|Dark black line shows Maximum Latewood Density [MXD], "a temperature proxy that is superior to that of TRW [Tree Ring Width]," and demonstrates proxy temperatures exceeded those of the year 2000 multiple times over the past 800 years. Grey line is a proposed new temperature proxy ["blue intensity"] which shows cooling over the past 300 years.|
Clim. Past Discuss., 9, 5227-5261, 2013
1Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden
2Bert Bolin Centre for Climate Research, Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden
3Department of Geography, Johannes Gutenberg University, Mainz, Germany
Abstract. At high latitudes, where low temperatures mainly limit tree-growth, measurements of wood density (e.g. Maximum Latewood Density, MXD) using the X-Ray methodology provide a temperature proxy that is superior to that of TRW [Tree Ring Width]. Density measurements are however costly and time consuming and have lead to experimentation with optical flatbed scanners to produce Maximum Blue Intensity (BImax). BImax is an excellent proxy for density on annual scale but very limited in skill on centennial scale. Discolouration between samples is limiting BImax where specific brightnesses can have different densities. To overcome this, the new un-exploited parameter Δ blue intensity (ΔBI) was constructed by using the brightness in the earlywood (BIEW) as background, (BImax − BIEW = ΔBI). This parameter was tested on X-Ray material (MXD − earlywood density = ΔMXD) and showed great potential both as a quality control and as a booster of climate signals. Unfortunately since the relationship between grey scale and density is not linear, and between-sample brightness can differ tremendously for similar densities, ΔBI cannot fully match ΔMXD in skill as climate proxy on centennial scale. For ΔBI to stand alone, the range of brightness/density offset must be reduced. Further studies are needed to evaluate this possibility, and solutions might include heavier sample treatment (reflux with chemicals) or image-data treatment (digitally manipulating base-line levels of brightness).