Author Kaufman: 'Tracing changes in global average temperature is important'

Lead author Darrell Kaufman and co-author Ellie Broadman analyzed data from the last 12,000 years to address the Holocene global temperature conundrum.

Their comprehensive assessment suggests that the global average temperature about 6,500 years ago was likely warmer and followed by a multi-millennial cooling trend, highlighting the need for more research to firmly resolve the conflict between climate models and proxy evidence.

Accurate climate models have long been essential in understanding the Earth's climate and formulating effective policies to combat the impacts of global warming. However, a new study published in Nature Today sheds light on a conundrum that puzzled climate scientists for years, known as the Holocene global temperature conundrum. 

Lead author Darrell Kaufman, a Regents’ professor in the School of Earth and Sustainability, and University of Arizona postdoctoral researcher Ellie Broadman, a co-author who worked on this study while earning her Ph.D. at NAU, conducted an in-depth analysis to tackle this challenging issue. 

The Holocene global temperature conundrum revolves around the discrepancy between climate models and paleo-environmental "proxy" data. The proxy data, which includes evidence from oceans, lakes, and other natural archives, indicate a peak global average temperature about 6,500 years ago, followed by a cooling trend until the Industrial Revolution sparked human-caused warming. 

In contrast, climate models generally show increasing global average temperatures over the same period. The study delves into the critical importance of understanding this discrepancy, as Darrell Kaufman emphasizes, "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."

The researchers found that the global average temperature around 6,500 years ago was likely warmer than previously believed. This finding suggests that climate models might be underestimating vital climate feedback that can amplify global warming. 

Ellie Broadman added, "Our findings demonstrate the impact that regional changes can have on global average temperature. Environmental changes in some regions of the Earth can cause feedbacks that influence the planet as a whole. Understanding and capturing these regional changes and feedback is crucial in climate models."

The implications of this research are far-reaching, as it not only affects our understanding of historical climate trends but also has implications for predicting future climate scenarios. Climate models serve as the primary source for detailed quantitative climate predictions, making their accuracy vital for devising strategies to mitigate and adapt to climate change.

Kaufman highlighted, "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. Our review suggests that climate models are underestimating important climate feedbacks that can amplify global warming." 

The study's comprehensive global-scale assessment encompassed published studies from a variety of natural archives, climate model simulations, and forces that drove past climate changes, such as Earth's orbit, solar irradiance, volcanic eruptions, and greenhouse gases. 

Despite these efforts, uncertainties still linger.

The challenge arises from the complexity of reconstructing past temperatures accurately. Broadman, an expert in science communication, expressed the difficulty in conveying the findings to diverse audiences, including educators, policymakers, nonprofits, and scientists worldwide. 

She said, "Our work highlights that some of those regional changes and feedbacks are really important to understand and capture in climate models." 

The study calls for further research and development of analytical tools to capture global temperature trends on a larger scale. Kaufman's lab is already taking steps in this direction, testing the use of chemical reactions involving amino acids preserved in lake sediment as a new method for studying past temperature changes.

By combining this with new radiocarbon dating technology, the research team hopes to gain more insights into the reversal of the long-term cooling trend. 

In conclusion, the Holocene global temperature conundrum remains unsolved despite significant progress in recent years. The study's findings point to the importance of refining climate models and better understanding natural climate forcings and feedback to accurately predict future climate scenarios. 

As climate change continues to impact our planet, resolving this conundrum becomes even more crucial in shaping effective strategies to address its effects.