It unfortunately takes a little bit of time, but climate skeptics’ claims that observations don’t support climate model projections aren’t supported as more observations are made of the Earth system.  The latest instance: instead of using just climate projections, a pair of researchers have used observations to try to determine whether internal variability (natural year-to-year changes), self-acceleration (positive feedback loops), or external forcing were most the likely drivers of observed sea-ice retreat in the past 30 years.

The takeaway from this research: external forcing (CO2) is shown to be most responsible.  This is a good case of how science works: investigate multiple potential causal factors and let the observed data speak for themselves.

The captions for the figures below come directly from the paper.

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Figure 1. Evolution of Arctic sea-ice extent in (a) March and (c) September and the year-to-year changes in (b) March and (d) September. For this figure, an offset of +0.35 106 km2 in September and of 0.16  106 km2 in March has
been added to the entire original NSIDC dataset [Fetterer et al., 2002, 2010] to make the time series consistent with the original HadISST satellite time series during the period 1979–1996 [Met Office Hadley Centre, 2006].

The investigation included 60 years of robust sea ice data – from HadISST as well as NSIDC.  They used NSIDC in the satellite era because it provides a more consistent interpretation of the period.  As you can see, conditions in the Arctic started changing in the 1980s.  By 2000, both March and September conditions were different than conditions in the middle of the 20th century.  One big question when looking at this or similar time series data is whether the recent decline in sea ice is natural or not.  The following figure helps to answer that question quite definitively:

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Figure 2 [4 in paper]. Relationship between sea-ice evolution and various forcings. (a) Temporal evolution of solar irradiance [Fröhlich, 2000], AO-index [Thompson and Wallace, 1998], PDOindex [Mantua and Hare, 2002], and CO2 concentration (scaling with a 1.66 W/m2 equivalence for a 100 ppm increase [Intergovernmental Panel on Climate Change, 2007]). The thin lines denote monthly values, while thick lines denote averages over 1 year (CO2), 5 years (AO-index, PDO-index) and 10 years (solar irradiance). (b–e) September sea ice extent from 1953 until 2010 is plotted against annual mean values of the various forcings whenever data was available. The R2 values are calculated for a standard linear regression as indicated by the shading (2s).

The primary message of this figure is the takeaway from the research: the correlation between sea-ice extent and CO2 is a remarkably strong 0.84.  Moreover, no similar correlation was found between sea-ice extent and any other forcing: not irradiance, PDO-index, AO-index (shown in figure above), cosmic rays, volcanic eruptions, or poleward oceanic heat transport (not shown in paper).  In other words, the set of explanations that skeptics like to use to explain away a multitude of climate change effects has been shown to not explain the decline in sea-ice.  Irradiance has a positive, but much smaller relationship than does CO2 concentration.  The PDO- and AO-indexes have no statistical explanation for sea-ice extent.

The paper includes this important passage:

Note that the same reasoning allows us to conclude that changes in CO2 concentration are not the main driver for the observed sea-ice evolution in the Antarctic. With no clear trend in the sea-ice extent there, there is virtually no correlation with the increasing CO2 concentration. This underpins the fact that in the Antarctic, sea-ice extent is at the moment primarily governed by sea-ice dynamics. In contrast, in the Arctic the sea-ice  movement is constrained by the surrounding land masses and the thermodynamic forcing becomes more relevant there.

At this point, the rise in CO2 remains the leading explanation for the decline in Arctic sea-ice extent.  Since this work was based on statistical analyses of observational data, I eagerly await observational-type climate change skeptics to accept the work as valid.  It won’t happen, of course, as I’ve found that the vast majority of skeptics won’t accept any amount or type of evidence.  That’s because the real issue is based on values, which skeptics don’t want to discuss.  Instead, they use climate change as a proxy argument.

For the rest of us, this is an important result.  I truthfully do await scientific responses to this work.  It should be challenged on legitimate scientific grounds, if it is challenged at all.

Cross-posted at WeatherDem – the blog.