Archaeologists strive to improve climate modeling

A man rides a horse in front of the Egyptian pyramids.

Photo: Anne-Christine Poujoulat / AFP (Getty Images)

Models that project how the climate will change in the future rely heavily on information about what has happened in the past, including how humans have used the land. Scientists feed data into models to create algorithms that estimate everything from weather to vegetation to land use. But this approach has a fundamental flaw.

According to archaeologists – who have dedicated their lives to collecting data on world history – the models used by climatologists to describe past land use (called “Earth system models”) are, in many ways, superficial. Specifically, they tend to overlook the impact humans have had on the earth over the millennia, often mistakenly classifying the earth as untouched when it was not. That’s why more than 200 researchers from around the world have come together to create one of the world’s first archaeological climate databases. Although models of the Earth system progress without archaeological data, these scientists believe that the historical records they collect could radically improve the way we think about the future.

“It’s fair to say that these models never really took archaeological knowledge into account,” said Kathleen morrison, historical anthropologist at the University of Pennsylvania. She specializes in the use of pollen to reconstruct vegetation records and has spent decades understanding the links between changes in social life, land use and biodiversity in southern India over the years. 5000 years ago. When she came across an esteemed representation by a model of the places she had studied for so long, she was shocked.

“It was the ‘ah ha’ moment for me. All of the rice fields, pastures, canals, towns and villages that my colleagues and I studied seemed to be gone, and the model attributed those times and places to mostly natural vegetation, ”Morrison said. “I knew it was wrong. Many of us in archeology had never even heard of these models. But we knew we had a contribution to make.

Two of the main Earth system models used by scientists to understand prehistoric land cover and model the climate today are the Global Environment Historical Database or HYDE, and a less recent one called KK10 (the the name comes from the initials of the scientists who created it and the fact that it was released in 2010). These models use modern data on how the earth is used and what it is made of, and they “look back” on what they thought would have happened in the past. The retrospectives are based on estimates of past population density multiplied by “stock” values ​​to represent per capita land use for all regions of the world and all past periods, explained Morrison, but these models ignore that humans used the earth in the past in different ways. which are not at all analogous to the present.

“The models currently in use are problematic,” said Morrison. “But the nature of the error is not always the same.” Because models are “disabled” in different ways in different regions, there is no simple, one-size-fits-all solution to the problem.

For example, the models are based on assumptions about the amount of cropland per capita needed to feed people and they assume a constant in the amount of food needed and produced, although this has changed considerably over time, Emily hammer, another anthropologist from the University of Pennsylvania, said. The models also assume that people produce only enough food to survive, without overconsumption or waste.

Different groups of people have changed their land use practices, the crops they cultivate and their way of life – for example, whether they live in large, concentrated settlements or in scattered villages and hamlets. In 1500, for example, Western European farmers who grew wheat and barley used about 2.5 acres of land per person for food, Hammer explained. During the same period, farmers in southern China who grew paddy rice used more than 0.8 acres per person for food. The same “stock” values ​​cannot be included in the formula for both. Humans are complicated, and all the land use choices that different societies have made in the past have in turn affected the climate (yes, ancient human activities are probably influenced the climate, but not at the scale they do today).

Given these disparities and the fact that, said Hammer, “we actually had a lot of data on” different types of land use, the researchers launched the LandCover6k project in 2015. They aim to aggregate, synthesize and harmonize all the evidence of land use and make reconstructions of vegetation through time with real palaeoenvironmental and archaeological data, and not algorithms.

In a paper recently published in the journal PLOS One, the researchers set up a hierarchical database, a sort of eight-kilometer grid on the Earth’s surface that shows what humans were doing in a given square at any given time. “Were people farming in this area or were they harming animals, hunting and gathering. Did they burn the landscape? Did they produce pottery, which requires a lot of fuel to run the kilns? Hammer said. “Were they doing other kinds of technology, like making or smelting glass or plowing the earth?”

This builds on a previous project called ArchaeoGLOBE, which operated at a much coarser resolution scale of regions roughly the size of a country. “Achieving global coverage, especially in areas that have been the subject of less archaeological investigation, will be more difficult and will likely take many years,” Lucas stephens, said the creator of ArchaeoGLOBE. So far, iLandCover6k has only created maps for 6000, 4000 and 2000 years ago for Mesopotamia and the Arabian Peninsula.

These surveys will in turn inform climate modeling, making them more accurate by improving model inputs, such as per capita land use figures, according to Morrison. All of this data is crucial if scientists are to understand vegetation, and therefore climate, in the past – and what will follow as the planet warms.

“Our ultimate goal, however, is to develop maps based on data derived from archaeological evidence and to avoid models all together,” Morrison said. “This is a huge task that draws on over a century of research around the world, and this is just the first major explanation for this approach.”

Researchers who actively use projection models of land use are also eager for archaeological data to connect to algorithms and increase accuracy – but some say it’s been a long time and likely there won’t be any removal. complete long-term models. .

“It’s just a classification of things. It hasn’t really broken new ground yet, ”said Erle Ellis, an environmental scientist at the University of Maryland, Baltimore, of the LandCover6k database. “We held our breath to get the products out of this research. What they’re trying to do is a lot harder than creating a model, isn’t it? But there hasn’t been huge progress in this area. “

Ellis is also involved in LandCover6k research, but he pointed out that models like HYDE are already improve significantly. There is a classic phrase among modelers all models are wrong, but some are useful because a model is always an interpretation of the facts, not the facts themselves. “But this will also be true for local archaeological data [like Morrison’s]Ellis said. “There really isn’t the potential to have a 100% empirical world map of land use history. This will always involve some type of model. “

In fact, climate modeling is already adjusting to increasingly consider how humans have affected the earth throughout history. According to Stephens, some of the information from ArchaeoGLOBE is already being incorporated into the next version of HYDE. The same week LandCover6K newspaper came out, he and Ellis published research in Proceedings of the National Academy of Sciences using the latest version of the HYDE modeling system. They demonstrated that, while previous climate models used to say that 82% of Earth’s land in 6000 BCE was “wild”, the most up-to-date algorithm indicated that most of the Earth’s biosphere was showing signs of human transformation 12,000 years ago years.

“It really shocked us, didn’t it? It’s a lot more than we expected, ”said Ellis.

He added that the archaeologists to whom he provided these finds believe that this is also a reducing figure, and that entering more precise data will show that the land use was even more extensive. But help could be on the way as scientists build the archaeological database further.

“It will be exciting to see if, and if so, how our results differ from model-based expectations,” said Morrison, who acknowledged that there is still a lot of work to be done on LandCover6k. “We all have the same goal: to use an understanding of the past to meet the challenges of the present and the future.”

Sofia is a freelance science journalist working between Italy, UK and USA. His work has been published in Inverse, Quartz, Wired, The Guardian, National Geographic, Atlas Obscura, etc.

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Gail Mena

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