Secrets of a thousand summers revealed in growth rings of trees

THE STUDY of ancient trees and ancient ice is providing dramatic evidence that the world is now growing notably warmer than it has been for at least the last thousand years.

These so-called "proxy" climate indicators have been used by the scientists at the Climatic Research Unit (CRU) at the University of East Anglia in Norwich to produce a remarkable annual record of summer temperatures of the last millennium.*

Examining the growth rings found inside the trees, and the snowfall and melt rings visible when a core is taken deep into the ice, they have been able to piece together what summer was like for every year since well before the Norman conquest.

They can indicate, for example, that in 1106, whenHenry I was on the throne, it was swelteringly hot, and in 1601, when Elizabeth I had only two more years left to reign, it was decidedly chilly.

But more important than any such annual detail is their discovery that the 1990s have been hotter than any period in the whole 1,000 years, and that 1998 is the warmest year of all.

This has been deduced because Professor Phil Jones and his colleagues at the CRU, who have compiled the 1,000-year "proxy" record, are the very people who are in charge of maintaining and updating the modern instrumental record (now taken by accurate thermometers) which goes back to 1855. Professor Jones, one of the world's leading experts on the global temperature record, does not equivocate. "The work we have done with various proxy climate indicators would indicate that 1998 will probably be the warmest year of the millennium," he says

Much of data for the temperature reconstruction has been uncovered by his colleague, Dr Keith Briffa, whose academic interest is made clear by a notice on his office door: Tree Rings Tell It Like It Was.

Dr Briffa is one of the world's leading experts on deciphering climate signals from the concentric circles visible when a tree is cut across its trunk. Each ring represents a year's annual growth, and in general, the thicker and denser the ring, the more favourable that year's growing conditions.

In northern latitudes, warmth (or the lack of it) is usually the limiting factor: a wide ring means a hot summer. In drier regions, such as the south-western United States, the limiting factor is moisture, (as illustrated in the picture above). Ice cores are analysed in two main ways: for their melt layers, which record how warm a summer was by how much ice melted and refroze; and by analysis of their oxygen isotopes - the heavy isotope O18 is more present in years that were warm.

The scientists at the CRU have used 17 separate sets of data to build up summer average temperatures for both hemispheres from 1000 to 1991, with 10 of them used to produce their northern hemisphere record. Five of these are from tree rings, from northern Sweden, western Siberia, Alberta and Manitoba in Canada, and Idaho in the US.

Three data sets are from ice cores, one from Spitzbergen and two from Greenland, and the remaining two are written records of temperature compiled for central England and central Europe, which go back to the 17th and 16th centuries respectively.

The scientists are quick to point out that there are many uncertainties, especially in the earlier part of the record up to about 1500, which relies on only four data sets, three from tree rings and one from ice cores. But they have found that when they come to the more recent period, when temperatures were recorded accurately by scientific instruments, the "proxy" record corresponded closely with the "instrumental" one.

"We can learn an enormous amount about the climate of the past from proxy indicators," Dr Briffa said. "All the time there is an increasing sophistication of the methods by which we can interpret the data." Another of the world's leading tree-ring experts, Professor Henri Grissino-Mayer from Valdosta State University, Georgia, is full of admiration for Professor Jones, Dr Briffa and their colleagues, and their thousand years of temperature history.

"They have uncovered a major modern disruption of the temperature record," he says. "By putting the tree-ring record alongside the modern instrumental record, these guys have discovered that the temperatures we are experiencing now are unprecedented for the last 1,000 years.

*High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures. PD Jones, KR Briffa, (Climatic Research Unit, University of East Anglia) TP Barnett (Scripps Institution of Oceanography, La Jolla, California) and SFB Tett (Hadley Centre for Climate Prediction and Research, Bracknell, Berkshire), The Holocene 8, 4 (1998) pp455-471.

AD1106 - 2nd Hottest Summer

In 1106, England was still being oppressed by the Normans 40 years after William the Conqueror gained the kingdom at the battle of Hastings. Scotland and Wales were still independent.

On the English throne was the Conqueror's third son, Henry I. He succeeded his brother William Rufus who was killed in 1100.

Forests still covered much of Britain: there were still wolves and wild boars.

Dr Keith Briffa, of the Climatic Research Unit, says the extreme heat could have been from a powerful El Nino, the climatic event blamed for this year's warmth.

AD1601 -The

Coolest Summer

In 1601, Elizabeth I was near the end of her reign.

Shakespeare had probably just written Twelfth Night.

The summer was chilly. Research indicates it was the coldest summer of the millennium in the hemisphere.

John Stowe's Annals of England, 1605, remarks of 1601: "The month of June was very cold. Frosts every morning." As England wasusing the Gregorian calendar, some frosts were in July.

In 1600, Huaynaputina, a volcano in Peru, erupted, spewing dust and debris into the stratosphere, blocking out the sun's warmth.