2. Climate projections rely on models.
What is a model?
CGE13208 Climate Change: Threats, Mitigation and Adaptation
3. 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.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
4. Earth’s Climate System
Model - representing all changes/interactions
by mathematical equations
CGE13208 Climate Change: Threats, Mitigation and Adaptation
Components of global climate system
(bold), their processes and
interactions (thin arrows) and some
aspects that may change (bold arrows)
5. Operational Climate Model
The 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.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
6. Climate model – a mathematical representation of the climate
system based on physical, biological and chemical principles.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
7. Horizontal and vertical resolution
CGE13208 Climate Change: Threats, Mitigation and Adaptation
9. Improvement in climate models
(1) Increase in model spatial resolution
1990
2007
CGE13208 Climate Change: Threats, Mitigation and Adaptation
10. Model output example - Sea Surface Temperature
High resolution ocean model coupled with atmospheric
model (Princeton University, ocean 0.1 deg; atmosphere
0.5 deg)
CGE13208 Climate Change: Threats, Mitigation and Adaptation
11. 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
CGE13208 Climate Change: Threats, Mitigation and Adaptation
16. 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.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
17. A critical factor for climate “projection” - Emission
Scenarios
One 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.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
18. 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.
CGE13208 Climate Change: Threats, Mitigation and Adaptation
19. Total global annual CO2 emissions from all sources for
six emission scenarios
CGE13208 Climate Change: Threats, Mitigation and Adaptation
21. How good are the models - validation
CGE13208 Climate Change: Threats, Mitigation and Adaptation
22. James Hansen’s 1988 projections
CGE13208 Climate Change: Threats, Mitigation and Adaptation
A: exponential
increase in
GHG
B: slowed down, but
stable increase in
GHG
C: GHG emission
decreases after
2000
23. Some model projections are
conservative
CGE13208 Climate Change: Threats, Mitigation and Adaptation
24. 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.
25. 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
26. Regional changes in annual average surface
temperature
(1986-2005 to 2081-2100)
RCP 2.6 RCP 8.5
28. 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
39. 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
46. 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
47. 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)
50. 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 models
Urbanization effect included
51. Projection of very hot days and cold
days in Hong Kong
Parameter
(annual
number)
1980-
1999
Average
(observed)
Projection
2050-2059 2090-2099
Lower
limit
average
Upper
limit
Lower
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
52. 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
53. Rainfall projection in Hong Kong
Under high emission scenario, annual rainfall will increase
by 180mm (compared with 1986-2005 normal)
54. 高溫室氣體濃度情景下...
Extreme dry year – annual r/f <1289mm; Extreme wet year – annual rainfall >3168mm
Period
Extremely dry years Extremely wet years
1885 - 2005 2 3
2006 - 2100 2 12
(相片來源：水務署)
(相片來源：渠務署)
56. 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 3.7
5 3.1 3.4 3.9
10 3.3 3.5 4.1
20 3.4 3.6 4.2
50 3.5 3.8 4.4
56
Storm surge of typhoon
Rising sea level increases the risk of storm surge
57. 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.
58. 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.
59. • 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 :
