Does COVID-19 pandemic help?
It is well-understood that climate change is caused by release of greenhouse gases into the atmosphere, a phenomenon also known as carbon emission. It is estimated that owing to COVID-19, the global carbon emissions in 2020 may drop by 4-7 percent. While the unprecedented COVID-19 pandemic has not come to an end, and the world is seeing major downturn of economy, does the COVID-19 pandemic help to alleviate the negative impacts of climate change? According to the World Meteorological Organisation (WMO), such a level of reduction of emissions does not mean that carbon dioxide concentration in the atmosphere will go down1. On the contrary, the carbon dioxide will continue to accumulate and the level will just increase a little less than without this reduction. As a matter of fact, the monthly mean carbon dioxide concentration measured at Mauna Loa in Hawaii in May 2020 was the highest in record for the month of May2. Accompanying this, the global mean temperature in May 2020 was also the highest in record for the month of May. As such, the Secretary-General of WMO said that the economic and industrial downturn as a result of the Coronavirus pandemic was not a substitute for concerted and coordinated climate action.
What causes climate change?
Accumulation of greenhouse gases in the atmosphere is the cause of climate change. Greenhouse gases act to trap infrared radiation coming from the Earth, thereby keeping the Earth “warm”. In fact, without greenhouse gases, the Earth would have had an average temperature of -18°C instead of the present 15°C.
So, what is the problem with greenhouse gases? Actually, it is the ever-increasing release of greenhouse gases into the atmosphere that causes the problem. If we looked at the historical CO2 concentration in the atmosphere, the CO2 concentration varied between 150 ppm and 300 ppm in the past 800,000 years until in the past decade or so when the CO2 concentration abruptly shot up and reached around 410 ppm. This sudden rise in CO2 concentration after the industrialisation era is caused by human activities such as burning of fossil fuels.
According to Intergovernmental Panel on Climate Change (IPCC), it is “95 percent certain that humans are the main cause of current global warming”3. There is scientific evidence to support this claim. Scientists performed computer climate models to simulate the variation of global mean surface temperature in the past based on two different scenarios. One scenario considered only natural factors such as variation of Earth’s orbit, past solar activity and volcanic activity. The other scenario considered not only natural factors, but also human factors such as changes in greenhouse gas concentration, land use and man-made pollution. The result was that the simulation based on natural factors alone was not able to reproduce the observed global warming. The simulation based on both natural and human factors matched well with actual observations. This suggested that human factors contributed to global warming.
What are the impacts of climate change?
As mentioned above, accumulation of more greenhouse gases in the atmosphere helps trap more infrared radiation from the Earth, thus leading to general increase in the global mean temperature. As a matter of fact, the annual global mean surface temperatures in the past 6 years, namely from 2014 to 2019, are the top 6 highest annual global mean surface temperatures on record. It speaks for itself that the Earth is warming up.
(Source: UK MetOffice)
With increase in air temperature, the atmosphere can hold more moisture according to physical law. More moisture in the atmosphere means heavier downpour when it rains. Global warming causes the ocean to warm up as well, thereby expanding its volume. Global warming also leads to melting of glaciers and ice caps, particularly the Greenland and Antarctica ice sheets. These factors together cause a rise in global mean sea level.
Studies reviewed that the number of tropical cyclones in the Western North Pacific will not increase as a result of global warming. Despite this, tropical cyclones can develop more intensely under global warming, and therefore potentially bringing greater risks from their high winds. This, coupled with the trend of rising global mean sea level, will increase the threat of storm surge from tropical cyclone to coastal cities like Hong Kong.
Storm surge is a rise in sea level due to the combined effects of high winds and low atmospheric pressure associated with tropical cyclones. High winds from a tropical cyclone help push sea water towards the coast, thus piling up water against the coast. On the other hand, the rather low pressure near the centre of a tropical cyclone has an effect of sucking up sea level. These two effects in tandem cause momentary rise in sea level, which is storm surge. This rise is on top of the normal astronomical tide and the effect of climate change. People in Hong Kong may recall the occasion of Super Typhoon Mangkhut in 2018, the maximum storm surge in the harbour then was 2.35 metres, the highest so far on record in Hong Kong. Many low-lying coastal areas in the territory suffered from inundation.
In case the time of the maximum storm surge coincides with the time of the highest astronomical tide, the sea level will rise higher, bringing inundation to a greater area near the coast. With the effect of global warming, the mean sea level will continue to rise in future in background. We may expect more frequent and widespread inundation caused by storm surge in future.
Impacts of Climate Change to Hong Kong - Temperature
Under the combined effect of climate change and urbanisation, the annual mean temperature in Hong Kong exhibited an increasing trend, just like other cities around the world. There was an average rise of 0.13°C per decade from 1885 to 2019. The rate of increase became faster in the latter half of the 20th century at an average increasing rate of 0.21°C per decade during 1990-2019.
In the scenario where the current high rate of greenhouse gas emission continues in future, Hong Kong will expect much hotter conditions in the coming century. Let’s first take a look at “hot night”. It refers to a day with a minimum temperature of 28°C or above. The average annual number of hot nights in Hong Kong in 1986-2005 was 18. This number is projected to soar to 149 nights by the end of the 21st century, representing a 7-fold increase! Now look at “very hot day”. This is a day with a maximum temperature of 33°C or above. The average annual number of very hot days in 1986-2005 was 9. This number is forecast to shoot up to 112 by the end of the 21st century, representing a 11-fold increase! Then take a look at “cold day”. It is a day with a minimum temperature of 12°C or below. The average annual number of cold days in 1986-2005 was 15. This will plummet to a bare 1 day by the end of the 21st century. If we continue the current high rate of greenhouse gas releases to the atmosphere, we will face frequent unbearably very hot weather in future.
Impacts of Climate Change to Hong Kong - Rainfall
Global warming favours heavier rain. In the high rate of greenhouse gas emission scenario, the number of extremely wet years (with annual rainfall exceeding 3,168 mm) in Hong Kong will increase from 3 in 1885-2005 to about 12 in 2006-2100. Meanwhile, the number of extremely dry years (with annual rainfall less than 1,289 mm) in Hong Kong would remain about the same at 2. Thus, we would witness a shift towards wetter years in future. More heavy rain will increase the risk of rain-related hazards such as thunderstorm, landslip and flooding. That means, global warming may trigger more multi-hazard situations. That is really worrying.
Impacts of Climate Change to Hong Kong – Sea Level
As a coastal city, Hong Kong is prone to the negative impact of storm surge from tropical cyclone. In past history of Hong Kong, the highest casualty from tropical cyclone was due to storm surge. The typhoons in 1906 and 1937 both brought significant storm surge to the territory, resulting in more than 10,000 deaths in each case!
Based on the tide data collected in the Victoria Harbour from 1954 to 2019, the mean sea level in the harbour rose at a rate of 31 mm per decade. In the scenario of high rate of emission of greenhouse gases, the mean sea level will increase by about 0.98 m above the average of 1986-2005 by the end of the 21st century. Such a rise in sea level will increase the risk of inundation to the low-lying coastal areas in Hong Kong due to approaching tropical cyclones in future.
Join Hands to Combat Climate Change
The 13th goal of United Nation’s Sustainable Development Goal4 is to “take urgent action to combat climate change and its impacts”. Even though greenhouse gas emissions are forecast to drop in 2020 owing to travel bans and economic downturn associated with the COVID-19 pandemic, this reduction in emission is only temporary. Climate change is not on pause. Adopted in 2015, the Paris Agreement called for joint efforts to keep a global temperature rise this century well below 2°C above pre-industrial levels, and better keep it within 1.5°C. We are now more than 1.1°C above pre-industrial level. More efforts need to be devoted to achieve this goal. So, let’s join hands together to reduce carbon emission and to cut down consumption of electricity and water. Every bit counts!
- IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.
Author: Dr Cho Ming CHENG
Director of the Hong Kong Observatory
Dr Cheng graduated with a BSc degree in Physics in 1985 from the University of Hong Kong and subsequently received a PhD degree in Physics in 1989. He joined the Hong Kong Observatory in 1989 as a Scientific Officer. Since the 1990s, Dr Cheng has been involved in weather forecasting, TV weather reporting, hydrometeorology, weather radar and satellite meteorology, terminal Doppler weather radar, LIDAR, and aerodrome meteorological observing system, etc. In 2011, he was promoted to the post of Assistant Director of the Hong Kong Observatory, overseeing the weather forecasting and warning branch which was responsible for development in public weather services, Hong Kong Observatory website, “MyObservatory”mobile app, research in forecasting techniques, etc. In 2020, Dr Cheng was promoted as the Director of the Hong Kong Observatory.