Climate change around the world has been caused by the buildup of greenhouse gases. These gases are primarily produced by the burning or heating of materials that contain carbon, including fossil fuels like coal, oil, and natural gas. When these materials are burned or heated, they emit, or release, gases that build up in the upper atmosphere, where they trap heat, like a greenhouse for plants. Over time, this has caused average global temperatures to rise. That, in turn, has created a whole range of dangerous problems which are threatening the planet. Each year, 51 billion tons of greenhouse gases are emitted into Earth’s atmosphere. That’s a big number, and the problem can seem overwhelming.

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 carbon emissions into five primary categories, which we call the Grand Challenges. These 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, they are also very interconnected. Let’s explore each of the Grand Challenges, their links to each other, 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 because 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 is that we already have many solutions. Renewable energy like solar and wind are becoming cheaper and more widely available, and nuclear power is a promising option. The bad news is that 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, we will be using more and more electricity.

By 2050, global electricity use could triple. This is 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 some less prosperous 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. Even if we were able to make all this stuff with zero-carbon electricity, we would still run into one major obstacle.

Arguably, the single biggest problem in any of the five Grand Challenges is that, 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 product. Cement is the key ingredient in concrete, and other than water, concrete is the most-used substance in the world. 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 these cities will 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. 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 threatens global food supplies. So we need to reduce emissions resulting from how we produce food while also ensuring that everyone, including the most vulnerable, has 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 problem. Methane, a potent greenhouse gas, is produced by fertilizer and cow burps, 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. Some experts estimate 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 the way humans live their lives. 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. Today in the United States, transportation is the largest source of greenhouse gas emissions.

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 the production of electric cars. But this Grand Challenge isn’t just about moving ourselves around, it is also about the large-scale movement of goods around the world. That’s bad news because ships, planes, and trucks are very difficult to electrify—they 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 heat waves 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 more able to withstand extreme weather. As for buildings that already exist, these can be upgraded by improving insulation and heating systems. We already have many of the innovations we need in this category, especially heat pumps. Heat pumps are more energy-efficient than gas furnaces, and they’re becoming more affordable. 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, they are all linked to each other. This can be both challenging and helpful. On the one hand, it’s hard to predict how changes in one Grand Challenge will influence another Grand Challenge. But on the other hand, if we solve one problem—like producing zero-carbon 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 would be most helpful.