RELATED POST: ANTARCTIC SEA ICE 1979-2018

RELATED POST: GEOLOGICAL FEATURES OF THE ARCTIC

RELATED POST: ARCTIC SEA ICE BIBLIOGRAPHY

RELATED POST: TIDAL CYCLES BIBLIOGRAPHY

ABSTRACT: Satellite radiometry and visual imagery of Arctic sea ice since 1979 have allowed us to track its dramatic seasonal cycle in which 70% of the March maximum sea ice area is gone by September. This extreme seasonal cycle is taken to imply that Arctic sea ice area is sensitive to ambient temperature. The satellite data also show a long term year to year decline in Arctic sea ice in every season concurrent with global warming. This concurrence has led to the assumption that the observed long term decline in sea ice is a response to global warming. This study is a test of that hypothesis. Arctic sea ice data for each calendar month are tested for the responsiveness of sea ice area to global warming at annual and five-year time scales using detrended correlation analysis. Satellite measurements of lower troposphere temperature over the north polar region are used as the relevant measure of global warming. It is found that of the twelve calendar months only two contain both statistically significant sea ice loss and statistically significant correlation of the rate of sea ice loss with global warming. These results do not constitute convincing evidence of correlation required to support the assumption that sea ice decline is driven by global warming.

It is likely that the observed loss in sea ice area is a more complex phenomenon possibly with roles for winds, ocean currents, geothermal heat, and natural multi-decadal variability of lunar nodal cycles and other ocean characteristics not measured and not fully understood. Global warming may play a role in what may be a complex multivariate phenomenon but the data do not show that global warming drives year to year changes in Arctic sea ice area or that the decline can be halted or moderated by taking climate action.

A map of undersea geological activity in the Arctic shown below was provided by geologist James Edward Kamis in a blog about the Arctic Ocean [LINK] . In the map, red triangles with black outline are locations of active submarine volcanoes and areas marked with red cross hatching indicate mantle plumes (rocks being pushed out of the earth’s mantle that often melt when they reach lower pressure areas). These geological features imply that geothermal activity is extensive in the Arctic and that they may well play a role in sea ice dynamics. These effects are not considered in the standard climate change interpretation of changes in Arctic sea ice.

1. Arctic sea ice area undergoes an intense seasonal cycle reaching a winter maximum extent of 13 million square kilometers (MSK) in March and a summer minimum of 4 MSK in September on average in the study period 1979-2017/8. The seasonal cycle in sea ice area is synchronized with the seasonal cycle in air temperature in an almost perfect inverse relationship. This correlation is generally accepted as sufficient evidence that sea ice is responsive to air temperature – melting in the warmth of summer and freezing in the cold of winter (Serreze, 2011). The observed seasonal relationship is the basis for the assumption that the observed decline in Arctic sea ice area since satellite measurements began in 1979 is attributable to anthropogenic global warming (AGW), and that it could accelerate AGW due to a loss of albedo if the Arctic becomes ice free (Perovich, 2007) for a number of summer months (Cavalieri, 2012) (Comiso, 2002) (Comiso, 2008) (IPCC, 2014) (Parkinson, 2002) (Serreze, 2007) (Winton, 2006). These concerns have specific application in the September minimum sea ice area in the Arctic where the lowest extents observed have occurred in the most recent decade (Liu, 2013) (Overpeck, 2005) (Winton, 2006).
2. This study is a statistical test of the hypothesis that global warming explains changes in Arctic sea ice area at annual or 5-year time scales. It was carried out as soon as the July 2018 data for Arctic sea ice area and satellite based measurements of lower tropospheric temperature in the North Polar region became available. Both Arctic and Antarctic sea ice areas are studied. The annual time series of mean monthly sea ice area are studied for each calendar month separately. Unusually low September minimum sea ice area in the Arctic in 2007, 2012, and 2016 created a great interest in the study of sea ice (NSIDC, 2016) (NASA, 2016) (Vidal, 2016). The year 2016 is also considered notable for its low sea ice area in the winter maximum month of March (NASA, 2016) and it re-kindled the alarming prospect of an ice-free Arctic in summer both as a sign of dangerous climate change and also as a positive feedback that could accelerate climate change (Liu, 2013) (Overpeck, 2005) (Winton, 2006) (Wang-Overland, 2009) (Wang-Overland, 2012) (Wang-Overland, 2013). In this work, we find a statistically significant decline in sea ice area in the Arctic for the calendar months of June to October with the rate of decline lowest in June and graducally rising to the highest in October. No statistically significant trend in Arctic sea ice area is found for the other calendar months. The relationship between global warming and sea ice area at annual and 5-year time scales for each of the twelve calendar months is studied with detrended correlation analysis. Of the months with a statistically significant full span OLS sea ice loss, only June and October show statistically significant detrended correlations with global warming at these time scales. It is noteworthy that neither of the seasonal extremum months of March and September shows a statistically significant detrended correlation between global warming and sea ice area.
3. Since the Little Ice Age ended in the mid-19th century, there has been a strong and steady warming trend (Overpeck, 1997) that is generally assumed to be human-caused by way of fossil fuel emissions that is thought to cause atmospheric carbon dioxide concentration to rise with that in turn causing warming by way of a “greenhouse gas effect” of these changes in atmospheric composition (Callendar, 1938) (Revelle, 1983) (Keeling, 1977) (Hansen, 1981) (Lacis, 2010). With regard to the adverse and possibly dangerous effects of human-caused global warming and climate change, a great deal of attention has been given to the changes observed in the Arctic Ocean. The reason for the focus on the Arctic and particularly Arctic sea ice, is three-fold. First the Arctic is warming faster than the rest of the world with a steep and alarming rate of loss in September’s seasonal minimum sea ice area from year to year. Secondly, the loss of Arctic sea ice is also a loss of ice albedo which suggests a positive feedback loop with catastrophic runaway global warming. Thirdly, the effect of Arctic warming on the jet stream may imply more harmful impacts of climate change than otherwise possible.
4. Sea ice consists of free floating floes in constant motion driven by wind and ocean currents. Sea ice area is defined as a contiguous surface area of the sea, measured in millions of square kilometers (MSK), where at least 15% of the sea surface consists of floating ice. On average, the dispersion of sea ice within a sea ice area varies from 50% to 80% (Munshi, Trends in polar sea ice area, 2015). Both sea ice area and the degree of dispersion within the extent can be estimated in the brightness data of passive microwave images taken from satellite mounted instruments (Comiso, 1997). Sea ice area corrected for the degree of dispersion is reported as sea ice Area (NSIDC, 2016).
5. An intense interest in polar sea ice area in the climate change era derives from the proposition that the multi-year decline in sea ice Area serves as measure of the impact of anthropogenic global warming (AGW) (Lacis, 2010) (Hansen, 1981) (IPCC, 2014) (Comiso, 2002). An additional consideration is the reduction in albedo due to lost polar sea ice could accelerate AGW and further complicate its effects on the climate system (Comiso, 2008) (Winton, 2006) (Perovich, 2007) (Serreze, 2007).
6. Sea ice Area undergoes a deep seasonal cycle in both poles. In the Arctic, it reaches a summer minimum of ≈4 MSK or less in September rising to a winter peak of ≈12 MSK in March in the average seasonal cycle. The seasonal cycle is reversed in the Antarctic where the summer minimum is reached in February (2 MSK) with a winter maximum of in September (14.5 MSK). The amplitude of these seasonal changes is much greater than the differences implied by long term declining trends. (Parkinson, 2002) (Cavalieri, 2012) (Munshi, Trends in polar sea ice area, 2015). Because of these large seasonal changes, trend analyses of sea ice area are usually restricted to the summer minimum and winter maximum months. The study of the other calendar months, when undertaken, must be carried out one month at a time so that long term trends are not confounded by the relatively stronger seasonal changes.
7. The generally assumed link between global warming and Arctic sea ice area subsumes a causal relationship in which global warming causes declining sea ice. The observed decline in sea ice area and the observed rise in surface temperature are assumed to be causally related so that the interpretation of these changes proceeds in terms of what appears to be obvious dynamics in which warming causes loss of Arctic sea ice. This relationship seems obvious in particular because the very high rate of warming in the Arctic region compared with other global regions (Munshi, Arctic Sea Ice Bibiliography, 2018). Yet, it is well known that correlation among time series data are often spurious because the effect of long term trends may be falsely interpreted as a responsiveness at the time scale of interest (Prodobnik, 2008) (Munshi, Spurious Correlations in Climate Science, 2018).
8. Satellite radiometry and visual imagery for polar ice area are provided as monthly mean values by the National Snow and Ice Data Center of the USA (NSIDC, 2016). These data are available both in dispersed (Extent) and in concentrated (Area) form in millions of square kilometers (MSK). Here, only the (Area) measures of sea ice area is used because the dispersed extent is confounded by the degree of dispersion. The primary surface temperature data used in this study are the monthly mean temperature anomalies above ocean surfaces in the North Polar Oceans within 60N to 82.5N. The data are lower troposphere temperatures (LTT) measured with satellite mounted instruments. They are provided as monthly mean anomalies from January 1979 to July 2018 by the University of Alabama Huntsville (Christy/Spencer, 2018).
9. Trend analysis is carried out separately for each calendar month. The testable implication of the theory that the declines in sea ice area can be attributed to warming is a negative correlation between monthly mean temperature and monthly mean sea ice area for each month on a year to year basis at the annual time scale. A 5-year time scale is also tested. Hypothesis tests for these correlations are set up with the null hypothesis H0: ρ≥0 against HA: ρ<0. This form of the hypothesis test derives from a theory that implies a negative correlation. Each comparison is made at a maximum false positive error rate of α=0.001 in accordance with “Revised standards for statistical evidence” published by the National Academy of Sciences to address an unacceptable rate of irreproducible results in published research (Johnson, 2013) (Siegfried, 2010). The effect of multiple comparisons on the overall study-wide maximum false positive error rate is estimated using Holm’s procedure (Holm, 1979).
10. Data for Arctic Sea Ice area in millions of square kilometers is provided by the National Snow and Ice Data Center of the Federal Government of the USA as both daily and monthly means from 1979 onwards (NSIDC, 2018). These data are taken by polar orbiting satellites that began measurements in December 1978 using radiometry as well as imagery in the visible spectrum. These measurements are considered to be the most reliable estimates of sea ice area possible as of this writing. The NSIDC reports sea ice area as EXTENT and also as AREA. The EXTENT measure, previously known as “dispersed extent”, reports the total area of the Arctic where at least 15% of the sea surface is occupied by sea ice. The AREA measure, previously known as “concentrated extent”, is the net ice area of the extent with areas of visible sea water removed from the extent measure. Only the AREA measure is used in this study because the EXTENT measure is confounded by the intervening variable having to do with the degree of dispersion that is of no interest in terms of the research question.
11. Figure 7 is a tabulation of full span trends in sea ice area and temperature anomalies for each calendar month along with tests for statistical significance. The tests for significance are carried out at α=0.001. They show that the observed sea ice declines are statistically significant in the five summer and fall months of June to October. The declines observed in the data for the other calendar months can’t be interpreted as phenomena that require a cause and effect explanation because the observed trend could have been generated by randomness. The warming trends appear to be clearer but with the exception of July. Generally, the rate of warming is greater in winter and spring than in summer. However, the greatest rate of sea ice decline is seen in summer and fall with much more gradual sea ice declines in winter and spring. Only the five summer and autumn months of June to October contain statistically significant declines in sea ice area.
12. Figure 8 is a tabulation of the results of detrended correlation analysis that addresses the research question of whether changes in sea ice area in each calendar month are responsive to the global warming temperature trend for that calendar month. The test is carried out at two time scales – annual and 5-year. At the annual time scale there is only one hypothesis and that is whether sea ice area is responsive to temperature at an annual time scale. This portion of the study is simply an update of a previous work with more recent data (Munshi, Responsiveness of sea ice extent to warming, 2016). At the 5-year time scale, three different hypotheses are tested. They are (1) whether the 5-year moving average sea ice area is responsive to the 5-year moving average temperature, (2) whether the 5-year rate of decline in sea ice is responsive to the 5-year moving average temperature, and (3) whether the 5-year decline in sea ice is responsive to the 5-year rate of warming.
13. The results of detrended correlation analysis tabulated in Figure 8 show that four statistically significant correlations are found at an annual time-scale. They are the winter months of January and February, June in summer and October in autumn. Of these months only June and October show a statistically significant decline in sea ice (Figure 7). As a way of comparison, the 2016 study had also found four statistically significant correlations at an annual time scale but they were for the months of February, April, June, and July (Munshi, Responsiveness of sea ice extent to warming, 2016). The expectation was that a greater number of significant results would be found at the 5-year time scale but exactly four significant results were found with the months of January, February, and October in common with the annual time scale and with the addition of the month of December not found in the annual time scale. Of these months only the month of October shows a statistically significant declining trend in sea ice (Figure 7).
14. Curiously, of the five months (June to October) with statistically significant declines in sea ice area (Figure 7), only one (October) shows a correlation with global warming at the five-year time scale whereas there were two at the one-year time scale. Thus, of the twelve calendar months studied only two months are found that contain both statistically significant losses in sea ice and statistically significant correlation of the rate of sea ice loss with the relevant measure of global warming. These results do not constitute convincing evidence of the correlation required to support the assumption that sea ice decline is driven by global warming.
15. It is likely that the observed loss in sea ice area is a more complex phenomenon possibly with a role for winds, ocean currents, geothermal heat, and natural multidecadal variability of ocean characteristics not measured and not fully understood. Global warming may play a role in what may be a complex multivariate phenomenon but the data do not show that global warming drives year to year changes in Arctic sea ice area. More importantly, the results do not imply that the observed decline in Arctic sea ice area can be halted or moderated by taking climate action. 
16. The geological features of the Arctic that may play a role in sea ice extent variation are described in a related post [LINK] . 

[LIST OF POSTS ON THIS SITE]

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