By trapping heat from the sun, greenhouse gases have kept Earth's climate habitable for humans and millions of other species. But those gases are now out of balance and threaten to change drastically which living things can survive on this planet—and where.

Atmospheric carbon dioxide—the most dangerous and prevalent greenhouse gas—is at the highest levels ever recorded. Greenhouse gas levels are so high primarily because humans have released them into the air by burning fossil fuels. The gases absorb solar energy and keep heat close to Earth's surface, rather than letting it escape into space. That trapping of heat is known as the greenhouse effect.

The roots of the greenhouse-effect concept lie in the 19th century, when French mathematician Joseph Fourier calculated in 1824 that Earth would be much colder if it had no atmosphere. In 1896, Swedish scientist Svante Arrhenius was the first to link a rise in carbon dioxide gas from burning fossil fuels with a warming effect. Nearly a century later, American climate scientist James E. Hansen testified to the United States Congress that “The greenhouse effect has been detected and is changing our climate now."

Today, climate change is the term scientists use to describe the complex shifts, driven by greenhouse-gas concentrations, that are now affecting our planet’s weather and climate systems. Climate change encompasses not only the rising average temperatures we refer to as global warming but also extreme weather events, shifting wildlife populations and and habitats, rising seas, and a range of other impacts.

Governments and organizations around the world, such as the Intergovernmental Panel on Climate Change (IPCC), the United Nations body that tracks the latest climate change science, are measuring greenhouse gases, tracking their impacts, and implementing solutions.

Major Greenhouse Gases and Sources

Carbon dioxide (CO2): Carbon dioxide is the primary greenhouse gas; it is responsible for about three-quarters of emissions. It can linger in the atmosphere for thousands of years. In May 2019, carbon dioxide levels reached 414 parts per million at Hawai'i's Mauna Loa Atmospheric Baseline Observatory, the highest monthly average ever recorded. Carbon dioxide emissions mainly come from burning organic materials: coal, oil, gas, wood, and solid waste.

Methane (CH4): The main component of natural gas, methane is released from landfills, natural gas and petroleum industries, and agriculture (especially from the digestive systems of grazing animals). A molecule of methane doesn't stay in the atmosphere as long as a molecule of carbon dioxide—about 12 years—but it is at least 84 times more potent over two decades. It accounts for about 16 percent of all greenhouse gas emissions.

Nitrous Oxide (N2O): Nitrous oxide occupies a relatively small share of global greenhouse gas emissions—about six percent—but it is 264 times more powerful than carbon dioxide over 20 years, and its stay in the atmosphere exceeds a century, according to the IPCC. Agriculture and livestock—including fertilizer, manure, and burning agricultural residues—along with burning fuel, are the biggest sources of nitrous oxide emissions.

Industrial gases: Fluorinated gases, such as hydrofluorocarbons, perfluorocarbons, chlorofluorocarbons, sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3) have heat-trapping potential thousands of times greater than CO2 and stay in the atmosphere for hundreds to thousands of years. Accounting for about two percent of all emissions, they're used as refrigerants, solvents, and in manufacturing, sometimes occurring as byproducts.

Other greenhouse gases include water vapor (H2O) and ozone (O3). Water vapor is actually the world's most abundant greenhouse gas, but it is not tracked the same way as other greenhouse gases because it is not directly emitted by human activity and its effects are not well understood. Similarly, ground-level or tropospheric ozone (not to be confused with the protective stratospheric ozone layer higher up) is not emitted directly but emerges from complex reactions among pollutants in the air.

Effects of Greenhouse Gases

Greenhouse gases have far-ranging environmental and health effects. They cause climate change by trapping heat, and they also contribute to respiratory disease from smog and air pollution. Extreme weather, food-supply disruptions, and increased wildfires are other effects of climate change caused by greenhouse gases. The typical weather patterns we've grown to expect will change; some species will disappear; others will migrate or grow.

How To Reduce Greenhouse-Gas Emissions

Virtually every sector of the global economy, from manufacturing to agriculture to transportation to power production, adds greenhouse gases to the atmosphere, so all of them must evolve away from fossil fuels if we are to avoid the worst effects of climate change. Countries around the world acknowledged this reality with the Paris Climate Agreement of 2015. The changes will be most important among the biggest emitters: Twenty countries are responsible for at least three-quarters of the world's greenhouse gas emissions, with China, the United States, and India leading the way.

The technologies for ramping down greenhouse-gas emissions already exist, for the most part. They include swapping fossil fuels for renewable sources, boosting energy efficiency, and discouraging carbon emissions by putting a price on them. The world technically has only one-fifth of its "carbon budget"—the total is 2.8 trillion metric tons—remaining in order to avoid warming the planet more than 1.5 degrees Celsius (2.7 degrees Farenheit). Halting the trends in motion will require more than just phasing out fossil fuels. In fact, the paths to halting global temperature increases of 1.5 or 2 degrees Celsius (2.7 or 3.6 degrees Farenheit), the two goals outlined by the IPCC, rely in some way on adopting methods of sucking CO2 from the sky. Those include planting trees, conserving existing forests and grasslands, and capturing CO2 from power plants and factories.

Carbon Dioxide Levels Are at a Record High. Here's What You Need To Know.

Human activity is largely responsible for the high levels of greenhouse gases through the release of fossil fuels burned in the atmosphere, like this power plant in Juliette, Georgia, United States.

carbon dioxide
Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

climate
Noun

all weather conditions for a given location over a period of time.

Noun

gradual changes in all the interconnected weather elements on our planet.

emission
Noun

discharge or release.

emission
Noun

discharge or release.

fossil fuel
Noun

coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

Noun

increase in the average temperature of the Earth's air and oceans.

greenhouse gas
Noun

gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.

methane
Noun

chemical compound that is the basic ingredient of natural gas.

nitrous oxide
Noun

greenhouse gas used in medicine and the manufacture of rockets. Also known as laughing gas or happy gas.

ozone
Noun

form of oxygen that absorbs ultraviolet radiation.