# Heatwave increases nighttime light intensity in hyperdense cities of the Global South: a double machine learning study

## TL;DR

This study finds that heatwaves in cities like Delhi and São Paulo lead to increased nighttime economic activity, as measured by light intensity.

## Contribution

The novel use of double machine learning to analyze the impact of heatwaves on nighttime light intensity in hyperdense Global South cities.

## Key findings

- Heatwaves lasting at least 2 days significantly increase nighttime light radiance in Guangzhou, Cairo, and São Paulo.
- The increase in nighttime light intensity diminishes when heatwaves last longer than 2 days.
- The study provides insights for improving urban resilience to nighttime effects of heatwaves.

## Abstract

Heatwaves, intensified by climate change and rapid urbanization, pose significant threats to urban systems, particularly in the Global South, where adaptive capacity is constrained. This study investigates the relationship between heatwaves and nighttime light (NTL) radiance, a proxy of nighttime economic activity, in four hyperdense cities: Delhi, Guangzhou, Cairo and São Paulo. We hypothesized that heatwaves increase nighttime activity. Using a double machine learning (DML) framework, we analysed data from 2013 to 2019 to quantify the impact of heatwaves on NTL while controlling for local climatic confounders. The results show a statistically significant increase in NTL radiance for Guangzhou, Cairo and São Paulo when a heatwave event lasts at least 2 days, indicating a rise in nighttime activities. However, when we extend the definition of the heatwave beyond the 2-day threshold, such an increase in the NTL values is reduced. We derive insights to improve resilience to the nighttime effects of heatwaves in urban areas.

This article is part of the theme issue ‘Urban heat spreading above and below ground’.