Lecture 8: Climate change and future water resources Flashcards
What is the grren house effect
omg
What are the most important GHG
CO2, Ch4, N20
What are the observed changes in aerosols
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.
What is the observed changes in LST and AT
bb
What are the potential direct changes due to chain in surface air temperature
bb
what is the importance of atmospheric water vapour
• 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
what is the clausius Clapeyron relationship
• 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.
what is the water vapour positive feedback effect
bb
what does it lead to?
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)
detection and atrribution of observed changes?
• 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
what evidence is there of the intensification of the hydrological cycle?
• 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
what are the changes to the global water vapour
• 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.
changes in global precipitation
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.
what are the changes in global runoff and streamflow
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.
Changes in global evaporation?
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)
what are the changes in global soil moisture?
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
what are the attribution of changes in ET
• 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.
What are the attribution of changes in streamflow
• 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, …
change in extreme
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
changes in global precipitation extremes
bb
changes in global record-breaking precipitation extremes?
bb
what is the observational evidence for relationship with clausius-clapeyron relationsip
b
summary of changes in global hydrological extremes
• 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.
what are the changes driven by changes in circulation
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).
change in the width of the tropical belt
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).
observational evidence of poleward movement of mid-latiitude storm track
bb
Regional Example. Observed Drying in the
Mediterranean
• 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.
Drivers of Future climate change
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.
How Do We Project Future Climates?
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
what processes do climate models represent?
progression of climate models?
bb
the resolution of climate models has increased dramatically with increased computing power
bb
the resoltuion of climate models has increased dramatically with increased computing power
bb
Expected Future Changes in the Water Cycle
• 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…
Hydrological Sensitivity and Climate Elasticity
• 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.
Hydrological Sensitivity and Climate Elasticity
part 2
• 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
Future Projected Changes in the Hadley Cell
• 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
Future Changes in Precipitation
Percentage changes in Seasonal Precipitation by 2081-2100
Relative to 1986-2005 for the RCP8.5 Scenario
bb
what is future change in ET
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
future changes in soil moisture
bb
future changes in runoff
• 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
Generalisation of projected changes in major components of the hydrologic cycle by the end of the 21st century
bb
Climate Change and Droughts
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.
Climate Change and Extreme Precipitation (Floods)
• 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.
Summary of Projected Changes
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, …
Uncertainties in Future Projections of Climate
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
Attribution of Uncertainty to 3 Sources
• 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
Uncertainties in Future Projections of Drought
• 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.
Quantifying Impacts of Climate Change on WR
• 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
Downscaling Climate
Model Outputs into
Something Useful for
Impacts
• 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
Bias Correcting Climate Models to Ensure that They are
Representative of Local Climate
• 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
Bias Correcting Climate Models to Ensure that They are
Representative of Local Climate
bb
Example of Estimating Future Climate Impacts on
Water Resources
• 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.
Future Changes in Water Demand – Driven
Primarily by Population Growth
bb
Regional Changes in Population Growth
• 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
Projected Water Stress by Country by
the mid 21st Century
• 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.
Connections between Future Drivers and
Responses in Water Resources
bb
