When people talk about the consequences of global warming, the first thing that usually gets discussed is rising sea levels. However, the rising temperatures come with some significant direct consequences as well. Rising temperatures by themselves will lead to an increase in dangerous heatwaves, but they will also allow more water to remain in the vapor phase in the air. This increased humidity will make the human body’s removal of heat by perspiration more difficult. The National Weather Service (NWS) quantifies this using a measure called the heat index. A recent paper written by a research team led by UK scientist Tom Matthews used a modeling study to analyze changes in heat index as temperature changes, and came to the unfortunate conclusion that heat index will increase more quickly than temperature. Second, due largely to the urban heat island effect, cities will warm up faster than their surrounding areas. This puts large groups of people at a heightened risk of exposure to dangerous heat. A paper from this past month, written by a team led by UC-Irvine professor Simon Papalexiou, uses the existing temperature record to show that the hottest temperature of the year at different locations has been increasing overall at a slightly higher rate than the annual mean temperature, but this rate increases dramatically for many cities. Taken together, these papers show an increasingly serious vulnerability of the world’s major urban centers to disruptive and dangerous hot spells.
The NWS defines an excessive heat warning when the heat index, not the temperature itself, exceeds 105ºF for at least two days. (The Papalexiou paper identifies this number as 115ºF, but that is a misprint.) The heat index combines temperature with humidity, which is the amount of water vapor in the atmosphere. The more water vapor in the air at a given temperature, the harder it is for the human body to shed heat via perspiration. In other words, you will have a harder time cooling down on a high-humidity day than you will on a day with the same temperature but low humidity. The heat index then is essentially the temperature that dry air would need to be to make the body’s potential for overheating equivalently likely to the actual weather conditions. The amount of water vapor the atmosphere holds increases exponentially with temperature, which means that a general increase in temperature will produce an even greater increase in the heat index. Matthews and his colleagues used a computer simulation of how heat index varies with temperature to predict that the global heat stress burden — defined as the surface area over which the heat index exceeds the 105ºF threshold, multiplied by the number of days per year that area exceeds the threshold and the number of people within the area— will rise at a rate that is three times larger than the rise in global mean temperature. Under a temperature rise of 1.5ºC (2.7ºF) relative to pre-industrial temperatures (the recommended limit adopted by the Paris Agreement), the heat stress burden will be nearly 6 times greater than it was during the reference period from 1979-2005. At 2ºC warming, the heat stress burden will increase by a factor of 12.
The cities of Karachi, Pakistan (with a population of nearly 15 million) and Kolkata (formerly Calcutta), India (with a metropolitan area that contains 14 million people) suffered under a killer heat wave in 2015 where temperatures exceeded their warmest values in at least 36 years. The Matthews paper predicts that under 1.5ºC warming, Kolkata would see one day a year on average with a heat index as high as what was experienced in 2015, and Karachi would see one such day every 3.7 years. A number of major cities have the potential for large effects, but none more so than Lagos, Nigeria; its heat stress burden has the potential to be increased by a factor of 1000, given both rising temperatures and likely population increases, just from a global temperature rise of 1.5ºC. Furthermore, a rise of 1.5ºC will expose 350 million more people globally to heat stress at least once a year.
Papalexiou and his team used data from 8848 stations in the Global Historical Climatology Network — the same temperature database on which NASA and NOAA base their temperature records — to assess trends in the hottest temperature of the year in a given region, along with their statistical significance. They found that 80% of the land area with sufficient data has a positive trend in the hottest temperature of the year over the last half century, with nearly half of that area having a linear trend in excess of 0.20ºC per decade. The global mean temperature over that time has increased at a rate of about 0.18ºC per decade, but globally the mean hottest temperature of the year at a given location has increased by about 0.25ºC per decade. The largest area of significant increase extends across central and eastern Europe, although some other parts of the world show large increases too.
The study becomes most unsettling when it discusses changes in the hottest temperature of the year in large cities. For example, over the last fifty years, the hottest temperature of the year in the city of Paris has risen by an average of 0.96ºC per decade. Over the past thirty years, Houston’s hottest temperature of the year has risen by 0.99ºC per decade. The primary cause of the large rise in temperatures in cities relative to warming globally is the urban heat island effect; cities absorb more radiation than the countryside does, and are warmer as a result. This effect needs to be carefully accounted for when assessing the warming of the globe as a whole, but more than half the world’s population lives in urban areas.
(For updates on new posts, please click the "Follow" button.)
Illuminating,as always,Scott!
ReplyDelete