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Atmospheric concentrations of greenhouse gases

Earth’s temperature is a balancing act

Graph displaying that models accounting solely for natural factors understate current climate trends by ~1 degree F, compared to models that include human factors, which accurately predict observed temperatures.

Models that account only for the effects of natural processes are not able to explain the warming observed over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.

Click the image to view a larger version.Earth’s temperature depends on the balance between energy entering and leaving the planet’s system. When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When absorbed energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including:

These factors have caused Earth’s climate to change many times.

Scientists have pieced together a record of Earth’s climate, dating back hundreds of thousands of years (and, in some cases, millions or hundreds of millions of years), by analyzing a number of indirect measures of climate such as ice cores, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in Earth’s orbit around the sun.[2]

This record shows that the climate system varies naturally over a wide range of time scales. In general, climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations.[2]

Recent climate changes, however, cannot be explained by natural causes alone. Research indicates that natural causes do not explain most observed warming, especially warming since the mid-20thcentury. Rather, it is extremely likely that human activities have been the dominant cause of that warming.[2]

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Radiative Forcing

Radiative forcing is a measure of the influence of a particular factor (e.g. GHGs, aerosols, or land use changes) on the net change in Earth’s energy balance. On average, a positive radiative forcing tends to warm the surface of the planet, while a negative forcing tends to cool the surface.

GHGs have a positive forcing because they absorb energy radiating from Earth’s surface, rather than allowing it to be directly transmitted into space. This warms the atmosphere like a blanket. Aerosols, or small particles, can have a positive or negative radiative forcing, depending on how they absorb and emit heat or reflect light. For example, black carbon aerosols have a positive forcing since they absorb sunlight. Sulfate aerosols have a negative forcing since they reflect sunlight back into space.

NOAA’s Annual GHG Index, which tracks changes in radiative forcing from GHGs over time, shows that such forcing from human-added GHGs has increased 27.5 percent between 1990 and 2009. Increases in CO2 in the atmosphere are responsible for 80% of the increase. The contribution to radiative forcing by CH4 and CFCs has been nearly constant or declining, respectively, in recent years.

The greenhouse effect causes the atmosphere to retain heat

When sunlight reaches Earth’s surface, it can either be reflected back into space or absorbed by Earth. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). Greenhouse gases like water vapor (H2O), carbon dioxide (CO2), and methane (CH4) absorb energy, slowing or preventing the loss of heat to space. In this way, GHGs act like a blanket, making Earth warmer than it would otherwise be. This process is commonly known as the “greenhouse effect.”

The role of the greenhouse effect in the past

Over the last several hundred thousand years, CO2 levels varied in tandem with the glacial cycles. During warm “interglacial” periods, CO2 levels were higher. During cool “glacial” periods, CO2 levels were lower.[2] The heating or cooling of Earth’s surface and oceans can cause changes in the natural sources and sinks of these gases, and thus change greenhouse gas concentrations in the atmosphere.[2] These changing concentrations are thought to have acted as a positive feedback, amplifying the temperature changes caused by long-term shifts in Earth’s orbit.[2]