Causes of Global Warming

Fossil Fuels

What are Fossil Fuels?

Coal, crude oil, and natural gas are all considered fossil fuels because they were formed from the fossilized, buried remains of plants and animals that lived millions of years ago. Because of their origins, fossil fuels have a high carbon content. Some common fossil fuels include oil, coal, and natural gas.


Crude oil, or petroleum (literally “rock oil” in Latin), is a liquid fossil fuel made up mostly of hydrocarbons (hydrogen and carbon compounds). Oil can be found in underground reservoirs; in the cracks, crevices, and pores of sedimentary rock; or in tar sands near the earth’s surface. It’s accessed by drilling, on land or at sea, or by strip mining in the case of tar sands oil and oil shale. Once extracted, oil is transported to refineries via supertanker, train, truck, or pipeline to be transformed into usable fuels such as gasoline, propane, kerosene, and jet fuel—as well as products such as plastics and paint.

Image of offshore rig, low light and cloudy sky in background.

Petroleum products supply about 37 percent of U.S. energy needs, with the transportation sector consuming the most. U.S. oil consumption in 2016 was 10 percent below the record high of 2005 and only 3 percent higher than during the 1973–74 embargo by the Organization of the Petroleum Exporting Countries (OPEC)—despite the U.S. economy tripling in size in the decades since. However, oil use has increased modestly for the past four years, as relatively low gasoline prices have fueled a rise in vehicle miles traveled and renewed interest in SUVs and light trucks. Still, U.S. consumption of petroleum products is forecast to decrease, at least through 2035, as fuel efficiency standards lead to cleaner-running vehicles. Continued strengthening of clean car and fuel economy standards remains critical for reducing oil consumption.

On the production side, the United States has experienced a decade long upswing. Production growth is due in large part to improvements in horizontal drilling and hydraulic fracturing, technologies that have created a boom in U.S. shale oil and natural gas extraction. While horizontal drilling enables producers to drill down and outward—thus reaching more oil or gas from a single well—hydraulic fracturing (also known as fracking) is designed to extract oil or natural gas from unyielding rock, including shale and other formations. Fracking involves blasting huge quantities of water mixed with chemicals and sand deep into a well, at pressures high enough to fracture rock and enable the oil or gas to escape. This controversial method of extraction creates a host of environmental and health problems, including air and water pollution.


Coal is a solid, carbon-heavy rock that comes in four main varieties differentiated largely by carbon content: lignite, sub-bituminous, bituminous, and anthracite. Nearly all of the coal burned in the United States is sub-bituminous or bituminous. Found in abundance in states including Wyoming, West Virginia, Kentucky, and Pennsylvania, these coal types are middle of the pack in terms of carbon content and the heat energy they can produce. Regardless of variety, however, all coal is dirty. Indeed, in terms of emissions, it’s the most carbon-intensive fossil fuel we can burn.

Image of strip mining to extract coal from the land.

Coal is extracted via two methods: Underground mining uses heavy machinery to cut coal from deep underground deposits, while surface mining (also knownas strip mining) removes entire layers of soil and rock to access coal deposits below. Strip mining accounts for about two-thirds of coal sourced in the United States. Although both forms of mining are detrimental to the environment, strip mining is particularly destructive, uprooting and polluting entire ecosystems.

Coal and the power plants that burn it account for less than a third of U.S. electricity generation, down from more than half in 2008. Cleaner, cheaper alternatives—including natural gas, renewables like solar and wind, and energy-efficient technologies—make coal far less economically attractive. Today, coal-fired power plants continue to close, despite the Trump administration’s promises of a revived industry. Future demand for coal is expected to remain flat or to fall as market forces propel alternative energy sources forward.

Natural Gas

Image of the fracking of natural gas.

Composed mostly of methane, natural gas is generally considered either conventional or unconventional, depending on where it’s found underground. Conventional natural gas is located in porous and permeable rock beds or mixed into oil reservoirs and can be accessed via standard drilling. Unconventional natural gas is essentially any form of gas that is too difficult or expensive to extract via regular drilling, requiring a special stimulation technique, such as fracking.

In the United States, the development and refinement of processes like fracking have helped make the country the world’s top producer of natural gas since 2009—and the biggest consumer of it, too. Abundant in the United States, natural gas covers nearly 30 percent of U.S. energy needs and is the largest source of energy for electricity. Forecasts suggest it will become an even greater part of the U.S. energy mix through 2050, threatening to exacerbate air and water pollution.


Forests can be found from the tropics to high-latitude areas. They are home to 80% of terrestrial biodiversity, containing a wide array of trees, plants, animals and microbes, according to the World Bank, an international financial institution. Some places are especially diverse — the tropical forests of New Guinea, for example, contain more than 6% of the world’s species of plants and animals.

Deforestation in the Amazon Rainforest.

Forests provide more than a home for a diverse collection of living things; they are also an important resource for many around the world. In countries like Uganda, people rely on trees for firewood, timber and charcoal. Over the past 25 years, Uganda has lost 63% of its forest cover, Reuters reported. Families send children — primarily girls — to collect firewood, and kids have to trek farther and farther to get to the trees. Collecting enough wood often takes all day, so the children miss school.

According to a 2018 FAO report, three-quarters of the Earth’s freshwater comes from forested watersheds, and the loss of trees can affect water quality. The UN’s 2018 State of the World’s Forests report found that over half the global population relies on forested watersheds for their drinking water as well as water used for agriculture and industry.

Deforestation in tropical regions can also affect the way water vapor is produced over the canopy, which causes reduced rainfall. A 2019 study published in the journal Ecohydrology showed that parts of the Amazon rainforest that were converted to agricultural land had higher soil and air temperatures, which can exacerbate drought conditions. In comparison, forested land had rates of evapotranspiration that were about three times higher, adding more water vapor to the air.

Trees also absorb carbon dioxide, mitigating greenhouse gas emissions produced by human activity. As climate change continues, trees play an important role in carbon sequestration, or the capture and storage of excess carbon dioxide. Tropical trees alone are estimated to provide about 23% of the climate mitigation that’s needed to offset climate change, according to the World Resources Institute, a nonprofit global research institute.

Deforestation not only removes vegetation that is important for removing carbon dioxide from the air, but the act of clearing the forests also produces greenhouse gas emissions. The Food and Agriculture Organization of the United Nations says that deforestation is the second-leading cause of climate change. (The first is the burning of fossil fuels.) In fact, deforestation accounts for nearly 20% of greenhouse gas emissions.



Before reaching our plates, our food is produced, stored, processed, packaged, transported, prepared, and served. At every stage, food provisioning releases greenhouse gases into the atmosphere. Farming in particular releases significant amounts of methane and nitrous oxide, two powerful greenhouse gases. Methane is produced by livestock during digestion due to enteric fermentation and is released via belches. It can also escape from stored manure and organic waste in landfills. Nitrous oxide emissions are an indirect product of organic and mineral nitrogen fertilisers.

Agriculture accounted for 10% of the EU’s total greenhouse-gas emissions in 2012. A significant decline in livestock numbers, more efficient application of fertilisers, and better manure management reduced the EU’s emissions from agriculture by 24% between 1990 and 2012.

However, agriculture in the rest of the world is moving in the opposite direction. Between 2001 and 2011, global emissions from crop and livestock production grew by 14%. The increase occurred mainly in developing countries, due to a rise in total agricultural output. This was driven by increased global food demand and changes in food-consumption patterns due to rising incomes in some developing countries. Emissions from enteric fermentation increased 11% in this period and accounted for 39% of the sector’s total greenhouse-gas outputs in 2011.

Given the central importance of food in our lives, a further reduction of greenhouse-gas emissions from agriculture remains quite challenging. Nevertheless, there is still potential to further reduce the greenhouse-gas emissions linked to food production in the EU. A better integration of innovative techniques into production methods, such as capturing methane from manure, more efficient use of fertilisers, and greater efficiency in meat and dairy production (i.e. reducing emissions per unit of food produced) can help.

In addition to such efficiency gains, changes on the consumption side can help to further lower greenhouse-gas emissions linked to food. In general, meat and dairy products have the highest global footprint of carbon, raw materials, and water per kilogramme of any food. In terms of greenhouse-gas emissions, livestock and fodder production each generate more than 3 billion tonnes of CO2 equivalent. Post-farm transport and processing account for only a tiny fraction of the emissions linked to food. By reducing food waste and our consumption of emission-intensive food products, we can contribute to cutting the greenhouse-gas emissions of agriculture.

In line with projected population growth and changes in dietary habits in favour of higher meat consumption, the global demand for food is expected to grow by up to 70% in the coming decades. Agriculture is already one of the economic sectors with the largest environmental impact. This substantial increase in demand will unsurprisingly create additional pressures. How can we meet this increasing global demand while at the same time reducing the impacts of European food production and consumption on the environment?

Reducing the amount of food produced is not a viable solution. The EU is one of the world’s largest food producers, producing around one eighth of the global cereal output, two thirds of the world’s wine, half of its sugar beet, and three quarters of its olive oil. Any reduction in key staples is likely to jeopardise food security in the EU and in the world, and increase global food prices. This would make it harder for many groups around the world to access affordable and nutritious food.

Producing more food out of the land that is already used for agriculture often requires heavier use of nitrogen-based fertilisers, which in turn release nitrous oxide emissions and contribute to climate change. Intensive agriculture and fertiliser use also release nitrates to the soil and to water bodies. Although not directly linked to climate change, high concentrations of nutrients (especially phosphates and nitrates) in water bodies cause eutrophication. Eutrophication promotes algae growth and depletes oxygen in the water, which in turn has severe impacts on aquatic life and water quality.

Whether in Europe or the rest of the world, meeting the growing demand for food by using more land would have serious impacts on the environment and the climate. The areas most suitable to agriculture in Europe are already cultivated to a large extent. Land, especially fertile agricultural land, is a limited resource in Europe and across the world.

Converting forest areas into agricultural land is also not a solution as this process is a source of greenhouse-gas emissions. Similar to many other land-use changes, deforestation (currently occurring mainly outside the European Union) also puts biodiversity at risk, further undermining nature’s ability to cope with climate change impacts (such as absorbing heavy rainfall).