From NOT A LOT OF PEOPLE KNOW THAT

By Paul Homewood

A melting patch of ice in the Rockies is uncovering trees that were growing 7000 years ago:

Significance

Recent warming has decreased snow and ice cover and increased the elevation of most subalpine treelines around the world. A mid-Holocene (c. 5,950 to 5,440 cal y BP) whitebark pine forest preserved within a perennial ice patch in the Greater Yellowstone Ecosystem provides insights into the consequences of past climate change and ecosystem dynamics. Mid-Holocene treeline expanded ~180 m above its modern elevation when warm-season temperatures were similar to mid-to-late-20th-century conditions. Treeline elevation was subsequently lowered due to periods of cooling related to increased volcanism, but primarily from declining summer insolation during the late Holocene. As current ice-patch temperatures exceed the warmest mid-Holocene conditions, we expect regional loss of ice patches and possibly renewed upslope treeline expansion.

Abstract

Climate-driven changes in high-elevation forest distribution and reductions in snow and ice cover have major implications for ecosystems and global water security. In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of a high-elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pinus albicaulis) trees, located ~180 m in elevation above modern treeline, that date to the mid-Holocene (c. 5,950 to 5,440 cal y BP). Here, we used this subfossil wood record to develop tree-ring-based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice-patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 °C (±0.2 °C), until a multicentury cooling anomaly suppressed temperatures below 5.8 °C, resulting in stand mortality by c. 5,440 y BP. Transient climate model simulations indicate that regional cooling was driven by changes in summer insolation and Northern Hemisphere volcanism. The initial cooling event was followed centuries later (c. 5,100 y BP) by sustained Icelandic volcanic eruptions that forced a centennial-scale 1.0 °C summer cooling anomaly and led to rapid ice-patch growth and preservation of the trees. With recent warming (c. 2000–2020 CE), warm-season temperatures now equal and will soon exceed those of the mid-Holocene period of high treeline. It is likely that perennial ice cover will again disappear from the region, and treeline may expand upslope so long as plant-available moisture and disturbance are not limiting.

https://www.pnas.org/doi/10.1073/pnas.2412162121

The paper incorrectly claims that recent warming is a new phenomenon, but provides no data to back this up.

However many tree line studies have conclusively shown that tree lines in the Rockies were much higher than now as recently as the Middle Ages.

HH Lamb : Climate, History and the Modern World

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