The year 2019 started with extreme temperatures in the two Hemispheres. In the Southern Hemisphere, Australia experienced an extraordinary heatwave and the hottest January on record with temperatures exceeding 49oC in parts of South Australia. In the Northern Hemisphere, the "polar vortex" brought bitterly cold temperatures to the Midwest of the United States in January. According to media reports, temperatures in Chicago fell to -30oC, even colder than parts of Antarctica. One could anticipate that climate change deniers will take this opportunity to cast doubt on the human-caused global warming whenever there are extreme cold weather events somewhere in the world (CMS: Is Global Warming Really at Work?). The usual deniers' tactic is to conflate short-term weather fluctuations and long-term climate trends. However, short-term fluctuations in weather say nothing about long-term trends in climate (CMS: The ABC's of Climate). As rightly pointed out by climate experts in the United States, we need to look at the big picture instead of confining our view to a short time horizon and a small area on the Earth's surface. Globally speaking, there is a lopsided ratio of hot records to cold records at land-based stations in the last 12 months, roughly about 8.3 to 1 . In a longer term, almost every corner on Earth experienced a warming trend in the past 117 years (Figure 1).
What about the polar vortex? Is it undergoing any changes? As illustrated in Figure 2 for the case of the Northern Hemisphere, the polar vortex is the tight band of swirling winds in the upper atmosphere that confines cold air to the Arctic. If the polar vortex is weak, the westerly jet stream will be weakened and become wavier, favouring southward incursions of Arctic air mass, which will bring bitterly cold weather to the mid-latitudes (CMS: Polar Vortex). A recent study reveals that weak states of the polar vortex have become more persistent in the past four decades  against the backdrop of global warming. Scientists put the blame on the diminishing Arctic sea ice because the uncovered ocean releases more heat to the atmosphere aloft, thereby changing the atmospheric circulation. The diminishing Arctic sea ice also contributes to the amplified warming of the Arctic through the "climate-albedo" feedback mechanism: less sea ice -> darker ocean surface exposed -> more solar heat absorbed -> further sea ice loss. The Arctic is now warming at a rate twice the global average. Such differential warming causes the temperature contrast between the polar region and the equator to decrease, which in turn weakens the polar vortex and the westerly jet stream, making the westerly jet stream wavier and favouring outbreaks of frigid air from the north pole to the south (Blog: Warm Arctic, Cold Continents). A recent study also reveals that a warm Arctic means colder and snowier winters in Northeastern United States .
While more research is needed to further prove or disprove the aforementioned theory, there is no time for us to play a waiting game in the midst of climate change. The atmospheric concentration of carbon dioxide in 2017 reached a record high of 405.5 ppm and keeps on rising. Global carbon dioxide emissions in 2018 are set to hit an all-time high. If we want to achieve the goals of the Paris Agreement, significant reduction of carbon emissions is required. As pointed out by the UN Emissions Gap Report 2018, the mitigation ambitions agreed at the Paris climate summit need to be roughly tripled if we want to contain global temperature rise to within 2oC. The mitigation ambitions would need to increase fivefold if we want to limit global warming to within 1.5oC. It's high time to step up our efforts to combat climate change.
S M Lee, F C Sham
 Bitterly cold extremes on a warming planet: Putting the Midwest's late January record cold in perspective
 Winter cold extremes linked to high-altitude polar vortex weakening
 Warm Arctic means colder, snowier winters in Northeastern U.S.
Figure 1 Warming trend during 1901-2017
(Data source: National Aeronautics and Space Administration, U.S.)
Figure 2 A stable (left) and a weak (right) polar vortex
One should still recall the widespread damages in Hong Kong brought by Mangkhut on 16 September 2018. It was fortunate that there was no fatality in Hong Kong on that day. According to the Observatory's records, there was no fatality for the past three "Signal No. 10" storms including Mangkhut. However, a recent unfortunate incident happened on 31 October 2018 when Severe Tropical Storm Yutu was located about 600 km southeast of Hong Kong and the Standby Signal No. 1 was in force. A young man was tragically drowned in the rough seas while surfing in Shek O that afternoon. According to press reports, 29 people were killed or missing when tropical cyclone warning signals were in force since 2000. About 40% of these incidents were associated with water activities. Among them, eight were engaging in swimming, surfing or diving; three were swept away while fishing; another one, an off-duty fireman, tragically lost his life while attempting to save a swimmer in rough seas.
Actually, the threats of tropical cyclone are not limited to the associated inclement weather (e.g. high winds, squally heavy rain, storm surge, etc.) when it comes close. A tropical cyclone can also inflict casualties when it is located at hundreds of kilometres away from us. Huge waves are usually whipped up by high winds associated with tropical cyclones. These waves could travel much further away from the storm and they are known as swells. Affected by the outer subsiding air of a distant tropical cyclone, local weather may be deceptively fine with light winds. In such situations, the awareness of the public on the effect of the weather would usually be lowered. As swells travel at speeds much faster than the movement of tropical cyclone, when the storm is still at a distance, the severe swells generated by it could have already reached Hong Kong. Even worse is that the wave heights would increase suddenly when these swells enter shallow water of the shore and attack the shoreline with a surprise, posing hazards to people close to the shoreline or engaging in water sports activities. Therefore, when tropical cyclone warning is in force, we should be on the alert and pay attention to the announcement and swell information provided by the Hong Kong Observatory. Members of the public should also stay away from the shoreline and avoid engaging in water sports activities.
Figure 1 Cases of death or missing during tropical cyclone passages since 2000
Figure 2 Swells can pose hazards to people close to the shoreline or engaging in water sports activities
HUI Kin-chung and CHEUNG Sze-yuen
B. Y. Lee: Caught red-handed
CHAN Chik-cheung: Swells from distant typhoons
Hong Kong Observatory video: Tropical cyclone swells
Breaking news of the Arctic just cannot stop in recent years. For example, average Arctic land surface temperature in 2017 was the second highest on record. Arctic sea ice extents for January and February 2018 were both record lows for the respective month. Furthermore, the "last ice area" of northern Greenland was broken up for the first time this year.
Very old and thick sea ice exists along the shores of northern Greenland. Sea ice over the region normally persists in summer. Scientists believed that this region would be the "last ice area", i.e. the last area where sea ice could still be found in the Northern Hemisphere under global warming. However, the anomalous warmth in northern Greenland this February and August caused the "last ice area" to open up twice1. According to media reports, the opening up of the "last ice area" in February 2018 was the first time on record (Figure 1)2 while the second opening up in August was highly unusual. Some experts described the situation as "scary", which prompted scientists to re-examine which areas of Arctic are capable of withstanding the effects of climate change.
The size of reflective surface of sea ice is one of the critical factors that affect the Earth's temperature. Reduction of sea ice weakens the Earth's ability to reflect sunlight, enabling the Earth to absorb more heat which, in turn, promotes further melting of ice and snow in a vicious circle. Studies pointed out that accelerated warming in the Arctic might have profound impacts on mid-latitude weather, causing longer droughts or heat waves over the region3. On the other hand, the progressive loss of sea ice will very likely cause the global polar bear population to decline by more than 30% over the next 40 years (Figure 2). The Arctic sea ice extent usually reaches its annual minimum around mid-September. In the last 40 years, the monthly Arctic sea ice extent in September exhibits a significant decreasing trend (Figure 3). In addition, very old sea ice in the Arctic has nearly vanished (region in red in Figure 4). If the trend persists, the Arctic Ocean will be ice-free in summer by mid-21st century.
Sea ice plays an important role in maintaining Earth's temperature. Present-day global mean temperature was approximately 1oC above pre-industrial levels, leading to sea level rise, melting of sea ice and more extreme weather. If the sea ice continues to decrease and the climate continues to warm, the polar bear will not be the only victim.
F. C. Sham
Figure 1 Sea ice cover near northern Greenland opened up on 26 February 2018 (red ellipse).
(Photo courtesy: US National Snow and Ice Data Center, NSIDC)
Figure 2 All polar bears across the Arctic face shorter sea ice season.
(Photo courtesy: Mario Hoppmann)
Figure 3 Significant decrease in September Arctic sea ice extent
Figure 4 Very old sea ice in the Arctic has nearly vanished.
1. Unusually warm winter breaks up sea ice in the Arctic
2. Thick Arctic sea ice off Greenland breaks up for first time on record
3. The influence of Arctic amplification on mid-latitude summer circulation