Greenhouse effect
Greenhouse gases create the GHG effect
Greenhouse gases in the atmosphere block infrared radiation
from escaping directly from the surface to space.
This leads to warmimg the Earth’s surface
and the lower layers of an atmosphere
The earth’s climate is driven by a continuous flow of energy from the sun. This energy arrives mainly in the form of visible light. About 30% is immediately scattered back into space, but most of the remaining 70% passes down through the atmosphere to warm the earth’s surface.
The earth must send this energy back out into space in the form of infrared radiation. Being much cooler than the sun, the earth does not emit energy as visible light. Instead, it emits infrared, or thermal radiation. “Greenhouse gases” in the atmosphere block infrared radiation from escaping directly from the surface to space. Infrared radiation cannot pass straight through the air like visible light. Instead, most departing energy is carried away from the surface by air currents and clouds.
The main greenhouse gases are water vapour, carbon dioxide (CO2), ozone (O3), methane (CH4), nitrous oxide (N2O), and halocarbons and other industrial gases. Apart from the industrial gases all of these gases occur naturally. Together, they make up less than 1% of the atmosphere. This is enough to produce a “natural greenhouse effect” that keeps the planet some 30°C warmer than it would otherwise be — essential for life.
Levels of all key greenhouse gases (with the possible exception of water vapour) are rising as a direct result of human activity. Emissions of carbon dioxide (mainly from burning coal, oil, and natural gas), methane and nitrous oxide (due to agriculture and changes in land use), ozone (generated by the fumes in automobile exhausts) and long-lived industrial gases such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs) are changing how the atmosphere absorbs energy. The result is known as the “enhanced greenhouse effect”.
The climate system must adjust to rising greenhouse gas levels to keep the global “energy budget” in balance. In the long term, the earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate must change somehow to restore the balance between incoming and outgoing energy.
This adjustment will include a “global warming” of the earth’s surface and lower atmosphere. But this is only part of the story. Warming up is the simplest way for the climate to get rid of the extra energy. But even a small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).