Sunday, January 17, 2010

7. The Ominously Quiet Sun

The Intergovernmental Panel on Climate Change (IPCC) and many of its supporters have tried to discount the sun's influence on the climate by arguing that changes in solar luminosity are not enough to explain the observed temperature changes. This flimflam is yet another example of a true but irrelevant fact, since it is not the tiny variations in brightness that matter, but the solar activity, as indicated by sunspots (i.e., the relatively dark, planet-sized blotches in the photosphere, the outer layer of the sun).


Under normal conditions, the number of sunspots follows a well-known cycle with an average period of around 11 years. You can check the current number at spaceweather.com, where you will find a daily photo of the sun from SOHO (the Solar and Heliospheric Observatory, a NASA/ESA spacecraft that maintains a constant watch from L1, the point where the gravity of the Earth just balances that of the sun). A new cycle, the 24th since records have been kept, was supposed to begin in early 2007, but it did not happen. A few tiny sunspots occasionally appeared, but generally lasted less than 24 hours.  There were 266 spotless days in 2008 and 260 in 2009, making them the 4th and 5th most spotless years since the daily record began in 1849. To find two sequential years with so few spots, we have to go back to the Dalton Minimum, a period from 1790 to 1830 when solar activity was abnormally low.

There have been one or two larger spots in the last few weeks, but it is too early to tell whether the new cycle is finally getting under way. In any case, there is no doubt that we are in the deepest and longest solar minimum in two centuries. Moreover, the solar wind is the weakest it has been since measurements in space began, 50 years ago.


When the sun is active – i.e., when there are many sunspots – the magnetic field expands and the solar wind increases. One of the results is that cosmic rays from the galaxy are deflected, so that fewer of them reach the Earth. The clear connection between sunspots and the flux of cosmic ray neutrons is shown in the figure. (Note that the right-hand scale for neutron counts is inverted: the flux is minimum when the sunspots are maximum, and vice versa.)


Many authors have suggested that ionization due to cosmic ray showers that stop in the lower atmosphere triggers nucleation of low clouds that reflect sunlight back into space, leading to global cooling. We now have direct evidence for this effect.

Every now and then, the sun exhibits a coronal mass ejection (i.e., it throws off a large cloud of magnetized plasma). If the cloud passes near the Earth, the flux of galactic cosmic rays drops rapidly, on a scale of hours, and returns to normal over the next few days. This phenomenon is known as a Forbush decrease (after the physicist who first studied the effect in the late 1930s). A recent analysis of independent satellite and terrestrial measurements during 26 Forbush decreases between 1987 and 2007 has shown quite clearly that the global coverage of clouds and their liquid water content decreased during each event.

Whether or not cloud formation is the mechanism connecting sunspots and climate, periods of low solar activity have always been associated with really frigid weather, worldwide. The rout of Napoleon's army during the retreat from Moscow in 1812 was due to the remarkably severe winter, and thus to the Dalton Minimum. The Maunder Minimum, a 70-year period with very few sunspots from 1645 to 1715, was even more extreme: there were skating parties on the Thames in winter, and people could walk across the sea-ice from Manhattan to Staten Island.

Sunspots have been observed sporadically for at least two thousand years, but consistent records only date from the start of telescopic astronomy in the 17th century. Before that, proxy measurements indicate that these frigid periods occur every few centuries.

[The radioactive isotope carbon-14 (a.k.a. 14C) has a half-life of 5700 years, so trace amounts are still found in atmospheric CO2 only because it is replenished by cosmic ray neutrons, which collide with 14N atoms in the upper atmosphere and transmute them into carbon and hydrogen (14C and 1H). The amount of 14C found in a tree ring (after correction for radioactive decay) is a measure of the atmospheric concentration when the ring was formed, from which the average cosmic ray flux can be inferred, and hence the solar activity.]

In particular, it appears that the Medieval Climate Optimum, the warm period from 800 to 1300 AD, was a time of high solar activity, and that the Little Ice Age was related to the Spörer Minimum (from 1460 to 1550) and the Maunder and Dalton Minima.

The evidence is thus quite strong that the cooling trend that began in 2002 is due to the quiet sun – and the pattern so far is ominously similar to the beginning of the Dalton Minimum. The sunspot number will presumably increase during the next few years, reaching a maximum around 2015. If  that peak is weak (a few tens of sunspots instead of hundreds), we can expect severe global cooling during the following two or three decades. 

Solar physicists are already discussing an appropriate name for this potential calamity. The probable choice is the Eddy Minimum, after the astronomer Jack Eddy, who died last June but published a landmark paper on this subject in 1976. 

The effects could be much worse than anything warming might do: for example, they could include catastrophic damage to the wheatfields in Canada and the USA that have become the breadbasket of the world. Prudence therefore demands that planning for climate change include consideration of how we might adapt to a much colder world.

1 comment:

Anonymous said...

waiting for next post