In South Asia, heatwaves and humid heat stress will be more intense and frequent during the 21st century, but there is room for cautious optimism and a need for urgent actions
Intergovernmental Panel on Climate Change (IPCC) has recently published the Sixth Assessment Report (AR6) of the Working Group I, which has focused on the physical science basis of climate change. This report has clearly mentioned that recent changes in the climate are widespread, rapid, and intensifying, and unprecedented in thousands of years.
It concludes unequivocally that human influence has warmed the climate at a rate that is unprecedented in at least the last 2,000 years. The scale of recent changes across climate systems is unprecedented over many centuries to many thousands of years. For example, carbon dioxide (CO2) concentration is highest in at least two million years; sea level rise is fastest in at least 3,000 years; arctic sea ice area is lowest in at least 1,000 years; glacier retreat is unprecedented in at least 2,000 years.
It is unquestionable that human activities are causing climate change, making extreme climate events, including heat stress (which includes heatwaves), heavy rainfall, and droughts more frequent and severe. In recent decades, many changes in the climate system became larger in direct relation to increasing global warming.
Extreme heatwave became more frequent and more intense, heavy rainfall became more frequent and more intense, droughts have increased in some areas, fire weather (condition of dry, hot and wind) became more frequent. At the same time, the oceans warmed, acidifying and losing oxygen.
Changes we see in the frozen areas of the planet, such as global retreat of glaciers since the 1990s, 40% decrease in Arctic sea ice since 1979 and decrease in spring snow cover since the 1950s. Changes in climate are intensifying and already affecting every region on Earth in multiple ways, and these changes will increase with further warming.
Changes to the water cycle and monsoons will be observed as with the warmer temperature atmosphere can hold more water and more and faster evaporation. This will cause heavier precipitation, floods in some areas, while intensifying dry seasons and droughts in some other areas.
This assessment report now more confidently states that observed warming is driven by emissions from human activities, with greenhouse gas warming, partly concealed by aerosol cooling. Every ton of CO2 emissions adds to global warming which will impact the climate.
Under all emissions scenarios, global surface temperature will continue to increase until at least the mid-century. Global warming of 1.5°C will exceed in the coming two decades unless there is a deep reduction in carbon dioxide (CO2) and other greenhouse gas emissions.
Ocean and land currently sink about 56% of CO2 emission which are projected to be less effective under higher emission scenarios. Changes in several climatic impact-drivers (such as trend, event or extreme) would be more widespread at 2°C compared to 1.5°C global warmings and even more widespread and/or pronounced for higher warming levels.
Human activities affect all the major climate system components, with some responding over decades and others over centuries. Many such changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, particularly changes in the ocean, ice sheets and global sea level.
Occurrences of low-likelihood events but high outcomes, such as ice sheet collapse, abrupt ocean circulation changes cannot be ruled out due to global warming. Some compound extreme events such as flood events caused by a combination of sea level rise and extreme rainfall events will be prevalent as the climate warms.
In South Asia, it is clearly evident the mean surface temperature has increased since the pre-industrial period, and this trend will continue over the coming decades. In South Asia, heatwaves and humid heat stress will be more intense and frequent during the 21st century.
Marine heatwaves threaten global biodiversity and ecosystem have become more frequent and will continue to increase over the 21st century. Although South Asian monsoon has weakened in the second half of the 20th century due to anthropogenic aerosol forcing, in the long-term, summer monsoon precipitation will increase.
The average and heavy precipitation will increase over much of Asia, which would lead to more flooding. In the mountainous areas, glaciers and seasonal snow duration are declining, and this trend will continue in the future.
Relative sea level around Asia has increased faster than the global average, with coastal area loss and shoreline retreat. In the open ocean, acidification, changes in sea ice, and deoxygenation are detectable in many areas.
The surface of the Indian Ocean has warmed faster than the global average. Relative sea level around South Asia will continue to rise, which will increase coastal inundation, flooding, erosion and salinity in the coastal regions. The proportion of intense tropical cyclones, peak wind speeds and precipitation will increase with increasing global warming.
Restricting human-induced global warming below a specific level needs limiting cumulative CO2 emissions so that net zero greenhouse gas emissions can be reached by the middle of the century. To limit global warming, strong, rapid, and sustained reductions in CO2, methane, and other greenhouse gases are necessary. This would reduce not only the consequences of climate change but also improve air quality. Climate we experience in the future depends on our decisions now!
AKM Saiful Islam is Professor, Institute of Water and Flood Management (IWFM), Bangladesh University of Engineering and Technology (BUET). Contact: [email protected]