There is a correlation between humid climates and tonal languages [1], [2]. And there is some evidence that there have been shifts in tonal complexity within l

There is a correlation between humid climates and tonal languages [1], [2]. And there is some evidence that there have been shifts in tonal complexity within languages over time [3]. [1] https://www.mpg.de/8879447/tonal-languages-institutes https://www.mpg.de/8879447/tonal-languages-institutes [2] http://www.pnas.org/content/112/5/1322.abstract http://www.pnas.org/content/112/5/1322.abstract [3] http://languagelog.ldc.upenn.edu/nll/?p=4906 http://languagelog.ldc.upenn.edu/nll/?p=4906