Guest post by Renee Hannon
The Arctic and Antarctic regions are different and yet similar in many ways. The Arctic has ocean surrounded by land and the Antarctic is a continent surrounded by water. Both are cold, glaciated and located at Earth’s poles some 11,000 miles apart. While sea ice has been retreating in the Arctic, it has been relatively stable in the Antarctic. This post examines surface temperature trends, solar insolation, and CO2 at the polar Arctic and Antarctic regions during the Holocene interglacial period.
Holocene Polar Temperature Trends are Out of Phase
The Holocene interglacial started about 11,000 years ago after termination of the previous glacial period. It is commonly described as consisting of an early Holocene climate optimum from approximately 10,000 to 6,000 years before present (BP, before 1950). This optimum is followed by a pronounced cooling in the mid-late Holocene referred to as the Neoglacial period which culminates in the Little Ice Age (LIA) around 1800 years AD (Lui, 2014).
Past Holocene temperature anomalies are typically estimated from ice core proxies. This post uses Arctic temperature anomalies from Agassiz-Renland isotope data corrected for elevation by Vinther, 2009 and Antarctic temperature anomalies from Dome C ice core proxies calculated by Jouzel, 2001. Temperatures are presented as anomalies relative to present day average polar temperatures. Time is shown as both years AD/BC and years before present, BP. Years BP (yr BP) is the key reference in the text. Datasets used are referenced at the end of the post.
Arctic and Antarctic Holocene temperature anomalies are shown in Figure 1. Arctic temperature anomalies show a prominent climate optimum from 10,000 to 6,000 yr BP with a brief cold interruption around 8,200 yr BP. The Neoglacial cooling is also evident where Arctic temperature anomalies steadily cool from 6,000 yr BP to the LIA as described in the literature.
The Antarctic seems to be a bit more contrary from the simple Climate optimum and Neoglacial description. Antarctic does exhibit a temperature high or optimum from about 11,500 to 9000 yr BP. Masson, et. al, 2000, examined all existing Antarctic ice core records which confirm a widespread early Holocene climate optimum during this time. This early optimum is followed by cooling temperatures to a minimum around 8000 yr BP. Most core sites studied by Masson in the Antarctic display this cool minimum. Masson also recognizes a secondary Antarctic late warm optimum between 6,000 and 3,000 yr BP.
Compared to the Arctic, the Antarctic shows a much-abbreviated early climate optimum that ends just after the Arctic climate optimum begins. While the Arctic stays warm during the Holocene climate optimum 10,000 to 6000 years BP, the Antarctic experiences a cold period from 9000 to 6000 yr BP. While the Arctic shows progressive cooling during the “Neoglacial period”, the Antarctic is experiencing a second warming trend. Therefore, the Holocene climate optimum and Neoglacial period better describe the Northern Hemisphere, not the Antarctic region. The two polar hemispheres do not warm and cool together and underlying long term trends appear to be out of phase after the Antarctic early Holocene optimum.
Polar Temperature Trends are Synchronous with Local Solar Insolation
It has long been recognized that Northern Hemisphere (NH) summer solar insolation influences reconstructed ice core temperatures (Laskar, 2004). Figure 2 shows the strong Northern Hemisphere summer insolation during the early Holocene synchronous with the Arctic temperature climate optimum. In the early Holocene, northern summer insolation reaches a maximum about 9,000 years ago. Northern insolation becomes progressively weaker during the mid-late Holocene coeval to the Arctic Neoglacial cooling trend.
In this post, Northern insolation refers to the Northern Hemisphere summer (June-August) and Southern insolation refers to the Southern Hemisphere summer (December-February). Figure 2 shows both Northern and Southern summer insolation at 65 degrees latitude which are out of phase during the Holocene. In the early Holocene, Northern summer was at perihelion when Earth is closest to the sun around 9,000 yr BP, and southern summer occurred when Earth was farthest from the sun. While Northern insolation progressively declines during most of the Holocene, Southern insolation progressively increases.
Today, northern, and southern summers are reversed from the Early Holocene. Presently, the southern summer occurs near perihelion when Earth is closest to the sun and northern summer occurs when Earth is farthest from the sun. In the future, northern insolation will continue to decline, and southern insolation which is currently strong will begin to decline.
How do Antarctic temperature trends relate to solar insolation? Masson, 2000, states that the early Antarctic climate optimum occurs at the same time as the Northern Hemispheric summer insolation optimum around 10,000 years BP. Although correct, this appears to be the only time Antarctic temperature trends display any resemblance to Northern insolation as shown in Figure 2. When Northern insolation is at an optimum around 9,000 yr BP, Antarctic temperatures are cooling towards a minimum. When Northern insolation is declining during the mid-late Holocene, Antarctic temperatures are warming or flat after the 8,200 yr BP cold period. During most of the Holocene, warming Antarctic temperature trends seems to be more aligned with increasing Southern insolation.
Holocene polar temperature trends appear to be largely synchronous and in the same direction as their local summer solar insolation. For most of the Holocene, Antarctic and Arctic temperature trends appear out of phase with each other just as Northern and Southern summer insolation are out of phase. The role of local insolation may be a strong influence on the underlying millennium-scale polar temperature trends in the polar regions.
CO2 is Synchronous with Antarctic Temperature Trends
Antarctic ice core data are routinely used as proxies for past CO2 concentrations. Antarctic CO2 data is the key dataset for paleoclimate CO2 trends during interglacial and glacial periods for the Southern Hemisphere. Surprisingly, Antarctic CO2 data is frequently used in Northern Hemisphere studies as well as compared to instrumental CO2 global trends (Ahn and Brooks, 2013, Kohler, 2011, NOAA, 2020).
Many technical articles and research from the mid-1990’s reached the hypothesis that CO2 gas in Greenland ice core bubbles were enriched by acid-carbonate chemical reactions and therefore, are unreliable (Anklin, 1995, Barnola, 1995). This theory was put forward because CO2 measurements from Greenland ice cores are more variable and generally 20-30 ppm higher than Antarctic CO2 measurements. As a result, Greenland CO2 datasets are not used in scientific studies to understand the Northern and Southern hemisphere’s interactions with and sensitivity to greenhouse gases under various climatic conditions. For a more detailed discussion on CO2 from Arctic Greenland cores, refer to my previous post here.
Figure 3 shows CO2 data plotted with Antarctic and Arctic temperature anomalies for the Holocene. CO2 tends to be synchronous with Antarctic temperatures and out of phase with Arctic temperatures. In the early Holocene, CO2 reached 270 ppm around 11,500 yr BP. CO2, then gradually decreased to a minimum of 255 ppm around 8,000 yr BP. During most of the Holocene since 8,000 yr BP, CO2 has been increasing. This increase in CO2 parallels the increase in Antarctic temperature trends and is contrary to Arctic temperature trends which decreased during this time.
It is difficult to compare the elevated CO2 records from the present to the past since CO2 data from the Northern Hemisphere during the Holocene is not publicly available. The scant Greenland CO2 data that is publicly available shows CO2 is generally 20-30 ppm higher than Antarctic CO2 and as high as 375 ppm in the Holocene (Neftel, 1982 and Barnola, 1995). Even today, Barrow and South Pole observatories have seasonal amplitude differences resulting in CO2 being 12-15 ppm higher at Barrow than in the South Pole during northern winter months almost 60% of the year (NOAA, 2020).
Holocene Polar Correlations
Holocene polar temperature anomalies over the past 10,000 years are compared to Northern insolation, Southern insolation and CO2 using Pearson’s correlation shown in Figure 4.
The Arctic and Antarctic temperature underlying millennium trends have a negative correlation confirming they are mostly out of phase. A strong positive correlation occurs between Arctic temperature anomaly trends and Northern insolation as expected. And the Arctic temperature trends shows a strong negative correlation with Southern insolation, no surprise there. Arctic temperature trends also show a strong negative correlation with CO2.
Antarctic temperature trends show a positive correlation with Southern Hemispheric insolation as well as with CO2 trends. These correlations are all strong, above 0.74. CO2 shows a surprisingly strong correlation with Southern insolation of 0.92. Interestingly, the Holocene Antarctic temperature anomaly trends show a strong negative correlation with Northern insolation.
Antarctic temperature proxy data is key to understanding paleoclimate trends and climatic conditions. The Southern Hemisphere is vastly under-represented in proxy data compared to the Northern Hemisphere for the Holocene and contains only about 10-15% of the proxy records. As seen in these polar comparisons, the Southern extratropics behave very differently than the Northern Hemisphere.
In conclusion, the often-overlooked Antarctic dances to a different beat than the Arctic. Antarctic and Arctic underlying temperature trends are mostly opposite in phase during the Holocene. Holocene polar temperatures trends are largely synchronous and in the same direction as their local summer solar insolation over the past 10,000 years suggesting local insolation influences the secular temperature trend of the polar regions.
The Holocene CO2 trends measured from Antarctic ice cores are coeval with Antarctic temperature trends and out of phase with Arctic temperature trends. Despite being routinely used in climate research, Antarctic CO2 is probably not representative of global and/or Arctic CO2 trends.
Acknowledgements: Special thanks to Donald Ince and Andy May for reviewing and editing this article.
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Masson, V., Vimeux, F., Jouzel, J., Morgan, V., Delmotte, M., Ciais, P., Hammer, C., Johnsen, S., Lipenkov, V. Y., Mosley- Thompson, E., Petit, J.-R., Steig, E., Stievenard, M., and Vaik- mae, R.: Holocene climate variability in Antarctica based on 11 ice cores isotopic records, Quaternary Res., 54, 348–358, 2000. https://is.muni.cz/el/1431/jaro2015/Bi8300/39087998/Masson_etal2000_climate_Antarctica_ice-core.pdf
Neftel, A, H Oeschger, J. Schwander, B. Stauffer, and R. Zumbrunn, Ice core sample measurements give atmospheric CO2 content during the past 40,000 years. Physics Institute, University of Bern. Nature Vol. 295, 1982. https://www.researchgate.net/publication/230889363_Ice_core_sample_measurements_give_atmospheric_CO2_content_during_the_past_40000_yr.
NOAA, Global Monitoring Laboratory, 2020. Visualization comparing instrumental CO2 data to past Antarctic ice core CO2 data. https://www.esrl.noaa.gov/gmd/ccgg/trends/history.html
NOAA, Global Monitoring Laboratory, 2020. Graph of Barrow, Mouna Loa, and South Pole CO2 data showing seasonal and global trends. https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_trend.html
Zhengyu Liu, Jiang Zhu, Yair Rosenthal, Xu Zhang, Bette L. Otto-Bliesner, Axel Timmermann, Robin S. Smith, Gerrit Lohmann, Weipeng Zheng, OliverElison Timm. The Holocene temperature conundrum. Proceedings of the National Academy of Sciences Aug 2014, 111 (34) E3501-E3505; DOI:10.1073/pnas.1407229111, https://www.pnas.org/content/111/34/E3501
Arctic Agassiz-Renland temperature proxy by Vinther, 2009. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/vinther2009greenland.txt
Dome C temperature proxy by Jouzel, 2001. https://www1.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/epica_domec/edc96-iso-45kyr.txt
Solar Insolation by Laskar, J., 2004. http://vo.imcce.fr/insola/earth/online/earth/online/index.php.
Dome C CO2 data by Bereiter, 2016. https://www1.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/antarctica2015CO2.xls