Estimating sea ice decline due to natural variability

From, a new paper examines the contribution of natural cycles the AMO, AO and AMOC to the reduction in Arctic sea ice.  A Japanese and UK collaboration used five CMIP3 coupled general circulation models with pre-industrial forcing to investigate the impact of the AO, AMO and AMOC on March and September sea ice extent.

Figure 1. Normalized AMO index (Enfield et al 2001), AO index (calculated from Allan and Ansell (2006)), September and March SIE (Meier et al 2007). The time series were detrended (apart from the AO index), then passed through a 10 year low pass filter.

Negative correlations of sea ice extent and the AMO and AMOC, but not the AO, were significant at the 90% level indicating that the increasingly positive values of the AMO over the period of satellite ice observations from 1979 up to ~2001 may contribute, at least in part, to the decline of sea ice.  The predictive capability and correlation of the models with observation differed depending on their handling of the AMO and the northward transport of ocean heat, affecting the flow of Atlantic water into the Arctic.

The group then used a simple linear model to predict the sea ice extent over 1979-2010 and an extended period 1953-2010, using the five models and the AMO index to estimate the AMO driven component.  Depending on the model used, 5-30% of the sea ice loss during the satellite era (1979-2010) is attributable to the natural cycle of the AMO. Over the longer period from 1953 the proportion was smaller and the authors suggest that,  despite increased observational uncertainty in the pre-satellite era, this “is more likely to be representative of the anthropogenically forced component.”  This says even more about the lack of predictive capability of the models.

Sources of multi-decadal variability in Arctic sea ice extent
J J Day et al 2012 Environ. Res. Lett. 7 034011

The observed dramatic decrease in September sea ice extent (SIE) has been widely discussed in the scientific literature. Though there is qualitative agreement between observations and ensemble members of the Third Coupled Model Intercomparison Project (CMIP3), it is concerning that the observed trend (1979–2010) is not captured by any ensemble member. The potential sources of this discrepancy include: observational uncertainty, physical model limitations and vigorous natural climate variability. The latter has received less attention and is difficult to assess using the relatively short observational sea ice records. In this study multi-centennial pre-industrial control simulations with five CMIP3 climate models are used to investigate the role that the Arctic oscillation (AO), the Atlantic multi-decadal oscillation (AMO) and the Atlantic meridional overturning circulation (AMOC) play in decadal sea ice variability. Further, we use the models to determine the impact that these sources of variability have had on SIE over both the era of satellite observation (1979–2010) and an extended observational record (1953–2010). There is little evidence of a relationship between the AO and SIE in the models. However, we find that both the AMO and AMOC indices are significantly correlated with SIE in all the models considered. Using sensitivity statistics derived from the models, assuming a linear relationship, we attribute 0.5–3.1%/decade of the 10.1%/decade decline in September SIE (1979–2010) to AMO driven variability.

It’s Open Access, so read the rest here: doi:10.1088/1748-9326/7/3/034011

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7 Responses to Estimating sea ice decline due to natural variability

  1. Bloke down the pub says:

    In the anthropogenic signal, do they make any distinction between CO₂ and soot pollution?

  2. Doug Proctor says:

    What they really say is that their modelling gives 30% of the decline to the AMO, and 70% to other factors not modelled.

    The conclusion is another argument from ignorance. It would hold consideration more if we believed that the current decline in sea ice extent were a new, never-before event. Since we know that it is not, and has not been for a great deal of time, then what they are saying is that 70% of the current decline is attributable to unknown factors which may be the same as the factors that lead to significant extent decline in prior events.

    Geologists are excellent skeptics because we know that our thoughts of how things connect, how things came about, are exercises in creating internally consistent stories. These stories may or may not reflect reality, but they allow us to make predictions in the detail; observation and test follow. Most of the time we have to adjust our thoughts. Even when we succeed at first.

    You don’t have to be right to drill an oil well. You just have to drill a well where the oil is. Similarly, you don’t have to be right to predict a sea-ice decline. You just have to make a prediction when the ice is declining.

    • Verity Jones says:

      I am kicking myself as I had a lot more to say about this paper, but didn’t get back to it on Sunday…or Monday, and of course it is now all forgotten.

      @Bloke down the pub
      to my chagrin I don’t remember now – I’d have to read the link again.

      @Doug Proctor “What they really say is that their modelling gives 30% of the decline to the AMO, and 70% to other factors not modelled.” Exactly. I was going to link to Danish ice maps of the 1930s-1950s which show a high level of uncertainty but where it is possible to compare what was known with the information for the same areas today.

  3. mwhite says:

    They’re static at the moment, problems with bad weather and ice

    • Verity Jones says:

      I see they’ve started moving again. It is getting late in the season and cooling rapidly. This could be worth following – and I don’t mean that with any sense of schadenfreude.

  4. Verity Jones says:

    Just what the world needs – sigh!

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