Planetary Freshwater Boundary ‘Dramatically’ Transgressed: New Research

  • Earth’s operating systems have remained in relative balance for thousands of years, allowing civilization to flourish. However, humanity’s actions have resulted in the transgression of multiple planetary boundaries, resulting in the destabilization of these vital operating systems.
  • This week, scientists announced that humanity has crossed the planetary limit of fresh water. Other boundaries already crossed are climate change, integrity of the biosphere, biogeochemical cycles (pollution by nitrogen and phosphorus), change in the earth system and new entities (pollution by synthetic substances).
  • In the past, the limit of fresh water was defined only by “blue water” – a measure of humanity’s use of lakes, rivers and groundwater. But scientists have now extended that definition to include “green water” – precipitation, evaporation and soil moisture.
  • Scientists say soil moisture conditions are changing from boreal forests to the tropics, with abnormally dry and wet soils now common, potentially altering the biome. The Amazon, for example, is getting much drier, which could cause it to reach a tipping point from rainforest to savannah, releasing large amounts of stored carbon.

Mankind’s alteration of the water cycle has pushed the world further than a safe operating space for the continuation of life on Earth, scientists say. A reassessment of the planetary boundary for fresh water which now includes precipitation, soil moisture and evaporation – so-called ‘green water’ – found the boundary was ‘significantly transgressed’ , the situation likely to worsen before any reversal in the trend is observed. Previously, researchers had only considered rivers, lakes and groundwater in their assessments.

According to researchers from the Stockholm Resilience Center in collaboration with colleagues from Germany, the Netherlands, Finland, Austria, Australia, the United States and Canada. The results were published this week in the journal Nature reviews Earth and environment.

“This is a wake-up call that we need to stop the way we change green water,” says lead author Lan Wang-Erlandsson of the Stockholm Resilience Center at Stockholm University. “We are profoundly altering the water cycle,” she says, noting that this destabilization of the Earth system is now affecting the health of the entire planet, making it significantly less resilient to environmental shocks.

The nine planetary boundaries, counterclockwise from top: climate change, biosphere integrity (functional and genetic), earth system change, freshwater use, biogeochemical fluxes (nitrogen and phosphorus), ocean acidification, atmospheric aerosol pollution, stratospheric ozone depletion and the release of new chemicals. In 2022, scientists announced the boundary transgression of freshwater features and new features. Image courtesy of J. Lokrantz/Azote based on Steffen et al. 2015 (via Stockholm Resilience Center).

Based on the results, water is now the sixth boundary to be breached, of the nine identified by the Planetary Boundaries Framework. Published in 2009 and updated regularly, the framework delineates a safe operating space for humanity, beyond which civilization could collapse and life as we know it might be altered. The other borders already transgressed are climate change, integrity of the biosphere, biogeochemical flows (pollution by nitrogen and phosphorus), change in the earth system, and also since 2022, new entities, which include pollution by plastics and other human-made substances.

Until now, the freshwater limit was considered to be inside the safety zone. The so-called “freshwater use” limit was based on authorized human consumption and set at 4,000 km3/year of water used and not returned as runoff. It assessed water extracted from rivers, lakes and groundwater, known as “blue water”.

The updated assessment uses soil moisture in the root zone of plants to measure the limit of “green water”, because it is directly influenced by human pressures and because it has a direct impact on a range of large-scale ecological, climatic, biogeochemical and hydrological dynamics.

Dried soils in Pinang Tunggal, Malaysia during the 2014 drought. Researchers are seeing alarming changes in soil moisture from boreal forests to the tropics. Image by Photo credit: Marufish on VisualHunt.

During droughts, for example, plants can maintain photosynthesis and transpiration by accessing soil moisture, but once these moisture levels drop below a critical threshold, vegetation mortality increases, especially for plants like tropical trees which generally do not have alternative drought coping strategies. The study highlights that soil moisture anomalies in the root zone are also key drivers of the terrestrial carbon cycle, and that changes in soil moisture under a high carbon emissions scenario could risk transforming lands from a net carbon sink to a carbon source by the middle of this century.

Evidence of this escalating process can already be found in the diminishing resilience of critical ecosystems such as the Amazon and Congolese rainforests, which store vast amounts of carbon and provide immense biodiversity. Both of these biomes are considered vital to Earth’s operating systems, but could be pushed past environmental tipping points by freshwater boundary transgressions.

“The Amazon rainforest depends on soil moisture for its survival. But there is evidence that parts of the Amazon are drying up. The forest is losing soil moisture due to climate change and deforestation,” says Arne Tobian, co-author of the new assessment and a PhD student at the Stockholm Resilience Center and the Potsdam Institute for Climate Impact Research. “These changes potentially bring the Amazon closer to a tipping point where large parts could shift from rainforest to savanna-like states,” he adds.

A line of dead and damaged trees after a fire in the Amazon rainforest of western Brazil during the 2010 drought. When drier than normal conditions occur, fires from open edges encroach on forests and burn dry and stressed trees. In a wetter past, rainforests were much more resilient and resistant to drought and fire. Image courtesy of NASA/JPL-Caltech.

The new assessment found the phenomenon to be global, with soil moisture shifting from boreal forests to the tropics, from farmlands to forests. Increasingly, abnormally wet and dry soils are commonplace. Extreme weather events induced by climate change lead to an increase in severe droughts and floods, while changes in land use for agriculture and other purposes can cause soils to dry out.

“Water is fundamental to every living organism on Earth,” says Wang-Erlandsson. “There are so many things that are interconnected, that are now changing in unprecedented ways,” she adds, noting that impacts on the water cycle are driven by multiple human actions far beyond consumption levy. “It is massively affected by climate change, land management, land degradation, etc. It is complex and intertwined with our human activities; in everything we do,” she explains.

“This latest scientific analysis shows how we humans are pushing green water far beyond the variability Earth experienced for many thousands of years during the Holocene period,” the co-author concludes. assessment Johan Rockström, director of the Potsdam Institute for Climate Impact Research. and professor at the Stockholm Resilience Centre.

Fires burn in the Amazon in August 2020 next to Kaxarari Indigenous Territory in Labrea, Amazonas State, Brazil. Logged forests are intentionally cleared in the Amazon by land grabbers and others to clear land for cattle ranches or cropland. Deforestation and climate change combine to increase drought and decrease soil moisture in the biome. Image by Christian Braga/Greenpeace.

With six out of nine planetary boundaries already crossed, the resilience of Earth’s operating system as a whole is now quite low, warns Wang-Erlandsson. Continued deterioration in the functioning of Earth systems will increase the risk of regional environmental regime shifts. Humanity must act to reverse these growing changes and return to a safe zone once again, she says.

“Reducing the risks of green water change on the Earth system now requires immediate water action to address climate change, deforestation and land degradation,” says co-author Ingo Fetzer. evaluation and researcher at the Stockholm Resilience Centre.

Based on the results, “current global trends and trajectories of increasing water use, deforestation, land degradation, soil erosion, air pollution and climate change must be quickly halted and reversed to increase the chances of remaining in the safe operating space”.

Quote:

Wang-Erlandsson, L., Tobian, A., van der Ent, RJ et al. A planetary boundary for green water. Nat Rev Land Approx. (2022).

Banner image: Farmland with windmill to extract declining groundwater, eight years after drought in the Northern Cape Province of South Africa. Until now, the planet’s freshwater limit was considered to be within the safe zone, based on allowable human consumption – an assessment that only considered water extracted from rivers, lakes and groundwater, called “blue water”. Updated research that also includes “green water” – precipitation, evaporation and soil moisture – indicates that the boundary has been significantly transgressed. Image by Petro Kotze.

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A sheep carcass on a cattle farm in Kenhardt, South Africa, eight years after a record-breaking extreme drought. The researchers found that the Earth’s water cycle was profoundly altered by human activity, making the planet less resilient to environmental shocks. Image by Petro Kotze.

Amazon Destruction, Amazon Drought, Amazon Rainforest, Climate, Climate Change, Climate Change And Severe Weather, Climate Science, Environment, Severe Weather, Freshwater Ecosystems, Precipitation, Research, Rivers, Water, Crisis water, water shortage

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