Ice reveals mysteries behind medieval murder

Assassinated: Thomas Becket was beheaded in Canterbury Cathedral on 29 December 1170. © Getty

Is our history written in the air? Scientists have dug up evidence in the Swiss Alps that shows how the death of an archbishop in the 12th Century led to a drop in medieval air pollution.

Breaking news! Thomas Becket, the Archbishop of Canterbury, has been brutally murdered in his cathedral by four knights acting on behalf of the king of England.

This was the gruesome story that shocked medieval Europe 850 years ago. The Pope excommunicated the king, Henry II, and made Thomas Becket a saint. Appalled by what had been done in his name, the king did penance and walked barefoot to Canterbury, his monks whipping him as he went.

And now, a new discovery in the Colle Gnifetti glacier in the Swiss-Italian Alps sheds new light on the full impact of this political assassination.

Scientists have used highly sensitive lasers to examine tiny slices of ice that froze during the reign of Henry II. Trapped there is a record of the air pollution in Europe during the middle ages.

We think of air pollution as a modern problem. But in the 12th Century, lead mining in northern England produced toxic air that blew across Europe and left traces of lead in the glacial ice. This metal was used in the roofs and windows of the finest medieval buildings. So, high levels of air pollution give historians a snapshot of an industrious and prosperous country.

They compared the rise and fall in pollution with historical accounts and discovered that, in 1170, there was a sharp drop in the levels of lead in the air. The same year that Thomas Becket was slain in Canterbury cathedral.

His death horrified Europe and plunged England into a political and economic crisis. Taxes went unpaid; the mines fell silent. And the air cleared above the Colle Gnifetti glacier.

Ten years later, King Henry II was repairing his damaged reputation with the church by paying for the construction of monastic buildings, all needing lead for their roofs and stained glass windows.

Sure enough, the ice record shows lead pollution soared during those years. Scientists are hailing it as an incredible discovery. As well as Becket’s death, the pollution levels show other historical events and political crises. The archaeologist Christopher Loveluck says, “We can see the deaths of King Henry II, Richard the Lionheart, and King John there, in the ancient ice.”

So, is our history written in the ice and in the air?

Silent witness

Some say, yes, history is everywhere – if we know where to look. Long before we started putting plastic in the oceans and carbon dioxide in the atmosphere, humans were leaving their mark on the environment. Frozen in the ice is the trace of 12th-Century air that can tell a story of murder, religion, and the rise and fall of kings.

Others are less sure. This is just one clue, one piece of evidence, that forms part of a bigger narrative. We can only make sense of the traces of lead in the glacier because of the rich historical record from the period. These written accounts tell us the full story and, without them, we wouldn’t know what these new discoveries mean.

You Decide

  1. Who do you think was to blame for the death of Thomas Becket?
  2. Does history have to be written down to be truly believable?


  1. Use the expert links to research the events around the murder of Thomas Becket. On paper, create a newspaper front page with Becket’s murder as the main story, with an attention-grabbing headline and picture. Write the news story, including key events and interviews with witnesses and important people.
  2. The global lockdown in response to the coronavirus pandemic has caused air pollution to drop around the world. Imagine you are an archaeologist, 850 years in the future, and have discovered evidence of the clean air in 2020. Write an imaginary letter to a scientist friend, giving your first theories about your incredible discovery.

Some People Say...

“A historian is often only a journalist facing backwards.”

Karl Kraus (1874-1936), Austrian writer and journalist

What do you think?

Q & A

What do we know?
History is much more than a list of past events. It requires detective work to piece together clues and build theories of what happened and why. Traditionally, historians have looked at documents written at the time. But archaeologists can fill in gaps with material evidence, like the lead deposits in the Swiss glacier.
What do we not know?
No matter how much evidence we have, we can never be absolutely certain about historical events. Without a time machine to go back and check the facts, we can only interpret the clues and signs that have survived. Historical accounts can be unreliable, but so can deposits of lead pollution frozen in ice. Some scientists think the pollution came from German mines and not English ones, or that movement in the snow and ice makes them of no practical use.

Word Watch

Thomas Becket
He was a friend of Henry II who made him Archbishop to increase his power over the church. However, Becket defended the church and fell out with the king. In an angry outburst, the king famously said, “Will no one rid me of this turbulent priest?” Hearing the king, four of his knights rode off and did just that.
A very serious medieval punishment, excommunication meant Henry II no longer had the support of the church and had no authority to be king. In the religious world of 12th-Century Europe, on death, he could expect to go to hell.
Henry II
King of England from 1154 to 1189.
Punishment inflicted on oneself as an expression of regret and repentance.
A large dense body of ice that builds up over many centuries.
A chemical element with the symbol Pb and atomic number 82. It is a heavy metal that is denser than most common materials. When freshly cut, lead is silvery with a hint of blue; it tarnishes to a dull, grey colour when exposed to air.
Hard working.
Monastic buildings
Monasteries were at the centre of medieval life. They were vast complexes of churches, hospitals, schools, and manufacturing buildings.
Stained glass windows
Windows of small pieces of coloured glass arranged to form patterns or pictures, traditionally held together by strips of lead.


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