Cognitive consequences: Study reveals startling link between climate change and shrinking human brain size

A recent study conducted by cognitive scientist Jeff Morgan Stibel from the Natural History Museum in California has uncovered a surprising link between climate change and a decline in the size of the human brain.

By analysing climate records and human remains spanning a 50,000-year period, Stibel's research sheds light on how humans develop and adapt in response to environmental stress, according to Science Alert. The findings, published in a paper, underscore the importance of understanding the impact of climate change on human brain size and, ultimately, human behaviour.

Stibel's investigation involved examining 298 Homo specimens from the past 50,000 years, alongside natural records of global temperature, humidity, and rainfall.

What does the study reveal?

The results revealed a significant reduction in average brain size during periods of warmer climate, compared to cooler ones. Stibel's earlier research on brain shrinkage served as the impetus for this study, as he sought to uncover the underlying causes behind this phenomenon.

In an interview with PsyPost's Mane Kara-Yakoubian, Stibel emphasised the critical need to comprehend changes in the human brain over time. He expressed his disappointment at the lack of research on this subject, stating, "We know the brain has grown across species over the past few million years, but we know very little about other macro-evolutionary trends."

To gather data, Stibel collected information on skull sizes from ten published sources, resulting in 373 measurements taken from 298 human bones spanning 50,000 years.

To estimate brain sizes accurately, he adjusted for geographical region and gender by including estimations of body size.

To account for potential dating errors, Stibel categorised the fossils into groups based on their age and conducted his analysis using four different fossil age spans.

Additionally, he compared brain size to four climate records, including temperature data from the European Project for Ice Coring in Antarctica (EPICA) Dome C, which provides precise temperature measurements spanning over 800,000 years.

Over the past 50,000 years, the Earth has experienced various climate fluctuations, including the Last Glacial Maximum, a period characterised by consistently colder temperatures until the end of the Late Pleistocene. Following this, the Holocene period witnessed rising average temperatures, leading up to the present day.

Stibel's analysis revealed a consistent pattern of changing brain size in Homo, correlated with fluctuations in climate as temperatures rose and fell.

Notably, humans experienced a significant decline in average brain size, amounting to just over 10.7 percent, throughout the Holocene warming period, which occurred approximately 17,000 years after the last glacial maximum.

According to Stibel, brain size changes tend to occur thousands of years after climate changes, suggesting that species-level adaptation often requires multiple generations. This evolutionary pattern manifested within a relatively brief period of 5,000 to 17,000 years, raising concerns about the potential detrimental effects of ongoing global warming on human cognition.

Stibel argues that even a slight reduction in brain size among contemporary humans could have significant physiological implications that are not yet fully understood.

Furthermore, the analysis revealed that humidity and rainfall levels also influenced brain growth, albeit to a lesser extent compared to temperature. Dry spells exhibited a weak correlation with slightly larger brain volumes.

While the study establishes a clear relationship between climate change and differences in brain size, it acknowledges that climate alone does not account for all observed variations.

Stibel suggests that other factors such as ecosystem elements, indirect climate effects (such as, vegetation and net primary production), or non-climate factors like culture and technology could also contribute to changes in brain size.