The speed of carbon rise changes an ocean tipping point and an ancient forest collapse
New modeling places an Atlantic current collapse at 2 degrees under today's pace of carbon rise, while a 61 per cent forest loss 56 million years ago shows how rapid carbon change reached land.
Science··Midday
At today's pace the Atlantic current fails near 2 degrees
René van Westen and colleagues at Utrecht University ran one climate model under three rates of carbon dioxide increase and reported the result in Nature Climate Change. One scenario raised the gas by half a part per million a year; a second ran five times faster; a third ran ten times faster and close to the present rate. Under the slowest path the Atlantic Meridional Overturning Circulation stayed intact beyond 5 degrees Celsius of warming. Under the fastest path it collapsed near 2 degrees Celsius. Van Westen says that threshold could arrive around 2060 if emissions are not cut sharply. The mechanism is stratification: under fast warming, surface water loses density far more quickly than the deep ocean, so the layers stop mixing and the sinking that drives the current fails. The model omits Greenland ice-sheet melt and methane. David Armstrong McKay at the University of Sussex cautions that higher-complexity models still disagree enough about circulation stability that the finding is not yet a real-world projection.[1]
Forest canopy fell 61 per cent 56 million years ago
The same week's land evidence comes from fossil leaf cells in sediment cores from the Hanna Basin in Wyoming. Regan Dunn of the Natural History Museum of Los Angeles County and colleagues, writing in Science, show that forest canopy first rose and then fell 61 per cent within a few thousand years during the Paleocene-Eocene Thermal Maximum. The carbon dioxide released in that warming about 56 million years ago is thought to be close to what humanity would emit in total by the end of this century under a pessimistic scenario. Global average temperature rose by 5 to 9 degrees Celsius, and the climate stayed altered for more than 150,000 years. Because plants grow sun leaves and shade leaves differently, the team reconstructed canopy density — the leaf area index — from the shape of fossilised leaf cells. Canopy rose at the start, then dropped 61 per cent within a few thousand years and stayed 35 per cent lower for tens of thousands of years until the maximum ended. Dunn says the first carbon pulse probably came from a large volcanic eruption in carbon-rich north Atlantic sediments and initially boosted growth, until temperatures passed plant limits. The study argues that heat overwhelmed the fertilising effect of carbon dioxide.[2]
Pace shifts both the ocean threshold and its land counterpart
The two developments track the same gas in different settings: one models today's Atlantic circulation, the other recovers a forest canopy from 56 million years ago. In van Westen's runs the collapse threshold shifts from beyond 5 degrees under a slow build-up to about 2 degrees under a fast one. Dunn's cores show a rapid carbon surge also left a measurable land collapse: canopy first rose, then fell 61 per cent within a few thousand years and stayed low for tens of thousands of years. Both put weight on pace as well as total release. On the ocean side the result comes from three scenarios in a single model; Greenland melt and methane are omitted, higher-complexity models still disagree, and McKay says the finding is not yet a real-world projection. On the land side the evidence is canopy density rebuilt from leaf-cell shape and the reading that heat overran carbon dioxide fertilisation. Read together, when carbon rises quickly the ocean threshold can move to lower warming, while the ancient forest loss shows how the same kind of pace hit the land. If today's emission pace is not cut sharply, the model threshold points toward around 2060; in the deep past the climate stayed altered for more than 150,000 years.[1], [2]
