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Thursday, 23rd October 2014

Record High Carbon Dioxide Concentration

In September 2014, the World Meteorological Organization (WMO) released the annual Greenhouse Gas Bulletin[1], announcing that the atmospheric concentration of carbon dioxide (CO2) in 2013 had set a new record of 396 ppm (Figure 1), 42% higher than the pre-industrial level. The annual increase in 2013 was 2.9 ppm, the largest year-to-year change since 1984. At the current rate of increase, the global average CO2 concentration is expected to exceed 400 ppm in the next couple of years.

Figure 1.

Figure 1.      Global average atmospheric CO2 concentration (source: WMO)

Analyses of Antarctic ice cores have shown that Antarctic temperature (Note 1) and atmospheric CO2 concentration in the past 800,000 years go hand-in-hand (Figure 2). Variations of CO2 concentration and Earth's temperature are controlled by natural factors including Earth's orbital changes, volcanic eruptions and changes in solar activity. However, such natural variations have apparently been overwhelmed by the human-caused emission of greenhouse gases in recent centuries. As a result, the current CO2 concentration has risen to a level unprecedented in the past 800,000 years.

Figure 2.

Figure 2.      Variations of Antarctic temperature and atmospheric CO2 concentration estimated
from Antarctic ice cores (source: US National Academy of Sciences)

To estimate possible temperature changes in the future, climate scientists have designed a number of greenhouse gas concentration scenarios (Figure 3) and computed the projected global temperature in the 21st century under these scenarios. It is most alarming that the current CO2 emission has a strong tendency to follow the trajectory of the worst scenario of high greenhouse gas concentration, which will lead to a global average temperature rise of 4oC (compared to the pre-industrial level) by the end of this century. Scientists have warned that a world warmer by 4oC would be marked by intense heat waves, extreme precipitation, and damaging sea level rise.

Figure 3.

Figure 3.      CO2 emissions associated with different greenhouse gas concentration scenarios
shown in coloured lines. Historical observations are shown in black.
(source: Global Carbon Project)

If we still want to meet the "2oC target" (Note 2), immediate, substantial and sustained reduction of CO2 emissions is required. We cannot afford to procrastinate as the climate system also needs a finite amount of time to re-adjust to the actions we take for a better outcome. "We have the knowledge and we have the tools for action to try to keep temperature increases within 2oC to give our planet a chance and to give our children and grandchildren a future," said Michel Jarraud, WMO Secretary-General. That future is now in our hands.

S M Lee and H W Tong

1. Climate change FAQ: How do scientists reconstruct the temperature tens of thousands of years ago?

2. At its meeting held in Luxembourg in 1996, European Union indicated that the global mean surface temperature increase should not exceed 2oC above the pre-industrial level to avoid the risk of severe climate change impacts on human and ecological systems. See "Climate change FAQ: What is the "2oC Target"?"

[1] World Meteorological Organization Press Release No. 1002


Wednesday, 3rd September 2014

What You Need to Know about Thunderstorm Warning

"The Observatory issued thunderstorm warning at one o'clock this afternoon......". During summer, you may often come across such warning messages on radio, the Observatory's website or mobile application "MyObservatory". However, on some occasions, you were out there in the sun without any sign of thunder or raindrop. You might ask doubtfully, "Where have all the thunderstorms gone?"

As a matter of fact, it is common to see sunshine and shower concurrently in Hong Kong, especially during the heat of the summer. In inland areas that heat up more quickly in the day, showers and thunderstorms are easily triggered due to high temperature. Sometimes, this kind of thunderstorms is limited in size, and thus weather at a distance may remain fine without showers or thunderstorms (Figure 1). When the Observatory issues the thunderstorm warning, it aims at alerting the public of the threat posed by thunderstorms within the territory. Where possible, the Observatory provides additional information on the specific areas affected by thunderstorms in the warning bulletin and the weather report (Figure 2). Under the situation as illustrated in Figure 1, people in the urban areas are not aware of the thunderstorms that are witnessed by people in Sai Kung. On some occasions, thunderstorms are affecting only part of the territory but they are forecast to evolve quickly so that there is a chance of them spreading across the territory or affecting more places in Hong Kong. For safety reason, the Observatory will not specify the areas affected by thunderstorms in the warning. This eliminates the need to update the warning frequently, which may cause confusion to the public.

Figure 1

Figure 1      Radar images showing the rapid growth and decay of thunderstorms. Lightning locations
are denoted by white squares.

Figure 2

Figure 2      Members of the public can check the thunderstorm warning and the affected
areas using "MyObservatory".

How should the public respond when thunderstorm warning is in force? If you are located within the affected area as mentioned in the warning, please take appropriate precautions such as staying indoors or stop engaging in water sports. For details, please visit the Observatory's webpage on thunderstorm warning: Whenever you want to know if there are any thunderstorms close by that may affect your outdoor activities, please take a look at the lightning location map in the Observatory's website ( or use the mobile application "MyObservatory" to check the location of lightning associated with thunderstorms. By keeping an eye on the news of thunderstorms and making good use of the Observatory's online information, members of the public can better plan and engage in outdoor activities.

The growth and decay of thunderstorms can be very rapid and sporadic. It is still a challenge to forecast accurately their variation several hours ahead owing to the limitation of the existing science and technology. Therefore, the validity period of Observatory's thunderstorm warning will not be too long to minimize false alarm and reduce disruption to the public. The Observatory will keep close watch on the evolution of thunderstorms and extend the warning period if needed, until the thunderstorms completely dissipate to ensure public safety.

YC Kong


Friday, 20th June 2014

Anticipated Return of El Niño*

Four years after the previous El Niño event, significant warming signs re-emerged over the central and eastern equatorial Pacific in the past couple of months. The sea surface temperature in May 2014 has already exceeded the normal range over those regions, suggesting that another El Niño is brewing (by definition, an El Niño occurrence is confirmed on the basis of anomalously warm central and eastern equatorial Pacific persisting for five to six months). As the warming trend is expected to continue in the next few months as predicted by many climate models around the world, it is therefore likely that an El Niño event will become established later this year.

Incredibly, an anomaly in the ocean so far away can alter the atmospheric circulation worldwide through what is known as "teleconnection" mechanism. This in turn will affect the weather and seasonal climate in many parts of the world, including Hong Kong to a certain extent.

Will El Niño bring more rainfall?

People tend to associate El Niño with heavy summer (June-August) rainfall because a strong El Niño in 1997 coincided with the wettest summer (2358.6 mm) and the wettest year (3343.0 mm) on record in Hong Kong. However, the summer of 1983, another intense El Niño year, had below-normal seasonal rainfall of just over 900 mm. Looking at the data over a long period of time, the general relationship between El Niño and Hong Kong's summer rainfall remains inconclusive. An analysis of El Niño summer rainfall records during 1950-2013 shows that the percentage occurrence of a drier summer is even higher than that of a wetter summer (Figure 1), although the difference may not be considered statistically significant taking into account the limited records available.

On the other hand, El Niño's impact on winter (December-February) and spring (March-May) rainfall is more pronounced. Figure 2 shows the distribution of Hong Kong's winter rainfall under El Niño and normal conditions (i.e. neither El Niño nor La Niña, the opposite phase of El Niño) during 1950-2013. It can be seen that the odds of a wetter (drier) El Niño winter will noticeably increase (decrease) compared to a normal winter.

Will El Niño affect tropical cyclones?

Since 1961, there has not been one single tropical cyclone coming within 500 km of Hong Kong in April and May when El Niño is in place (see Figure 3), whereas in a La Niña or normal year, the tropical cyclone season in Hong Kong can begin as early as April, e.g. 1978. That is because tropical cyclones in El Niño years tend to form further east near the central North Pacific and hence are less likely to journey as far as the South China Sea in the early season. As the season progresses, other factors come into play and El Niño's impact on tropical cyclone behaviour then becomes less well-defined.

For more information on El Niño and La Niña, please visit the Observatory webpage:

S M Lee

* The warming of surface waters over the central and eastern equatorial Pacific Ocean usually peaks around Christmas, hence the name "El Niño" (Spanish for "the little boy" or "the Christ Child") coined for the phenomenon.

Figure 1

Figure 1      Distribution of Hong Kong's summer rainfall under El Niño and normal conditions during 1950-2013.
Normal condition refers to the situation of neither El Niño nor La Niña in place
during the whole season.

Figure 2

Figure 2      Distribution of Hong Kong's winter rainfall under El Niño and normal conditions during 1950-2013.
Normal condition refers to the situation of neither El Niño nor La Niña in place
during the whole season.

Figure 3

Figure 3      Average monthly number of tropical cyclones coming within 500 km of Hong Kong
under El Niño and normal conditions during 1961-2013. Normal condition refers
to the situation of neither El Niño nor La Niña in place during the month.


Last revision date: <23 Oct 2014>