Water:
• Increased evaporation and atmospheric water store. Positive feedback of water
vapour increases global temps, evaporation and transpiration.
• Increased precipitation in urban areas leads to flood risks.
• Water vapour is an atmospheric source of energy, leading to extreme weather
events.
• Storage in cryosphere shrinks and water transferred to oceans.
Carbon:
• Increase atmospheric store, decreased biosphere and ocean store.
• Increased decomposition and rate of transfer
• If aridity arises forests replaced by grasslands, reducing C in biomass.
• Boreal forest expands poleward.
• C released from permafrost.
• Ocean acidification limits C storage capacity
Case Study – California Drought
Human activity or long-term climate change?
Causes:
• Decades of over pumping groundwater has altered layers of clay beneath California’s
Central Valley, reducing the aquifer’s ability to store water. (NASA).
Positive feedback- if too much water is extracted compaction becomes irreversible,
and the soil’s ability to retain water is permanently diminished.
• Global warming contributions to climate change due to declines in spring snowpack.
Earlier snowmelt, associated with warmer temperatures, can lead to water supply
being increasingly out of phase with water demands.
• Natural variations in the La Nina weather pattern in the Pacific Ocean. Warm patch
of water in Western Pacific that stops storms forming over California (National
Oceanic and Atmospheric Administration).
2017 Snowfall and Negative Feedback:
• Much drought eliminated due to a persistent weather pattern of storm systems
rising the snowpack well above average. By February 2018, over 60% of the state
was in no level of drought.
• State of emergency lifted, and cutbacks ended.
• New figures from National Drought Monitors produced growing concern of another
dry winter as electronic sensors showed state-wide snow levels 27% below normal.
Impacts:
• Farmers in dry Central Valley alone lost $810 million in 2014 from idle fields.
• 102 million trees died 2011-2016 (US Forest Service).
• Positive Feedback- Highly stressed conifers in Sierra Nevada more susceptible to bark
beetles which kill trees (NASA)
Management:
• Pump groundwater from more shallow sand and gravel layers rather than clay which
is less easily recharged and more susceptible to compaction (NASA).
• Plastic shade balls float on reservoirs to prevent evaporation.
Other countries Management of drought:
• China’s ‘Great Green Wall’ along border of Gobi Desert to create largest artificial
forest and combat 3000km/yr desert growth.
Taking place until 2050 and will increase global forest cover by 10%.
• South Australia Managed Aquifer Recharge (MAR) to store water that can be
pumped to the surface to contribute to domestic water supplies/restore river flows
in times of low rainfall.
Uses river/storm water, roof run-off, treated wastewater etc enabling recovery in
summer’s high irrigation needs.
Unreliable source and limited aquifers available. Water contamination in aquifer by
geochemical interactions.