280 parts per million. That is 0.018% to 0.028%, or less than 3/100 of one percent. If the atmosphere was a $10 dollar bill, nitrogen would be $7.80 of that, oxygen would be $2.10, and the amount of carbon dioxide would be less than thirty cents.
The periods of low carbon dioxide content correspond to ice ages. The periods of higher CO2 are when the continent-sized glaciers melted. Even a small change has big effects.
In the last century, we have nearly doubled the amount of atmospheric carbon dioxide. It is now at about 400 parts per million. The Earth has not seen this level of atmospheric carbon dioxide since long before there were humans on the Earth.
This cannot help but warm the Earth’s climate. To claim otherwise is to claim physics doesn’t work on Earth. That’s all there is to it.
Why hasn’t the temperature of the Earth already been drastically altered? Because the Earth is big, and it takes many years for the full effect to be felt, and for all that excess carbon dioxide to absorb all the heat energy it can. As it is, surface temperatures have been soaring. 2014 was the hottest year in human history. 2015 was hotter. 2016 is likely to be hotter still.
And I haven’t discussed the amount of heat that has been absorbed by the oceans, and which has not been released into the atmosphere. Over 90% of the excess heat that the increased level of CO2 has trapped has been buried deep in the oceans, through mechanisms I’m not going to get into now. The point is, it’s happening, and there simply isn’t any way to intelligently or rationally argue that it isn’t.
This isn’t hard to understand. It is impossible to rationally deny. It is very hard to ignore, except by people who really want to do so.
Anyone who claims global warming isn’t happening is a person who is not just a global warming denier, but a physics denier, and a reality denier.
It’s that simple.
Corrected a few minor errors.