Rapidly declining remarkability of temperature anomalies may obscure public perception of climate change

Abstract

The changing global climate is producing increasingly unusual weather relative to preindustrial conditions. In an absolute sense, these changing conditions constitute direct evidence of anthropogenic climate change. However, human evaluation of weather as either normal or abnormal will also be influenced by a range of factors including expectations, memory limitations, and cognitive biases. Here we show that experience of weather in recent yearsrather than longer historical periods-determines the climatic baseline against which current weather is evaluated, potentially obscuring public recognition of anthropogenic climate change. We employ variation in decadal trends in temperature at weekly and county resolution over the continental United States, combined with discussion of the weather drawn from over 2 billion social media posts. These data indicate that the remarkability of particular temperatures changes rapidly with repeated exposure. Using sentiment analysis tools, we provide evidence for a "boiling frog" effect: The declining noteworthiness of historically extreme temperatures is not accompanied by a decline in the negative sentiment that they induce, indicating that social normalization of extreme conditions rather than adaptation is driving these results. Using climate model projections we show that, despite large increases in absolute temperature, anomalies relative to our empirically estimated shifting baseline are small and not clearly distinguishable from zero throughout the 21st century.climate change | perception | Twitter | baseline | temperature A nthropogenic climate change involves the shifting of weather beyond bounds historically experienced by communities and ecosystems. Global average temperatures are now significantly higher than preindustrial levels, an effect that cannot be explained without greenhouse gas emissions, while modeling studies have shown that local temperature anomalies will statistically emerge from the noise of natural variability in the relatively near term (1-4). However, how are temperatures that are extreme in a long-term, historical sense understood and interpreted by people exposed to them? Will increasingly unusual temperatures constitute direct, experiential evidence of a changing climate, or will changing conditions be rapidly normalized so that even large absolute temperature anomalies are not perceived as particularly unusual?Answers to these questions depend on how the subjective definition of normal temperatures evolves over time as the climate changes: What baseline do people use to evaluate the weather? In a nonstationary climate, the question of what the appropriate climate reference window should be is not obvious. Various baselines, ranging from the preindustrial period to the last 30 y, are used in the scientific literature, reflecting the inherent ambiguity in choosing a stable reference period in a nonstationary series (5, 6). The baseline actually used by nonscientists in evaluating weather as either normal or abnormal is even harder to theoretically...

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