Do Tree Stumps Indicate the HTM was Warmer than Today?

If you look at any reconstruction of global mean temperatures covering the entirety of the Holocene, you'll see that the 21st century is warmer than the mean reconstructed temperatures for any point during the Holocene. Now proxy-only reconstructions (Kaufman, Shakun, Marcott) show a warmer Holocene Thermal Maximum than data assimilation approaches (Osman, Erb), and the CI for the HTM is wide enough in Kaufman's reconstruction to allow a small possibility that some years were warmer during the HTM were than they are today, but the evidence is clear that the recent warming spike is unprecedented in the Holocene, and temperatures are likely warmer today than any time during the Holocene. 

Attempts by contrarian influencers to undermine this conclusion have been substantial but unconvincing and frequently disingenuous. For instance, Javier Vinós manipulated Marcott's proxy data to make the difference between the Little Ice Age (LIA) and the HTM about 2x warmer than what was warranted by the proxy data (I documented his dishonesty here). However, there are other attempts that I suspect have more to do with general ignorance than outright deception. Some point to the fact that retreating Arctic and alpine glaciers are revealing tree stumps that were buried when glaciers had advanced over what used to be forests, and these tree stumps can be carbon dated to the HTM, between 6,000 and 10,000 years ago. And this is actually true. One study[1] writes,

The rapidly receding glaciers in our time sometimes release well-preserved subfossil trees, which can be dated with 14C and dendrochronology. Surprisingly, up to 10,000-year old logs were found in the forefield of the Pasterze glacier, the largest glacier in the Austrian Alps... This indicated that in the Early Holocene these glaciers were even smaller than today, allowing trees to grow in an area still covered by ice today.

Contrarian influencers then argue that these these tree stumps indicate that the globe must have been warmer when these forests were alive, before they were covered by advancing glaciers. This argument may sound superficially convincing, but it actually suffers from at least two significant problems:

1. Orbital cycles, particularly obliquity, favor warm summers during the HTM in the Northern Hemisphere, particularly in the Arctic, but the opposite end of the globe averaged cooler. Changes in  summer insolation in the Arctic and NH alpine areas strongly impact tree line dynamics even down to mid-latitudes. Warm summers in these areas do not indicate that annual global mean temperatures were warmer or that the global reconstructions above are wrong. Even if some areas were warmer during the HTM than today, the evidence still indicates that the globe was cooler. Despite the fact that summer insolation is much lower now than during the HTM, these glaciers are now retreating. AGW is reversing the expected trend from changing summer insolation. 
2. It takes a long time for glacial ice to retreat in response to a warming signal. For any local temperature, there is an glacial extent that would be at equilibrium with it (glacial extent remains roughly constant with seasonal variability), but the fact that Arctic and alpine glaciers are retreating indicates that current glacial extent is not at equilibrium with current temperature, and temperatures are too warm for current glacial extent. It will take decades to centuries for the glacial extent to reach equilibrium with the current rapid warming signal. So the fact that tree stumps are being uncovered by retreating glaciers indicates that conditions are warmer today than when glaciers advanced to cover those trees, and it's possible annual temperatures are also warmer than when those forests were alive.

By way of illustration, if you take a pot of water and freeze it, then put it on a hot stove, the air around the pot of frozen water will be hot, but the ice doesn't immediately melt. It takes time for the ice to melt in response to the stove's heat. The air surrounding the pot of water is much hotter than the air in the freezer, but there will still be period of time when ice exists in the pot. Likewise, as climate warms around mountain glaciers, glaciers still experience winter snow accumulation in the accumulation zone, adding mass at higher elevations, and they still lose mass in the ablation zone during the summer. But as temperatures increase, winter accumulation is smaller and summer ablation is larger. The glacier will continue to lose mass until it reaches a new equilibrium to temperature. This lag is frequently ignored by contrarian influencers.

The lag between a warming signal and the extent of mountain glaciers is significant in this discussion, but the lag can be accounted for. A study was published in 2005 that reconstructed changes in global mean temperatures from glacier records.[2] The reconstruction actually replicates the "hockey stick" reconstructions from Michael Mann et al. And while this study only goes back to 1700, it does show that the glacial records support both proxy and instrumental evidence of past climate changes. Mountain glaciers simply do not contain evidence that the globe was warmer during the HTM, and they don't even clearly indicate that individual locations were warmer during the HTM than they are today.

There is a reason why every reconstruction of Holocene temperatures[3] indicates that the 21st century is warmer than the warmest years of the HTM. The authors of these studies are well aware of these tree stumps. They've been uncovered by retreating glaciers for decades. But the proxy evidence does not indicate that the global mean was warmer during the HTM than during the 21st century. Likewise, there's a reason why the geologists and glaciologists that study alpine glaciers and find these tree stumps do not share the convictions of contrarian influencers. In fact, in another post by Ryan Katz-Rozene, Ryan documents how geologists and glaciologists evaluate these data, and they all agree that the 21st century has been warmer than the HTM. Katz-Rozene asked Gregory Pederson, the lead author of a study[4] contrarian influencers use to claim that the HTM was warmer than today, to explain the implications of his study. Pederson responded,

This study has absolutely no bearing on the fact that today, Global Mean Surface Temperatures are higher than they've been throughout the span of Holocene. That fact is absolutely true.

There is no convincing evidence that the HTM was globally warmer than the 21st century. Rather, it seems clear that contrarian influencers are only superficially aware of the evidence and lack the skill to interpret it in light of all the evidence we have about the global climate during the Holocene. The best evidence we have is that globe in the 21st century is warmer than it has been in the last 120,000 years.

 

References:

[1] Walter Kutschera, Gernot Patzelt, Joerg M. Schaefer, Christian Schlüchter, Peter Steier and Eva Maria Wild. The movements of Alpine glaciers throughout the last 10,000 years as sensitive proxies of temperature and climate changes. EPJ Web Conf., 232 (2020) 02002. DOI: https://doi.org/10.1051/epjconf/202023202002

[2] Oerlemans J (2005) Extracting a Climate Signal from 169 Glacier Records. Science 29 Apr 2005: Vol. 308, Issue 5722, pp. 675–677 DOI: https://doi.org/10.1126/science.1107046

[3] Erb, M. P., McKay, N. P., Steiger, N., Dee, S., Hancock, C., Ivanovic, R. F., Gregoire, L. J., and Valdes, P.: Reconstructing Holocene temperatures in time and space using paleoclimate data assimilation, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2022-184, 2022.

[4] G.T. Pederson, D. Stahle, D.B. McWethy, M. Toohey, J. Jungclaus, C. Lee, J. Martin, M. Alt, N. Kichas,
N. Chellman, J.R. McConnell, & C. Whitlock, Dynamic treeline and cryosphere response to pronounced mid-Holocene climatic variability in the US Rocky Mountains, Proc. Natl. Acad. Sci. U.S.A. 122 (2) e2412162121, https://doi.org/10.1073/pnas.2412162121 (2025).

[5] Glen M. MacDonald, Andrei A. Velichko, Constantine V. Kremenetski, Olga K. Borisova, Aleksandra A. Goleva, Andrei A. Andreev, Les C. Cwynar, Richard T. Riding, Steven L. Forman, Tom W.D. Edwards, Ramon Aravena, Dan Hammarlund, Julian M. Szeicz, Valery N. Gattaulin, Holocene Treeline History and Climate Change Across Northern Eurasia, Quaternary Research, Volume 53, Issue 3, 2000, Pages 302-311, ISSN 0033-5894. https://doi.org/10.1006/qres.1999.2123. https://www.sciencedirect.com/science/article/pii/S0033589499921233

[6] Feng, M., Sexton, J. O., Wang, P., Montesano, P. M., Calle, L., Carvalhais, N., Poulter, B., Macander, M. J., Wulder, M. A., Wooten, M., Wagner, W., Elders, A., Channan, S., and Neigh, C. S. R.: Northward shift of boreal tree cover confirmed by satellite record, Biogeosciences, 23, 1089–1101, https://doi.org/10.5194/bg-23-1089-2026, 2026.