Thursday is Earth Day, a day that a decade or two ago transitioned from being a weirdo-hippie celebration to being an opportunity for businesses to sell things made out of wood instead of plastic. There’s probably a term for the tipping point at which something goes from a national joke to a national marketing opportunity, but if there is, I don’t know it.

This quick flip was useful for politicians uninterested in addressing the central threat the Earth faces. Instead of there being a day when Americans are reminded that the world is warming and that the effects and scale of that warming are unclear, there is a day in which people can talk about how good trees are. Trees are good; no offense to trees. But it’s a lot harder to sell things that are focused on addressing the buildup of gas in the atmosphere than it is to sell a thing that will protect a sea turtle. So the latter gets a lot of attention and the former less so.

I’ve found over the years that there is a pretty widespread lack of familiarity with how climate change actually works and what contributes to it. So, in the interest of selling nothing more than a subscription to a newspaper, I decided it was worth putting together something of a primer on the subject, centered on how it is measured.

The natural place to begin is by explaining how the warming process works.

If you live in a house in a cold-weather climate, you are probably aware that your attic is insulated. That’s to keep heat in your house and prevent it from escaping into the outside air. The Earth is not similarly insulated, so heat often radiates off the planet’s surface and into space.

Sometimes, though, that heat radiates into the atmosphere, where it is absorbed by one of the countless gas molecules floating around up there, a layer of protection that has an insulating effect. Some of those molecules, excited (literally) by the absorbed energy, then release the energy back out. Sometimes it’s released up toward space. Sometimes it strikes another molecule. And sometimes it heads back toward Earth.

This is oversimplified, but you get the point. Heat headed for space ends up being redirected back toward the Earth. And the more molecules floating around in the atmosphere, the more likely it is that one will absorb heat before it escapes and the more likely it is that the heat will be aimed back down.

That’s the most basic element of warming. There are knock-on effects: Warming air holds more water vapor which can absorb more heat, warming temperatures can thaw permafrost that then releases methane into the atmosphere. But the central issue is the buildup of those gas molecules.

Scientists track the density of those molecules in the atmosphere. The National Oceanic and Atmospheric Administration has an online tool that shows the densities of carbon dioxide, methane, nitrous oxide and sulfur hexaflouride in the atmosphere. In the past several decades, each has climbed higher.

You’ll notice we flagged a particular point on the methane graph. Methane is a particularly potent greenhouse gas, one that traps more radiation than carbon dioxide. According to the Environmental Protection Agency, “the comparative impact of CH4″ — methane — “is 25 times greater than CO2 over a 100-year period.” So remember that September 2006 point on that graph for later.

It’s important to note that the measurements above track global concentrations. If a molecule of carbon dioxide is released in Paris, it contributes to that total as surely as one released in Pittsburgh. Over the past 60 years, the amount of carbon dioxide released by the United States as a percentage of the global total annually has decreased as other countries, particularly China, have seen big increases in emissions. (The data below come from the Global Carbon Atlas.)

This is why climate change is a global problem. Curtailing emissions entirely in the United States would neither remove the existing molecules from the atmosphere or mean that no new molecules would be added. It’s why activists and politicians targeting the issue have embraced international compacts such as the Paris climate accord in an effort to instantiate international rules aimed at cutting emissions across the board. The goal, at the very least, is to slow the rate at which gas is emitted into the atmosphere.

So let’s look at the sources and types of emissions in the United States. EPA data for 2019 show that most of the emissions in this country come from transportation and electricity generation: burning gasoline in cars and burning coal in power plants, both of which release carbon dioxide (among other things). Carbon dioxide made up more than three-quarters of emissions in 2019. (Notice that agriculture makes up 10 percent of emissions. We’ll come back to that, too.)

If you’d looked at the EPA’s website in 2016 (which you still can, thanks to the Internet Archive), you’d have seen that electricity generation was a larger contributor to carbon dioxide emissions than transportation by a 37 percent to 31 percent margin. Now, those percentages have flipped: Transportation is responsible for 35 percent of carbon dioxide emissions and electricity generation only 31 percent.

One reason for that is the change in how electricity is generated in the United States. Until 2009 or so, coal was the most common generation method for producing electricity in power plants. But then that changed. Natural gas became much cheaper and it burned more cleanly than coal, releasing far less carbon dioxide. So electricity-generating facilities began switching over.

In the past 20 years or so, the amount of electricity generated from natural gas and renewable sources (wind, solar and hydroelectric) has climbed. The amount produced by nuclear plants has held steady. Coal generation has plunged.

A key driver of that dynamic was the surge in hydraulic fracturing. About 20 years ago, oil drillers figured out how to extract vast amounts of oil and gas from shale deposits underground: drill down and then sideways, injecting water at high pressure into the hole. The rock shatters and gas and oil are released. Areas of North Dakota, Oklahoma and Texas boomed and the market was suddenly saturated with natural gas.

After holding steady for decades, natural gas production in the United States began to surge in about 2006.

Natural gas is primarily methane. Remember that first graph showing the increase in atmospheric methane beginning in the same period? That’s not a coincidence. While natural gas burns more cleanly, a lot of it can escape into the atmosphere at drilling sites.

It’s a reminder of how complex this all is. Consider that shift in carbon dioxide emissions between electricity production and transportation. The electricity production emissions have dropped (thanks to the transition away from coal) but transportation emissions have recovered after the recession a decade ago. That’s a key reason the Biden administration has recently targeted emissions from automobiles.

Then there’s that chunk of emissions from agriculture. There are a lot of jokes about how cows contribute to climate change but — cows contribute to climate change! The amount of nitrous oxide released into the atmosphere is a function of practices such as fertilization. That methane, though, is from what the EPA tenderly refers to as “part of [animals’] normal digestive process,” meaning flatulence and belching. Pools of manure also contribute to methane emissions.

We come to the question that many have asked recently: Does eating less meat help the planet? The answer to that is similar to the answer to a lot of environment-related questions. It does, a little. But scaled up? If lots of people eat less meat or drive less? If they drive cars that emit less carbon dioxide? If they get their power from wind turbines? If that becomes the norm not just here but in China and India?

All of a sudden, the picture changes.