New study on Holocene conundrum suggests need for reexamination of both proxy data and climate models
In an open access paper in the Proceedings of the National Academy of Sciences, researchers from the US, China and Europe re-examine the “Holocene conundrum”—a contradiction between the results of reconstructed data records and model-based climate simulations. Marine and terrestrial proxy temperature records suggest global cooling during the Late Holocene period, following the peak warming of the Holocene Thermal Maximum (∼10,000-6,000 years (ka) before the present) until the rapid warming induced by increasing anthropogenic greenhouses gases. However, current climate models suggest a robust warming trend in the period—the opposite from the cooling in the reconstruction.
Based on their study, lead author Zhengyu Liu from the University of Wisconsin-Madison and colleagues from Rutgers University; the National Center for Atmospheric Research; the Alfred Wegener Institute for Polar and Marine Research; the University of Hawaii; the University of Reading; the Chinese Academy of Sciences; and the University of Albany suggest that there may be potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.
We have been building models and there are now robust contradictions. Data from observation says global cooling. The physical model says it has to be warming. The question is, “Who is right?” 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.—Zhengyu Liu
The Holocene temperature conundrum has important implications for understanding climate change and evaluating climate models, as well as for the benchmarks used to create climate models for the future. However it does not, the authors emphasize, change the evidence of human impact on global climate beginning in the 20th century.
Over the last 10,000 years, Liu says, we know atmospheric carbon dioxide rose by 20 parts per million before the 20th century, and the massive ice sheet of the Last Glacial Maximum has been retreating. These physical changes suggest that, globally, the annual mean global temperature should have continued to warm, even as regions of the world experienced cooling, such as during the Little Ice Age in Europe between the 16th and 19th centuries.
The three models Liu and colleagues generated took two years to complete. They ran simulations of climate influences that spanned from the intensity of sunlight on Earth to global greenhouse gases, ice sheet cover and meltwater changes. Each shows global warming over the last 10,000 years.
However, the bio- and geo-thermometers used last year in a study in the journal Science suggest a period of global cooling beginning about 7,000 years ago and continuing until humans began to leave a mark—a profound global warming trend.
In the Science study (Marcott 2013), the authors looked at data collected by other scientists from ice core samples, phytoplankton sediments and more at 73 sites around the world. The data they gathered sometimes conflicted, particularly in the Northern Hemisphere.
Because interpretation of these proxies is complicated, Liu and colleagues believe they may not adequately address 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. It’s a limitation the authors of last year’s study recognize.
With their current knowledge, Liu and colleagues don’t believe any physical forces over the last 10,000 years could have been strong enough to overwhelm the warming indicated by the increase in global greenhouse gases and the melting ice sheet, nor do the physical models in the study show that it’s possible.
The fundamental laws of physics say that as the temperature goes up, it has to get warmer.—Zhengyu Liu
Caveats in the latest study include a lack of influence from volcanic activity in the models, which could lead to cooling—though the authors point out there is no evidence to suggest significant volcanic activity during the Holocene—and no dust or vegetation contributions, which could also cause cooling.
Liu says climate scientists plan to meet this fall to discuss the conundrum.
Both communities have to look back critically and see what is missing. I think it is a puzzle.—Zhengyu Liu
The study was supported by grants from the US National Science Foundation, the Chinese National Science Foundation, the US Department of Energy, and the Chinese Ministry of Science and Technology.
Zhengyu Liu, Jiang Zhu, Yair Rosenthal, Xu Zhang, Bette L. Otto-Bliesner, Axel Timmermann, Robin S. Smith, Gerrit Lohmann, Weipeng Zheng, and Oliver Elison Timm (2014) “The Holocene temperature conundrum” PNAS doi: 10.1073/pnas.1407229111
Marcott SA, Shakun JD, Clark PU, Mix AC (2013) “A reconstruction of regional and global temperature for the past 11,300 years,” Science 339 (6124):1198–1201 doi: 10.1126/science.1228026