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How sensitively does organic carbon stored in soils react to changes in temperature and humidity?

Abstract

Carbon storage in soils is important in regulating atmospheric carbon dioxide (CO2). However, the sensitivity of the soil-carbon turnover time (τsoil) to temperature and hydrology forcing is not fully understood. Here, we use radiocarbon dating of plant-derived lipids in conjunction with reconstructions of temperature and rainfall from an eastern Mediterranean sediment core receiving terrigenous material from the Nile River watershed to investigate τsoilin subtropical and tropical areas during the last 18,000 years. We find that τsoil was reduced by an order of magnitude over the last deglaciation and that temperature was the major driver of these changes while the impact of hydroclimate was relatively small. We conclude that increased CO2 efflux from soils into the atmosphere constituted a positive feedback to global warming. However, simulated glacial-to-interglacial changes in a dynamic global vegetation model underestimate our data-based reconstructions of soil-carbon turnover times suggesting that this climate feedback is underestimated.

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• ScienceDaily article
• research cited

Stokes et al study

See also:

• Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?
• A Critical Look at UN IPCC's Emissions Accounting

cross-posted from: https://lemmy.sdf.org/post/34653379

China’s plans to build a massive hydro project in Tibet have sparked fears about the environmental impacts on the world’s longest and deepest canyon. It has also alarmed neighboring India, which fears that China could hold back or even weaponize river water it depends on.

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A hydroelectric project at a remote river gorge in eastern Tibet, an ecological treasure trove close to a disputed border with India. Indian politicians have reacted angrily, saying it gives China the ability to release destructive “water bombs” across the border in any future conflict. They are planning a retaliatory dam on their side of the border that experts say could be at least as environmentally destructive.

Two Chinese dams will barricade the Yarlung Tsangpo, the Tibetan name for the Brahmaputra River, as it is about to flow through the world’s longest and deepest river canyon — think the Grand Canyon on the Colorado River, only three times as deep. Projected to cost $137 billion, the scheme will be the world’s biggest single infrastructure project, with almost three times the generating capacity of the world’s current largest hydroelectric dam, China’s Three Gorges on the Yangtze River.

Chinese ecologists say the canyon is one of the most precious biodiversity hotspots on the planet, containing some of Asia’s tallest and most ancient trees as well as the world’s richest assemblage of large carnivores, especially big cats. But India’s anger is geopolitical. Pema Khandu, the chief minister of Arunachal Pradesh, the Indian state immediately downstream, called the project “a big threat” that could dry up the river through his state during routine operation and potentially be weaponized to unleash a flood in which, he said, hundreds of thousands could lose their lives.

[...]

The stakes are high, with tensions over scarce water resources in the region rising. India last month suspended its adherence to a treaty in operation for 65 years to share with Pakistan the waters of another great South Asian river, the Indus. Meanwhile the 30-year-old Ganges Water Treaty between India and Bangladesh is set to expire next year, with India widely accused of violating its terms.

“Weaponizing water is a perilous strategy that may backfire,” says Mehebub Sahana, an environmental geographer at the University of Manchester. “The weakening of water diplomacy in South Asia is not just a regional threat; it endangers global climate security.”

Tibet, part of China since 1951, is the water tower of Asia. Its vast glaciers sustain major rivers on which more than 1.3 billion people in 10 countries depend for drinking, irrigating crops, and hydropower. China, already the world’s leading producer of hydroelectricity, sees more dams on these rivers as a key to reducing its carbon emissions.

[...]

Technical details about the project have yet to be published. But Chinese government media say it will have a generating capacity of 60,000 megawatts, almost 30 times that of the Hoover Dam. But the two proposed dams don’t need to be even as high as the Hoover Dam, says Gamble. “This is more a mega-project than a mega-dam.” The site’s unique geography will do the work, as the water rushes downward for thousands of feet through 12-mile-long tunnels to deliver unprecedented power to turbines at the bottom of the canyon, before discharging the flow back into the river close to the border with India. “Indian soldiers will overlook the project from their bunkers,” says Gamble.

Indian scientists believe that operating the dams to meet China’s electricity needs will change the river’s strongly seasonal flow. “Reduced water flow in the dry season, coupled with sudden releases of water during monsoons, could intensify both water scarcity and flooding, endangering millions,” says Sahana.

The project could also impact sediment flows in the river. Erosion in the canyon currently supplies 45 percent of the total volume of sediment that flows downstream on the Brahmaputra, says Robert Wasson, a geomorphologist at James Cook University, in Australia. Bypassing the canyon could reduce sediment supply to the lower reaches and damage the river’s vast delta, says Sahana. “Any disruption to the balance of sediment could accelerate coastal erosion and make the already low-lying [delta] area more vulnerable to sea-level rise.” But this outcome is far from clear, says Wasson, as too little is known about sediment movement on the river.

[...]

The geopolitics of international rivers in South Asia has long been fraught. India itself has often been accused of being an upstream bully — notably on the Indus River, which flows out of the Himalayas and through India to Pakistan.

[...]

Last month, tensions soared again when India unilaterally suspended its adherence to the treaty, as part of its retaliation for a terrorist attack. Pakistan’s prime minister Shehbaz Sharif responded by warning that if India tried to block the river’s flow it would be met with “full force and might.”

The parallel between this standoff on the Indus and the threat posed to India by the Chinese project on the Brahmaputra is compelling, but not exact. There is no treaty governing the management of the Brahmaputra, for instance. But the power of upstream countries over their downstream neighbors is central to both disputes. In each case, the hydrological and political stakes are high in a region with a troubling history of belligerent rhetoric, unilateral actions on shared rivers, and taking up arms over disputed waters.

cross-posted from: https://slrpnk.net/post/21852805

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Global temperature leaped more than 0.4°C (0.7°F) during the past two years, the 12-month average peaking in August 2024 at +1.6°C relative to the temperature at the beginning of last century (the 1880-1920 average). This temperature jump was spurred by one of the periodic tropical El Niño warming events, but many Earth scientists were baffled by the magnitude of the global warming, which was twice as large as expected for the weak 2023-2024 El Niño. We find that most of the other half of the warming was caused by a restriction on aerosol emissions by ships, which was imposed in 2020 by the International Maritime Organization to combat the effect of aerosol pollutants on human health. Aerosols are small particles that serve as cloud formation nuclei. Their most important effect is to increase the extent and brightness of clouds, which reflect sunlight and have a cooling effect on Earth. When aerosols – and thus clouds – are reduced, Earth is darker and absorbs more sunlight, thus enhancing global warming. Ships are the main aerosol source in the North Pacific and North Atlantic Oceans. We quantify the aerosol effect from the geographical distribution of sunlight reflected by Earth as measured by satellites, with the largest expected and observed effects in the North Pacific and North Atlantic Oceans. We find that aerosol cooling, and thus climate sensitivity, are understated in the best estimate of the United Nations Intergovernmental Panel on Climate Change (IPCC).

Global warming caused by reduced ship aerosols will not go away as tropical climate moves into its cool La Niña phase. Therefore, we expect that global temperature will not fall much below +1.5°C level, instead oscillating near or above that level for the next few years, which will help confirm our interpretation of the sudden global warming. High sea surface temperatures and increasing ocean hotspots will continue, with harmful effects on coral reefs and other ocean life. The largest practical effect on humans today is increase of the frequency and severity of climate extremes. More powerful tropical storms, tornadoes, and thunderstorms, and thus more extreme floods, are driven by high sea surface temperature and a warmer atmosphere that holds more water vapor. Higher global temperature also increases the intensity of heat waves and – at the times and places of dry weather – high temperature increases drought intensity, including “flash droughts” that develop rapidly, even in regions with adequate average rainfall.

Polar climate change has the greatest long-term effect on humanity, with impacts accelerated by the jump in global temperature. We find that polar ice melt and freshwater injection onto the North Atlantic Ocean exceed prior estimates and, because of accelerated global warming, the melt will increase. As a result, shutdown of the Atlantic Meridional Overturning Circulation (AMOC) is likely within the next 20-30 years, unless actions are taken to reduce global warming – in contradiction to conclusions of IPCC. If AMOC is allowed to shut down, it will lock in major problems including sea level rise of several meters – thus, we describe AMOC shutdown as the “point of no return.”

We suggest that an alternative perspective – a complement to the IPCC approach – is needed to assess these issues and actions that are needed to avoid handing young people a dire situation that is out of their control. This alternative approach will make more use of ongoing observations to drive modeling and more use of paleoclimate to test modeling and test our understanding. As of today, the threats of AMOC shutdown and sea level rise are poorly understood, but better observations of polar ocean and ice changes in response to the present accelerated global warming have the potential to greatly improve our understanding.

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Climate models have a history of underestimating the cooling effect of aerosol pollution.

Related: Will Brazil’s President Lula wake up to the climate crisis? (commentary)

Tropical deforestation was found to cause large reductions in precipitation using a range of observation-based datasets1. However, the limitations of satellite-based space-for-time statistical analysis have hindered understanding of the roles of reshaped mesoscale atmospheric circulation and regional precipitation recycling at different scales. These effects are considered nonlocal effects, which are distinct from the local effects governed by deforestation-induced reductions in evapotranspiration (ET). Here we show reversed precipitation responses to Amazon deforestation across wet and dry seasons. During the wet season, deforested grids experienced a noteworthy increase in precipitation (0.96 mm per month per percentage point forest loss), primarily attributed to enhanced mesoscale atmospheric circulation (that is, nonlocal effect). These nonlocal increases weaken with distance from deforested grids, leading to significant precipitation reductions in buffers beyond 60 km. Conversely, during the dry season, precipitation decreases in deforested grids and throughout all analysis buffers, with local effects from reduced ET dominating. Our findings highlight the intricate balance between local effects and nonlocal effects in driving deforestation-precipitation responses across different seasons and scales and emphasize the urgent need to address the rapid and extensive loss of forest in the Amazon region.

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There was a time when an ecologist’s education was not complete without the mud of a marsh on their boots or the scent of damp earth after a rainforest downpour. Increasingly, however, the discipline is moving indoors. A paper published in Trends in Ecology & Evolution by Masashi Soga and Kevin J. Gaston highlights a disconcerting trend: the decline of fieldwork in ecological research and education.

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