Question: I read somewhere that lead deposits from Roman times (approx 2,000 years ago) have been discovered in ice cores approx 400 metres below the current ice surface in Greenland. As a non-scientific person, I am asking a very basic question: if the above information is correct, then that suggests that around 400 metres of ice has accumulated in the past 2,000 years which suggests to me that the earth has been cooling significantly over the past 2,000 years. I would be delighted if you could shed some light on this line of thinking.
-Tony Scotland
Hi Tony, thank you for your question. In cold regions more snow falls (accumulates) than melts (ablates) in the warm season, causing the snow to build up and compact into ice, which can be seen as annual layers in Greenland’s ice cores. In general, glaciers receive more ice in their upper reaches and lose more ice in their lower reaches, resulting in accumulation and ablations zones. Greenland’s ice cores are taken from the accumulation zone, where the ice growth from each year is preserved. Temperatures in Greenland only exceed freezing during July and August, resulting in net growth of ice in the accumulation zone. Therefore, global cooling is not required for the accumulation of ice in Greenland, and the evidence of Roman activity would be expected at a depth of around 400m [1].
Here’s some further reading:
This paper by McConnell et al. explains the evidence of lead in ice cores in more depth
For more on changes to the Greenland ice sheet have a look at Tom Slater’s recent article here
And here are some extra resources on mass balance and accumulation and ablation
Hopefully this answers your question,
Holly