Dorothy behind the curtain (Part 2)

Guest post by Peter Morcombe  (link to Part 1)

Global warming or cooling effects should be magnified at high latitudes so it makes sense to look for temperature trends in the Arctic and Antarctic.  Ice sheets provide excellent temperature proxies going back more than 700,000 years (Vostok) in the southern hemisphere and over 50,000 years in Greenland (GISP/GRIP).

Ten years ago, Richard Alley used data from the GISP2 ice cores to explore abrupt climate change events such as the “Younger Dryas” 12,500 years ago: http://www.ncdc.noaa.gov/paleo/pubs/alley2000/alley2000.html

The GISP2 data has previously been related to more recent warm and cold periods.  See: http://www.foresight.org/nanodot/?p=3553 and http://www.rationaloptimist.com/blog/best-shot and currently http://wattsupwiththat.com/2010/12/28/2010%E2%80%94where-does-it-fit-in-the-warmest-year-list/

Unlike some other temperature proxies, the GISP2 ice core data does not require that we ignore or re-write history but it is a challenge to link up with current instrumental records.   While NOAA scientists have no data that can bridge the gap between the end of GISP2 (1905) and the present day, they did suggest some other sources of information such as the Danish Meteorological Institute (DMI) station data for Greenland going back 140 years: http://www.dmi.dk/dmi/tr10-05.pdf

Using the DMI data, temperature anomaly plots were constructed for Greenland going back to 1873, thus providing some overlap with the GISP2 ice core data.

The correlation between these anomalies surprised me given the distances between the sites (right).

The anomalies for the 8 coastal stations were combined and averaged over a 10 year running period.  As expected, the resulting plot had a strong upward trend reflecting recent “Global Warming”.

The temperature increase over the period is 2.3 ^oC compared to the IPCC’s ~0.8 ^oC for the same period (see AR4).  These numbers seem to fit the idea that temperature changes are magnified at high latitudes but there are a couple of problems.  Firstly, the “Hockey Stick” increase ended in 1938 and temperature declined sharply before rising again in the late 1990s.  Given the relentless increase in CO2 concentrations, other factors must be overwhelming that effect in order to produce a prolonged cooling.  Secondly, the rapid temperature increase over the last 12 years may be coming to an end.  No doubt these changes will be dismissed as “local” but over the last several thousand years Greenland has cooled and warmed in step with locations at lower northern latitudes.

But I really wanted to look back further in time. In Part 1 showed that temperatures around central Greenland in the period 1988-94 averaged -29°C and showed some correlation with coastal stations.  Actually the temperature at the GISP site was recorded only for 1989-1995 with many missing months.  A best estimate for temperatures during the most complete years gave an average of -30.8°C and this allows for a look back further in time.  So, tacking the above anomaly plot onto the GISP2 records produced a “Hockey Stick” that should delight Michael Mann.

Looking even further back quite a different picture emerges with temperatures declining over the long term:

The GISP2 ice core data corresponds closely with major historical events.  The Minoan Warm Period (~1,300 B.C) is followed in sequence by two Roman Warm Periods, the Dark Ages, the Medieval Warm Period (MWP) and the Little Ice Age.  Modern times appear to be warmer than the MWP, at least in this part of Greenland, but short of the temperatures reached in the Minoan Warm Period.  The DMI points out that its data is unadjusted, so there may be some Urban Heat Island (UHI) effect at work, particularly with regard to the Nuuk station.

While temperatures have been rising since 1850 there have been warmer periods during the last 9,000 years and presumably prior to the formation of the ice sheet at the GISP2 location ~70,000 years ago.  Abrupt temperature change, both up and down seems to be the rule during the period covered by the GISP2 data and this is hard to explain in terms of atmospheric CO2 concentration given its relatively leisurely and monotonic rate of change.

[Note – I found I’d previously been sent a paper with data from the GRIP and DYE3 Greenland ice cores. GRIP agrees with GISP2, while the DYE3 site is warmer.  Both show up the warm blip in the 1930s – Verity]

DOI: 10.1126/science.282.5387.268
Science 282, 268 (1998);
D. Dahl-Jensen, et al.
Past Temperatures Directly from the Greenland Ice Sheet