Tuesday, July 18, 2006 ... Deutsch/Español/Related posts from blogosphere

Solar forcing

One of the most obvious phenomena that are important for determining the climate on the Earth is the solar activity. The Sun is important because we receive 99.998% of energy from this rather well-known star (the rest are mostly geothermal heat sources). Recall from the text

how those averaged 342 Watts per squared meter (one quarter of 1370 W/m^2, the solar constant) coming from the Sun are absorbed and/or reflected.

If the solar constant changes, the equilibrium temperature of the Earth changes, too. You can use the Stefan-Boltzmann law to have a rough idea about the effect. Because the heat (power) emitted by the Earth goes like

  • IR emission = const T^4

where T is the temperature in Kelvins, you can see that each change of the temperature by 0.2% (around 0.6 Kelvins because we live in nearly 300 Kelvins) requires a 0.8% change of the solar constant because of the fourth power; this relation is the black body value of temperature sensitivity. 0.8 percent from 342 W/m^2 is about 3 W/m^2, way too much. In fact, we should count these 0.8 percent from the actual IR emission only, which is about 2/3 of those 342 W/m^2, more precisely 235 W/m^2. Still, you might need a change of 2 W/m^2 or so to explain the warming by 0.6 Kelvins during the 20th century.

The real "bare" change of the solar constant in the last 100 years was probably smaller, about 0.4 W/m^2. However, Nir Shaviv in

proposed a mechanism, including a calculation, that attributes a very important amplifying role to the reduced cosmic ray flux. (See also astro-ph/0209252.) The reduced intensity of the cosmic rays decreases the formation of the low-level clouds (through a suppression of their condensation cores). This change allows more solar rays to reach the surface of our planet which results in warming. With this mechanism accounted for, Shaviv changes the bare estimate of 0.2 Kelvin degrees resulting from the 0.4 W/m^2 increase of the solar constant to an improved estimate of 0.4 Kelvin degrees.

The reason why I write about these mechanisms right now is that

has just posted a text about the solar forcing. You should notice that the calculated increase of the temperature - written as 0.4 or 0.5 degrees, according to the exact source - is a majority or nearly all of the observed 20th century warming which was around 0.6 degrees.

Surely, some numbers will be recalculated, modified. New terms will be added and others will be ignored in the future. But the main message of this exercise is that there are definitely other terms that are likely to be as important as the greenhouse effect or even more important. It would be completely irrational to pick one particular term - the greenhouse effect of the carbon dioxide - and create new ministries responsible for this single term.

The full answer to an important question - for example, what temperatures do we expect in 2050 - has many other terms and even the sign of the sum may differ from the sign of one individual term. When you get an idea about the sign of one term in a sum, you are still pretty far from a full calculation.

In 2003, Nir Shaviv and Ján Veizer studied the effect of galactic cosmic rays on the

and found that 2/3 of the changes could be attributed to the passages of the solar system through the spiral arms of the Milky Way. Assuming that the rest is attributed to carbon dioxide, they obtain 0.5 Kelvins as the likely value of the climate sensitivity, with the probability that the value exceeds 2 Kelvins being below 1%.

Some people might be interested in the citation counts: the Shaviv-Veizer paper has 38 citations right now while its main competitor,

has 25 citations now, despite the heavy omnipotent bias promoting CO2. But whoever wants to have a strong opinion should actually try to read both of these papers and perhaps even the 8-citation vague and critical paper by RealClimate.ORG's Rahmstorf et al.

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