Heating Up or Cooling Down? Scientists Study Past Climate Change Conundrum

Is the world warming or cooling? What does climate change actually entail? A new study of past climate trends reveals that, in contrast to a previous study, there's been a consistent global warming trend over the course of our current geological epic rather than a period of global cooling before human influence.

Our current geological period is called the Holocene, and this particular warming trend is called the Holocene temperature conundrum. It has important implications for understanding climate change and evaluating climate models, as well as for the benchmarks used to create models for the future.

"The question is, 'Who is right?'" said Zhengyu Liu, one of the researchers, in a news release. "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 20^th century. During this time period, the massive ice sheet of the Last Glacial Maximum steadily retreated. These physical changes suggest that the mean global temperature should have continued to warm. Yet last year's study seemed to suggest a period of global cooling about 7,000 years ago and continuing until humans began to leave a mark. Something didn't add up.

That's why Liu and colleagues created three models, running simulations of climate influences that spanned 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.

It's possible that samples that were 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 reveals 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.