Figure 1
A new analysis of an ice core from Dome Fuji in Antarctica, along with climate simulation results, shows a high degree of climate instability within glacial periods with intermediate temperatures. This instability was attributed primarily to global cooling caused by a reduced greenhouse effect.
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Figure 1. Left: Ice Core from the Dome Fuji station. Right: Oxygen isotope ratio (an indicator of air temperature) and dust flux (an indicator of atmospheric particle concentrations) during the past 720,000 years obtained from the Antarctic Dome Fuji ice cores. The triangles at the bottom of the chart indicate the locations of warming peaks in the Antarctic as abstracted by this study.
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A research group, comprising 64 researchers from the National Institute of Polar Research, Tohoku University and other organizations, analyzed atmospheric temperatures and dust from the past 720,000 years using an ice core obtained at Dome Fuji in Antarctica. Results indicate that when intermediate temperatures occurred within a glacial period, the climate was highly unstable and fluctuated. A climate simulation also revealed that the major cause of the observed climate instability was global cooling due to a decline in the greenhouse effect.
Climate instability impacts both the Earth's natural environment and human society. To understand how global warming could affect climate instability, it is important to identify periods of similar instability in the past and to study their causes. Unfortunately, little progress has been made to improve our documenting and understanding of climate instability prior to the last glacial period.
The research group analyzed the Second Dome Fuji ice core (Fig. 1 left) that was obtained as part of the Japanese Antarctic Research Expedition (JARE) between 2003 and 2007. Their team reproduced fluctuations in the air temperature and dust in the Antarctic from the past 720,000 years (Fig. 1 right).
They combined this with data from the Dome C ice core to obtain highly robust paleoclimate data. They examined the data, discovering that for the past 720,000 years, the intermediate climate within glacial periods was marked by frequent climate fluctuations (Fig. 2).
This raised a question: Why does the most instability occur when there is an intermediate climate during a glacial period, rather than during an interglacial period, such as we are currently experiencing, or during the coldest part of a glacial period?
The research group used a climate model to first reproduce three types of background climate conditions--the interglacial period, intermediate climate within a glacial period, and the coldest part of a glacial period. They then performed a simulation that added the same quantity of fresh water to the northern part of the North Atlantic Ocean in each of the three climate conditions. The simulation results indicated that the response to freshwater inflow is maximized during the intermediate climate that occurs within glacial periods, causing the climate to become unstable (Fig. 3 A-C).
An important factor affecting climate instability is the vulnerability of Atlantic deep water circulation during global cooling resulting from a decrease in the atmospheric carbon dioxide concentration (Fig. 3 D-E). Until now, the primary factor for climatic instability was thought to be the existence and instability of continental ice sheets in the Northern Hemisphere. But this experiment has revealed that carbon dioxide is another important factor, determining not only the average state of the climate, but also the long-term stability of the climate.
These results also suggest that future stability in the present interglacial period, which has continued for more than 10,000 years, is not guaranteed.
The study results have been published in the online journal, Science Advances.
Contact:
Shuji Aoki
Center for Atmospheric and Oceanic Studies,
Graduate School of Science
Tohoku University
Email: [email protected]
Figure 2 (below left): Relationship between the frequency of climate fluctuations with temperatures in the Antarctic during the past 720,000 years obtained from analysis of Antarctic ice cores (black spots), as well as results for the final glacial period based on ice cores from Greenland (red squares). During the warm interglacial periods, and the coldest portion of a glacial period, the frequency of climate fluctuations was low. But during periods of intermediate temperatures within a glacial period, climate fluctuations occurred frequently and the climate was unstable.
Figure 3 (below right): Figure 3. Temperature deviations following the continuous addition of fresh water to the northern part of the North Atlantic for 500 years as simulated by the Coupled Atmosphere-Ocean Global Climate Model (MIROC), which was used to reproduce the atmospheric carbon dioxide concentrations and ice cover corresponding to three different climate conditions (A: an interglacial period, B: intermediate climate within a glacial period, and C: the coldest part of a glacial period).
During the intermediate climate within glacial periods, the response was dramatic, with the Northern Hemisphere cooling and the southern hemisphere warming. Sensitivity tests using artificial conditions were also performed (D, E) in which the atmospheric carbon dioxide concentrations and ice-sheet coverage in the Northern Hemisphere from the interglacial periods were switched with those from the intermediate climate within glacial periods. The atmospheric carbon dioxide concentration appears to play a major role in increasing climate instability.
Figure 2
Figure 3
