期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2021
卷号：118
期号：45
DOI：10.1073/pnas.2108783118
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Understanding of climate “tipping points” is extremely limited. Their representation within Earth System models (ESMs) is completely unconstrained because of a lack of any actual occurrences of these events in recent times. As an example, most ESMs fail to simulate the rapid desertification of the Sahara during the Holocene. Here, we overcome this by tuning uncertain aspects of one ESM using the climate of 6,000 y ago. This approach reveals a hitherto-absent tipping point within this ESM that produces excellent agreement between the modeled and observed timing of abrupt change in North Africa. This demonstrates how paleoclimate information can be used to radically improve the representation of abrupt changes in the ESMs that are employed for future projections.
While paleoclimate records show that the Earth System is characterized by several different tipping points, their representation within Earth System models (ESMs) remains poorly constrained. This is because historical observations do not encompass variations large enough to provoke such regime changes, and paleoclimate conditions are rarely used to help develop and tune ESMs, which potentially ignores a rich source of information on abrupt climate change. A critical example is the early to mid-Holocene “greening” and subsequent rapid desertification of the Sahara, which most ESMs fail to reproduce, casting doubt on the representation of land–atmosphere coupling and monsoon dynamics. Here, we show that this greening and abrupt termination can be successfully simulated with one ESM after optimizing uncertain model components using both present-day observations and crucially mid-Holocene (6,000 y before present) reconstructions. The optimized model displays abrupt threshold behavior, which shows excellent agreement with long paleoclimate records that were not used in the original optimization. These results suggest that in order to realistically capture climate-system thresholds, ESMs first need to be conditioned with appropriate paleoclimate information.
关键词：tipping point; abrupt climate change; climate model
