Guest essay by Jim Steele
Director emeritus Sierra Nevada Field Campus, San Francisco State University and author of Landscapes & Cycles: An Environmentalist’s Journey to Climate Skepticism
I just finished reading the paper Influence Of Internal Variability On Arctic Sea-Ice Trends by Swart et al (2015) in the journal Nature Climate Change and discussed by Anthony here. The paper might be better titled a Statistical Justification For The Pause In Arctic Sea Ice Melt as they concluded, “Thus, pauses in sea-ice loss, such as seen over the past eight years, are not surprising and are fully expected to occur from time to time.” In other words, we should still trust the models and ignore skeptics who cherry-pick the current pause and thickening of sea ice.
They also determined, “according to the models there is about a one in three chance of a 7-year period with a positive sea-ice trend, despite strong anthropogenic forcing.” And to their credit they also reported that the enhanced sea ice loss between 2001 and 2007 was rare, but plausibly enhanced by natural variability concluding, “Thus, both the enhanced sea-ice loss during 2001–2007, and the recent period of near-zero trend are consistent with the supposition of internal climate variability onto the background of long-term radiatively forced sea-ice decline…”
However the “elephant” mired in the thickening Arctic ice was, if this paper was truly anything more than an excuse for the lack of an Arctic sea ice death spiral, and the “background of long-term radiatively forced sea-ice decline” is a global phenomenon, then why wasn’t their analysis extended to the condition of global sea ice and Antarctica? Why cherry-pick just the Arctic?
Without access to their models, I can’t directly ascertain their statistical probability of a pause in Antractica’s hypothesized sea ice decline, but their Figure 3B (below) suggests the probability is zero. The black line represents the modeled probabilities of a increasing pause‑lengths based on observational data. A probability of a 30‑year pause (or increase in sea ice) between 1979 and 2013 is clearly zero. The graph’s other colored lines are probabilities for pauses based on different projections of CO2 concentrations throughout the 21st century referred to as Representative Concentration Pathways. The Wiki graph at the end of the article illustrates the CO2 trajectory for each RCP.
From Figure 3 c: Probability of a pause as a function of pause length in the Historical-RCP4.5 experiment over 1979–2013 (black), and in the future over 2066–2100 under the RCP2.6 (blue), RCP4.5 (cyan) and RCP8.5 (red) experiments. The horizontal dashed line represents a probability of p = 0.05. A pause is a period with a trend ≥0. Only ice extents ≥1 x 106 km2 are considered.
Thus it is more than likely, the observed 30_year “pause” in Antarctica sea ice “decline” is NOT “consistent with the supposition of internal climate variability onto the background of long-term radiatively forced sea-ice decline”. It suggests the model’s supposition of long-term radiatively forced sea-ice decline is need of serious reconsideration!
Their study was strictly a statistical analysis, independent of the various causes that might be attributed to the changing sea ice patterns at either pole. So it doesn’t matter how many hypothetical reasons may be conjured up to explain Antarctica’s growing sea ice. Based on their analyses, the models’ inability to predict a 30‑year trend in growing Antarctica sea ice “despite strong anthropogenic forcing”, can not be explained by CO2 driven models or random variability. As I have argued before Antarctica sea ice growth is a better indicator of climate change and there are very good reasons to believe the loss of Arctic sea ice is better explained by ocean and atmospheric oscillations.