Arctic sea ice free (extent)

[Glenn Tamblyn]

The change will alter the anomalies, but it won't alter how much the anomalies have changed over time which is the key thing of interest.

This is just bringing NSIDC more into line with WMO standards for climatology. To fully match they would need to use 1961-1990 or 1991-2020 as their baseline - obviously not possible.

[Shared Humanity]

Which makes anomalies seem smaller and less frightening. "A new normal"

Yes and I am certain they know this is the effect. I've always felt a decade was adequate for calculating a baseline and, if we look at the area and extent loss as compared to this base, it looks very scary.

[crandles]

I've always felt a decade was adequate for calculating a baseline

For some climate things, a 30 year base period seems sensible to be able to tell how unusual something is. For other things you do not need as long as 30 years and for some events, particularly rare ones you need more than 30 years. However, when you know the climate is changing showing 3 decadal averages does seem a lot more useful than one 30 year average.

Why does
http://ocean.dmi.dk/arctic/plots/meanTarchive/meanT_2013.png
use a 50 year average of 1958 to 2002? 5 decadal averages would appear to me to be much better.

If you are only plotting anomalies only then making all records consistently use 1981 to 2010 as the base period would seem sensible as satellite records often start around 1979 so 1981 to 2010 seems the only consistent choice available.

[Juan C. García]

However, when you know the climate is changing showing 3 decadal averages does seem a lot more useful than one 30 year average.

I totally agree. Maybe the best way to see it is to see the IJIS/JAXA SIE graph. Right now NSIDC is giving the approximated average (discarding 1979 and 1980 or 2000) of the first two lines (80´s and 90´s). With the new standard, NSIDC is going to give the approximated average of the first three lines (80´s, 90´s and 2000´s). The graphs of 2007, 2011 and 2012 are too low in values, so it doesn`t matter if you use standard 1979-2000 or 1981-2010. The differences are clear.
But the true is that you can see AGW better if you have the three decadal lines, instead of just one.

[Laurent]

Scientists Predict Arctic Will Be Ice-Free By Summer 2050, So... We're All Screwed
http://www.huffingtonpost.co.uk/2014/06/17/global-warming-arctic-ice_n_5502396.html

Ice free in summer by 2050...really...that's strange, I am not seeing the volume decline the same way... not in 2014 but 2017 why not...

[wili]

Interesting to look back at our predictions. Probably few of us anymore would be predicting virtually ice free conditions by 2016.

Here, though is something on how the Arctic will likely manage to remain icefree throughout the year at some point:

"in warm “equable” climates the Arctic was blanketed by clouds in winter (picture horizon-to-horizon grey skies that most UK readers will be familiar with), which trapped warmth, keeping the Arctic largely ice-free. "

https://www.skepticalscience.com/Why_the_Miocene_Matters.html#commenthead

[AbruptSLR]

I suspect that many who follow this thread do not realize that anthropogenic aerosols have been both more effective at global cooling than previously projected; but as the linked reference shows anthropogenic aerosols have been more effective still at masking/damping Arctic Amplification even than it has been at masking/damping the increase in mean global surface temperature.  Thus as aerosols are cleaned-up (projected to occur very rapidly in China, which is only of the largest sources of aerosols) the Arctic will warm faster than previously estimated.  This means that looking at the old trend lines many not be a good indication of how much Arctic sea ice loss will occur in the coming couple of decades (certainly a seasonally ice free Arctic in 2017 is not in the cards at this time, but I think it could happen by 2030 to 2035):

Najafi, M.R., et al. (2015) Attribution of Arctic temperature change to greenhouse-gas and aerosol influences, Nature Climate Change, doi:10.1038/nclimate2524

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2524.html

Abstract: "The Arctic has warmed significantly more than global mean surface air temperature over recent decades, as expected from amplification mechanisms. Previous studies have attributed the observed Arctic warming to the combined effect of greenhouse gases and other anthropogenic influences. However, given the sensitivity of the Arctic to external forcing and the intense interest in the effects of aerosols on its climate, it is important to examine and quantify the effects of individual groups of anthropogenic forcing agents. Here we quantify the separate contributions to observed Arctic land temperature change from greenhouse gases, other anthropogenic forcing agents (which are dominated by aerosols) and natural forcing agents. We show that although increases in greenhouse-gas concentrations have driven the observed warming over the past century, approximately 60% of the greenhouse-gas-induced warming has been offset by the combined response to other anthropogenic forcings, which is substantially greater than the fraction of global greenhouse-gas-induced warming that has been offset by these forcings. The climate models considered on average simulate the amplitude of response to anthropogenic forcings well, increasing confidence in their projections of profound future Arctic climate change."
See also (both extract and image):

http://www.carbonbrief.org/blog/2015/02/aerosols-dampen-pace-of-arctic-warming-for-now-say-scientists/

Extract: "And the results suggest the cooling effect from aerosols is much larger in the Arctic than elsewhere in the world, Najafi adds. A separate study finds aerosols were responsible for offsetting around five per cent of global greenhouse gas warming between 1901 and 2010, and around 27 per cent for the shorter period of 1951 to 2010.