Global temperature has always varied...

Climate change deniers (I use that term for people who maintain that human actions are not affecting climate) invariably state that global surface temperatures of the earth have varied markedly throughout the geological history. This is, indeed, true. A thorough study about temperature variations on earth was recently published by Judd et al. in Science (Science 385 eadk 3705; Sept 20, 2024). Indeed, the study clearly shows that the global surface temperature has varied markedly during the past 500 000 000 years. Furthermore, the present temperature is at the lower end of the temperature variations. So, the climate change deniers can be happy, it is more likely that global temperatures increase than anything else. Or can they?

I would say not. One can now place the extinction waves in the temperature diagram. Before the present, there has been five massive extinction events in the earth’s history. Four out of five have occurred when the surface temperature has increased markedly and only one can be associated with temperature decrease. Why is a temperature increase more fatal to life than temperature decrease? One explanation is seen already when temperature tolerance of animals is estimated. They can survive long periods of time at temperatures much below optimal, but succumb rapidly at temperatures only a few degrees above optimal. Indeed, high activity cannot be achieved at high temperatures as the heat produced by the activity cannot be dissipated, but causes structural damage to proteins rendering them inactive. Thus, it is likely that if we now are faced with a temperature increase, another extinction event is likely to occur.

The study by Judd et al. clearly shows that temperature variations are much more tightly associated with carbon dioxide level than earlier estimated: increased carbon dioxide necessarily causes temperature increase and decreased carbon dioxide temperature decrease. This being the case, it is clear that the human-induced increase in carbon dioxide level will cause temperature increase. Since, further, the increase in carbon dioxide level has been thousands of times faster than the earlier changes, it is likely that the effects on extinctions are more pronounced than in earlier geological times.

Thus, although Judd et al. study clearly shows the global surface temperature has been higher than presently many times in geological history without human influence, the changes of the past  indicate the need for preventing carbon dioxide increase.