Eduardo Zorita sent me an interesting paper (Pollissar et al 2006, Solar modulation of Little Ice Age climate in the tropical Andes) hot off the press on June 1, 2006, co-authored by Bradley, which reported:
The intersection of the ELA [equilibrium line altitude] and pollen estimates indicate that during the LIA the Venezuelan Andes were both cooler (~3.2°C) and wetter (~208 mm/ yr, +22%) than present … Our data suggest considerable sensitivity of tropical climate to small changes in radiative forcing from solar irradiance variability.
They mention that "during most of the past 10,000 yr, glaciers were absent from all but the highest peaks in the Cordillera de Merida." Bradley has been one of the longest standing opponents of the LIA – has the leopard changed his spots?
You may recall that IPCC TAR notoriously concluded that the "conventional terms of “Little Ice Age” and “Medieval Warm Period” appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries. "
The terms “Little Ice Age” and “Medieval Warm Period” have been used to describe two past climate epochs in Europe and neighbouring regions during roughly the 17th to 19th and 11th to 14th centuries, respectively. The timing, however, of these cold and warm periods has recently been demonstrated to vary geographically over the globe in a considerable way (Bradley and Jones, 1993; Hughes and Diaz, 1994; Crowley and Lowery, 2000). … Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of “Little Ice Age” and “Medieval Warm Period” appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries.
A quick browse through the IPCC references shows how reliant their viewpoint is on the corpus of Hockey Team articles that I’ve discussed here. A longer discussion by Mann is here.
The preamble to Pollissar et al is oddly different, now characterizing the LIA as a "significant global event" and even citing MBH99 as a source:
During the past millennium, significant climatic fluctuations have occurred. Prominent among these is the Little Ice Age (LIA), recognized in historical records (e.g., ref. 1 -Pfister) and documented in proxy climate records from many locations (2 – Jones and Mann 2004). Although the LIA was a significant global event (3- MBH99), its causes and regional differences in the timing and climatic response remain unclear (2, 4 – Jones and Mann 2004, Crowley 2000).
The research strategy of Pollissar is quite different from the twin Hockey Team strategies of heavy reliance on tree rings and equally heavy reliance on mining for statistical correlations between tree rings and gridcell temperatures. Pollissar et al use what I would call more old-fashioned methods used by Lamb, such as changes in glacier elevations or changes in vegetation altitudes, to deduce past climates. However, they link such changes here to changes in lake sediments, which provide a continuous record of change over the past century – as opposed to relying on moraine locations to measure glacier advance and retreat. Their strategy briefly:
The presence of glaciers in the L. Mucubaji watershed increased the flux of inorganic sediment to the lake, producing a continuous lake-sediment record of glacier activity. The Mucubaji record is corroborated by analysis of sediments from L. Blanca (Fig. 1), a small, closed-basin lake at 1,620 m in unglaciated terrain. At this site, the flux of inorganic sediment to the lake increases during wet periods, producing a distinctive signature in the sediments that contrasts with drier periods. Both records are subsequently compared with nearby pollen histories that chronicle vegetation change in response to climate during the LIA (13).
Their Figure 2 shows a variety of proxy records. The two sediment series are the top two series in the panel below. I like the sharpness of Series B (Lago Blanca), the expression of which they interpret as glacier presence/absence, with onset in the 13th century and ending in the 19th century. The non-existence of the glacier in the MWP is very distinct in this record. Series B shows its maximum extent in the late 17th century, the period of greatest North Atlantic chill in Lamb’s view of the world. One wonders whether the dating of Series A might have stretched out a bit relative to Series B – the changes would be sharper with some perhaps plausible dating differences. Series D shows the pollen biome.
Original Caption: Fig. 2. Lake-sediment records from the Venezuelan Andes compared with indices of solar activity and additional tropical paleoclimate proxies. (A-C) Glacial advances, indicated by increases of sediment MS in L. Mucubaji (A) (vertical gray shading), coincide with an increase in precipitation, shown by higherMSin L. Blanca (B) and higher abundances of Cyperaceae (sedge) pollen in the Piedras Blancas peat bog located near to L. Mucubaji (C) (13). (D) Lowering of ecological zones and colder-wetter climate during the LIA is indicated by the hbiome (equivalent to the minimum estimated departure in ref. 15) from the Piedras Blancas site. (E) Minima in reconstructed solar irradiance (black line) (16) using the scaling of ref. 17 or maxima in 14C (gray line, inverted scale) (18) are coeval with glacier advances. The 14C record reflects solar modulation of the 14C production rate and is scaled to the reconstructed irradiance curve of ref. 16. (F) Annual record of latitude-weighted volcanic aerosol forcing (gray bars and left axis) (4) and 50-yr averages (line and right axis, multiplied by 4 to scale with the reconstructed solar irradiance and plotted at youngest age of the 50-yr window). (G) Wetter conditions are supported by the Punta Laguna, Mexico, d18O record of higher P/E during Mucubaji glacial advances (19). (H) Abundances of the foraminifer Globigerina bulloides in Cariaco Basin sediments are higher during glacial advances indicating stronger trade winds (20).
From these results, they conclude a glacier ELA (equilibrium line altitude) lowering of ~300 to ~500 m , suggesting that a modern equilibrium has not been reached. They report a similar lowering in pollen biome, with a gradual decrease in hbiome during the LIA, with an average LIA value of approximately ~220 m and a minimum of ~460 m near the LIA termination (Fig. 2D).
Using physical lapse rates rather than mining for statistical correlations between annual series of ring widths, they conclude that this lowering:
is equivalent to a temperature depression of 2.6-4.3°C, using the modern annual precipitation of 950 mm (Fig. 5).
They note that declines of about 2 deg C had been inferred for the Caribbean and attributed the greater change in the Venezuelan Andes to changed lapse rate:
The reconstructed LIA temperature depression in the high altitudes of the Venezuelan Andes is greater than that inferred for Caribbean sea-surface temperatures (SSTs) [~2°C (34-36)]. This result is likely a consequence of changes in adiabatic lapse rates due to cooling. Cooler tropical SSTs would reduce the absolute humidity of the lower troposphere and steepen the slope of the moist adiabat above the condensation level. This effect would lead to cooling at 4,500 m above sea level, which was ~1.5 times that at sea-level (Fig. 6), in agreement with our glacier- and pollen-temperature estimates.
Big numbers for an event local to the North Atlantic. They attribute these changes to solar modulation, noting that this implies a high sensitivity to solar. A couple of other tangential notes: Christy, in his recent presentation at the Marshall Institute mentioned in passing that the Rocky Mountain glaciers were all Neoglacial i.e. post-Holocene Optimum. I posted up last October some questions about the dating of the Kilimanjaro glacier. In my opinion, Thompson’s evidence for the existence of Kilimanjaro glacier in the Holocene Optimum is extremely weak and his very limited "evidence" can be readily interpreted in a way consistent with its non-existence at that time. If the Venezuela glaciers are transient, it certainly adds weight to the possibility that the Kilimanjaro glacier is transient.
P. J. Polissar, M. B. Abbott , A. P. Wolfe , M. Bezada, V. Rull and R. S. Bradley, 2006, Solar modulation of Little Ice Age climate in the tropical Andes Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0603118103 http://www.pnas.org/cgi/content/short/0603118103v1?rss=1
34. Winter, A., Ishioroshi, H., Watanabe, T., Oba, T. & Christy, J. (2000) Geophys. Res. Lett. 27, 3365–3368.
35. Watanabe, T., Winter, A. & Oba, T. (2001) Marine Geol. 173, 21–35.
36. Nyberg, J., Malmgren, B. A., Kuijpers, A.