Traditional Chinese Simplified Chinese

  • Where is the "Hiatus" in Global Warming?

  • Friday, 26th June 2015

In the past several years, climate deniers have been bleating about the slowdown of global warming since 1998, or the so-called "hiatus". Against the background of increasing atmospheric carbon dioxide concentration in recent decades, some deniers even claim that carbon dioxide does not cause global warming and hence there is no need to curb emissions. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change has explained that short-term trends, such as the trend over 1998-2012,  would be very sensitive to the start and end dates selected and would not in general reflect the long-term climate trend (e.g. the trend over 1951-2012).  Short-term temperature fluctuations of the globe can be strongly affected by natural variations of the climate system, such as the El Niño that we are experiencing now,  as well as by volcanic and solar activities.  In our earlier blog "Has global warming stopped?", we showed how climate deniers used cherry-picking tactics to magnify selected short-term trends in support of their claims while ignoring the big picture.

Estimating the Earth's temperature has always been a challenge to climate scientists because of differences in evolving observing practices, changes in instrumentation, methods and locations of measurement, as well as the uneven and insufficient coverage of temperature measurement, especially over the oceans. Some studies have suggested that natural variations, such as the increase in heat uptake by the oceans, has an important role to play in determining the global temperature in the past couple of decades. Recent developments however have enabled scientists to come up with a better estimate of global average temperature: over land, a much larger surface temperature dataset (viz. the International Surface Temperature Initiative databank [1]) has been released and the number of stations available for analysis has doubled; while over the oceans, surface temperature observations on buoys have increased and at the same time a new method has been developed to improve the correction of the systematic bias of ship data.

Figure 1 shows the old and new data analyses of the US National Oceanic and Atmospheric Administration (NOAA) [2] that depict a global warming trend since the 1950s and, based on the analyses, Figure 2 compares the old and new warming rates over different periods of time. While the long-term overall warming rates in NOAA's old and new analyses are rather similar (0.117 and 0.129 oC/decade respectively) during the period of 1951-2012, the warming rate for the period of 1998-2012 has more than doubled from 0.039 oC/decade in the old analysis to 0.086 oC/decade in the new analysis. Even more intriguing is the fact that if the latest data of 2013 and 2014 are included in the new analysis, the warming rate over 1998-2014 would become 0.106 oC/decade, bringing it even closer to the long-term overall warming rate. As such, the fragility of the so-called "hiatus" claim of significant warming slowdown is evident for all to see as it has obviously failed to withstand the test of time!

Figure 1

Figure 1      NOAA's old and new analyses of global temperature anomaly (data source: NOAA)

Figure 2

Figure 2      NOAA's analyses of global warming rate over different periods of time (data source: NOAA)

S M Lee


[1] Release of the International Surface Temperature Initiative's Global Land Surface Databank

[2] Science publishes new NOAA analysis: Data show no recent slowdown in global warming

  • How to interpret the tropical cyclone forecast track map?

  • Monday, 22nd June 2015

The Hong Kong Observatory (HKO) has recently extended its tropical cyclone track forecast from 3 days to 5 days. What kind of useful information can we draw from the enhanced forecast track map?

In fact, the benefits brought by the extended forecast tracks are pretty obvious. For example, in early May this year (2015), when Tropical Cyclone Noul developed over the western North Pacific and moved west towards Luzon, people started to pay attention to it and discussed whether it would directly affect Hong Kong. The 3-day forecast track issued by HKO on the morning of 7 May indicated that Noul would move westward steadily, with the tendency to enter the South China Sea (Figure 1). On the other hand, the then 5-day forecast track still under testing at HKO suggested that Noul would recurve and track towards Taiwan in the next 4 to 5 days (11 and 12 May), leaving little direct threat to the weather of Hong Kong. Actually, Noul started to recurve before reaching Luzon and then skirted across the seas east of Taiwan (Figure 2). This demonstrates that an extension of the range of forecast track could help people know earlier about the future possible movement of the tropical cyclones and respond accordingly. In the case of Noul, people staying in Hong Kong should not too worry about the situation, but people planning to go to Taiwan should make timely preparations.

Figure 1

Figure 1      The 3-day and 5-day forecast tracks of Tropical Cyclone Noul

Figure 2

Figure 2      Actual track of Noul

Are the extended forecast tracks reliable?

The ability of HKO issuing longer forecast tracks can be attributed to the considerable improvement in the track forecast accuracy achieved in recent years. According to the results of objective verification, the accuracy of the fifth-day forecast position of the tropical cyclones is now comparable to that of the third-day forecast first introduced in 2003. In fact, the position error for the 24 hour forecasts was still close to 200 km in the year of 2000 and before. Yet, this error has persistently fallen below 100 km from 2010 onwards. The skill of the track forecasts issued by HKO is indeed on par with that of the major forecast agencies in the region [1]-[3].

How about the yellow shaded area covering the forecast track?

Weather changes invariably. Just like other weather forecasts, errors are always inherent at some level in the tropical cyclone forecast tracks and perfect forecast could hardly be achieved. In general, the accuracy of the track forecasts declines (viz. the growth of the forecast error) as the forecast range increases. In other words, the confidence of forecasters in the forecast position for day 5 should be less than that for day 1. To help people appreciate the forecast uncertainties in the track forecasts, the Observatory has introduced the "Potential Track Area" (the yellow shaded area in Figure 1) to indicate the area within which the tropical cyclone will possibly be located. The "Potential Track Area" is constructed by joining five different circles centred at the 24, 48, 72, 96 and 120-hour forecast positions respectively. The sizes of the circles are defined from the error statistics in the past years such that the tropical cyclone is expected to fall within the respective circle at each forecast hour with a probability of about 70%. This means that in 10 times of similar track forecast, about seven times the tropical cyclone centre will be located within the indicated area. The presentation of track forecast uncertainties in the form of probability has now been widely adopted by the forecast agencies around the world.

Figure 3

Figure 3      The area within 500 km from Hong Kong

The "Potential Track Area" essentially reflects the current level of skills in tropical cyclone track forecasting. At present, the radius of the circle which corresponds to the fifth-day forecast amounts to 500 km. People should fully understand and take into account the inherent uncertainties when using the track forecasts. For example, if the Observatory predicts that a tropical cyclone will make landfall over Hong Kong five days from now, the tropical cyclone could still possibly end up making landfall over as far west as the City of Haikou on the Hainan Island, or over the City of Xiamen in the Fujian Province in the east. Furthermore, there exists some 30% chance that the tropical cyclone will be passing at a farther distance away from Hong Kong.

Information not provided by the forecast track map

While a tropical cyclone only appears as a symbol on the forecast track map, the associated inclement weather such as strong winds, heavy rain, rough seas, swells, or storm surge will definitely affect beyond a single point and reach as far as hundreds of kilometres away from the storm centre, and may even be reaching well beyond the "Potential Track Area". In addition to the movement track, the extent of area that will be impacted by the tropical cyclone also depends on other factors such as the size of the circulation, intensity, structure and propagation speed of the storm. Due to the same reason, it is not easy at all to deduce the local weather solely from the forecast track. People should constantly pay attention to the latest warnings or reports about the tropical cyclone issued by the Observatory.

Chan Sai-tick


[1] Activities of the RSMC Tokyo - Typhoon Center in 2014

[2] Member Report of China (2014)

[3] Annual Tropical Cyclone Report 2013, Joint Typhoon Warning Center

  • Seven Questions and Answers about Earthquakes

  • Tuesday, 5th May 2015

We are deeply saddened by the severe earthquake which struck Nepal on 25 April 2015, killing more than seven thousands of people hitherto (5 May 2015). Apart from levelling houses and destroying some renowned historical buildings, the earthquake even touched off a deadly avalanche in the Himalayas. It was the most powerful earthquake that hit Nepal since 1934 [1]. Continued multiple aftershocks then occurred, with two of them reaching a magnitude of above 6.0.

Figure 1

Figure 1      Mr Mok Hing-yim, Observatory's Senior Scientific Officer (Head of the Observatory's Geophysics,
Time and Marine Meteorological Services) giving an account of the earthquake in Nepal.

1. What was the cause of this earthquake in Nepal ?

Nepal is located along the boundary of the Indian-Australian tectonic plate and the Eurasian tectonic plate (Figure 2), which belongs to an active seismic belt. The earthquake this time was caused by a collision between the two aforementioned tectonic plates (Figure 3).

Figure 2

Figure 2      Diagram showing the global distribution of tectonic plates (the location of
Nepal is indicated by the red arrow).

Figure 3

Figure 3      Schematic diagram illustrating the collision between the Indian-Australian tectonic plate
and the Eurasian tectonic plate.

2. The US Geological Survey said the Nepal's earthquake was 7.9 magnitude while there were some reported a magnitude of 8.1. It was also noticed that the magnitude was amended at a later time of reporting. What was the reason behind it ?

As different seismological monitoring centres receive data from different seismograph stations, their computed magnitudes may have slight differences. Besides, for stronger earthquakes, data from a larger number of seismograph stations and also seismograph stations located further away from the epicentre are normally required to obtain a more accurate analysis. Therefore the magnitude of stronger earthquake is often subject to amendments upon the reception of data from seismograph stations further away from the epicentre and recalculation.

Figure 4

Figure 4      Global Seismographic Network (till February 2015) [2]. IRIS, IDA and USGS in the legend are the abbreviations of
Incorporated Institutions for Seismology Research, International Deployment of Accelerometers and US Geological
Survey respectively. The orange star over China denotes Hong Kong Po Shan Seismological Station.

3. What are the scales to quantify the intensity of an earthquake ?

Different countries or regions in the world adopt a certain earthquake intensity scale to quantify the tremor at different locations. In Hong Kong, the Modified Mercalli Scale (MMS) [3] is adopted. The MMS is scaled into 12 different levels, which are determined from the feel of tremor by human beings as well as the effects and severity of damages on building structures, etc. (Figure 5). The intensity of an earthquake at a certain location is related to both the magnitude of the earthquake and the distance of that location from the epicentre.

Figure 5

Figure 5      Modified Mercalli Scale (MMS).

4. How to monitor earthquakes and are they predictable ?

We set up seismograph station and utilise seismometers (Figure 6) to detect the seismic waves generated by earthquakes (Figure 7). Parameters such as origin time, epicenter, magnitude and depth of the earthquake can then be computed using the data collected from a network of seismograph stations. However, there is no reliable way yet to predict earthquakes even with the current state-of-the-art scientific knowledge and technology.

Figure 6

Figure 6      Broadband seismometer used for detecting global earthquakes.

Figure 7

Figure 7      Seismic waveforms recorded by seismograph stations for the Nepal M7.9 earthquake on 25 April 2015.

5. What are the threats associated with an earthquake and the precautionary measures ?

The main threats of earthquakes include the collapse of buildings, landslides, breakdown of electricity supplying systems, fire hazards caused by the leakage of gaseous fuels, etc. Strong submarine earthquakes may trigger tsunamis, posing severe threats to coastal residents. When earthquake occurs, the first thing is to keep calm and then seek refuge as appropriate. Safety rules can be found in the Hong Kong Observatory's webpage.

Figure 8

Figure 8      Safety rules during the earthquake. For details please refer to the Observatory's webpage.

6. What was the world's deadliest earthquake recorded in recent years ?

If the casualties brought by tsunami triggered by an earthquake were also taken into account, the disastrous magnitude 9.1 earthquake that occurred west of northern Sumatra over the Indian Ocean on 26 December 2004 and its associated tsunami resulted in a death toll of over 220 thousands, which was the world's deadliest earthquake in recent years [4].

Figure 9 (i)

Figure 9 (ii)

Figure 9      Aftermath of the tsunami in Thailand generated by the magnitude 9.1 earthquake in the Indian Ocean in 2004
(Photos: Courtesy of Dr Wong Wing-tak).

7. Is Hong Kong vulnerable to earthquake ?

Most earthquakes of the world occur along the boundaries of crustal tectonic plates. Hong Kong lies within the Eurasian Plate and is at a distance from the nearest plate boundary (Figure 10). It is not located at an active seismic belt. According to the assessment of the Geotechnical Engineering Office of the Civil Engineering and Development Department, the earthquake risk in Hong Kong is low to medium. Notwithstanding this, members of the public should know more about earthquakes and understand the safety rules during and after earthquakes, especially when they travel in areas where earthquakes frequently occur.

Figure 10

Figure 10      Position of Hong Kong within the global tectonic plates (indicated by the red arrow).

Further information

The Observatory always attaches great importance to the work on earthquake monitoring. Under the trend of globalisation, many people often travel to places outside Hong Kong and the messages of earthquakes occurring around the world are vital to the public. The Observatory endeavours to enhance the dissemination of information on earthquakes. The global Quick Earthquake Messages service was launched in 2011 ( and The information is also accessible on the mobile app "MyObservatory", the Observatory's webpage and Weibo, providing the public with timely information on global earthquakes.

Terence Kung


[1] Wikipedia

[2] IRIS webpage

[3] Hong Kong Observatory's webpage

[4] USGS webpage