Electricity
Electricity is the largest single source of the world's greenhouse gas emissions, producing about a quarter of the global total. But 75% of greenhouse gas emissions hail from other sources.
With global carbon emissions having increased by around 90% since 1970, the world cannot place its confidence in renewable sources alone.
Agriculture
Between farming and deforestation, agriculture is responsible for around 24% of greenhouse gas emissions.
The main offender from this category is cattle, which produce more carbon dioxide emissions than most single countries, save for China and the United States.
But other elements of farming that can be dangerous as well. The use of nitrogen fertilizer, for instance, releases nitrous oxide, a greenhouse gas that's 300 times more potent than carbon dioxide. In October, Gates and a cohort of billionaires invested millions in a genetically modified bacteria that could mitigate this problem.
Manufacturing
The manufacturing industry is close behind in terms of emissions, producing around 21% of the global total.
When fossil fuels are burned to create things like steel or cement, they release harmful levels of carbon dioxide into the atmosphere. In recent years, scientists have also discovered that plastics release greenhouse gases as they break down in the ocean.
"Look at the plastic, steel, and cement around you," Gates wrote. "All of it contributed to climate change."
Transportation
The fourth challenge, transportation, contributes another 14% to global emissions.
While plenty of attention has been given to low-emission cars, burning fossil fuels for other forms of transportation such as airplanes, trucks, and cargo ships has even more damaging effects.
The shipping industry alone produces about as much greenhouse gas as Germany and Britain, and is expected to account for 17% of all emissions by 2050.
Buildings
While buildings represent just 6% of global emissions, this number is likely to skyrocket as more people move to cities and demand additional building stock.
Within buildings, everyday fixtures such as heaters and lights soak up significant amounts of energy, while appliances like refrigerators and air conditioners release a potent form of greenhouse gas known as HFCs. To tackle these challenges, Gates recommends installing more energy-efficient windows and insulation systems.
Though small on their own, these elements represent a powerful collective threat to environmental safety.
