The latest report of IPCC Working Group I, Climate Change 2021: The Physical Science Basis, has clearly mentioned that climate change is already affecting every region on Earth in multiple ways. Climate impacts are already more widespread and severe than expected. The observed mean surface temperature increase has clearly emerged out of the range of internal variability compared to 1850-1900.
Impact of global warming on heatwaves
Heat extremes have increased while cold extremes have decreased, and these trends will continue over the coming decades. It is indisputable that human activities are causing climate change, making extreme climate events, including heatwaves, heavy rainfall, and droughts, more frequent and severe.
It is now univocal that Earth’s climate is changing; global temperatures have already risen by about 1.1 °C from pre-industrial times, and it warns that the 1.5 °C threshold is likely to be reached by 2040. This report projects with medium confidence that heatwaves and humid heat stress will be more intense and frequent during the 21st century.
The total number of days per year with a maximum temperature exceeding 35ºC will increase with the increase of global warming. The latest IPCC Working Group II report mentioned that risks would escalate quickly with higher temperatures, often causing irreversible impacts of climate change. We have already seen that millions of people in many parts of the South Asian region are experiencing a brutal heatwave that is threatening lives and livelihoods, and there is no relief in sight.
Possible causes of the heatwave in India and flash floods in Bangladesh
The Indian Meteorological Department (IMD) reported that the month of March this year was the hottest in 122 years since they started maintaining records. Bangladesh Meteorological Department (BMD) mentioned that very little rainfall has been observed in March this year compared to the long-term average. This year, western disturbances were weaker, which would cause little pre-monsoon rainfall in north-western and central India. On the other hand, anticyclones with an area of high atmospheric pressure led to hot, dry weather over parts of western India in March, according to the media reports.
Interestingly, anticyclones formed over the Bay of Bengal push moisture-laden air towards the northeast direction of Bangladesh and cause heavy precipitation over the Meghalaya and Tripura hills. According to the Flood Forecasting and Warning Center (FFWC) of the Bangladesh Water Development Board (BWDB), more than 1300 mm of precipitation was recorded during the first two weeks at the Cherrapunji station in Meghalaya, India.
Due to this intense heavy rainfall, we have observed flash floods in northeast Bangladesh which are located in the foothills of Meghalaya. The standing Boro paddy crops were damaged in Sunamganj, Sylhet, and Netrokona districts of Bangladesh due to unusual early flash floods in the first week of April.
Some scientists mentioned that the pressure pattern associated with La Niña conditions, which are currently presiding over the Pacific, has persisted longer than expected. This, along with warm waves coming from the Arctic, has caused heat waves to form. Such prolonged La Niña conditions and their impact on dryness and heatwave during Spring and Summer seasons in Bangladesh and India are unexpected.
Cities and climate change
Urban areas are home to more than 50% of the world’s population and are the site of most of its built assets and economic activity. By 2050, the population in urban areas is expected to increase by 2.5 to 3 billion and comprise two-thirds of the world population.
For the next three decades, nearly 70 million residents will move to urban areas every year. The majority of these new residents will live in small- to medium-sized cities in the developing world. Urban centres and cities are warmer than the surrounding rural areas due to what is known as the urban heat island effect. This urban heat island effect results from several factors, including reduced ventilation and heat-trapping due to the close proximity of tall buildings, heat generated directly from human activities, the heat-absorbing properties of concrete and other urban building materials, and the limited amount of vegetation.
The difference in observed warming trends between cities and their surroundings can partly be attributed to urbanization.
Despite having a negligible impact on global annual mean surface-air warming, urbanization has exacerbated the effects of global warming in cities. Urbanization has exacerbated changes in temperature extremes in cities, in particular for nighttime extremes.
Three main factors contribute to amplifying the warming of urban areas:
1) Urban geometry: Tall buildings close to each other absorb and store heat and also reduce natural ventilation
2) Human activities: Due to heat released from domestic and industrial heating or cooling systems, running engines, and other sources
3) The materials that make up cities: These materials are very good at absorbing and retaining heat and then re-emitting that heat at night
The urban heat island effect is further amplified in cities that lack vegetation and water bodies. Urbanization alters the water cycle, generating increased precipitation over and downwind of cities and increasing surface runoff intensity. Urbanization can also induce phenomena such as the urban dryness island, referring to conditions where lower humidity values are observed in cities relative to more rural locations and to slower wind speed compared to adjacent suburbs and countryside.
Both sea levels and air temperatures are projected to rise in most coastal settlements. The combination of extreme sea level, increased by both sea-level rise and storm surge, and extreme rainfall or river flow events will increase the probability of flooding. An increase in pluvial flood potential in urban areas where extreme precipitation is projected to increase, especially at high global warming levels.
Future urbanization will amplify the projected air temperature change in cities regardless of the characteristics of the background climate, resulting in a warming signal on minimum temperatures that could be as large as the global warming signal. Compared to the present day, large implications are expected from the combination of future urban development and more frequent occurrence of extreme climate events, such as heatwaves, with more hot days and warm nights adding to heat stress in cities.
Climate change adaptation for sustainable cities
Impact assessments and adaptation plans in cities require high-spatial-resolution climate projections along with models that represent urban processes, ensemble dynamical and statistical downscaling, and local impact. Some cities are within regions that have already exceeded 1.5°C and have been forced to adapt, affording them experiences that can be shared with, adapted for, or replicated in other cities.
But city action alone will not be sufficient. Limiting warming to 1.5°C will require immediate action within and across sectors, as well as multilevel governance. It will require rapid and far-reaching systems transitions in energy and industry, land use and ecosystems, and urban and infrastructure linked to the implementation of the Sustainable Development Goals. Cities offer many of the most readily available, feasible, and cost-effective options for these transitions.
Dr AKM Saiful Islam is Professor and Director of the Institute of Water and Flood Management (IWFM) at Bangladesh University of Engineering and Technology (BUET), Dhaka. Email: [email protected].
