Lecture 8: Climate change and future water resources

Question 1

What is the grren house effect

Answer 1

omg

Question 2

What are the most important GHG

Answer 2

CO2, Ch4, N20

Question 3

What are the observed changes in aerosols

Answer 3

Seasonal average aerosol optical depth (AOD) trends at 0.55 μm for 1998–2010
using SeaWiFS satellite data (Hsu et al., 2012). It is very likely that AOD has
decreased over Europe and the eastern USA since the mid 1990s and increased
over eastern and southern Asia since 2000. In the 2000s dust-related AOD has been
increasing over the Arabian Peninsula and decreasing over the North Atlantic Ocean.

Question 4

What is the observed changes in LST and AT

Answer 4

bb

Question 5

What are the potential direct changes due to chain in surface air temperature

Answer 5

bb

Question 6

what is the importance of atmospheric water vapour

Answer 6

• There is very little water held in the atmosphere - only about 0.001% of the total
Earth’s water volume.
• But it has an oversized impact on our climate and water resources
• Water vapor is the most important greenhouse gas in the atmosphere.
• Heat radiated from Earth’s surface is absorbed by water vapor molecules in the
lower atmosphere.
• The water vapor molecules, in turn, radiate heat in all directions. Some of the
heat returns to the Earth’s surface

Question 7

what is the clausius Clapeyron relationship

Answer 7

• Atmospheric water vapor drives many important meteorological phenomena
(notably precipitation).
• The Clausius–Clapeyron equation describes the relationship between
temperature and water vapor, and is important because it describes how water
vapor increases with global warming
• For water vapor under typical atmospheric conditions (near standard
temperature and pressure) the relationship is

equaition on blackboard

• The August-Roche-Magnus formula provides a very good approximation, using
pressure in hPa and temperature in Celsius:

• This implies that saturation water vapor
pressure changes approximately
exponentially with temperature under
typical atmospheric conditions.
• Hence the water-holding capacity of the
atmosphere increases by about 7% for
every 1 °C rise in temperature.

Question 8

what is the water vapour positive feedback effect

Answer 8

bb

Question 9

what does it lead to?

Answer 9

intensification of the HC
1. Increases in temperature increase the
water holding capacity of the atmosphere
2. Evaporation
increases
3. Precipitation events
become heavier and
fewer
4. Leads to
longer dry spells
in between the
fewer storms
and due to
increased ET

This is the intensification of the hydrological cycle: faster ET, heavier P, longer dry spells.
(Plus changes in temperature dependent cold-season processes)

Question 10

detection and atrribution of observed changes?

Answer 10

• Detection is the identification of trends against the background of natural
variability
• Attribution is the scientific identification and quantification of changes in the
climate and the hydrological cycle due different plausible factors
• For climate, these could be volcanoes, solar activity, GHGs, land use, …
• For the water cycle, these are changes in the climate itself, the response of
vegetation, land use, …
• Methods for attribution include statistical evaluation of observed data and model
experiments to isolate different potential driving factors

Question 11

what evidence is there of the intensification of the hydrological cycle?

Answer 11

• There are several lines of evidence for the intensification of the
hydrological cycle.

• These include global but often regional changes in the following
variables:
• Water vapor
• Precipitation
• Evapotranspiration
• Extreme rainfall / floods
• Dry spells / droughts

• Many factors ensure that detecting such changes is difficult:
• The complexity of the climate system
• Natural variability
• Regional differences
• Lack of data

Question 12

what are the changes to the global water vapour

Answer 12

• It is very likely that global near
surface air specific humidity has
increased since the 1970s.
• However, during recent years the
near surface moistening over
land has abated (medium
confidence).
• As a result, fairly widespread
decreases in relative humidity
near the surface are observed
over the land in recent years.

Question 13

changes in global precipitation

Answer 13

Generally there has been a shift to more rain than snow. When averaged over the land areas of the
mid-latitudes of the NH, all datasets show a likely overall increase in precipitation (medium
confidence since 1901, but high confidence after 1951). For all other zones one or more of data
sparsity, quality, or a lack of quantitative agreement amongst available estimates yields low
confidence in characterisation of such long-term trends in zonally averaged precipitation.

Question 14

what are the changes in global runoff and streamflow

Answer 14

About one-third of the
200 largest rivers show
significant trends in
streamflow for 1948–
2004.

E.g. The Congo,
Mississippi, Yenisey,
Paraná, Ganges,
Colombia, Uruguay
and Niger.

The rivers having
downward trends (45)
outnumber those with
upward trends (19).

Decreases in streamflow are found over many low and midlatitude
river basins such as the Yellow River in northern
China since 1960s where precipitation has decreased.

Overall confidence is low for an increasing trend in global
river discharge during the 20th century.

Question 15

Changes in global evaporation?

Answer 15

Globally ET has generally increased
over land mainly because of higher
temperature and water holding capacity
(see later)
• A slight decline in recent years is due to
lower soil moisture from lower
precipitation (and higher ET)

Question 16

what are the changes in global soil moisture?

Answer 16

Changes for 1980-2010
based on three independent
datasets:
1. Satellite retrievals (ESA)
2. Reanalysis data (ERAInterim)
3. Land surface model data
(GLDAS-Noah)

The changes are uncertain
and difficult to attribute –
precipitation, vegetation,
warming, land use change,
…
There are no direct SM
observations globally

Question 17

what are the attribution of changes in ET

Answer 17

• A general increasing trend of ET is apparent in all estimates of ET (satellites, models, empirical)
• Climate has the major influence on ET
• Globally, rising CO2 ranks second, giving decreasing trends in canopy transpiration and ET,
especially for tropical forests and high-latitude shrubland.
• Increasing nitrogen deposition slightly amplified global ET via enhanced plant growth.
• Land-use-induced ET responses, are minor globally, but pronounced locally.

Question 18

What are the attribution of changes in streamflow

Answer 18

• Changes in streamflow have been observed over many parts of the world,
particularly increases in the northern hemisphere
• Potential drivers of these changes: climate, human management, aerosols, CO2,
land use changes, …

Question 19

change in extreme

Answer 19

Changes in extremes are important
because of their impacts on water
resources and demand, and on hazard
risk

Change can occur due to:

a) Shifts in the mean. For example, if
the climate gets generally hotter then
there will be more extreme hot days
b) Shifts in variance. For example, if
precipitation becomes more variable,
then there will be more droughts and
floods
c) Changes in both mean and variance.
This can lead to even greater
increases in the frequency of high or
low extremes

Question 20

changes in global precipitation extremes

Answer 20

bb

Question 21

changes in global record-breaking precipitation extremes?

Answer 21

bb

Question 22

what is the observational evidence for relationship with clausius-clapeyron relationsip

Answer 22

b

Question 23

summary of changes in global hydrological extremes

Answer 23

• There is strong evidence that warming has lead to changes in temperature
extremes—including heat waves—since the mid-20th century.
• Changes in extremes for other climate variables are generally less coherent than
those observed for temperature, owing to data limitations and inconsistencies
between studies, regions and/or seasons.
• However, increases in precipitation extremes, for example, are consistent with a
warmer climate.
• Analyses of land areas with sufficient data indicate increases in the frequency
and intensity of extreme precipitation events in recent decades, but results vary
strongly between regions and seasons.

• Heavy Precipitation
• Evidence for increases is most compelling for increases in heavy
precipitation in North America, Central America and Europe,
• But in some other regions—such as southern Australia and western Asia—
there is evidence of decreases.

• Drought
• Likewise, drought studies do not agree on the sign of the global trend, with
regional inconsistencies in trends dependent on how droughts are defined.
• However, indications exist that droughts have increased in some regions
(e.g., the Mediterranean) and decreased in others (e.g., central North
America) since the middle of the 20th century.

Question 24

what are the changes driven by changes in circulation

Answer 24

A major feature of the tropics and subtropics is the Hadley Cell. This is the largescale
movement of air in the troposphere, with rising air at the ITCZ and sinking air
outside the tropics in areas known as subtropical high-pressure cells. Humid air
rises at the ITCZ, typically forms deep cumulonimbus clouds as it goes to the top of
the troposphere, and then the air heads poleward and sinks in subtropical areas
(i.e., subtropical highs).

Question 25

change in the width of the tropical belt

Answer 25

Can be measured in different ways: the width of the Hadley circulation; the separation of
the Northern and Southern Hemisphere subtropical jet-stream cores; the width of the
region of frequent high tropopause levels; and the width of the region with tropical
column ozone levels (Northern Hemisphere only).

Question 26

observational evidence of poleward movement of mid-latiitude storm track

Answer 26

bb

Question 27

Regional Example. Observed Drying in the

Mediterranean

Answer 27

• Trends in winter precipitation from
gauges across the Mediterranean
indicate drying across many parts
(bottom panel)
• Multiple global precipitation datasets
show this also (top panel)
• What are the consequences to water
resources in the region?
• Climate model experiments were
used to attribute the drying by
switching on and off anomalies inseas
surface temperatures (SSTs) around
the world
• Experiments indicate that drying is
caused by increased warming in the
tropical oceans.

Question 28

Drivers of Future climate change

Answer 28

Projected CO2, CH4, and N2O
emissions over the 21st century

These have been developed for
different scenarios called
Representative Concentration
Pathways (RCP)

These represent plausible
pathways to reach different
levels of radiative forcing

RCP85 is the highest RCP with
a radiative forcing of 8.5 W/m2
by the end of the 21st century.
This is business as usual.

RCP26 is the lowest – this
represents a mitigation
scenario where we do
something about climate
change.

Question 29

How Do We Project Future Climates?

Answer 29

scenarions of external forcings:
• Greenhouse Gases
• Aerosols
• Volcanoes
• Solar Activity
• Land use change

climate models
-Multiple centers
around the world
e.g. Hadley Center in
the UK

future climate projections
-Simulations of future
climates for multiple
scenarios and models

Question 30

what processes do climate models represent?

progression of climate models?

Answer 30

bb

Question 31

the resolution of climate models has increased dramatically with increased computing power

Answer 31

bb

Question 32

the resoltuion of climate models has increased dramatically with increased computing power

Answer 32

bb

Question 33

Expected Future Changes in the Water Cycle

Answer 33

• Expected continuation of direct temperature
effects – e.g. glacier melt, snow cover decline,
more rain than snow, saturation vapour pressure
increases with temperature, …
• Temperature-vapour changes should lead to net
increases in rainfall, surface evaporation and
plant transpiration.
• But projected future changes in the water cycle
are far more complex than projected
temperature changes.
• Some regions of the world will be subject to
decreases in hydrologic activity while others will
be subject to increases.
• There are important local seasonal differences
among the responses of the water cycle to
climate change as well.
• Plus human management…

Question 34

Hydrological Sensitivity and Climate Elasticity

Answer 34

• Hydrological sensitivity is a measure of how the water cycle responds to climate
change
• One way of quantifying this is the concept of climate elasticity of the hydrological
cycle, which is the ratio of the change in the variable of interest (e.g. Q, R or ET) to
the change in the climate variable (e.g. T or P).

e(R,P) = dR/dP

Hydrological Sensitivity and Climate Elasticity
• Many other ways to estimate e

• This can be calculated from
observations by calculating
annual changes in P and R.

Question 35

Hydrological Sensitivity and Climate Elasticity

part 2

Answer 35

• The elasticity of runoff to precipitation, e(R,P) > 1, and typically ranges from 1-3. This
means, for example, that a doubling of precipitation will lead to at least a doubling of
runoff
• In humid regions (PET/P < 1), a change in P mostly transforms into R
• In arid regions (PET/P > 1) a change in P mostly transforms into ET
• e(R,T) is typically up to 0.1 per oC
• The effects of global warming induced precipitation changes in runoff will be greater
than the effects of increased ET due to increased temperature

• But in reality way more complicated because of:
• regional differences
• CO2 fertilization
• Changes in vegetation seasonality
• Wind speed, cloudiness, …
• And the overall Feedbacks between land ET and global P and T

Question 36

Future Projected Changes in the Hadley Cell

Answer 36

• One of the most important
expected changes is in the
overall circulation of the
atmosphere
• The figure shows projected
changes by the end of the
century in the locations of the
northern and southern edges of
the Hadley Cell under the
RCP4.5 scenario.
• Each climate model (circle)
shows a different change that
depends on their sensitivity to
GHG forcings
• Overall, the expectation is an
expansion of the Hadley cell
with associated drying in the
sub-tropics and wetting in higher
latitudes

Question 37

Future Changes in Precipitation
Percentage changes in Seasonal Precipitation by 2081-2100
Relative to 1986-2005 for the RCP8.5 Scenario

Answer 37

bb

Question 38

what is future change in ET

Answer 38

Increases or decreases over land are largely driven by changes in precipitation. Prominent areas of decrease
over land in southern Africa and northwestern Africa along the Mediterranean.
• Annual mean evaporation increases over land in the northern high latitudes are consistent with the increase in
precipitation and the overall warming that would increase potential evaporation.
• Other factors - increased atmospheric CO2 promotes stomatal closure and reduced transpiration, which can
potentially yield increased runoff. There is potential for substantial feedback between vegetation changes and
regional water cycles, though the impact of such feedback remains uncertain

Question 39

future changes in soil moisture

Answer 39

bb

Question 40

future changes in runoff

Answer 40

• Decreases in southern Europe, the Middle East, and southwestern USA
• The large decreases in runoff in southern Europe and southern Africa are consistent with changes in the
Hadley Circulation and related precipitation decreases and warming-induced evapotranspiration increases
• Increases in Southeast Asia, tropical East Africa and at high northern latitudes - The high northern latitude
runoff increases are likely under RCP8.5 and consistent with the projected precipitation increases
• In snowmelt dominated regions, the timing of peak runoff in the spring is expected to change

Question 41

Generalisation of projected changes in major components of the hydrologic cycle by the end of the 21st century

Answer 41

bb

Question 42

Climate Change and Droughts

Answer 42

The risk of future
agricultural drought
episodes is increased in
the regions of robust soil
moisture decrease
described earlier

• The consecutive dry-day index (CDD) is the length of the longest period of
consecutive days with precipitation less than 1 mm.
• Patterns are similar to projected changes in both precipitation and soil moisture.
• Substantial increases in this measure of meteorological drought are projected in the
Mediterranean, Central America, Brazil, South Africa and Australia while decreases
are projected in high northern latitudes.

Question 43

Climate Change and Extreme Precipitation (Floods)

Answer 43

• Consistently, climate models project future episodes of more intense precipitation in
the wet seasons for most of the land areas,
• Especially in the NH and its higher latitudes, and the monsoon regions of the world,
and at a global average scale.
• Globally averaged end of 21st century changes over land range from 5% (RCP2.6) to
20% (RCP8.5) more precipitation during very wet 5-day periods.
• Implies that the probability of floods will increase, although regional differences and
local factors make the uncertainties large.

Question 44

Summary of Projected Changes

Answer 44

Drivers of change:
• Direct temperature impacts on snow and ice
• Local thermodynamic processes driving intensification of hydro cycle
• Changes in circulation
• Warmer oceans causes more ET and potentially more moisture
transport to lands

Climate models project:
• Precipitation increase in the near-equatorial regions, which tend to
be wet in the present climate.
• In subtropical land areas— places that are already relatively dry—
precipitation is projected to decrease during the 21st century.
• Mid-latitude storm track are projected to move poleward along with
the poleward edge of the subtropical dry zones
• More extremes – extreme precipitation, floods and droughts
• Regions that may face water resources issues – Mediterranean,
southern Africa, southwestern US/northern Mexico, southeast
Australia, …

Question 45

Uncertainties in Future Projections of Climate

Answer 45

Future projections are uncertain
because of:

1. Natural (internal) variability –
we do not know what the
climate will be from year to
year or decade to decade
2. Model differences – climate
models provide simplified
representations of the realworld
processes – we don’t
know which one is most
realistic
3. Uncertain GHG emissions –
we do not know what
emissions pathway we will
take in the future

Question 46

Attribution of Uncertainty to 3 Sources

Answer 46

• For temperature and precipitation most uncertainty is due to emissions uncertainty,
then model uncertainty and then internal variability
• The uncertainties become larger over time
• Uncertainties from the models is larger for precipitation, because precipitation is very
difficult to simulate

Question 47

Uncertainties in Future Projections of Drought

Answer 47

• Fractional uncertainty,
defined as uncertainty
divided by the change since
2006

• For different regions and
different variables (SPI12
represents meteorological
drought; SMA represents
agricultural drought)

• Uncertainty depends on:
• The variable
• The time period
• The region

• The Mediterranean shows
the least uncertainty (most
certain) results.

Question 48

Quantifying Impacts of Climate Change on WR

Answer 48

• How do we estimate the impact on water resources? Climate models are
generally too coarse and do not simulate all processes that are relevant.
• The traditional approach has been to downscale climate model outputs of
precipitation and temperature and use these to drive hydrological or water
resources models (also other impact models)
• Downscaling is translating data to higher resolution – for example, taking climate
model outputs at 100km resolution and translating this 5km resolution

Question 49

Downscaling Climate
Model Outputs into
Something Useful for
Impacts

Answer 49

• Downscaling	takes	coarse	resolution	
GCM	output	and	translates	it	to	the	
scale	of	impacts	as	represented	by	
impact	models
• Impact	models	can	be	a	hydrology	or	
water	resources	model,	an	energy	
model,	a	crop	model,	a	model	of	
tourism,	…

• Downscaling	can	be	done	in	one	of	
two	ways:
• Dynamical	downscaling	which	
uses	a	higher	resolution	regional	
climate	model	embedded
• Statistical	downscaling	which	
uses	a	statistical	relationship	
between	the	coarse	scale	GCM	
output	and	the	fine	scale	data

Question 50

Bias Correcting Climate Models to Ensure that They are

Representative of Local Climate

Answer 50

• Models are wrong – they have biases and errors compared to the real world
• They may overestimate historic temperature or precipitation
• They may show the wrong relationships between, for example, precipitation and
evapotranspiration
• These biases can be removed (somewhat) through bias-correction methods

Question 51

Bias Correcting Climate Models to Ensure that They are

Representative of Local Climate

Answer 51

bb

Question 52

Example of Estimating Future Climate Impacts on

Water Resources

Answer 52

• We would like to know how climate
change will impact the hydrology of
a lake that is important for water
resources
• We can use a hydrological model
forced by climate model future data
and compare with historic
simulations
• The climate model could be
downscaled and bias corrected to
the scale of the lake
• We can examine multiple future
climate scenarios and use multiple
climate models to understand the
uncertainties.

Question 53

Future Changes in Water Demand – Driven

Primarily by Population Growth

Answer 53

bb

Question 54

Regional Changes in Population Growth

Answer 54

• Rapidly growing populations will
drive increased consumption by
people, farms and companies.
• More people will move to cities,
further straining supplies.
• An emerging middle class could
demand more water-intensive
food production and electricity
generation.
• Sub-Saharan Africa is expected
to have the largest increase in
population

Question 55

Projected Water Stress by Country by

the mid 21st Century

Answer 55

• The world’s demand for water is likely to surge in the next few decades.
• But it’s not clear where all that water will come from.
• Climate change is expected to make some areas drier and others
wetter.
• As precipitation extremes increase in some regions, affected
communities face greater threats from droughts and floods.

Question 56

Connections between Future Drivers and

Responses in Water Resources

Answer 56

bb