Reprint 1149

Global Warming - Present and Future Challenges

C.M. Shun & S.M. Lee

Symposium on eCO2 Management -

A Paradigm Shift

25 May 2012

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Global Warming - Present and Future Challenges

SHUN Chi Ming

Director

Hong Kong Observatory

cmshun@hko.gov.hk

LEE Sai Ming

Acting Senior Scientific Officer

Hong Kong Observatory

smlee@hko.gov.hk

Abstract

The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate

Change (IPCC) clearly indicated that the increase in the atmospheric greenhouse gas

concentration due to human activities are very likely responsible for the observed global

warming since the middle of the 20th

century. In this paper, we will review the current

situation of global temperature, atmospheric carbon dioxide (CO2) concentration and CO2

emission. We will also discuss some of the impacts of a warming climate, and highlight the

work by the IPCC Working Group III in studying the mitigation potential of renewable

energy.

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1 A WARMING WORLD

1.1 Temperatures in 2011

The year 2011 would probably be remembered by the public as a rather cold year in

Hong Kong as we experienced two La Niña events in the same year, one near the start of the

year while the other near the end. Previous studies show that La Niña is usually associated

with cold weather in Hong Kong during the winter months [18]. We also experienced the

coldest January since 1977 with 14 cold days, doubling the normal figure of 7. December

2011 was also cold with a mean temperature almost 1℃ below normal. Although the

average temperature of 2011 in Hong Kong was slightly below normal with reference to the

30-year (1981-2010) climatology, an examination of Hong Kong's annual temperatures in La

Niña years (Fig. 1) reveals that the temperature has been on a rising trend in the recent

decades.

In March 2012, the World Meteorological Organization (WMO) released its

statement on the status of the global climate in 2011 [37], announcing that 2011 ranks as the

world's 11th warmest year in records dating back to 1880. The 10-year average temperature

for 2002-2011 equals that for 2001-2010 as the warmest 10-year period on record. The

global climate was influenced by La Niña, which has a cooling effect through heat exchange

between the atmosphere and the cooler-than-normal equatorial Pacific ocean. Yet, global

temperatures in 2011 were warmer than any previous moderate/strong La Niña year (Fig. 2).

1.2 Undeniable evidence of global warming

Temperature data around the world are collected and disseminated by the national

meteorological services of WMO members and collaborating research institutions. These

data feed three main global climate data and analysis centres, which develop and maintain

homogeneous global climate datasets based on peer-reviewed methodologies. The WMO

global temperature analysis is principally based on these three complementary datasets.

One dataset is the combined dataset in the UK maintained jointly by the Hadley Centre of the

Met Office and the Climatic Research Unit, University of East Anglia. The other two datasets

are maintained in the US: at the National Oceanic and Atmospheric Administration (NOAA)

and the Goddard Institute of Space Studies (GISS) of the National Aeronautics and Space

Administration (NASA) respectively. As shown in Fig. 3, these three datasets clearly show

the long-term increasing trend of global temperatures in the last century especially in the

recent decades.

In October 2011, a team of independent scientists including Saul Perlmutter, the

winner of the 2011 Nobel Prize in Physics, statisticians, physicists and climatologists,

published on the Internet the results of an independent study entitled "Berkeley Earth Surface

Temperature" [3]. The aim of the study was to resolve criticism of temperature analyses

raised by climate change skeptics. The investigators of the study examined 1.6 billion

temperature records from 39,000 stations, more than five times of the stations considered in

previous climate studies. To merge existing surface station temperature datasets into a new

comprehensive dataset, the investigators developed new and more advanced statistical

algorithms to process the temperature data, taking nearly all available data into account, with

due consideration given to data quality, instead of simply dropping short and discontinuous

data records. What they found was a rise in the average world land surface temperature of

approximately 1℃ since the mid-1950s. This finding agrees very well with the previous

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studies of the three global centres mentioned above (Fig. 3). The Berkeley study also

concluded that the urban heat island effect does not contribute significantly to the average

land surface temperature rise because urban regions of the Earth only amount to less than 1%

of the land area. The comprehensive dataset and statistical algorithms resulted from the

study are all open to public scrutiny (http://berkeleyearth.org/).

It is well known that El Niño/Southern Oscillation (ENSO) can affect atmospheric

temperatures by exchanging ocean heat content with the atmosphere; aerosols from volcanic

eruption can reduce sunlight reaching the ground and therefore lowering the temperature [12];

and variation in solar activity can also affect the global temperature. Foster and Rahmstorf

[7] recently analyzed five datesets of global temperatures for the period 1979-2010: three

surface temperature datasets from NASA's GISS, NOAA and the Hadley Centre, and two

lower troposphere temperature datasets based on satellite observations, and identified the

impact of ENSO, volcanic aerosols and solar activity on the surface and lower troposphere

temperature by statistical methods. With the effects of ENSO, volcanic activity and solar

variability removed, they found a clear and consistent warming signal in the five adjusted

datasets (Fig. 4), without any cessation or slowing down in warming in the last decade.

The summer sea ice melting season in the Arctic normally ends in September.

According to the US National Snow and Ice Data Center (NSIDC), the Arctic sea ice extent

in September 2011 reached the second lowest on record since measurements began in 1979

[24]. The lowest extent was recorded in 2007, when both the atmospheric and oceanic

conditions were favourable for melting of Arctic sea ice. The continuing trend of decreasing

extent is obvious in Fig. 5, indicating the continuation of our warming climate. Fig. 6a and

6b show the satellite-derived image of Arctic perennial ice (ice that has survived at least one

summer) in 1980 and 2012 respectively. It is evident that the perennial ice has declined

significantly in the last 30 years. Moreover, the multi-year ice (i.e. thicker and older ice) is

diminishing at a rate of -15.1 percent per decade, faster that the younger and thinner ice at the

edges of the Arctic Ocean's floating ice cap [23].

2 IMPACTS OF A WARMING CLIMATE

2.1 Extreme weather events

In a changing climate, a relatively small shift in the climatic distribution can result

in substantial changes in the frequency of extreme events [13]. Over the last 50 years, there

were widespread changes in extreme temperatures and precipitation around the world. In

many places, hot days/nights and heat waves as well as heavy rain have become more

frequent, and cold days/nights have become rarer [1, 9, 14, 32, 36]. The Special Report on

Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation

(SREX) released in late March 2012 by the IPCC [16] and post-AR4 studies have reaffirmed

the assessments made in the IPCC AR4 about the general trend towards more warm extremes

and precipitation extremes, the general trend towards less cold extremes, and the

anthropogenic influence in these observed changes. A recent study shows that the

anomalously warm summers of 2003 in western and central Europe and of 2010 in eastern

Europe and Russia broke the 500-year long seasonal temperature record over 50% of Europe.

Using simulations from global climate models, Zwiers et al. [38] detected both anthropogenic

influence and the combined influence of anthropogenic and natural forcing in temperature

extremes at the global scale over land area (Fig. 7). Hansen et al. [11] investigated the

systematic shift of seasonal temperature distribution towards the warm side and the change in

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probability of unusually warm seasons in recent decades. They highlighted the emergence

and importance of the "3-sigma" (i.e. 3 standard deviations) warmer-than-climatology

extremely hot events in recent summers. Consistent with a warming climate, the probability

of these "3-sigma" events increased significantly in recent decades (Fig. 8). The global land

area covered by these "3-sigma" events in recent summers was more than 10 times several

decades ago. Examples of these "3-sigma" extremely hot events include the heat waves in

Texas and Oklahoma in 2011 and in Moscow in 2010. Rahmstorf et al. [28] also

investigated the role of a warming climate in the 2010 Moscow heat wave and concluded that

there was a probability of 80% that the July 2010 high temperature record was due to climate

warming.

Nevertheless, not every region in the world exhibits temperature trend fully

consistent with the overall behaviour. In recent years, cold winters were experienced in

some parts of the northern continents. Scientific understanding of such phenomenon has

recently advanced. Liu et al. [21] found that a warming Arctic with diminishing sea ice – a

clear manifestation of global warming – can cause more occurrences of blocking patterns in

the atmospheric circulation over the Northern Hemisphere. In winter, these blocking

patterns will lead to more cold air outbreaks from the Arctic to mid-latitudes, thus bringing

cold conditions to the affected areas. These findings reconcile with the study of Cohen et al.

[5] which showed that in the past decades the extratropics of the Northern Hemisphere

experienced significant warming trends for all seasons except winter, even though the overall

annual trend was for a rising temperature. These findings are also corroborated by the

results of Francis and Vavrus [8] which reveal that the warming Arctic with decreasing sea ice

has played an important role in affecting the large-scale atmospheric circulation to favour the

occurrence of persistent weather patterns, which are usually associated with weather extremes

such as cold spells and heat waves.

Observed changes in precipitation extremes were less spatially coherent and

statistically significant compared with observed changes in temperature extremes.

Post-AR4 studies with more regional results have updated the assessments of AR4. Overall

speaking, more locations show an increase rather than a decrease in extreme precipitation,

although there are wide regional and seasonal variations. A recent example in extreme

precipitation is the heavy downpour in Seoul in July 2011, in which more than 300 mm of

rainfall was received in a single day, the highest since 1907 [37]. New research since the

AR4 has provided more evidence of anthropogenic influence on various aspects of the global

hydrological cycle relevant to extreme precipitation changes.

Post-AR4 studies have used more simulations of global climate models to

investigate extreme temperature and rainfall projections for the 21st century in terms of

various extreme indices, return value and return period. Fig. 9, adapted from Orlowsky and

Seneviratne [25], shows a projected annual and seasonal changes in three indices for daily

maximum temperature for 2081-2100 with respect to 1980-1999. It depicts an increasing (a

decreasing) trend of warm (cold) index towards the end of the current century. Kharin et al.

[17] projected the return period of late 20th

-century 20-year return values of the annual

maximum of the daily maximum temperature (Fig. 10). Under certain greenhouse gas

emission scenarios, a 1-in-20 year extremely hot day event in the late 20th

century is likely to

become a 1-in-2 to 1-in-5 year event by the end of the 21st century. Meanwhile, a 1-in-20

year extreme daily rainfall event is likely to become a 1-in-5 or 1-in-15 year event in many

regions of the world (Fig. 11). A decrease in return period implies more frequent

occurrences of the extreme event.

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Here in Hong Kong, studies of past occurrences of extreme temperature and rainfall

events have also found that cold episodes have become rarer while very hot days and heavy

rain events are becoming more frequent over the past 120 years or so [33]. The

corresponding return periods of heavy rain and very hot days are decreasing while that of

very cold days is increasing. The trends in temperature extremes that have been observed

during the 20th

century are expected to continue into the 21st century with a significant

increase in the number of very hot days and a significant decrease in the number of cold days

[19]. The rainfall in Hong Kong in the 21st century may become more variable and extreme.

The number of rain days is expected to decrease while the daily rainfall intensity and the

number of days with extreme rainfall will increase [20].

2.2 Sea level rise

The World Glacier Monitoring Service (WGMS) and its predecessor organizations

have been compiling and disseminating standardized data on glacier fluctuations for more

than a century. The Glacier Mass Balance Bulletin No. 11 (2008-2009) [34] published by

WGMS in late 2011 confirmed the total disappearance of the Chacaltaya glacier in Bolivia,

which was previously predicted to last until 2015 [2]. Melting of glaciers should be

expected in a warming world. In order to produce long-term statistics for analysis, the

WGMS monitors a set of 37 reference glaciers in ten mountain ranges continuously for the

period 1980-2009. Its statistics show that the 37 glaciers, on average, are losing more and

more weight during the 30-year period. When mountain glaciers melt, part of the water will

flow to the oceans.

Earlier in 2011, a NASA-funded satellite study [22] revealed that the Greenland

and Antarctic ice sheets are losing mass at an accelerating pace (Fig. 12), three times faster

than that of mountain glaciers and ice caps. The study compared two independent

measurement techniques to track the ice mass on Greenland and Antarctica, and found the

two techniques in good agreement, both for total amount and rate of mass loss, over the

period from 1992 to 2009. It also found that the Greenland and Antarctic ice sheets lost a

combined mass of 475 gigatonnes a year on average, which can raise the global sea level by

an average of 1.3 millimeters a year. Melting of mountain glacier and ice sheets on

Greenland and Antarctica, together with thermal expansion of water will cause the sea level

to rise. The IPCC AR4 estimated that global sea level rise relative to the 1980-1999 average

could reach 0.59 m in the late 21st century. Some recent research studies found that future

sea level rise may be higher than the IPCC AR4 projection [27, 29]. According to the

SREX [16], it is very likely that mean sea level rise will contribute to upward trends of

extreme coastal high water levels in future.

3 CURRENT SITUATION OF CO2 EMISSION AND CONCENTRATION

Greenhouse gases (GHG) trap the incoming solar radiation within the Earth’s

atmosphere, causing it to warm. The WMO, through its Global Atmosphere Watch

Programme, coordinates the observations of GHG in the atmosphere through a network of

stations located in more than 50 countries. According to the WMO's Greenhouse Gas

Bulletin released in late 2011 [35], the amount of GHG in the atmosphere had reached a new

high in 2010 since pre-industrial time. CO2 is the single most important man-made GHG in

the atmosphere and contributes about 64% to the total increase in climate forcing by GHG.

Since the start of the industrial era in 1750, its atmospheric abundance has increased by 39%

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to 389 parts per million (number of molecules of the gas per million molecules of dry air) in

2010. This is also believed to be the highest atmospheric CO2 concentration in the past

800,000 years as revealed by ice coring studies in Antarctica [26].

Increase in anthropogenic CO2 emission is primarily due to combustion of fossil

fuels, deforestation and changes in land-use. According to the preliminary estimates made

by the US Carbon Dioxide Information Analysis Center (CDIAC), 2010 saw a record high

global CO2 emissions from fossil fuel combustion and cement manufacture [4]. The 2010

figure exceeded the previous record made in 2008 (Fig. 13). The slight drop in 2009 is

believed to be due to the global financial and economic crisis that started in 2008 [10].

4 FUTURE CHALLENGES

Since the United Nations Framework Convention on Climate Change (UNFCCC)

entered into force in 1995, the Conference of the Parties (COP) to the UNFCCC has been

meeting annually to assess progress in dealing with climate change. The 17th

Conference of

the Parties (COP17) was held in Durban, South Africa late last year. The outcomes of

COP17 included a decision by the Parties to adopt a universal legal agreement on climate

change as soon as possible, no later than 2015, and to have it implemented from 2020. We

will have to wait a few more years, hopefully, before a global agreement on CO2 emission is

reached. Yet, even if the Parties would agree to implement a very stringent restriction on

CO2 emission or even to manage to halt CO2 emission, the CO2 already in the atmosphere

will continue to linger around for many decades to come.

4.1 Renewable energy

As combustion of fossil fuel is a major source of GHG emission, renewable energy

naturally becomes a potential substitute for fossil-fuel-based energy if reduction of GHG

emission is to be achieved. The IPCC Working Group III has recently released the Special

Report on Renewable Energy Sources and Climate Change Mitigation (SRREN) [15]. The

SRREN provides an assessment and thorough analysis of renewable energy technologies and

their current and potential roles in the mitigation of GHG emissions. Based on an extensive

assessment of scientific literature, the SRREN indicates that renewable energy has a large

potential to mitigate GHG emissions.

In countries like Germany, renewable energy deployment has already been put into

practice in daily lives. According to a recent media report [39], there are already eight

million people in Germany totally relying on renewable energy and a total of 74 "green"

cities and villages. These figures are expected to further increase. For example, China has

pledged to reduce CO2 emissions per unit of GDP by 40 to 45 percent by 2020 compared to

2005 levels and the increasing use of renewable energy. According to the United Nations

Environment Programme (UNEP) [31], China invested in 2010 US$48.9 billion (+28% from

2009) in renewable energy and has become the world leader. Similar increases in other

Asian developing countries were also reported (India: +25%; others excluding China and

India: +31%).

4.2 What can we do as an individual?

In Hong Kong, GHG emission per capita was 6 tonnes of CO2-e in 2008 [6],

against the world average of about 5 tonnes of CO2 per capita [30]. To combat climate

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change and global warming, we believe every individual has his/her share of responsibility to

reduce the carbon footprint of Hong Kong. The Observatory will continue its research on

climate change and will engage the community, stakeholders and other users of climate

services to provide the best available and reliable scientific information to support their

decision-making in mitigating the effects of climate change. We will also sustain our active

public education activities to promote the public's awareness on global and local climate

change. We have produced an educational package on climate change which can be used in

classroom teaching. We have organized a speaker team on climate change to conduct talks

for schools and organizations. The climate change FAQs on our website

(http://www.hko.gov.hk/climate_change/climate_change_e.htm) are regularly updated with

new entries and there is a collection of blog articles on climate change authored by

Observatory’s colleagues. Just recently, we have released an online climate quiz which

helps promote climate science. Participants will learn more about climate and climate

change while having fun. All in all, everyone will have a part to play and to work together

in combating climate change and in pursuing greener living. The Observatory will continue

to be a proactive, supportive and credible partner to work with everybody to make a

difference.

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Fig. 1 Annual temperature (℃) of Hong Kong in La

Niña years

Fig. 2 Annual global temperature anomaly, La Niña in

blue, others in red. (source: WMO)

Fig. 3 Annual land-surface average temperature

(source: Berkeley Earth Surface Temperature)

Fig. 4 Temperature data of 1979-2010 with influences

from ENSO, volcanoes and solar variation removed

(source: Foster and Rahmstorf [7])

Fig. 5 Average monthly Arctic sea ice extent of September 1979-2011

(source: National Snow and Ice Data Center, University of Colorado)

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Fig. 6a Perennial Arctic sea ice in 1980 (source:

NASA)

Fig. 6b Perennial Arctic sea ice in 2012 (source:

NASA)

Fig. 7 Estimated return periods (years) and their 5% and 95% uncertainty limits for 1960s 20-year return values

of annual extreme daily temperatures in the 1990s climate. ANT refers to model simulated responses with only

anthropogenic forcing and ALL is both natural and anthropogenic forcing. Error bars are for annual minimum

daily minimum temperature (red: TNn), annual minimum daily maximum temperature (green: TXn), annual

maximum daily minimum temperature (blue: TNx), and annual maximum daily maximum temperature (pink:

TXx), respectively. Grey areas have insufficient data. Source: Zwiers et al. [38].

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Fig. 8 Frequency of occurrence of local temperature anomalies in units of the local standard deviation.

Anomalies are relative to the 1951-1980 base period. (Source: Hansen et al. [11])

Fig. 9 Projected annual and seasonal changes in three indices for daily Tmax for 2081-2100 with respect to

1980-1999, based on 14 GCMs contributing to the CMIP3. Left column: fraction of warm days (days in which

Tmax exceeds the 90th percentile of that day of the year, calculated from the 1961-1990 reference period);

middle column: fraction of cold days (days in which Tmax is lower than the 10th percentile of that day of the

year, calculated from the 1961-1990 reference period); right column: percentage of days with Tmax >30°C. The

changes are computed for the annual time scale (top row) and two seasons (December-January-February, DJF,

middle row, and June-July-August, JJA, bottom row) as the fractions/percentages in the 2081-2100 period

(based on simulations for emission scenario SRES A2) minus the fractions/percentages of the 1980-1999 period

(from corresponding simulations for the 20th century). Warm day and cold day changes are expressed in units of

standard deviations, derived from detrended per year annual or seasonal estimates, respectively, from the three

20-year periods 1980-1999, 2046-2065, and 2081-2100 pooled together. Tmax >30°C changes are given directly

as differences in percentage points. Color shading is only applied for areas where at least 66% (i.e., 10 out of 14)

of the GCMs agree on the sign of the change; stippling is applied for regions where at least 90% (i.e.,13 out of

14) of the GCMs agree on the sign of the change. Adapted from Orlowsky and Seneviratne [25].

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Fig. 10 Projected return period (in years) of late 20th-century 20-year return values of the annual maximum of

the daily maximum temperature. The bar plots show results for regionally averaged projections for two time

horizons, 2046-2065 and 2081-2100, as compared to the late 20th century (1981-2000), and for three different

SRES emission scenarios (B1, A1B, A2). Results are based on 12 GCMs contributing to the CMIP3. The

‘Globe’ analysis (inset box) displays the projected return period (in years) of late 20th-century 20-year return

values of the annual maximum of the daily maximum temperature computed using all land grid points. Adapted

from the analysis of Kharin et al. [17].

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Fig. 11 Projected return period (in years) of late 20

th century 20-year return values of annual maximum

24-hour precipitation rates. The bar plots show results for regionally averaged projections for two time horizons,

2046-2065 and 2081-2100, as compared to the late 20th

century (1981-2000), and for three different SRES

emission scenarios (B1, A1B, A2). Results are based on 14 GCMs contributing to the CMIP3. The ‘Globe’

analysis (inset box) displays the projected return period (in years) of late 20th

-century 20-year return values of

annual maximum 24-hour precipitation rates computed using all land grid points. Adapted from the analysis of

Kharin et al. [17].

Fig. 12 Total ice sheet mass balance between

1992 and 2009, as measured for the cumulative sum of

Greenland and Antarctica, in gigatonnes per year.

(source: NASA/JPL-UC Irvine-Utrecht

University-National Center for Atmospheric Research)

Fig. 13 Estimates of global carbon emission from

fossil-fuel combustion and cement manufacture

(source: CDIAC)

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