Earth's energy balance

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Earth's energy balance


FAQ 1.1, Figure 1. Estimate of the Earth’s annual and global mean energy balance. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation.

About half of the incoming solar radiation is absorbed by the Earth’s surface.

This energy is transferred to the atmosphere by:

bulletwarming the air in contact with the surface (thermals), 
bullet evapotranspiration 
bulletlongwave radiation that is absorbed by clouds and greenhouse gases.

The atmosphere in turn radiates longwave energy back to Earth as well as out to space.

Source: Kiehl and Trenberth (1997).


 
 

What Determines Earth’s Climate?

The climate system consists of :

bulletthe atmosphere,
bulletland surface,
bulletsnow and ice,
bulletoceans and other bodies of water, (store nine times as much of the sun’s heat as do the atmosphere and land combined)
bulletliving things.

The Atmosphere is Affected by:

bulletvolcanic eruptions,
bulletsolar variations, and
bullethuman-induced changes in atmospheric composition.


Solar radiation powers the climate system.

There are three fundamental ways to change the radiation balance of the Earth:

  1. by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself);
  2. by changing the fraction of solar radiation that is reflected (called ‘albedo’;  by changes in cloud cover, atmospheric particles or vegetation); and
  3. by altering the longwave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations).


Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms.

The amount of energy  reaching the Earth’s atmosphere per square metre per second over the entire planet is one-quarter of  the amount of energy reaching the top of Earth’s atmosphere per second per square metre facing the Sun during daytime. ( 1,370 Watts) 

About 30% of the sunlight that reaches the top of the atmosphere is reflected back to space:

bullet2/3 of this reflectivity is due to clouds and small particles in the atmosphere known as ‘aerosols’.
bulletthe remaining 1/3 of the sunlight.  -- is reflected from snow, ice and deserts (Light-coloured areas of Earth’s surface – mainly )


The most dramatic change in aerosol-produced reflectivity comes when major
volcanic eruptions eject material very high into the atmosphere. Rain typically clears aerosols out of the atmosphere in a week or two, but when material from a violent volcanic eruption is projected far above the highest cloud, these aerosols typically influence the climate for about a year or two before falling into the troposphere and being carried to the surface by precipitation. Major volcanic eruptions can thus cause a drop in mean global surface temperature of about half a degree celsius that can last for months or even years. Some man-made aerosols also significantly reflect sunlight.

The energy that is not reflected back to space is absorbed by the Earth’s surface and atmosphere. This amount is approximately 240 Watts per square metre (W m–2). To balance the incoming energy, the Earth itself must radiate, on average, the same amount of energy back to space. The Earth does this by emitting outgoing longwave radiation. Everything on Earth emits longwave radiation continuously. That is the heat energy one feels radiating out from a fire; the warmer an object, the more heat energy it radiates. To emit 240 W m–2, a surface would have to have a temperature of around –19°C. This is much colder than the conditions that actually exist at the Earth’s surface (the global mean surface temperature is about 14°C). Instead, the necessary –19°C is found at an altitude about 5 km above the surface.

The reason the Earth’s surface is this warm is the presence of greenhouse gases, which act as a partial blanket for the longwave radiation coming from the surface. This blanketing is known as the natural greenhouse effect.

The most important greenhouse gases are water vapour and carbon dioxide. The two most abundant constituents of the atmosphere – nitrogen and oxygen – have no such effect.

Clouds, on the other hand, do exert a blanketing effect similar to that of the greenhouse gases; however, this effect is offset by their reflectivity, such that on average, clouds tend to have a cooling effect on climate (although locally one can feel the warming effect: cloudy nights tend to remain warmer than clear nights because the clouds radiate longwave energy back down to the surface).

Human activities intensify the blanketing effect through the release of greenhouse gases. For instance, the amount of carbon dioxide in the atmosphere has increased by about 35% in the industrial era, and this increase is known to be due to human activities, primarily the combustion of fossil fuels and removal of forests. Thus, humankind has dramatically altered the chemical composition of the global atmosphere with substantial implications for climate.

Why did Air-Surface temperature flatten 1999-2012 ?