What intervention by a travel through time in the past would have the greatest positive effect on the presence and the further development until 2050?
First You would think to stop some big mass death.
To stop the assassinator in Sarajewo June 1914
To stop the train travel from Lenin to Russia 1917
To stop Adolf Hitler before 1920
But all this would have brought only some million dead less, no deep change of the present problems until 2050. There is a much more fundamental intervention in the past to stop the big problems climate change, peak-oil, peak natural gas, peak coal.
A talk with Bismarck about how to finance social insurance
Finance social insurance by a coal tax. The higher efficiency at using coal as an advantage for the German industry.