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CHRISTCHURCH: More than 3.3 billion people live in the tropics, which is about 40% of the world’s population.
Despite some areas of wealth, such as Singapore, the tropics are also home to around 85% of the world’s poorest people and are therefore particularly susceptible to the impacts of climate change.
The tropics are expected to see rising temperatures and changes in rainfall, and the question is whether this could make this region uninhabitable. How would this happen?
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HEAT STRESS
Humans regulate their body temperature in warm conditions through sweat. Sweat evaporates and cools the skin.
But if the conditions are humid, sweating and evaporation are much less effective. Humans can survive and function at fairly high temperatures if humidity is low, but as humidity increases, our ability to function rapidly decreases.
This effect is measured by a heat stress index that shows the apparent temperature felt under different conditions of relative humidity.
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From a human health point of view, the wet bulb temperature is critical. This is the temperature that a thermometer covered with a damp cloth would measure and reflects the maximum amount of cooling that can be achieved through evaporation.
High wet bulb temperatures are more of a problem for human health than high absolute temperatures. Wet bulb temperatures above 35 degrees Celsius are life-threatening because they cause hyperthermia, which means that the body cannot cool down and the internal body temperature exceeds 40 degrees Celsius.
Clothes drying in the sun during a hot spell in Singapore. (File photo: Gaya Chandramohan)
Climate model predictions used by the Intergovernmental Panel on Climate Change (IPCC) for the period 2080-2100 suggest warming in the tropics of approximately 1.6 degrees Celsius in mid-range emissions scenarios and up to 3.3 degrees Celsius in high emission scenarios, with margin error of approximately 0.5 degrees Celsius in both predictions.
Different parts of the world respond in different ways to warming due to greenhouse gas emissions. Projected warming in the tropics accounts for about 40 percent of the expected temperature rise in the Arctic.
High-latitude regions, north or south of the Equator, heat up faster than the world average because excess heat in the tropics creates a temperature and pressure gradient. This drives heat to higher elevations and latitudes through an atmospheric circulation called a Hadley cell.
The stronger the gradient, the more heat is exported.
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HOT IN THE CITY
There is an additional factor: Urbanization. Singapore is a good place to observe real climate change in the tropics.
Records from Singapore indicate that temperatures have risen by 1.1 degrees Celsius for 42 years through 2014. This is almost twice the average global warming rate in recent decades and is contrary to expectations.
The difference appears to be due to a heat island effect caused by the city itself. This is important because changes in land use amplify background global climate change and put tropical cities at greater risk from extreme heat.
Since populations are concentrated in cities, this increases the risk to human health. The average temperature in Singapore is about 27 degrees Celsius, while Jakarta in Indonesia is slightly warmer.
File photo of the business district of Jakarta, Indonesia, May 2, 2019 (Photo: REUTERS / Willy Kurniawan)
At the scale of the predicted average annual temperature change, none of these cities would become uninhabitable. But even a small rise in temperature would make life more challenging.
This is exacerbated in at least some parts of the tropics, because total precipitation is increasing, suggesting a long-term increase in humidity. For example, the average rainfall in Singapore increased by more than 500 mm, from 2,192 mm in 1980 to 2,727 mm in 2014.
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DEADLY HEAT
People who work outdoors are at higher risk, as are vulnerable populations, including the elderly. Under the IPCC’s high emissions trajectory, heat-related deaths in Jakarta in August are expected to rise from around 1,800 in 2010 to almost 27,000 in 2050.
Even taking into account a significant increase in older people as Indonesia’s population ages, this means an excess of 15,000 deaths this month.
Estimates based on high-emission predictions for the tropics and mid-latitudes suggest a decline of around 40% in the ability to do manual labor during the warmest month by 2050.
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These impacts will be strongest in the seasonally humid tropics (such as the Northern Territory of Australia), where more extreme warming is expected than in the equatorial zone.
Predictions for Darwin, in northern Australia, suggest an increase in days with temperatures above 35 degrees Celsius from 11 days per year in 2015 to an average of 43 days in the mid-range emissions scenario for 2030 and an average from 111 (range 54-211) days to 2090.
In the highest emission scenario, an average of 265 days above 35 degrees Celsius could be reached by 2090.
In summary, although absolute temperatures are expected to rise more slowly in the tropics compared to higher latitudes and polar regions, the combination of heat and increased humidity will make life challenging, but not impossible.
James Shulmeister is a professor in the College of Earth and Environmental Sciences at the University of Canterbury. This comment first appeared on The Conversation.
