While the media circulates the talking points pre-release “leaked draft” of IPCC’s AR5 amongst themselves, there are a few nuggets of interest coming out here and there we can write about. One such nugget is contained in a series of bullet points on the Washington Post Capital Weather Gang in an article by Jason Samenow:
7) The 30 years from 1983-2012 was very likely the warmest 30-year period of the last 800 years.
That is an interesting statement, not so much for what it says, but for what it doesn’t say. A caveat; that’s likely the reporter’s summary, not the exact text from the IPCC “leaked draft”. IPCC verbiage tends to be a bit more bloated. But, I think it is a fair summary.
Bishop Hill points out what was said in IPCC’s AR4 in 2007:
Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years.
So, they’ve gone from saying warmest in the last 1300 years to the last 800 years. Where does that figure in on Mann’s hockey stick graph from AR3 in 2001?
Figure 1. The hockey stick graph as it appeared in the IPCC Third Assessment Report WG1 (2001) summary, Figure 2.20, Northern Hemisphere temperature reconstruction.
So basically what they are saying is that at the year 1200 (2000AD minus 800 years), temperatures were warmer (or at least equal to) temperatures today.
This is curious, because it looks like we are back to what the IPCC said in the first report in 1990. Notice the bump, peaking at 1200AD:
Figure 2: IPCC 1990 FAR chapter 7 fig 7.1(c) from http://www.ipcc.ch/ipccreports/far/wg_I/ipcc_far_wg_I_chapter_07.pdf
Compared and overlaid with Mann’s work, which was highly criticized for turning all the proxy data from 1000AD to 1900AD into a nearly straight flat shaft with an upturned blade at the end, that 1200AD bump looks like the elephant in the proxy samples room.
Figure 3: IPCC 1990, Mann 1999 and Moberg 2005 data overlaid.
- (red) IPCC hand-digitised from IPCC 1990 figure 7.1c (note: it has been assumed that the tickmarks on the y axis are in units of °C and that the middle of the three represents zero).
- (blue) Mann from ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/jones2004/jonesmannrogfig5.txt
- (black) Moberg from http://www.nature.com/nature/journal/v433/n7026/suppinfo/nature03265.html
- (green dashed line) Central England temperatures to 2007 overlaid from Jones et al. 2009 page 34 fig. 7. http://pubs.giss.nasa.gov/docs/2009/2009_Jones_etal_2.pdf
Somewhere, Hubert Lamb must be pleased that his work from IPCC’s FAR in 1990 showing a warmer Medieval Warm Period than the present is getting attention again. Steve McIntyre must also be smiling at this.
The question now is: will this inconvenient bump be flattened and sanitized in the final version of IPCC AR5?
UPDATE: WaPo’s Jason Samenow adds in comments –
I’m the author of the blog post on the IPCC report. My post just featured a handful of findings… it’s not at all comprehensive…just a teaser.
As I note in my post, I’ll dig deeper into the report once it’s finalized. As for the MWP, the IPCC says a couple things:
“Analyses of paleoclimate archives indicate that in the Northern Hemisphere, the period 1983–2012 was very likely the warmest 30-year period of the last 800 years (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence).”
“Continental-scale surface temperature reconstructions show, with high confidence, multi-decadal intervals during the Medieval Climate Anomaly (950−1250) that were in some regions as warm as in the late 20th century. These intervals did not occur as coherently across seasons and regions as the warming in the late 20th century (high confidence).”
The IPCC stresses these statements are draft and subject to change via the government review.
Thanks for reading…