lecture 10 climate change projections, with particular reference to hong kong

Climate projections, with particular reference to

Hong Kong

Lecture 10

LSGI1B02 Climate Change and Society

CGE13208 Climate Change: Threats, Mitigation and Adaptation

Climate projections rely on models.What is a model?


The one and only tool - Climate model Climate models are our attempts to represent Earth’s climate system,

so that we can better understand how it works, since we cannot conduct atmospheric experiment, let alone whole-earth experiments.

Climate models are based on well established principles in physics, chemistry and biology.

Climate models are constrained by observations in the real world, and can also help inform further observational efforts.

Even fairly simple climate models, like energy balance models, can help us understand and represent important processes in Earth’s climate system.


Earth’s Climate SystemModel - representing all changes/interactions by mathematical equations

Components of global climate system (bold), their processes and interactions (thin arrows) and some aspects that may change (bold arrows)


Operational Climate ModelThe movement of energy, air, water, etc. are represented as horizontal and vertical exchanges between the boxes. In this way, models represent parts of the climate system and the world. Models attempt to capture the very complex interactions between Earth’s components.


Climate model – a mathematical representation of the climate system based on physical, biological and chemical principles.


Horizontal and vertical resolution


Modeler’s choice – Resolution


Improvement in climate models (1) Increase in model spatial resolution

1990

2007


Model output example - Sea Surface TemperatureHigh resolution ocean model coupled with atmospheric model (Princeton University, ocean 0.1 deg; atmosphere 0.5 deg)


Modelers’ choice – parameterization – some processes are too small-scale or too complex to be explicitly represented in the model

1 km

e.g. convective cloud

Direct Concern: To predict convective rain

Feedback to larger Scales:• Deep convection “overturns” the

atmosphere, strongly affecting mesoscale dynamics

• Changes vertical stability• generates and redistributes heat• removes and redistributes moisture• makes clouds, strongly affecting

surface heating and atmospheric radiation


Parameterization - approximating complicated processes


Temporal resolution(time is dependent on space)


Improvements in climate models

Complexity of models increased

More physics & interactions added


Complexity paradox Every number in the model can be a little bit off the

true world.

If enough little things get a little off, it can translate into a large array of possibility of predictions : a complex model may be more realistic, but as we add more factors to the model the certainty of prediction may decrease even though our intuitive faith in the model increases.


A critical factor for climate “projection” - Emission ScenariosOne of the greatest uncertainties in climate modelling relates to human behaviour. Choices that people make can affect the climate. Modelers do not attempt to predict human behaviour. Instead, they use scenarios to explore the consequences of possible human choices. Each scenario includes different assumptions about future human factors.


Emission scenarios - “A” storylines A1 - A future world of very rapid economic growth,

global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies.

A2 – A very heterogeneous world, self-reliance and preservation of local industries, increasing population. Economic development regionally oriented, technological change more fragmented.


Total global annual CO2 emissions from all sources for six emission scenarios


Representative concentration pathways (RCPs) – replacing emission scenarios


How good are the models - validation


James Hansen’s 1988 projections A: exponential increase in GHG

B: slowed down, but stable increase in GHG

C: GHG emission decreases after 2000


Some model projections are conservative

Projected changes in extremes – near term Increases in duration, intensity and spatial

extent of heat waves. Frequency and intensity of heavy

precipitation events over land will increase. Air quality – higher temperature in polluted

regions will trigger feedbacks that will increase peak levels of O3 and PM2.5.

Global annual mean surface temperature

Under RCP6.0 and RCP8.5, temperature rise is likely to exceed 2oC by the end of the century

Only under RCP2.6 will temperature rise less than 2oC

Regional changes in annual average surface temperature(1986-2005 to 2081-2100)

RCP 2.6 RCP 8.5

Projected seasonal regional temperature changes (RCP 4.5, 2016 – 2035, relative to 1986-2005)

Temperature changes in Asia in the near term (2016-2035) and long term (2081-2100)

Under RCP8.5, temperature in South China coastal area would rise by 4C

Projected regional seasonal precipitation changes (RCP 4.5, 2016 – 2035, relative to 1986-2005)

Change in average precipitation (1986-2005 to 2081-2100)

RCP 2.6 RCP 8.5

(a) Latitudinal changes in precipitation (b) changes in precipitation minus evaporation (RCP4.5, 2016-2035, relative to 1986-2005)

The wet getting wetter, the dry drier(21st century precipitation change, NOAA/GFDL CM2.1)

More intense and frequent extreme precipitation五天連續雨量上升

五天連續雨量上升 Changes in 20-year-return-period events (1986-2005) will decrease in 2081-2100

How climate change influences monsoon rainfall

Changes in ENSO (El Nino Southern Oscillation) intensity for 21st century uncertain, but will dominate climate variability

Global sea surface temperature change (relative to 1986-2005)

Chances of “Day After Tomorrow” ? Changes in Atlantic Meridional Overturning Circulation

More extreme hot days, less extreme cold days

Future tropical cyclone activities,2081-2100 relative to 2000-2019

NW Pacific: Total no. and no. of super typhoons not certain Typhoon intensity and rainfall likely to increase

I : total annual no.

II : total annual no. of Cat 4/5 storms

III: mean lifetime of max intensity

IV: rainfall

Projected increase in MSL > AR4

為什麼比 AR4高 ? 有考慮冰蓋快速的變化對物理過程有更好的撑握

Scenario 2045-2065 2081-2100RCP2.6 0.17 - 0.32m 0.26 - 0.55mRCP4.5 0.19 - 0.33m 0.32 - 0.63mRCP6.0 0.18 - 0.32m 0.33 - 0.63mRCP8.5 0.22 - 0.38m 0.45 - 0.82m

Regional changes in MSL at 2081-2100 (relative to 1986-2005)

Under RCP8.5, the Arctic Ocean will be deprived of sea ice in Sept before mid-21st century

Same, RCP4.5 RCP8.5, sea ice extent at 2081-2100

Northern hemisphere spring snow cover extent change

Change in permafrost area

Climate projections for Hong Kong in the 21st century

Make use of global climate models, BUT • Relatively low spatial resolution (150 – 400 km)• May not accurately represent local or station level climate (complex

topography, coastal or island locations, etc.)

Downscaling is a way to obtain higher spatial resolution output based on GCMs (global climate model)

Downscaling technique

From Global to Local - Downscaling

Statistical downscaling - basic concept

Statistical downscaling – develop quantitative relationships between large scale predictors and the local predictand

Large scale climate observations (predictors)

Local scale climate observations (predictands)

Set up statistical relationships

(e.g. regression)

GCM large scale outputs (predictors)

Downscaling model

Downscaled outputs (predictands)

Projecting rainfall in Hong Kong – downscaling

Projecting temperature change in Hong Kong – downscaling

香港年平均氣溫未來推算

Medium to low emission – RCP4.5 High emission – RCP8.5

Projection based on 25 AR5 climate modelsUrbanization effect included

Projection of very hot days and cold days in Hong Kong

Parameter(annual number)

1980-1999Average

(observed)

Projection 2050-2059 2090-2099

Lower limit average Upper

limitLower limit Average Upper

limit

Hot night 16 58 96 125 87 137 175

Very hot day 9 20 51 81 29 89 131

Cold day 17 1 4 8 0 1 5

Extreme event return period in 1900

Return period in 2000

Hourly rainfall≥ 100mm 37 years 18 years

2-hourly rainfall≥ 150mm 32 years 14 years

3-hourly rainfall≥ 200mm 41 years 21 years

More frequent extreme precipitation

Rainfall projection in Hong Kong

Under high emission scenario, annual rainfall will increaseby 180mm (compared with 1986-2005 normal)

高溫室氣體濃度情景下 ...

Extreme dry year – annual r/f <1289mm; Extreme wet year – annual rainfall >3168mm

PeriodExtremely dry

yearsExtremely wet

years

1885 - 2005 2 3 2006 - 2100 2 12

(相片來源：水務署 ) (相片來源：渠務署 )

SPI (standardized precipitation index) projection

Extreme drought like the 1963 event is possible

1963年乾旱

過去觀測 projection

Sea level projected in IPCC AR5 to rise by 0.26-0.82m during 2081-2100

Return period (year)Extreme sea level rise (m)

Current sea level Sea level rising by 0.26m

Sea level rising by 0.82m

2 2.9 3.2 3.75 3.1 3.4 3.910 3.3 3.5 4.120 3.4 3.6 4.250 3.5 3.8 4.4

56

Storm surge of typhoon

Rising sea level increases the risk of storm surge

Key findings of the projections for Hong Kong in the 21st century

Temperature : the increasing trend will continue. The mean temperature in the decade 2090-2099 is expected to rise by 4 to 5 oC relative to the period 1980-1999.

Rainfall : will increase during the latter half of the 21st century with about 10% increase relative to the 1980-1999 average.

Sea level : The sea-level at the South China Sea including Hong Kong is likely to be close to the global average in the long run.

• According to IPCC AR5, the global average sea-level will rise by 0.29 to 0.82 m at the end of 21st century relative to the period 1986 to 2005.

• Recent studies by some research groups suggest higher projections

Uncertainties : there are still large uncertainties in the model simulation for the future climate, depending very much on the future forcing emission scenarios and local urbanization effect as well as the model characteristics/performance.

When interpreting the climate projection results, it is important to note that :

• Climate projection is very different from weather or seasonal forecasts. Climate projection involves assumptions in future socio-economic and technological developments and greenhouse gas emission scenarios and aims at describing the plausible change in the future climate from a long term perspective, rather than depicting the "day to day" or "year to year" variations in weather.

• Our knowledge about the Earth is not perfect, so our climate models cannot be perfect. Some complex feedback mechanisms are still not fully understood and represented in the IPCC AR4 models (e.g. soil-biosphere interaction, aerosols and clouds, carbon cycle, atmosphere-ice-ocean interactions, etc.)

• Although a majority of the model projections suggests in general consistent trends for the changes in the climate of the 21st century, inter-model differences are still rather large with a divergence in the projections for the future climate. This, to a certain extent, reflects that climate projection is still subject to various uncertainties in the model simulation of the future climate.

• future human behaviors - greenhouse gas & aerosols emission, urbanization, land use, etc.;

• our incomplete knowledge of the Earth system - climate natural variations, etc.;

• unforeseeable variations in solar and volcanic activities

• the choice of models and model skills;

• the downscaling methodology;

• the stability of the statistical downscaling relationships in the future;

• …………

Uncertainty is an integral part of climate change projections.

It could arise from :

lecture 10 climate change projections, with particular reference to hong kong

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