Greenhouse effect in the context of "Carl Sagan"

Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The modern-day rise in global temperatures is driven by human activities, especially fossil fuel (coal, oil and natural gas) burning since the Industrial Revolution. Fossil fuel use, deforestation, and some agricultural and industrial practices release greenhouse gases. These gases absorb some of the heat that the Earth radiates after it warms from sunlight, warming the lower atmosphere. Carbon dioxide, the primary gas driving global warming, has increased in concentration by about 50% since the pre-industrial era to levels not seen for millions of years.

Climate change has an increasingly large impact on the environment. Deserts are expanding, while heat waves and wildfires are becoming more common. Amplified warming in the Arctic has contributed to thawing permafrost, retreat of glaciers and sea ice decline. Higher temperatures are also causing more intense storms, droughts, and other weather extremes. Rapid environmental change in mountains, coral reefs, and the Arctic is forcing many species to relocate or become extinct. Even if efforts to minimize future warming are successful, some effects will continue for centuries. These include ocean heating, ocean acidification and sea level rise.

The atmosphere of Earth consists of a layer of mixed gas (commonly referred to as air) that is retained by gravity, surrounding the Earth's surface. It contains variable quantities of suspended aerosols and particulates that create weather features such as clouds and hazes. The atmosphere serves as a protective buffer between the Earth's surface and outer space. It shields the surface from most meteoroids and ultraviolet solar radiation, reduces diurnal temperature variation – the temperature extremes between day and night, and keeps it warm through heat retention via the greenhouse effect. The atmosphere redistributes heat and moisture among different regions via air currents, and provides the chemical and climate conditions that allow life to exist and evolve on Earth.

By mole fraction (i.e., by quantity of molecules), dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases (see Composition below for more detail). Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.

Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (CO₂), from burning fossil fuels such as coal, oil, and natural gas, is the main cause of climate change. The largest annual emissions are from China followed by the United States. The United States has higher emissions per capita. The main producers fueling the emissions globally are large oil and gas companies. Emissions from human activities have increased atmospheric carbon dioxide by about 50% over pre-industrial levels. The growing levels of emissions have varied, but have been consistent among all greenhouse gases. Emissions in the 2010s averaged 56 billion tons a year, higher than any decade before. Total cumulative emissions from 1870 to 2022 were 703 GtC (2575 GtCO₂), of which 484±20 GtC (1773±73 GtCO₂) from fossil fuels and industry, and 219±60 GtC (802±220 GtCO₂) from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2022, coal 32%, oil 24%, and gas 10%.

Carbon dioxide is the main greenhouse gas resulting from human activities. It accounts for more than half of warming. Methane (CH₄) emissions have almost the same short-term impact. Nitrous oxide (N₂O) and fluorinated gases (F-gases) play a lesser role in comparison. Emissions of carbon dioxide, methane and nitrous oxide in 2023 were all higher than ever before.

Greenhouse gases (GHGs) are the gases in an atmosphere that trap heat, raising the surface temperature of astronomical bodies such as Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). Human-induced warming has been increasing at a rate that is unprecedented in the instrumental record, reaching 0.27 [0.2–0.4] °C per decade over 2015–2024. This high rate of warming is caused by a combination of greenhouse gas emissions being at an all-time high of 53.6±5.2 Gt CO2e yr−1 over the last decade (2014–2023), as well as reductions in the strength of aerosol cooling.

The five most abundant greenhouse gases in Earth's atmosphere, listed in decreasing order of average global mole fraction, are: water vapor, carbon dioxide, methane, nitrous oxide, ozone. Other greenhouse gases of concern include chlorofluorocarbons (CFCs and HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons, SF
₆, and NF
₃. Water vapor causes about half of the greenhouse effect, acting in response to other gases as a climate change feedback.

In the atmosphere of Earth, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis, and oceanic carbon cycle. It is one of three main greenhouse gases in the atmosphere of Earth. The concentration of carbon dioxide (CO₂) in the atmosphere reached 427 ppm (0.0427%) on a molar basis in 2024, representing 3341 gigatonnes of CO₂. This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity.

The current increase in CO₂ concentrations is primarily driven by the burning of fossil fuels. Other significant human activities that emit CO₂ include cement production, deforestation, and biomass burning. The increase in atmospheric concentrations of CO₂ and other long-lived greenhouse gases such as methane increase the absorption and emission of infrared radiation by the atmosphere. This has led to a rise in average global temperature and ocean acidification. Another direct effect is the CO₂ fertilization effect. The increase in atmospheric concentrations of CO₂ causes a range of further effects of climate change on the environment and human living conditions.

Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than in the planetary average. This is commonly referred to as the ratio of polar warming to tropical warming. On a planet with an atmosphere that can restrict emission of longwave radiation to space (a greenhouse effect), surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict. The poles will experience the most cooling when the global-mean temperature is lower relative to a reference climate; alternatively, the poles will experience the greatest warming when the global-mean temperature is higher.

In the extreme, the planet Venus is thought to have experienced a very large increase in greenhouse effect over its lifetime, so much so that its poles have warmed sufficiently to render its surface temperature effectively isothermal (no difference between poles and equator). On Earth, water vapor and trace gasses provide a lesser greenhouse effect, and the atmosphere and extensive oceans provide efficient poleward heat transport. Both palaeoclimate changes and recent global warming changes have exhibited strong polar amplification, as described below.