Tree rings and isotopes

We know now that the Maunder Minimum is real. Maunder's data were reliable and a period when there were very few sunspots did indeed coincide with a period of very cold weather. We know he was right because there is other evidence to support him, evidence that was not available in his own day.

The supporting evidence comes from studies of tree rings and ice cores (see "Climates of the Past" on pages 37-43). Tree rings are made from the cells surrounding the trunk that are produced as new growth every year. Their thickness reflects the growing conditions and so the climate can be inferred from the rings produced by a representative sample of trees of a number of species. They provide an indirect, or proxy, climate record.

Wood also contains the radioisotope carbon-14 (14C). This is produced in the atmosphere by the bombardment of nitrogen atoms by cosmic radiation particles. Consequently, when the cosmic radiation intensifies more 14C is produced, and the air contains less when cosmic radiation is weak. When, in the 1940s, the American chemist Willard Frank Libby (1908-80) discovered that the rate of radioactive decay of 14C could be used to date biological material it was assumed that the proportion of 14C in the atmosphere is constant. This allowed scientists to compile a "Libby standard" relationship between age and the ratio of 14C to 12C in the sample. It was only later that the standard was found to be incorrect, because its underlying assumption was false. The proportion of 14C in the air is not constant. It changes, because the intensity of the cosmic radiation producing it changes. The dating calibration has now been corrected for this and earlier radiocarbon dates have been amended.

There was an advantage, however, to what might have seemed an embarrassing error. The error was discovered by comparing radiocarbon dates to tree-ring dates obtained from bristlecone pines (Pinus longaeva), which provide a continuous record over thousands of years. The comparison revealed variations in the proportion of atmospheric 14C, and this, in turn, allowed scientists to infer the intensity of cosmic radiation.

Ice cores provide another record, because 14C is one of a number of radioisotopes produced by cosmic-ray bombardment. There are also isotopes of nickel (59Ni, with a half-life of 0.1 million years), calcium (41Ca, half-life 0.11 million years), iron (60Fe, half-life 0.3 million years), chlorine (36Cl, half-life 0.31 million years), aluminum (26Al, half-life 1 million years), and beryllium (10Be, half-life 2.7 million years). Beryllium-10 is the isotope most widely used to measure past cosmic-ray intensity. Air containing these isotopes becomes trapped between snow grains, and some of them can be extracted from ice cores.

So the tree-ring climate record, together with written records, confirmed the period of cold weather. The radioisotopes confirmed the reduction in solar output. Together they showed that Maunder had been correct.

Despite the lack of recognition of his discovery, E. Walter Maunder was well known and widely respected for his other achievements. He became director of the Solar Division at the Royal Observatory, a Fellow of the Royal Astronomical Society, and he was editor of the Journal of the British Astronomical Association. He and his wife, Annie Scott Dill Russell (1868-1947), whom he met in 1891, when she arrived at the observatory to work as a "lady computer," collaborated in writing many popular articles on astronomy and the Sun.

More recently, many more sunspot minima have been found. The table lists some of them.

The link between sunspot minima and cold climatic episodes seems firmly established, as does the parallel link between sunspot maxima and warm periods. There is an even closer link between temperature and the length of the sunspot cycle. Although the sunspot cycle is usually described as having a period of 11 years, in fact this is an average around which the period varies. In 1890, for example, when the cycle was 11.7 years long, the average global temperature was 0.72°F (0.4°C) lower than usual, and in 1989, when the period was 9.8 years, the temperature was 0.45°F (0.25°C) warmer.

The inconstant Sun

SUNSPOT MINIMA

Name

Dates

Remarks

Damon

1880-1930

Average global temperature 0.9°F (0.5°C) lower.

Dalton

1795-1825

Includes 1813, the "year without a summer."

Maunder

1645-1715

Average global temperature 1.8-3.6'F (l-2°C) lower.

Sporer

1400-1520

Little Ice Age

Wolf

1280-1340 B.C.E.

End of Pueblo and Hohokam American periods.

Greek

330

Homeric

750

Egyptian

1370

Silver Lake

1870

Noachan

2850

Sumerian II

3290

Sumerian I

3570

Jericho

5190

Sahelan

5950

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment