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Was Earth heating up or cooling down before global warming?

We cannot know exactly how Earth’s climate was in the past, nor how it will be in the future. However, it is very important for scientists to try and predict what changes may take place so that policy- and decision-makers can develop ways to reduce and mitigate against the destructive effects of a warming planet. 

Scientists make use of climate system models to get a better idea of how weather patterns and climatic factors may change in the future. These are computer simulations that use real data on factors that drive the climate today to predict how weather patterns may change under different conditions, such as increases in the levels of greenhouse gases. 

Before using a climate model to predict what may happen in future, it is important that its accuracy in identifying past climatic conditions is assessed. This is done by comparing the model’s simulated conditions with climatic data derived from other sources, such as tree rings or pollen, or the stable isotope ratios in lake and ocean sediments or ice cores. These sources of data are known as proxies since they reflect past climatic conditions indirectly. 

Interestingly, when climatic models are used to predict the Earth’s temperature during the Holocene Period (the past 12,000 years), the predictions disagree with the evidence obtained from proxy sources. The paleo-environmental proxy data, including evidence from ocean and lake sediments, indicate that temperature peaked at 6,500 years ago and then gradually cooled down until humans started burning fossil fuels around 200 years ago. In contrast, the climate models generally show global average temperatures increasing gradually throughout the Holocene Period.

The disagreement on climatic conditions during this time period is known as the Holocene temperature conundrum. A new review article, published in the journal Nature, has now addressed this conflict between the findings of climate models and proxy evidence by analyzing a large quantity of available data on global temperature from the past 12,000 years. 

The researchers included lead author Darrell Kaufman, a Regents’ professor in the School of Earth and Sustainability at Northern Arizona University, and University of Arizona postdoctoral researcher Ellie Broadman, a co-author who worked on the study while earning her Ph.D. at NAU. The team aimed to identify whether the global average temperature 6,500 years ago was warmer, as indicated by proxy evidence, or colder, as simulated by models, than the temperature in the late 19th century, when the Industrial Revolution led to a significant increase in human-caused warming.

This comprehensive and systematic assessment concludes that the global average temperature 6,500 years ago was likely warmer and was followed by a multi-millennial cooling trend that ended in the 1800s. This finding supports the data derived from numerous proxy sources, rather than the conclusions derived from current climate models. However, the authors cautioned that uncertainty still exists, despite recent studies that have claimed to have resolved the conundrum.

“Quantifying the average temperature of the earth during the past, when some places were warming while others were cooling, is challenging, and more research is needed to firmly resolve the conundrum,” Kaufman said. 

“But tracing changes in global average temperature is important because it’s the same metric used to gauge the march of human-caused warming and to identify internationally negotiated targets to limit it. In particular, our review revealed how surprisingly little we know about slow-moving climate variability, including forces now set into motion by humans, that will play out as sea level rises and permafrost thaws over coming millennia.”

We know more about the climate of the Holocene, which began after the last major ice age ended, 12,000 years ago, than any other multi-millennial period. There are published studies from a variety of natural archives that store information about historical changes that occurred in the atmosphere, oceans, cryosphere and on land; studies that look at the forces that drove past climate changes, such as Earth’s orbit, solar irradiance, volcanic eruptions and greenhouse gases; and climate model simulations that translate those forces into changing global temperatures. All these types of studies were included in this review.

If the researchers are correct, and the proxy data gives a more accurate reflection of global temperature changes during this time, this implies deficiencies in the models and casts uncertainty on their use to forecast climatic conditions in the future. In particular, this would challenge climate scientists’ understanding of factors that naturally drive climate changes and feedbacks, and that are used in the development of climate models. 

If the researchers are incorrect, then scientists need to improve their understanding of the way in which proxy records indicate past temperature information, and further develop analytical tools to capture these trends on a global scale.

What is a certainty, however, is that the change in global average temperature in the past 6,500 years, be it upwards or downwards, has been gradual and has been very small – less than 1oC (1.8oF). This is less than the warming already measured in the last 100 years alone, most of which is related to human activities. 

Attempting to resolve the Holocene global temperature conundrum has been a priority for climate scientists in the last decade; Broadman remembers reading the initial paper on this topic when she started her Ph.D. in 2016. All the studies since then have added to the understanding of this issue. However, it is still not clear what factors might be driving a cooling climate in the past 6,500 years when levels of atmospheric greenhouse gases were slowly increasing and ice sheets were slowly retreating – both factors that would be expected to cause warming, not cooling.

The current review certainly highlights the fact that we don’t yet know the solution to this conundrum.

“One interesting takeaway is that our findings demonstrate the impact that regional changes can have on global average temperature. Environmental changes in some regions of the Earth, like declining Arctic sea ice or changing vegetation cover in what are now vast deserts, can cause feedbacks that influence the planet as a whole,” Broadman said. 

“With current global warming, we already see some regions changing very quickly. Our work highlights that some of those regional changes and feedbacks are really important to understand and capture in climate models.”

Additionally, Kaufman said, accurately reconstructing the details of past temperature change offers insights into climate’s response to various causes of both natural and anthropogenic climate change. The responses serve as benchmarks to test how well climate models simulate the Earth’s climate system.

“Climate models are the only source of detailed quantitative climate predictions, so their fidelity is critical for planning the most effective strategies to mitigate and adapt to climate change,” said Kaufman. “Our review suggests that climate models are underestimating important climate feedbacks that can amplify global warming.”

By Alison Bosman, Staff Writer

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