Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such as climate change and global warming. Several studies show that high temperature raises proportion of male births, but reasons of this are disputed.[1] Catalano et al. report that cold weather is an environmental stressor, and women subjected to colder weather abort frail male fetuses in greater proportion, thereby lowering birth sex ratios. But cold weather stressors also extend male longevity, thereby raising the human sex ratio at older ages.[2] The Catalano team finds that a 1 °C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year.