The sum of climate change and heat island has caused Mexico City to experience a temperature increase of 4 degrees, in the last century: the most dramatic increase (3 degrees) comes from the second phenomenon and only one from the first, exposed the coordinator of the Climate Change Research Program (PINCC), Francisco Estrada Porrúa.
"In 2015 we did not consider the heat island, only climate change. If we see global warming a city would be in the worst-case scenario in warming of one or 1.5 degrees, but by adding the heat island, some cities have already passed two or three degrees. It is said that in Mexico City the intensity of the heat island is 3 degrees Celsius and global warming has also increased by one degree. That is to say, in the last century, we have a warming of around four degrees," said the researcher of the Institute of Atmospheric Sciences and Climate Change.
Participating in the Seminar "Synergies between global and local climate change", organized by the PINCC, the economist said that heat island is a phenomenon experienced in urban areas where high temperatures are observed due to human activity, caused mainly by buildings, sidewalks, or asphalt, which absorb more heat and release it slowly.
Thus, some factors determine urban climate: global climate change, which alters planetary conditions and has a regional and local expression; geographical factors (latitude and longitude); dynamic processes such as El Niño or La Niña; atmospheric pollutants with aerosols and black carbon; and the urban heat island, he said.
"In a scenario of high emissions we see that the planet could warm between two or 4.5 degrees, and that is similar to what the big cities have warmed due to the heat island; hence the importance of looking at these joint effects," commented Estrada Porrúa.
He also holds a Ph.D. in Environmental Economics from the Free University of Amsterdam and added that 2021 was the sixth hottest year for 1.8 billion people, and this will continue to be the case because cities have an additional source of warming in the heat island.
Heat island effects
The expert pointed out that the warming in a large city, due to heat island, is similar to what would be obtained in the most drastic scenarios of climate change at a global level, that is, around 2.5 and 4.5 degrees, so he and his team undertook the task of reviewing what would happen if the two types of climate change were combined. This work was published in the journal Nature Climate Change in 2017.
They reviewed the information of almost 1,700 large cities on the planet to observe what would happen because more than 50 percent of the global population lives there, which produces more than 80 percent of the global GDP. So what happens in these cities has important implications for what happens in the nations and the planet.
As calculations move into the future, said Estrada Porrúa, it is revealed that the major metropolises will increase their temperature by four or five degrees more by 2050, and by the end of the century, some of the most populated ones could face dramatic changes of eight degrees Celsius or more.
Economically speaking, the heat island and the interaction with global and local climate change represent an important challenge, because if pollutant emissions are lowered with international agreements, the economic impacts are reduced.
However, nations would have to go from investing approximately 32 trillion dollars - for the mitigation of the derived problems - and if you consider the heat island the costs can go up to 82 trillion dollars; even for the most ambitious stabilization scenario, the costs are still up to 41 trillion dollars.
"This gives us a very clear message, we have to act globally, we have to lower greenhouse gas emissions if we do not want to suffer these impacts, but there is also a very important thing, we have to act locally because if not, we will have very large impacts anyway. There is such a concentration of population and wealth in the big cities that what happens there will affect the whole planet", he explained.
Among the options to avoid these losses, he recommended that nations change 50 percent of their roofs to green or cool roofs; replace the pavement to cool to lower the heat island.
In Mexico City, the intensity of the heat island shows that the municipalities with a lower development index and low per capita income are the ones with the highest heat island, with places that are above 4 degrees Celsius. When this information is crossed with the Atlas de Riesgos, it is observed that the pattern coincides with the risks of maximum temperatures.
Currently, ICAT and PINCC are developing a project to propose tools and strategies for decision-making on climate change, air quality, and sustainable development in the megalopolis of the Valley of Mexico, which considers socioeconomic aspects, emissions and air quality, land-use change, adaptation strategies and policies, rural and urban systems, and global and local climate change scenarios, concluded Estrada Porrúa.