What 4,500 years of frozen garbage can teach us about climate change

Ancient frozen garbage heaps dot Greenland’s coastlines, undisturbed for thousands of years. As permafrost thaws, that buried debris is resurfacing, leading scientists to ponder: what’s living in our ancestors’ trash?

Researchers from the University of Copenhagen and the Technical University of Denmark have spent years studying the bacteria preserved inside Greenland’s ancient middens: the frozen rubbish heaps left behind by Paleo-Inuit hunters, Norse farmers, and early Danish colonists stretching back 4,500 years. Their findings, published in Frontiers in Microbiology in June 2026, show that these long-frozen waste sites are remarkable microbial time capsules, harboring bacteria linked to food poisoning, botulism, and antibiotic resistance — some of which are still detectable centuries after the people who created them disappeared.

Middens are essentially ancient landfills: piles of animal bones, shells, human waste, and broken objects that accumulated over generations of daily life. In most parts of the world, they decompose quickly. However, in Greenland’s cold, wet conditions, the permafrost has acted as a natural freezer, preserving not just the objects but also the microbes.

The researchers collected 78 samples from seven midden sites across West and South Greenland — two Paleo-Inuit, four Norse, and one early Colonial — and compared them against 143 soil samples from untouched nearby land. In total, they identified 1,207 bacterial species, with individual middens containing nine to 202 distinct species, far richer communities than those in the surrounding pristine soils.

Key findings

• A 4,500-year microbial record: The seven midden sites span the full sweep of human settlement in Greenland, from the earliest Paleo-Inuit arrivals through Norse farmers to early Danish colonists. Each cultural layer left a distinct bacterial fingerprint, making the middens an unbroken biological record of who lived there and how they lived.
• Pathogens still present after centuries: The middens contained opportunistic pathogens, including Clostridium perfringens (linked to food poisoning and gas gangrene) and bacteria that can cause botulism. In some early Colonial-era midden layers, two species of Clostridium-related bacteria together made up 40–50% of all bacteria detected, a striking concentration in ancient frozen waste.
• Ancient antibiotic resistance genes detected: Genes conferring resistance to 17 classes of antimicrobials were found across all sites, with resistance to β-lactams (including penicillins) and tetracyclines dominating. The same resistance genes were detected in both ancient midden layers and contemporary soils, indicating that antibiotic resistance can persist in permafrost for centuries, long before modern antibiotics were invented.
• Norse livestock farming left the clearest mark: Norse middens, dating from the 10th-15th centuries, showed the strongest human and animal bacterial signatures, reflecting the introduction of domestic livestock such as cattle and sheep to Greenland. Paleo-Inuit middens, by contrast, had bacterial communities closest to those of natural soil, suggesting that the microbial imprint of human activity fades with time.
• Permafrost thaw is the key variable: Arctic temperatures are rising three to four times faster than the global average. As permafrost thaws and coastlines erode, midden material that has been frozen solid for millennia is being exposed and mobilized. How bacteria behave once that material enters waterways on a greater scale remains an open and urgent question.
• Routine monitoring recommended: The researchers call for microbiome sampling to become a standard part of Arctic archaeological fieldwork — not just to understand the past, but also to track what may be emerging from it as the frozen landscape continues to change.

Why it matters

The effects of this research stretch well beyond Greenland. Middens exist across the entire Arctic — in Alaska, Canada, Siberia, and Svalbard — and all of them sit in landscapes that are warming fast. What researchers have documented in Greenland is likely playing out elsewhere, in sites that haven’t yet been sampled. For Arctic communities that rely on these coastal environments for drinking water, fishing, and daily life, the question of what is thawing out of ancient ground is not an abstract one. This study offers a measured reassurance — the bacteria aren’t spreading widely, at least not yet. When researchers tracked midden bacteria into nearby eroding shoreline areas, they found that ancient microbial communities were rapidly replaced by native marine bacteria rather than establishing themselves in the wider environment. While this is reassuring, the researchers note that the central issue is how much warmer things get, and how quickly.

​Click here to read the full study.