Greenhouse Gases

The main constituents of the atmosphere are nitrogen (N2) with 78.09 percent, oxygen (O2) with 20.94 percent and Argon (A) with 0.93 percent, constituting already 99.96 percent of the dry atmosphere. The minor constituents absorbing strongly thermal infrared (heat) radiation are (if ranked according to importance in the undisturbed pre-industrial atmosphere):

1. Water vapour (H2O), responsible for nearly two-thirds of the greenhouse effect;

2. Carbon dioxide (CO2), responsible for about 20 percent;

3. Ozone (O3), responsible for about 7 percent;

4. Nitrous oxide (N2O), contributing only about 3 percent;

5. Methane (CH4) contributing less than 3 percent.

Some other naturally occurring gases, like carbon monoxide (CO), are weak greenhouse gases but are neglected here.

It is important to note that two of these five greenhouse gases are shortlived, namely water vapour and ozone, with lifetimes of about 9 days and hours to months, respectively. The other three gases are all called long-lived, although they differ strongly in their lifetimes. Methane needs about 12 years and nitrous oxide 120 years until their concentration would have fallen to about 37 percent, i.e. 1/e, if no emissions occurred. For carbon dioxide no single number can be given as the uptake into the ocean is a complex process that involves several time scales, e.g. sedimentation of organisms. About 200 years are needed to reach 1/e for the additional (anthropogenic) load.

Table 2.2 Recent changes of naturally occurring long-lived greenhouse gases due to human activities (IPCC, 2007a)

Species

Concentration

Year

1750

2005

Change since 1998

CO2 (ppm)

280

379 ± 0.65

+ 13

CH4 (ppb)

730

1,774 ± 1.8

+ 11

N2O (ppb)

270

319 ± 0.12

+ 5

The assessment of the consequences of an enhanced greenhouse effect (for concentration changes see table 2.2.) is made more complicated by the strong temperature dependence of the water cycle including the dominant greenhouse gas water vapour (see box. 2.3). Radiative transfer calculations with fixed atmospheric composition, except a doubling of carbon dioxide concentration, and allowing so-called convective adjustment in the troposphere, give a 1.2°C average warming of near surface air temperature. If water vapour reacts, like in so-called equilibrium models of general atmospheric circulation, the warming roughly doubles mainly due to two positive feedbacks, one by water vapour (already mentioned) and the other by the snow/ice-albedo1/temperature feedback. The big uncertainty still remaining for the sensitivity of the climate system to an enhanced greenhouse effect is due to the less well known feedback of clouds, where even the sign of the global mean effect is not known, although locally it is clear that more low, optically thick water clouds would

1 Albedo is the ratio between backscattered and incoming solar radiation flux density. Therefore it is zero for a black body and unity for a completely backscattering or reflecting surface. Typical values of natural surfaces like forests and grassland vary from about 10 to 20 percent, but can reach 90 percent for fresh powder snow.

dampen the enhanced greenhouse effect (negative feedback) and that cold but thin cirrus (ice clouds) in the upper troposphere would enhance it.

Box 2.3 Known Positive Feedbacks in the Water Cycle

The dominant cycle for the climate system is the water cycle. This dominance is due to several positive and potentially also negative feedbacks. Two positive feedbacks have to be named here caused by:

- The Clausius-Clapeyron equation

- The differences between the albedo of snow/ice and liquid water

Feedback 1: Assuming chemical equilibrium and the second law of thermodynamics we get for the change of water vapour pressure in the atmosphere dps at saturation for a temperature change dT dps ^

dT vT2

with

L = latent heat of evaporation v = specific volume of water vapour T = absolute temperature (K)

For atmospheric temperatures between +25°C at saturation (base of a tropical cumulus cloud) and -80°C at saturation (tropical cirrus cloud top) the saturation pressure ps increases from 6 to 20 percent per °C temperature change following this equation. Hence the water vapour pressure varies by up to four orders of magnitude between cloud base and cloud top of a severe tropical cumulonimbus. In other words: Nearly all water vapour in this column (~ 60 mm of precipitable water) will fall out as rain, for a convergent flow even more.

Therefore, the dominant greenhouse gas water vapour will show a positive feedback (i.e. it will amplify) when a warming is stimulated by a greenhouse gas concentration increase.

Feedback 2: The brightest and the darkest natural surface are composed of water: Fresh powder snow "reflects" (in reality mainly backscatters) about 85 percent of incoming solar radiation flux density while the ocean absorbs about 96 percent at blue skies and high sun; hence reflecting only 4 percent. Consequently the disappearance of snow or sea ice warms the lower atmosphere which leads to further melting nearby. This positive feedback is the key feedback in glacial cycles and is still important at present, as large parts of the Northern Hemisphere land and ocean are seasonally and smaller parts permanently covered by snow or snow on sea ice. This positive so-called snow/ice-albedo/temperature feedback is also fundamental for the inception of a glacial.

Was this article helpful?

0 0
Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment