Â鶹ÒùÔº

Rising global temperatures affect human activity in many ways. Now, a new study illuminates an important dimension of the problem: very hot days are associated with more negative moods, as shown by a large-scale look at social media postings.

Overall, the study examined 1.2 billion social media posts from 157 countries over the span of a year. The research finds that when the temperature rises above 95 degrees Fahrenheit, or 35 degrees Celsius, expressed sentiments become about 25% more negative in lower-income countries and about 8% more negative in better-off countries. Extreme heat affects people emotionally, not just physically.

"Our study reveals that rising temperatures don't just threaten physical health or economic productivity—they also affect how people feel, every day, all over the world," says Siqi Zheng, a professor in MIT's Department of Urban Studies and Planning (DUSP) and Center for Real Estate (CRE), and co-author of a new paper detailing the results. "This work opens up a new frontier in understanding how climate stress is shaping human well-being at a planetary scale."

The paper, "," is published today in the journal One Earth.

Social media as a window

To conduct the study, the researchers evaluated 1.2 billion posts from the social media platforms Twitter and Weibo, all of which appeared in 2019. They used a natural language processing technique called Bidirectional Encoder Representations from Transformers (BERT), to analyze 65 languages across the 157 countries in the study.

Each social media post was given a sentiment rating from 0.0 (for very negative posts) to 1.0 (for very positive posts). The posts were then aggregated geographically to 2,988 locations and evaluated in correlation with area weather. From this method, the researchers could then deduce the connection between extreme temperatures and expressed sentiment.

"Social media data provides us with an unprecedented window into human emotions across cultures and continents," Wang says. "This approach allows us to measure emotional impacts of climate change at a scale that traditional surveys simply cannot achieve, giving us real-time insights into how temperature affects human sentiment worldwide."

To assess the effects of temperatures on sentiment in higher-income and middle-to-lower-income settings, the scholars also used a World Bank cutoff level of gross national income per-capita annual income of $13,845, finding that in places with incomes below that, the effects of heat on mood were triple those found in economically more robust settings.

"Thanks to the global coverage of our data, we find that people in low- and middle-income countries experience sentiment declines from extreme heat that are three times greater than those in high-income countries," Fan says. "This underscores the importance of incorporating adaptation into future climate impact projections."

In the long run

Using long-term global climate models, and expecting some adaptation to heat, the researchers also produced a long-range estimate of the effects of extreme temperatures on sentiment by the year 2100. Extending the current findings to that time frame, they project a 2.3% worsening of people's emotional well-being based on high temperatures alone by then—although that is a far-range projection.

"It's clear now, with our present study adding to findings from prior studies, that weather alters sentiment on a global scale," Obradovich says. "And as weather and climates change, helping individuals become more resilient to shocks to their emotional states will be an important component of overall societal adaptation."

The researchers note that there are many nuances to the subject, and room for continued research in this area. For one thing, social media users are not likely to be a perfectly representative portion of the population, with young children and the elderly almost certainly using social media less than other people. However, as the researchers observe in the paper, the very young and elderly are probably particularly vulnerable to heat shocks, making the response to hot weather possible even larger than their study can capture.

The research is part of the Global Sentiment project led by the MIT Sustainable Urbanization Lab, and the study's dataset is publicly available. Zheng and other co-authors have previously investigated these dynamics using social media, although never before at this scale.

"We hope this resource helps researchers, policymakers, and communities better prepare for a warming world," Zheng says.