Accurate climate models play a critical role in climate science and policy, helping to inform policy and decision-makers around the world as they consider ways to slow the deadly effects of global warming and adapt to changes that are already occurring. To test their accuracy, the models are programmed to simulate past climates to see if they match with geological data. Model simulations can contradict the evidence. How do we know what is true?
A review article published today (February 15) in the journal Nature, addresses this conflict between models and evidence, known as the Holocene global temperature puzzle. Lead author Darrell Kaufman, professor in Regent University’s School of Earth and Sustainability, and University of Arizona doctoral student Ellie Brodman, Ph.D. To solve the puzzle at NAU, we analyzed a wide variety of available data from the past 12,000 years. The study builds on Kaufman’s work included in the Intergovernmental Panel on Climate Change’s latest major climate report. (IPCC) and examines whether the global average temperature 6,500 years ago was warmer, as evidenced by proxy evidence from natural archives of past climate information, or colder as it was modeled than in the late 19th century, When The Industrial Revolution led to a significant increase in anthropogenic warming.
This comprehensive assessment concludes that global average temperatures were probably warmer around 6,500 years ago, followed by a several thousand-year cooling trend that ended in the 1800s. However, they warned that uncertainty still exists despite recent research that claims to have solved the mystery.
“In the past, measuring Earth’s average temperature where some places warmed and others cooled has been a difficult task, and more research is needed to definitively address this issue,” Kaufman said. Said. “But tracking changes in global average temperature is important because it’s the same metric used to set internationally agreed targets for measuring and limiting human-caused warming. Specifically, our review includes anthropogenic forces that will act over the next millennium as sea levels rise and permafrost melts. showed how surprisingly little we know about slow climate change.”
what do we know
We know more about the Holocene climate, which began after the last great ice age, which ended 12,000 years ago, than most other millennia. Research has been published from various natural history archives that store information about historical changes in the atmosphere, oceans, cryosphere, and land; Studies looking at the forces driving climate change, such as Earth’s orbit, solar radiation, volcanic eruptions, and greenhouse gases; and simulating climate models that translate these forces into global temperature change. All such studies were included in this review.
The problem so far has been that our two key lines of evidence point in opposite directions. Paleo-environmental “proxy” data, which includes data from oceans, lakes and other natural archives, point to a peak in global average temperatures about 6,500 years ago, followed by a global cooling trend until humans started burning fossil fuels. Climate models generally show that global average temperatures have increased over the past 6,500 years.
If the proxies are correct, they point to flaws in the models and particularly suggest that climate feedbacks that could exacerbate global warming are underrepresented. If climate models are correct, the tools used to reconstruct paleo temperatures need to be sharpened.
We also know that whether the numbers are rising or falling, 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 measured in the last 100 years, most of which is anthropogenic. However, since global temperature change of any magnitude is particularly important in response to greenhouse gas changes, knowing whether temperatures were higher or lower 6,500 years ago is important for our knowledge of the climate system and for improving future climate projections.
what we don’t know
This study highlighted the uncertainties in climate models. If the authors’ interpretation that recent global warming preceded 6,500 years of global cooling is correct, then scientists’ understanding of natural climate forcing and feedback and how these are represented in models needs to be improved. If they’re wrong, scientists need to improve their understanding of the temperature signal in surrogate records and develop analytical tools to capture these trends on a global scale.
Trying to solve the Holocene global temperature conundrum has been a priority for climatologists over the past decade; Brodman remembers reading the first paper on the subject when he started working for his PhD. All research since then has contributed to the understanding of this topic and has brought scientists in the field closer to a comprehensive understanding. Recent research on this topic has attempted to adjust the representative data for their supposed shortcomings by adding plausible effects to climate models and mixing the proxy data with the climate model output, and drawing different conclusions about the cause of the puzzle. This review takes a step back in reviewing the problem with a comprehensive global assessment and shows that we don’t yet know the solution to this puzzle.
Developing widely applicable methods for measuring past temperature is already a priority for climatologists. For example, Kaufman’s lab is testing the use of chemical reactions involving preserved amino acids in lake sediments as a new method for studying past temperature changes. Combined with new radiocarbon dating technology from NAU’s Arizona Climate and Ecosystems Laboratory, this method could help determine whether global warming is reversing a long-term cooling trend.
Why is this important?
Brodman, whose work focuses on science communication, produced the figures accompanying the work. This is a critical way to communicate elusive results to an audience, and in climate science this audience is very diverse and includes educators, policymakers, nonprofits and scientists worldwide.
“An interesting implication is that our findings show the impact of regional changes on global average temperature. Environmental changes in parts of the world, such as reductions in Arctic sea ice or changes in vegetation in places that are now vast deserts, cause a feedback loop that affects the planet as a whole. “With the current global warming, we can already see that some regions are changing very quickly. Our work highlights that some of these regional changes and feedback are really important for understanding and capturing climate patterns.”
Additionally, according to Kaufman, accurately reconstructing the details of past temperature changes allows us to understand the climate’s response to various causes of climate change, both natural and anthropogenic. The answers serve as a benchmark for testing how well climate models simulate Earth’s climate system.
“Climate models are the only source of detailed quantitative climate predictions, so their accuracy is critical for planning the most effective climate change mitigation and adaptation strategies,” he said. “Our review shows that climate models underestimate important climate feedback that could exacerbate global warming.”
Source: Port Altele
