"arrhenius was not particularly alarmed by his discovery at the time. what changed that made later scientists become much more concerned about the climate effects of carbon dioxide?"

Arrhenius, who proposed the theory of global warming due to increased carbon dioxide (CO2) levels in the atmosphere in the late 19th century, did not raise significant alarm because he believed that any human-induced warming would take thousands of years. However, over the years, several factors contributed to the increased concern among scientists regarding the climate effects of carbon dioxide:

Advancements in Climate Science: As climate science advanced, scientists gained a better understanding of the mechanisms involved in climate change. Improved models and data indicated that the warming effects of CO2 could manifest more rapidly than initially thought.

Accelerated Industrialization: The rapid industrialization and increased burning of fossil fuels in the 20th century led to a substantial rise in atmospheric CO2 levels. This unprecedented increase in greenhouse gas emissions accelerated the warming potential and shifted the timeline for potential climate impacts.

Observational Evidence: The observation of actual changes in global climate patterns, such as rising temperatures, melting ice caps, and more frequent extreme weather events, provided empirical evidence supporting the idea that human activities were influencing the climate.

International Recognition: The scientific community, policymakers, and the public became increasingly aware of the global nature of climate change. International collaborations and organizations like the Intergovernmental Panel on Climate Change (IPCC) highlighted the urgency of addressing climate issues.

Public Awareness and Advocacy: Increased public awareness, environmental movements, and advocacy efforts brought attention to the potential consequences of climate change. This pressured governments and institutions to take more proactive measures in understanding and mitigating the impacts of rising CO2 levels.

In summary, the initial lack of alarm by Arrhenius was based on the understanding of his time. Subsequent advancements in science, coupled with the rapid increase in anthropogenic CO2 emissions and observed climate changes, led to heightened concern among later scientists and the broader community about the urgent need to address the climate effects of carbon dioxide.