Space.com has an interesting article about recent research by Santer et al. (2018, Science 361, 245) into how the seasonal cycle of temperature has changed. It’s a topic we’ve looked at recently for surface temperature in the USA. Santer et al. study it in the troposphere (both TLT, the lower troposphere temperature, and TMT, mid-troposphere temperature), and look for patterns over the whole globe.
Result: a human-caused signal is now evident.