Identifying Natural Contributions to Late Holocene Climate Change
Humlum, O., Solheim, J.-K. and Stordahl, K. 2011. Identifying natural contributions to late Holocene climate change. Global and Planetary Change 79: 145-156.
Humlum et al.'s work is full of details and discussion on how the authors have constructed their analytical models of temperature variations, fully accounting for the large signals from millennial, multicentennial and multidecadal scales of temperature changes covering the late Holocene period. The authors used both the Fourier and wavelet transform methods in extracting natural climatic signals from temperature records at Svalbard from 1912-2010 and at Central Greenland GISP2 site for the past 4000 years.
Figure 1 shows the reconstruction of the GISP2 temperature record for the past 4000 years using only a 3-period analytical model (here, they found optimal periods to be at 2804-year, 1186-year and 556-year). The model is somewhat successful in replicating the observed temperature in that it explains over 63% of the changes in the hind-casting period. In order to add scientific weight to their results, the authors also surveyed and extensively discussed the nature of the natural recurrent periods to known cyclical variations of Earth-Moon orbits, as well as to some of the known variations of solar magnetic activity and related solar outputs.
Figure 1. The Central Greenland surface temperature from GISP2 project for the past 4000 years (blue line) and the modeled temperature adopting only 3 periods at 2804-year, 1186-year and 556-year. The 3-period model was able to replicate most of the observed changes (with one major exception at around the warming peak of 3 to 400 AD) and forecasted a large cooling trend in contrast to the IPCC-predicted rising atmospheric CO2 scenario from computer climate models. Adapted from Harder et al. (2011).
The timing of the appearance of Humlum et al. (2011) is almost perfect in the sense that a new paper by Kobashi et al. (2011) provides solid confirmation in regard to the presence of large natural temperature variations in Greenland surface or snow-surface temperatures on multicentennial and multidecadal timescales. Figure 2 shows the intercomparison of three previous surface temperature reconstructions from the ice-core projects of GISP2, GRIP and NGRIP, in addition to the new reconstruction method using a ratio of argon and nitrogen isotopes from air bubbles. Of present significance, the main reconstructed temperature periods of 333-year, 210-year, 174-year and 59-year in the Kobashi et al. (2011) study are similar to the 366-year, 205-year, 168-179-year and 58-year periods found in the GISP2 records by Humlum et al. (2011).
Figure 2. Four independent reconstruction of surface temperature variations in Greenland over the past 4000 years including the new argon and nitrogen isotopic ratio method by Kobashi et al. (2011). This figure largely confirmed the choice by Humlum et al. (2011) in adopting GISP2 data for their construction of analytical model of Greenland temperature variations that captured millennial-scale and multicentennial-scale changes. Adapted from Kobashi et al. (2011).
Finally, the large cooling tendency of about 1.5°C predicted by Humlum et al. for the next 800 years, as shown in Figure 1, poses a serious challenge to IPCC climate projection scenarios that only consider rising atmospheric CO2 as the significant modulator of surface temperature changes in Greenland. The work of Humlum et al. suggests that Greenland will not melt away if atmospheric CO2 emissions are not severely reduced.
Additional Reference
Kobashi, T., Kawamura, K., Severinghaus, J.P., Barnola, J.-M., Nakaegawa, T., Vinther, B.M., Johnsen, S.J. and Box, J.E. 2011. High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters 38: 10.1029/2011GL049444.
