Each year, humanity releases 51 billion tons of greenhouse gases into Earth’s atmosphere. That’s a big number. It’s hard to wrap your head around this fact.

How do you visualize 51 billion tons of gases? Is it a lot? It sounds like a lot. The Earth’s atmosphere is about 5.5 quadrillion tons (that’s 15 zeroes). In that context, 51 billion (nine zeros) doesn’t seem like a lot. But consider this: A great white shark weighs about one ton. Imagine if we launched 51,000,000,000 sharks into the sky each year. They wouldn’t blot out the Sun, but that’s still a whole lotta sharks to have swimming above you!

Of course, we can simplify how we think about emissions without resorting to sharks. For example, one way to simplify any problem—and to begin identifying the best solutions—is to break it into smaller, more-manageable pieces. In this course, we divide emissions into the five primary economic sectors responsible for releasing carbon emissions. We call these categories the Grand Challenges. They are:

1. How we plug in (27 percent of 51 billion tons)
2. How we make things (31 percent of 51 billion tons)
3. How we grow things (19 percent of 51 billion tons)
4. How we get around (16 percent of 51 billion tons)
5. How we keep cool and stay warm (7 percent of 51 billion tons)

While each of these Grand Challenges is a complex problem on its own, there are webs of relationships and causation linking them to each other. Nearly everything we do as humans emits greenhouse gases. Something as simple as ordering a hamburger involves emissions across the Grand Challenges, from the electricity we use to cook, the grocery stores where we buy the meat, the methane cows burp, the gas-powered vehicles used for transportation, to the refrigerated buildings where it’s sold. It gets complex quickly.

That’s why we think breaking these processes into five categories helps us make sense of the big challenge of climate change. It lets us identify which parts of hamburger production and consumption are doing the most damage and which parts we need to address first. It can help us understand the big, structural changes we need as a society, as well as the small, everyday changes in behavior that you can make to help lessen your personal carbon footprint. Let’s explore each of the Grand Challenges, their interdependencies, and some obstacles that make decarbonizing so challenging.

How we plug in (27 percent of 51 billion tons)

The first Grand Challenge is electricity generation. This Grand Challenge may not have the highest percentage, but it’s the most important. If we don’t decarbonize electricity generation, progress made in other Grand Challenges won’t really matter. For example, if we replace all gas-powered cars with electric vehicles but still use coal to generate our electricity, we haven’t really fixed the problem. We’ve just shuffled the carbon around.

This Grand Challenge is a good-news/bad-news situation. The good news: We already have many solutions. Renewable energy like solar and wind are becoming cheaper and more widely available. Nuclear power produces 70 percent of France’s electricity. That’s the good news. The bad news: We’re going to need more electricity—a lot more. As populations rise, poverty falls, and we electrify things such as cars and industrial processes that had previously relied on fossil fuels. The upshot is that more people are using more electricity.

By 2050, global electricity use could triple. That’s a big problem, because in 2023, about 60 percent of global electricity production came from burning fossil fuels—especially coal and gas. Fossil fuels are the cheapest way to generate electricity. For poor countries or countries with large populations, such as China and India, coal-powered electricity is rising.

How we make things (31 percent of 51 billion tons)

The manufacturing processes we use to create concrete, steel, glass, and other materials produce almost a third of humanity’s annual carbon emissions. These processes are hugely energy intensive and right now, nearly all that energy comes from burning fossil fuels. But even if we were able to make all this stuff with zero-carbon electricity, you would still run into one major obstacle.

Arguably, the single biggest problem in any of the five Grand Challenges is this: For every ton of cement we make, we emit one ton of carbon dioxide (CO₂). That’s because we make cement from limestone using extreme heat, a chemical reaction that releases CO₂ as an unavoidable byproduct. Cement is the key ingredient in concrete, and other than water, concrete is the most-used substance in the world. And we’re going to need more of it. A lot more. As populations grow and cities expand, the world will need to build the equivalent of one new New York City every month for the next 40 years. And do you know what cities need a lot of? Concrete.

How we grow things (19 percent of 51 billion tons)

The processes that we use to produce the food we eat are huge emitters of greenhouse gases. And to feed a more populated world, we’re going to need to grow more food in the years to come. At the same time, climate change is threatening global food supplies. We need to both reduce emissions that are the result of how we produce food while also ensuring that the most vulnerable people have access to the food they need.

Historically, to make room for our crops and livestock, humans have cut down a lot of trees. Since 1990, the world has lost half a million square miles of forest cover—more than the total size of Texas and California combined! This is a problem because when a tree is removed from the ground, it releases the CO₂ stored inside it and in the soil underneath it.

Methane released in agriculture is also a big emitter. Methane is produced by fertilizer and bovine flatulence, and it has about 28 times the warming power of CO₂. That’s why cow burps are no laughing matter. There were over a billion cattle in the world in 2023, and they produced so much methane that if they were a country, they would rank third in greenhouse gas emissions. Breakthrough Energy estimates that by 2050 there could be another 500 million cows roaming the planet.

How we get around (16 percent of 51 billion tons)

Fossil fuels have transformed humanity. And one of the biggest changes is our mobility. The internal combustion engine moves us and our stuff around the world faster than ever. Gasoline and oil are incredibly powerful and shockingly cheap. One gallon of gas contains as much energy as 130 sticks of dynamite and costs less than soda. But that convenience has come at a great cost.

Globally, this Grand Challenge makes up a smaller part of the pie, but in the United States, transportation is the largest source of greenhouse gas emissions.

The transportation industry has made great strides in consumer electric vehicles. However, the Organization for Economic Cooperation and Development (OECD) predicts that demand for transportation will keep growing through at least 2050. That demand will mostly come from the aviation, trucking, and shipping sectors. This Grand Challenge isn’t just about moving ourselves around, but also about the large-scale movement of goods around the world. And that’s bad news, because those sectors are very difficult to electrify—ships, planes, and trucks are just too heavy and have to go too far to be battery-powered.

How we keep cool and stay warm (7 percent of 51 billion tons)

As climate change transforms our environment, how we heat and cool our buildings is becoming increasingly important. In many regions, air conditioning is becoming essential to survival, as deadly heatwaves strike more frequently. We’re going to need more air conditioners, but with the technology we have now, new units will create more emissions.

This Grand Challenge is linked to “How We Make Things” because one of the solutions to this is to design new buildings to be more energy efficient and able to withstand extreme weather. For buildings that already exist, we can retrofit, improving insulation and heating systems, for example. We already have many of the innovations we need in this category, especially heat pumps. Heat pumps are an energy-efficient alternative to gas furnaces, and they’re becoming more affordable thanks to government policies. Still, they run on electricity. If we fail to decarbonize the electric grid, all the heat pumps in the world won’t solve this problem.

Conclusion

The five Grand Challenges seem at first glance to be neat categories, each distinct from the other. But as you’ve read, there’s lots of complexity linking them to each other. This can be both challenging and helpful. On the one hand, it’s hard to predict how changes in one sector will influence another. But on the other hand, if we solve one problem—like producing zero-carbon fusion energy—it would help make the other Grand Challenges a lot easier to solve. This is what makes the five Grand Challenges such a helpful tool. By breaking the sources of emissions into separate categories, we reveal linkages and can identify which innovations are most critical to lowering emissions across the most sectors, most efficiently.