Is our climate cooling down or warming up? A new study of past climate data reveals that, despite previous belief, there has been a consistent global warming trend over the last 10,000 years, rather than a period of global cooling before humans intervened.
"We have been building models and there are now robust contradictions," lead study author Zhengyu Liu, of the University of Wisconsin-Madison, said in a news release. "Data from observation says global cooling. The physical model says it has to be warming."
Our current geological epoch is called the Holocene, and the problem Liu is describing is referred to as the Holocene temperature conundrum.
Determining the cause of these clashing findings has important implications for understanding climate change and evaluating climate models, as well as for the benchmarks used to create climate models for the future. However, the researchers stress that their recent study does not change the evidence of human impact on global climate beginning in the 20th century.
"The question is, 'Who is right?'" Liu said. "Or, maybe none of us is completely right. It could be partly a data problem, since some of the data in last year's study contradicts itself. It could partly be a model problem because of some missing physical mechanisms."
Over the last 10,000 years, atmospheric carbon dioxide rose by 20 parts per million before the 20th century, and the massive ice sheet of the Last Glacial Maximum has been gradually retreating. These ecological changes suggest that the annual mean global temperature should have continued to warm, even as regions of the world experienced cooling.
And yet, a study last year published in the journal Science indicates a period of global cooling began 7,000 years ago until human activity, such as the burning of fossil fuels, started heating things up.
Clearly both of these findings do not add up, so Liu and his team created three models, running simulations of climate influences that ranged from the intensity of sunlight on Earth to global greenhouse gases, ice sheet cover and meltwater changes. Each showed global warming over the last 10,000 years.
The authors say it's possible that samples collected for the previous study may not have adequately addressed the bigger picture. For example, biological samples taken from a core deposited in the summer may be different from samples at the exact same site had they been taken from a winter sediment.
The latest study proves that more research may be needed to determine exactly what sort of trends impacted our planet in the past. This, in turn, could help inform future climate models.
The findings are published in the journal Proceedings of the National Academy of Sciences.