Climate models show the earth was warmer, then cooled
Climate models are programmed to simulate past climates to assess their accuracy against geological evidence. However, conflicts can arise between the model simulations and the evidence, creating a conundrum. In a recent article published in Nature, lead author Darrell Kaufman, a professor in the School of Earth and Sustainability, and co-author Ellie Broadman, a postdoctoral researcher, analyzed a broad range of data from the last 12,000 years to address the Holocene global temperature conundrum. The study builds on Kaufman’s previous work that was included in the latest climate report by the Intergovernmental Panel on Climate Change (IPCC). The study examines whether the global average temperature 6,500 years ago was warmer or colder than the late 19th century when human-caused warming significantly increased during the Industrial Revolution.
The assessment indicates that the global average temperature around 6,500 years ago was likely warmer and was followed by a cooling trend that ended in the 1800s. However, uncertainty still exists, even with recent studies that claimed to have resolved the conundrum.
Kaufman explains that quantifying the earth’s average temperature during the past is challenging, especially because some places were warming while others were cooling. Nevertheless, tracing changes in global average temperature is essential since it is the same metric used to assess human-caused warming and to identify internationally negotiated targets to limit it.
Studying the Holocene period
The Holocene period, which began after the last major ice age 12,000 years ago, is the most studied multi-millennial period in terms of climate. Past published studies from various natural archives contain information about historical changes in the atmosphere, oceans, cryosphere, and on land. They also include studies that examine the factors that caused past climate changes, such as Earth’s orbit, solar radiation, volcanic eruptions, and greenhouse gases, as well as climate model simulations that translate those factors into changing global temperatures.
Climate models need to improve
The challenge, until now, has been that our two primary sources of evidence point in different directions. Paleo-environmental “proxy” data, which includes evidence from oceans, lakes, and other natural archives, show that there was a peak global average temperature around 6,500 years ago, followed by a global cooling trend until humans started burning fossil fuels. On the other hand, climate models generally show that global average temperatures have been increasing in the last 6,500 years.
If the proxy data is accurate, it indicates that there are deficiencies in the models and specifically suggests that climate feedbacks that can amplify global warming are not accurately represented. If the climate models are correct, then the tools used to reconstruct paleotemperatures need to be improved.
The temperate rise in the past century beats the cumulate rise for 6,500 years prior
It is also important to note that whether the numbers increase or decrease, the change in global average temperature over the past 6,500 years has been gradual, probably less than 1 degree Celsius (1.8 degrees Fahrenheit). This is less than the warming already observed in the last 100 years, most of which has been caused by human activities. Nevertheless, understanding whether temperatures were higher or lower 6,500 years ago is crucial for our knowledge of the climate system and to improve our forecasts of future climate, given that any change in global temperature, no matter how small, can have significant impacts, particularly in response to changes in greenhouse gases.
Resolving the Holocene global temperature conundrum
Climate scientists have been attempting to resolve the Holocene global temperature conundrum for the last decade, with all the studies since adding to the understanding of this issue. Recent studies have tried to adjust proxy data to account for their weaknesses, insert plausible forcings into climate models, and blend proxy data with climate-model output. However, this review article takes a step back to revisit the issue with a comprehensive global-scale assessment and shows that we do not yet know the solution to this conundrum.
Developing widely applicable methods for quantifying past temperature is a high priority for climate scientists. Kaufman’s lab is testing the use of chemical reactions involving amino acids preserved in lake sediment as a new method for studying past temperature changes. Combined with new radiocarbon dating technology from the Arizona Climate and Ecosystem lab at NAU, this technique could help determine whether global warming reversed a long-term cooling trend.
“Environmental changes in some regions of the Earth, such as declining Arctic Sea ice or changing vegetation cover in what are now vast deserts, can cause feedbacks that influence the planet as a whole. 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.
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.”
Image Credits
In-Article Image Credits
Climate change? Telly, one of the first villages in the Bandiagara escarpment in Mali via Wikimedia Commons by Ferfinand Reus with usage type - Creative Commons License. June 12, 2008Featured Image Credit
Climate change? Telly, one of the first villages in the Bandiagara escarpment in Mali via Wikimedia Commons by Ferfinand Reus with usage type - Creative Commons License. June 12, 2008
