Vital Climate Graphics - Update

THE GREENHOUSE EFFECT

10

Cooling or heating, a balancing act

Every year the sun delivers an average of 340 watts of energy to every square metre of the Earth. To produce this amount of energy we would need 440 million large electric power plants, each gener- ating 100 million watts of power (NASA). It would get uncomfortably hot on Earth with all this energy, but fortunately for us, the amount of heat we receive from the sun is balanced by heat radiated back into space by the atmosphere. Radiative forcing is the change in the balance between radiation coming into the atmosphere and radiation going out. A positive radiative forcing tends on average to warm the surface of the Earth, and negative forcing tends on average to cool the surface. Greenhouse gases, for example, produce positive radiative forcing – they trap outgoing terrestrial (infrared) radiation, which causes a temperature rise at the Earth’s surface – the “greenhouse effect”. In contrast, negative radiative forcing from clouds and aerosols, which can reflect back into space, acts as a cooling mechanism.

The enhanced greenhouse effect Greenhouse gases are a natural part of the atmo- sphere. Without these gases the global average temperature would be around -20ºC. The problem we now face is that human actions – particularly burning fossil fuels (coal, oil and natural gas) and land clearing – are increasing their concentrations. The more of these gases there are, the more heat is trapped. This is known as the enhanced green- house effect. Naturally occurring greenhouse gas- es include water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases that are not naturally occurring include hydro-fluorocar- bons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6), which are generated in a variety of industrial processes. On average, about one-third of the solar radiation that hits the Earth is reflected back into space. The land and the oceans mostly absorb the rest, with the remainder trapped in the atmosphere. The solar radiation that strikes the Earth’s surface heats it up, and as a result infrared radiation is emitted.

The Greenhouse effect

A T M O S P H E R E

Some of the infrared radiationpasses through the atmosphere and is lost in space Net outgoing infrared radiation: 240 Watt per m 2

Some solar radiation is reflected by the atmosphere and Earth’s surface Outgoing solar radiation: 103 Watt per m 2

Solar radiationpasses through the clear atmosphere Incoming solar radiation: 343 Watt per m 2

S

U

E

O

H

G

N

A

E

S

E

E

R

S

G

Some of the infrared radiation is absorbed and re-emitted by the greenhouse gas molecules. The direct effect is the warming of the Earth’ssurfaceand the troposphere.

Net incoming solar radiation: 240 Watt per m 2

Surface gains more heat and infrared radiation is emitted again

Solar energy is absorbed by the Earth’s surface and warms it...

... and is converted into heat causing the emission of longwave (infrared) radiation back to the atmosphere

168 Watt per m 2

E A R T H

UnitedNationsEnvironmentProgramme /GRID-Arendal

Sources:OkanaganUniversityCollege,University ofOxford,EPA, IPCC.

Radiative forcing The radiative forcing from the increase in anthropogenic greenhouse gases since the pre-industrial era is positive (warming) with a small uncertainty range; that from the direct effects of aerosols is negative (cooling) and smaller; whereas the negative forcing from the indirect effects of aerosols (on clouds and the hydrologic cycle) might be large but is not well quantified. Key anthropogenic and natu- ral factors causing a change in radiative forc- ing from year 1750 to year 2000 are shown in this figure, where wide, colored bars mark the factors whose radiative forcing can be quantified. Only some of the aerosol effects are estimated here and denoted as ranges. Other factors besides atmospheric constitu- ents -- solar irradiance and land-use change -- are also shown. Stratospheric aerosols from large volcanic eruptions have led to important, but short-lived, negative forcings (particularly during the periods 1880-1920 and 1960-1994), which are not important over the time scale since the pre-industrial era and not shown. The sum of quantified factors in the figure is positive, but this does not include the potentially large, negative forcing from aerosol indirect effects.

Global mean radiative forcing (Wm -2 ) Anthropogenic and natural forcing of the climate for the year 2000, relative to 1750

Greenhouse gases

3

Halocarbons

Aerosols + clouds

N 2

O

2

Black carbon from fossil fuel burning

CH 4

1

Warming

Tropospheric ozone

Mineral Dust

CO 2

Solar

Contrails Cirrus Aviation

0

Stratospheric ozone

Organic carbon from fossil fuel burning

Land use (albedo only)

Biomass burning

Sulphate

-1

Cooling

-2

Aerosol indirect effect

The height of a bar indicates a best estimate of the forcing, and the accompanying vertical line a likely range of values. Where no bar is present the vertical line only indicates the range in best estimates with no likelihood.

LEVEL OF SCIENTIFIC UNDERSTANDING

UnitedNationsEnvironmentProgramme /GRID-Arendal