The United States Environmental Protection Agency says that Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2012, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth’s carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle—both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution. 
The main human activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO2. The main sources of CO2emissions in the United States are described below.
- Electricity. Electricity is a significant source of energy in the United States and is used to power homes, business, and industry. The combustion of fossil fuels to generate electricity is the largest single source of CO2 emissions in the nation, accounting for about 38% of total U.S. CO2emissions and 31% of total U.S. greenhouse gas emissions in 2012. The type of fossil fuel used to generate electricity will emit different amounts of CO2. To produce a given amount of electricity, burning coal will produce more CO2than oil or natural gas.
- Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods is the second largest source of CO2 emissions, accounting for about 32% of total U.S. CO2 emissions and 27% of total U.S. greenhouse gas emissions in 2012. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.
- Industry. Many industrial processes emit CO2 through fossil fuel combustion. Several processes also produce CO2 emissions through chemical reactions that do not involve combustion, for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 14% of total U.S. CO2 emissions and 12% of total U.S. greenhouse gas emissions in 2012. Note that many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production.
Carbon dioxide is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals. However, emissions and removal of CO2 by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere.
In the United States, since 1990, the management of forests and non-agricultural land has acted as a net sink of CO2, which means that more CO2 is removed from the atmosphere, and stored in plants and trees, than is emitted. This sink offset about 15% of total emissions in 2012 and is discussed in more detail in the Land Use, Land-Use Change, and Forestry section.
To find out more about the role of CO2 warming the atmosphere and its sources, visit the Causes of Climate Changepage and the Greenhouse Gas Indicators page in the Science section.
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Emissions and Trends
Carbon dioxide (CO2) emissions in the United States increased by about 5% between 1990 and 2012. Since the combustion of fossil fuel is the largest source of greenhouse gas emissions in the United States, changes in emissions from fossil fuel combustion have historically been the dominant factor affecting total U.S. emission trends. Changes in CO2 emissions from fossil fuel combustion are influenced by many long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2012, the increase in CO2 emissions corresponded with increased energy use by an expanding economy and population, and an overall growth in emissions from electricity generation. Transportation emissions also contributed to the 5% increase, largely due to an increase in miles traveled by motor vehicles.
U.S. Carbon Dioxide Gas Emissions, 1990-2012
Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012.
Going forward, CO2 emissions in the United States are projected to grow by about 1.5% between 2005 and 2020. 
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Reducing Carbon Dioxide Emissions
The most effective way to reduce carbon dioxide (CO2) emissions is to reduce fossil fuel consumption. Many strategies for reducing CO2 emissions from energy are cross-cutting and apply to homes, businesses, industry, and transportation.
Examples of Reduction Opportunities for Carbon Dioxide
||Examples of How Emissions Can be Reduced
||Improving the insulation of buildings, traveling in more fuel-efficient vehicles, and using more efficient electrical appliances are all ways to reduce energy consumption, and thus CO2 emissions.
||Reducing personal energy use by turning off lights and electronics when not in use reduces electricity demand. Reducing distance traveled in vehicles reduces petroleum consumption. Both are ways to reduce energy CO2 emissions through conservation.Learn more about What You Can Do at Home, at School, in the Office, and on the Road to save energy and reduce your carbon footprint.
||Producing more energy from renewable sources and using fuels with lower carbon contents are ways to reduce carbon emissions.
|Carbon Capture and Sequestration
||Carbon dioxide capture and sequestration is a set of technologies that can potentially greatly reduce CO2 emissions from new and existing coal- and gas-fired power plants, industrial processes, and other stationary sources of CO2. Learn more.
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*Carbon dioxide’s lifetime is poorly defined because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.
The world would warm by 4°C by the end of this century if we do not take concerted action now. The World Bank report “Turn Down the Heat: Climate Extremes, Regional Impacts, and the Case for Resilience“ is a result of contributions from a wide range of experts from across the globe. The report follows “Turn Down the Heat: Why a 4°C Warmer World Must be Avoided”, released in November 2012. This new report outlines an alarming scenario for the days and years ahead—what we could face in our lifetime.
The scientists tell us that if the world warms by 2°C—warming which may be reached in 20 to 30 years—that will cause widespread food shortages, unprecedented heat-waves, and more intense cyclones. In the near-term, climate change, which is already unfolding, could batter the slums even more and greatly harm the lives and the hopes of individuals and families who have had little hand in raising the Earth’s temperature. Today, our world is 0.8°C above pre-industrial levels of the 18th century.
We could see a 2°C world in the space of one generation. The first Turn Down the Heat report was a wake-up call. This second scientific analysis gives us a more detailed look at how the negative impacts of climate change already in motion could create devastating conditions especially for those least able to adapt. The poorest could increasingly be hit the hardest. For this report, we turned again to the scientists at the Potsdam Institute for Climate Impact Research and Climate Analytics. This time, we asked them to take a closer look at the tropics and prepare a climate forecast based on the best available evidence and supplemented with advanced computer simulations. With a focus on Sub-Saharan Africa, South East Asia and South Asia, the report examines in greater detail the likely impacts for affected populations of present day, 2°C and 4°C warming on critical areas like agricultural production, water resources, coastal ecosystems and cities.
The result is a dramatic picture of a world of climate and weather extremes causing devastation and human suffering. In many cases, multiple threats of increasing extreme heat waves, sea-level rise, more severe storms, droughts and floods will have severe negative implications for the poorest and most vulnerable. In Sub-Saharan Africa, significant crop yield reductions with 2°C warming are expected to have strong repercussions on food security, while rising temperatures could cause major loss of savanna grasslands threatening pastoral livelihoods.
In South Asia, projected changes to the monsoon system and rising peak temperatures put water and food resources at severe risk. Energy security is threatened, too. While, across South East Asia, rural livelihoods are faced with mounting pressures as sea-level rises, tropical cyclones increase in intensity and important marine ecosystem services are lost as warming approaches 4°C. Across all regions, the likely movement of impacted communities into urban areas could lead to ever higher numbers of people in informal settlements being exposed to heat waves, flooding, and diseases.
Here, we take a look at the world’s five biggest polluters, according to EDGAR http://edgar.jrc.ec.europa.eu/overview.php?v=intro&sort=des9
This picture from January 2013 shows two men walking in Beijing’s dense smog. (AFP/Getty Images)
China overtook the United States as the world’s biggest carbon dioxide polluter in 2006, and has topped the list ever since.
In 2012, the communist country pumped an estimated 9.8 billion tons of CO2 into the world’s atmosphere from the burning of fossil fuels such as oil and coal. And while emissions grew at a slower pace in 2012 than previous years, China still accounted for 70 percent of the global increase in CO2 emissions that year.
China’s smog levels are notorious. But air pollution is soaring to new heights due to the country’s rapid industrialization, reliance on coal power and increased car ownership for a booming population.
It doesn’t help that environmental laws are often ignored, according to green activists.
2. United States
Morning commuters travel the 210 freeway near Pasadena, Calif. (David McNew/AFP/Getty Images)
As stated above, America’s carbon dioxide emissions had been going down for years. The decline can be partly attributed to the economic recession, improved energy efficiency, and the shale-gas boom.
Emissions reached a 20-year low of just under 5.2 billion metric tons in 2012.
But new figures show CO2 emissions actually increased 2 percent in 2013 as more utilities turned to coal for energy after a steep rise in natural gas prices.
Emissions are still 10 percent below 2005 levels, but they were once down 12 percent. The Obama administration set a goal of being 17 percent below 2005 levels by 2020.
Suddenly, that goal looks like a steeper climb.
Indian commuters walk up a foot bridge in New Delhi on Jan. 31, 2013. (Prakash Singh/AFP/Getty Images)
India’s carbon dioxide emissions shot up 7.7 percent in 2012, with those from coal growing at an even faster pace of 10.2 percent.
That despite the South Asian country recording its lowest GDP growth — 4 percent — in a decade.
One in three Indians currently live in “critically-polluted” areas. And of the 180 cities monitored by India’s Central Pollution Control Board, just two — Malapuram and Pathanamthitta in Kerala — have what are considered “low” levels of air pollution.
A Russian woman wears a face mask to protect herself from smoke in Moscow. (Natalia Kolesnikova/AFP/Getty Images)
Russia’s carbon dioxide emissions plunged after the collapse of Soviet-era smokestack industries, but it remains the world’s fourth largest CO2 polluter at 1.7 billion metric tons in 2012.
Hundreds of cities currently exceed pollution limits, with Moscow, St. Petersburg and the far-northern Siberian city of Norilsk among the worst offenders.
Two cities — Norilsk and the central Russian city of Dzerzhinsk — made the environmental group Blacksmith Institute’s list of the world’s most polluted places this year.
The suffocating smog that blanketed swathes of China hit parts of Japan in March 2013. (Kazuhiro Nogi/AFP/Getty Images)
Japan recently watered down its target to cut carbon dioxide emissions despite the 1.3 million metric tons of CO2 it produced in 2012.
The new target, announced in November, reverses course from a goal set four years ago and now allows a 3.1 percent increase in emissions from 1990 levels rather than seeking a 25 percent cut.
It reflects the country’s increased reliance on fossil fuels in the aftermath of the Fukushimanuclear disaster in 2011.