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Flickering sun switched climate

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19.12.2001

Europe’s Little Ice Age coincided with low solar activity.
Pieter Brueghel’s painting ’The Census at Bethlehem’.

A solar slump may have chilled the Northern Hemisphere.

The flickering sun may cause rapid climate change, according to a new comparison of climate records. A 200-year cold snap 10, 300 years ago seems to have coincided with a passing slump in the sun’s activity¹.

Svante Bjorck of Lund University in Sweden and colleagues looked at sediments in Lake Starvatn on the Faroe Islands and in the Norwegian Sea, the width of growth rings in ancient German pine trees, and ancient ice drilled from deep within the Greenland ice sheet.

Each of these indicates how some characteristic of the environment has changed over the 11,000 or so years since the last ice age ended. The chemical composition of the ice core, for example, shows how the temperature of the atmosphere has changed. The distance between tree-rings reflects the average ambient temperature during each successive growing season.

Ice and trees show that the climate became suddenly colder about 10,300 years ago, then gradually warmed again over the ensuing century. Other records from the Californian coast and Tibet suggest that the cold snap may have been felt throughout the Northern Hemisphere, and perhaps worldwide.

Bjorck and colleagues propose that a weakening of solar activity may have caused this mini chill. It coincided, they find, with a large increase in the amount of beryllium-10 trapped in Greenland ice - evidence of a solar flicker.

This radioactive form of beryllium is produced when cosmic rays from space collide with nitrogen and oxygen atoms in the atmosphere. The magnetic field around the Earth protects the planet from cosmic rays. This field is stronger when the sun is more active - emitting more ultraviolet radiation and displaying more sunspots - so fewer cosmic rays can penetrate.

The proposed relationship between solar activity and climate change is controversial, partly because some have tried to pin modern-day global warming on it rather than on a human-induced greenhouse effect.

There is evidence, however, linking changes in solar activity to climate fluctuations in the more recent past. Abnormally high activity around AD 1100-1250, for example, has been mooted as the cause of a period of warming in medieval Europe. And the ’Little Ice Age’ between the sixteenth and eighteenth centuries coincided with a period of low solar activity.

Core values

The most pronounced climate swings, such as ice ages, happen slowly and last a long time - 100,000 years or so. Gradual, periodic changes in the shape of the Earth’s orbit around the sun are thought to trigger these larger-scale changes.

Evidence of shorter-term climate change has been observed before in ice-core records from Greenland and Antarctica. Apparently the global average temperature can switch between today’s mild climate and ice-age frigidity in just a few decades.

Sudden shifts are thought to be mostly due to ocean circulation. When ice sheets melt at the end of an ice age, the oceans get an injection of fresh water. By making seawater less salty and therefore less dense, this can suppress the conveyor-belt circulation that normally carries warm water from the tropics to the poles. Deprived of this source of heat, the high latitudes grow cold.

References

1. Bjorck, S. et al. High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger. Geology, 29, 1107 - 1110, (2001).

PHILIP BALL | Source: © Nature News Service

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