The high northern latitudes of the Arctic—seen as the canary in the coal mine for modern climate change—are warming at an outsized rate compared with elsewhere on the planet. Already, experts predict that the Arctic Ocean might be ice free during summer months in as little as 40–50 years. The trend has researchers concerned that resulting feedbacks, especially reductions in Earth’s albedo as ice increasingly melts, may lead to rapid changes in the global climate.
To understand how the future could play out, scientists look back to other warm periods in Earth’s history. Despite the Arctic’s critical role in Earth’s climate, however, data about the sea ice and climate history of the region are limited. Here Stein compiles a review of the existing literature on Arctic climate from the late Mesozoic era (about 150 million to 66 million years ago) through the ongoing Cenozoic era.
In the late Mesozoic, Earth’s atmosphere was characterized by much higher atmospheric greenhouse gas concentrations and much higher average temperatures than today. Then, during the past 50 million years or so, the planet experienced a dramatic long-term cooling trend, culminating in the glacial and interglacial cycles of the past 2.5 million years and the most recent ongoing interglacial period, in which rapid anthropogenic warming is occurring.
Much of the data presented in the review are from the International Ocean Discovery Program’s Expedition 302, called the Arctic Coring Expedition (ACEX), which was the first scientific drilling effort in the permanently ice covered Arctic Ocean. Examining geological records from sediment cores offers insights into previous climates on Earth and helps scientists disentangle natural and human-caused effects in the modern climate. The author combines and compares grain size, marine microfossil, and biomarker data from the ACEX sediment cores with information from terrestrial climate data, other Arctic and global marine climate records, and plate tectonic reconstructions to create a history of Arctic conditions reaching back into the Cretaceous period.
The results reveal numerous periods of warming and cooling, but overall, the planet’s temperature has mirrored trends in atmospheric carbon dioxide, with the transition from the warm Eocene to the cooler Miocene coinciding with a drop in carbon dioxide concentrations from above 1,500 to below 500 parts per million over a period of roughly 25 million years.
Although late Miocene climate and sea ice conditions might have been similar to those proposed to be in our near future, the rate of change in the late Miocene was very different from today. Whereas the ongoing change from permanent to seasonal sea ice cover in the central Arctic Ocean, strongly driven by anthropogenic forcing, is occurring over a timescale of decades, the corresponding change in the late Miocene probably occurred over thousands of years.
The author also highlights that as much as the sediment data reveal, there are also gaps in the understanding of the record. A long interval in which sedimentation rates slowed to a crawl during the early Cenozoic era, for example, presents challenges to scientists analyzing the Arctic climate history during the Miocene, Oligocene, Eocene, and Paleocene epochs. The cause of this slowdown remains a mystery to researchers, which, the author notes, emphasizes the importance of securing additional sediment cores from the Arctic on future scientific drilling expeditions to help fill the holes in the timeline. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003433, 2019)
—David Shultz, Science Writer