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11.4.1 Water
11.4.1.1 Climate Change and the Water Cycle
Freshwater-related risks of climate change increase significantly with increasing greenhouse gas (GHG) concentrations (robust evidence, high agreement)
The general warming of the earth associated with increases in atmospheric concentrations of GHGs will result in higher evaporation of surface water and more water
vapor in the earth’s atmosphere. This intensifies the earth’s natural water cycle
impacting precipitation (rain, snow, hail, and sleet) and the supply of freshwater
water resources. Intensifying the earth’s natural water cycle will also mean more
intense storms resulting in increased flooding risks, and, at the other end of the
spectrum, some areas will see more extended periods between precipitation result in
more frequent drought conditions.
So far there are no widespread observations of changes in flood magnitude and frequency
due to anthropogenic climate change, but projections imply variations in the frequency of
floods (limited evidence, medium agreement). Flood hazards are projected to increase in
parts of South, Southeast, and Northeast Asia; tropical Africa; and South America (limited
evidence, medium agreement). Since the mid-20th century, socioeconomic losses from
flooding have increased mainly due to greater exposure and vulnerability (high confidence).
Global flood risk will increase in the future partly due to climate change (limited evidence,
medium agreement).
Once precipitation falls, its movement can also be affected by climate change. Higher
temperatures usually result in less snowfall and more rainfall in winters and rapid thawing
of ice and snow in springs leading to higher rates of runoff and streamflow earlier in the
year and lower rate later. ‘In regions with snowfall, climate change has altered observed
streamflow seasonality, and increasing alterations due to climate change are projected.’
(robust evidence, high agreement).
Further,
because nearly all glaciers are too large for equilibrium with the present climate, there is a
committed water resources change during much of the 21st century, and changes beyond
the committed change are expected due to continued warming; in glacier-fed rivers, total
meltwater yields from stored glacier ice will increase in many regions during the next
decades but decrease thereafter (robust evidence, high agreement). Continued loss of glacier ice implies a shift of peak discharge from summer to spring, except in monsoonal
catchments, and possibly a reduction of summer flows in the downstream parts of glacierized catchments.
Water storage will change in various ways in response to changes in precipitation
and movement.
“Climate change is projected to reduce renewable surface water and groundwater resources
significantly in most dry subtropical regions (robust evidence, high agreement). In contrast,
water resources are projected to increase at high latitudes. Proportional changes are typically one to three times greater for runoff than for precipitation.” Further, “for each degree
of global warming, approximately 7% of the global population is projected to be exposed
to a decrease of renewable water resources of at least 20% (multi-model mean)” Such
changes will have impacts on water storage infrastructure such as dams and reservoirs.
P. Saundry
11.4.1 Water
11.4.1.1 Climate Change and the Water Cycle
Freshwater-related risks of climate change increase significantly with increasing greenhouse gas (GHG) concentrations (robust evidence, high agreement)
The general warming of the earth associated with increases in atmospheric concentrations of GHGs will result in higher evaporation of surface water and more water
vapor in the earth’s atmosphere. This intensifies the earth’s natural water cycle
impacting precipitation (rain, snow, hail, and sleet) and the supply of freshwater
water resources. Intensifying the earth’s natural water cycle will also mean more
intense storms resulting in increased flooding risks, and, at the other end of the
spectrum, some areas will see more extended periods between precipitation result in
more frequent drought conditions.
So far there are no widespread observations of changes in flood magnitude and frequency
due to anthropogenic climate change, but projections imply variations in the frequency of
floods (limited evidence, medium agreement). Flood hazards are projected to increase in
parts of South, Southeast, and Northeast Asia; tropical Africa; and South America (limited
evidence, medium agreement). Since the mid-20th century, socioeconomic losses from
flooding have increased mainly due to greater exposure and vulnerability (high confidence).
Global flood risk will increase in the future partly due to climate change (limited evidence,
medium agreement).
Once precipitation falls, its movement can also be affected by climate change. Higher
temperatures usually result in less snowfall and more rainfall in winters and rapid thawing
of ice and snow in springs leading to higher rates of runoff and streamflow earlier in the
year and lower rate later. ‘In regions with snowfall, climate change has altered observed
streamflow seasonality, and increasing alterations due to climate change are projected.’
(robust evidence, high agreement).
Further,
because nearly all glaciers are too large for equilibrium with the present climate, there is a
committed water resources change during much of the 21st century, and changes beyond
the committed change are expected due to continued warming; in glacier-fed rivers, total
meltwater yields from stored glacier ice will increase in many regions during the next
decades but decrease thereafter (robust evidence, high agreement). Continued loss of glacier ice implies a shift of peak discharge from summer to spring, except in monsoonal
catchments, and possibly a reduction of summer flows in the downstream parts of glacierized catchments.
Water storage will change in various ways in response to changes in precipitation
and movement.
“Climate change is projected to reduce renewable surface water and groundwater resources
significantly in most dry subtropical regions (robust evidence, high agreement). In contrast,
water resources are projected to increase at high latitudes. Proportional changes are typically one to three times greater for runoff than for precipitation.” Further, “for each degree
of global warming, approximately 7% of the global population is projected to be exposed
to a decrease of renewable water resources of at least 20% (multi-model mean)” Such
changes will have impacts on water storage infrastructure such as dams and reservoirs.
P. Saundry