Radiosondes show that after decades of cooling, the lower stratosphere is now warming

Mears, C., Fujiwara, M., Jeannet, P., Thorne, P., Bodeker, G., Haimberger, L., Hervo, M., Popp, C., Romanens, G., Steinbrecht, W., Stübi, R., and Van Malderen, R.