| 1. |
- Rockström, Johan, 1965-
(författare)
-
On-farm agrohydrological analysis of the Sahelian yield crisis : rainfall partitioning, soil nutrients and water use efficiency of pearl millet
- 1997
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sub-Saharan Africa is presently experiencing a steadily aggravated food security crisis. This crisis is a result of a rapidly growing population combined with insignificant yield increases of the major cereal crops maize, millet and sorghum, during the last decades. The situation is most severe in semi-arid regions where water is a major constraint on food production. The thesis addresses the human impact on crop yield development during the last century and the biophysical causes underlying the gap between extremely low on-farm yields and potential yields of pearl millet (Pennisetum glaucum (L.) Br.) in the semi-arid Sahel region in West Africa.Results are presented from two studies in Niger. The first analyses the driving forces behind the agricultural crisis in an agrarian system in the region of Zarmaganda. The second study analyses rainfall partitioning and crop water use efficiency in a farmer's field, located along a catena characteristic of the Sahel, in the Samadey watershed.Water scarcity in food production in semi-arid tropics is not necessarily a result of the low annual rainfall levels (ranging from 200 - 600 mm), but rather a result of a high rainfall variability between and within years and an extremely high evaporative demand of the atmosphere (600 - 900 mm during the 3-4 month rainy season). In addition to these two climatic modes of water scarcity, humans induce water scarcity with land degradation, which results in lowered infiltration and reduced soil water holding capacity.The Zarmaganda study shows that increased population pressure has forced a shift in the agrarian system during the last century from a system based on long bush fallows, to a "nutrient-mining" system with short (< 10 years) or abandoned fallows. This development is proposed to be the most significant structural transition of dryland agriculture in the Sahel region since its introduction.The results from the Samadey experiment indicate that low on-farm yields are caused by water and soil fertility constraints. Crop water scarcity is caused by rainfall erraticness, leading to episodic droughts even during average rainfall years, and runoff production as a result of soil surface crusting. Non-fertilised millet had a grain yield of 347 - 422 kg ha-1 compared to 526 - 697 kg ha-1 for the fertilised crop, for the 3 studied rainy seasons 1994-96. Substantial volumes of surface overland flow were measured. Observed inflow of sheet flow as runon from degraded upstream zones, corresponded to an additional 20 - 50 mm of water if distributed over the 8.5 ha field. Runoff production on a plot scale (15 x 6 m) amounted to 10 - 13 % of the annual rainfall. The study shows that runon and runoff flow can constitute significant components of the on-farm water balance.A systematic slope gradient was observed where yields decreased 35 - 40 % from the downslope to the upslope position along the 315 m gently sloping catena. This is explained by decreased soil water availability, caused by surface overland flow, and a low absolute but significant relative decrease in soil fertility, when moving upslope. The effect of this soil water gradient was that the upslope crop suffered more from episodic droughts, which hit the crop during panicle initiation in 1994, during grain filling in 1995, and during flowering in 1996. Spatial variability of soil water was high, with the mean infiltration around neutron probe access tubes ranging from 0.43 - 1.13 times rainfall.The variation of infiltration over time was high, due to rapid changes in crust coverage and the effect of rainfall intensity on runoff. This explains how a location in the field could function both as a runoff and a runon zone.Estimates from water balance modelling of seasonal water flow partitioning for runoff and runon producing zones in the field, indicate very low productive water losses by plant transpiration. These losses account for only 4 - 9 % of the available water (rainfall + runon) for the non-fertilised crop, and 7 - 24 % for the fertilised crop. This is due to the low leaf area observed in the farmer's field (maximum LAI = 0.44 m2 m-2 for the non-fertilised crop). Soil evaporation was high, amounting to 32 - 50 % of the available water, and conservative, presenting insignificant reductions with increased leaf area (LAI < 1 m2 m-2 for all treatments). Deep percolation was high, amounting to 140 - 250 mm. Large non-productive losses of water, together with low yields, resulted in low water use efficiencies (WUE), with evapotranspirational WUE = 6000 - 8000 m3 ton-1 grain, and rainfall WUE = 12,000 - 16,000 m3 ton-1 grain.Effects of rainfall erraticness, resulting in an unfavourable distribution of rainfall over time, explains why a crop that uses such a small proportion of the available water, in an environment with substantial deep percolation, still suffers from water scarcity.Large volumes of runon and the frequent dry spells, indicate the possibility of increasing yields with rainwater harvesting techniques for supplemental irrigation. The risk of crop failure would then be reduced, which possibly could increase the incentive for the farmer to invest in soil fertilisation. Such soil and water interactions would enable a long-term win-win solution for the farming system.The high spatial and temporal variability of soil nutrients and water availability should be considered as a non-negotiable part of the on-farm reality, which could constitute an opportunity, not only a constraint, for increased crop yields. Moreover, the development of site specific farming strategies, inspired by development in temperate regions, and cropping practices adapted to a variable toposequence availability of water, inspired by farmers in less arid tropics, could possibly be interesting in the semi-arid Sahelian savannah.
|
|
| 2. |
- Rockström, Johan, 1965-, et al.
(författare)
-
Safe and just Earth system boundaries
- 2023
-
Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 619:7968, s. 102-111
-
Tidskriftsartikel (refereegranskat)abstract
- The stability and resilience of the Earth system and human well-being are inseparably linked, yet their interdependencies are generally under-recognized; consequently, they are often treated independently. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice). The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future.
|
|
| 3. |
- Rockström, Johan, 1965-, et al.
(författare)
-
The planetary commons : A new paradigm for safeguarding Earth-regulating systems in the Anthropocene
- 2024
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - 0027-8424 .- 1091-6490. ; 121:5
-
Tidskriftsartikel (refereegranskat)abstract
- The Anthropocene signifies the start of a no-analogue trajectory of the Earth system that is fundamentally different from the Holocene. This new trajectory is characterized by rising risks of triggering irreversible and unmanageable shifts in Earth system functioning. We urgently need a new global approach to safeguard critical Earth system regulating functions more effectively and comprehensively. The global commons framework is the closest example of an existing approach with the aim of governing biophysical systems on Earth upon which the world collectively depends. Derived during stable Holocene conditions, the global commons framework must now evolve in the light of new Anthropocene dynamics. This requires a fundamental shift from a focus only on governing shared resources beyond national jurisdiction, to one that secures critical functions of the Earth system irrespective of national boundaries. We propose a new framework—the planetary commons—which differs from the global commons framework by including not only globally shared geographic regions but also critical biophysical systems that regulate the resilience and state, and therefore livability, on Earth. The new planetary commons should articulate and create comprehensive stewardship obligations through Earth system governance aimed at restoring and strengthening planetary resilience and justice.
|
|
| 4. |
- Tobian, Arne, 1992-, et al.
(författare)
-
Climate change critically affects the status of the land-system change planetary boundary
- 2024
-
Ingår i: Environmental Research Letters. - 1748-9326. ; 19:5
-
Tidskriftsartikel (refereegranskat)abstract
- The planetary boundaries framework defines a safe operating space for humanity. To date, these boundaries have mostly been investigated separately, and it is unclear whether breaching one boundary can lead to the transgression of another. By employing a dynamic global vegetation model, we systematically simulate the strength and direction of the effects of different transgression levels of the climate change boundary (using climate output from ten phase 6 of the Coupled Model Intercomparison Project models for CO2 levels ranging from 350 ppm to 1000 ppm). We focus on climate change-induced shifts of Earth's major forest biomes, the control variable for the land-system change boundary, both by the end of this century and, to account for the long-term legacy effect, by the end of the millennium. Our simulations show that while staying within the 350 ppm climate change boundary co-stabilizes the land-system change boundary, breaching it (>450 ppm) leads to critical transgression of the latter, with greater severity the higher the ppm level rises and the more time passes. Specifically, this involves a poleward treeline shift, boreal forest dieback (nearly completely within its current area under extreme climate scenarios), competitive expansion of temperate forest into today's boreal zone, and a slight tropical forest extension. These interacting changes also affect other planetary boundaries (freshwater change and biosphere integrity) and provide feedback to the climate change boundary itself. Our quantitative process-based study highlights the need for interactions to be studied for a systemic operationalization of the planetary boundaries framework.
|
|
