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Humans have been farming, burning forests and grazing livestock for thousands of years. Understanding this past land use is key to making accurate predictions of global change in the future. But how do you gather data about what was happening before the existence of satellites and GIS? Although not yet perfect, the so-called HYDE dataset is attempting to do just this, and has become a major resource for scientists who forecast global change. Creator Kees Klein Goldewijk, researcher at Utrecht ľ�ϸ���Ӱ�� and PBL, is now going further to build bridges with other disciplines to fill in the missing pieces of the puzzle.

For the past 20 years, William Ruddiman, palaeoclimatologist and Professor Emeritus at the ľ�ϸ���Ӱ�� of Virginia has worked on a hypothesis that posits pre-industrial age humans have been raising greenhouse gas levels in the atmosphere since the rise of agriculture at the start of the Holocene some 10,000 years ago.

“This has been controversial because it is of course very hard to find the data to understand the long term, pre-industrial revolution impact of burning, deforestation, farming, and other human-related land use practices on global climate,” says Kees Klein Goldewijk, an integrated global environment assessment researcher at Utrecht ľ�ϸ���Ӱ�� and the Netherlands Environmental Assessment Agency (PBL).

If we can accurately reproduce the past we can show that we better understand the underlying processes, and this gives more credibility to the way we are modelling how human land use contributes to climate change in the future.

Understanding the past to predict the future

Such understanding is critical, however.  “If we can accurately reproduce the past,” he explains, “we can show that we better understand the underlying processes, and this gives more credibility to the way we are modelling how human land use contributes to climate change in the future”.

To do this, Klein Goldewijk has developed the History Database of the Global Environment (HYDE), which is trying to reconstruct global human land use for the last 10,000 years. At first he started more short term. “We’ve had high quality satellite and GIS data for the last twenty years, and fifty years before that I could use data from UN bodies, national statistics offices and large international organisations”. But going back further was more challenging. “There was just no data,” he says.

HYDE now a historical land use benchmark

To get around this he had to come up with rules for the probability that agriculture was taking place somewhere at a given point in time. Using information such as precipitation, temperature, fertility of the soil, distance to water bodies or slope he is able to fairly accurately highlight regions already known to be ancient agricultural hotspots. HYDE is now included as a historical land use benchmark in many of the models using climate and land use for IPCC reports. “But it’s like a jigsaw puzzle,” says Klein Goldewijk. I have some parts of the puzzle but I’m still missing many pieces.”

Archaeologists, historians, anthropologists and geographers sharing vast empirical data

Academics with little previous link to global climate models are now coming together to paint a better picture of historic land use. “These archaeologists, historians, anthropologists and geographers know everything there is to know about their specific historic case studies. They can pinpoint with empirical data if, where and when people were engaging in agriculture. It’s invaluable data - they have the missing pieces of the puzzle.”

Human transformation of the Earth started much earlier than previously thought

co-authored by Klein Goldewijk compared this new data to the HYDE dataset. One main finding was that HYDE was too conservative – the new data showed that human transformation of the Earth started much earlier than previously thought, supporting Ruddiman’s hypothesis that humans have been altering the climate for much longer than the since industrial revolution.

Understanding the future impact of the actions we take today

Klein Goldewijk will continue to use this new disciplinary data to update the HYDE database, an ongoing effort coordinated by the international PAGES (Past Global Changes) Landcover6K working group.

“Archaeologists didn’t realise how valuable their data would be for global climate models,” says Klein Goldewijk. “Things are moving forward now that they are starting to share their data. A deeper understanding of the past is helping us understand the future impact of the actions we take today. And with this we will be able to make better informed policy decisions that minimize the impacts of climate change.”

Further reading

Stephens, L., Fuller, D., Boivin, N., Rick, T., Gauthier, N., Kay, A., ... & Denham, T. (2019). . Science, 365(6456), 897-902.

Klein Goldewijk, K., Beusen, A., Doelman, J., and Stehfest, E. (2017). . Earth Syst. Sci. Data, 9, 927-953.