This post is an answer to Peter Ellis from
a discussion we and others have about the IPCC- definition of an ice-free Arctic as "when sea ice extent is less than 10 6 km 2 for at least five consecutive years". I thought this discussion is more appropriate in this thread. Neven, if I was wrong, feel free to move this.
Well maybe - but I am not convinced, even more so as you, for some odd reason, keep mentioning a conspiracy here.
Alleging that changes are silently being made to IPCC reports for political reasons, coupled with comments like "I'm wondering about the IPCC integrity process" look, sound and smell like conspiracy theory to me. Call it what you will.
1) The figure caption reads as follows:
Figure SPM.7: ... (b) Northern Hemisphere September sea ice extent (5 year running mean) ... The dashed line represents nearly ice-free conditions (i.e., when
sea ice extent is less than 10 6 km 2 for at least five consecutive years). ...
So first they say "5 year running mean" and 9 lines later (in the original caption) they all of a sudden change to "five consecutive years"?
Exactly my point. The first draft said "5 year running mean", and that's what the graph shows. In the final draft, the bracketed part "(i.e. when sea ice extent is.... etc)" was added, without changing the graph or any of the other associated text. It's bracketed, and starts with 'i.e.' - both signs that it is simply an attempt to clarify the foregoing text, not to alter its meaning.
Hi Peter,
after some sleep I think you might be right. But the fact remains, that the addition "for at least five consecutive years" does alter the meaning of the foregoing text "5 year running mean" and not clarify it. Or am I wrong on this?
And as, according to P-maker, figure captions "may be changed by the secretariat in consultation
with the scientists afterwards" I find it weird that a Scientist involved in the IPCC process can't distinguish between a 5 year running mean and 5 consecutive years.
Anyhow. I asked aunt Google for help and - generous as she is - she came up with some infos that might shed some more light on the matter.
- First I found this article in
Planet Earth from August 2015 about an ice-free summer in the Arctic by Prof. Ed Hawkins.
- Who is Dr. Ed Hawkings? He is "Climate scientist in the National Centre for Atmospheric Science (NCAS) at the University of Reading. IPCC AR5 Contributing Author." (from
here about an ice-free summer in the Arctic by Prof. Ed Hawkins.
As it turns out he has contributed to the TS of AR5, WGI. So my guess is he should know why they added "5 consecutive years" I might be wrong tough). On page 21 he writes:
The Intergovernmental Panel on Climate Change (IPCC) concluded it was likely that the Arctic would be reliably ice-free in September before 2050, assuming high future greenhouse-gas emissions (where ‘reliably ice-free’ means five consecutive years with less than a million square kilometres of sea ice). Individual years will be ice-free sometime earlier – in the 2020s, 2030s or 2040s – depending on both future greenhouse- gas emissions and the natural erratic fluctuations.
- Second: In a more recent article on
Climate depot he is quoted(last paragraph):
Dr Hawkins said the Intergovernmental Panel on Climate Change, the UN’s climate science advisory body, had forecast that the Arctic would be “reliably ice-free”, meaning more than five consecutive years below one million sq km, by the mid-21st century.
Dr Hawkins said: “Putting a precise date on when we see the first days or weeks that are ‘ice-free’ is unwise because of the chaotic nature of the climate system and uncertainties in future greenhouse gas emissions.”
So the idea was, he says, to find a measure of ‘reliably ice-free’, whatever that means. And, accroding to this at least, "5 consecutive years" seems to be meant literally and not as an explanation of a 5 year running mean (Or am I reading this wrong again?)
One thing is sure: By his own admission, this definition will change the date for an ice-free Arctic from the 20s, 30s to later decades.
But are "5 consecutive years" more reliable as a 5 year running mean or a single year? And what is the scientific basis of this definition? I have no clue - I hope others do. The best thing would probably be to ask Dr Hawkins himself.
Any native speaking volunteers out there?