B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 87
Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Weather
We were at FL360 and encountering light to moderate chop. The seatbelt signs were on; and the Flight Attendants were seated; and we had slowed to Mach .76. ATC had indicated the rides would be better at FL380 and the FMC showed that would be within our safe operating envelope for the aircraft weight. We received clearance to climb to FL380 and initiated a climb. As we were climbing; the turbulence began to get worse instead of better; so we checked the cruise performance and found that if the rides were not better at FL380 we would have to descend back to FL360 or lower. At this time our altitude was about FL370 and we encountered severe turbulence. The airspeed began fluctuating wildly and I disconnected the autothrottles. Our vertical speed began fluctuating plus or minus about 1500 fpm and the autopilot disengaged. We were able to receive clearance back to FL360 and eventually FL340. There were no injuries reported from any passengers or Crew; and we continued to our destination without further incident.We had reports leading us to believe that the rides would be smoother at our planned cruise altitude of FL380.  There was no indication to us that the turbulence would become worse. The only way we could've avoided severe turbulence is if we had more accurate information.
B737 First Officer reported climbing to FL380 from FL360 to find a smoother ride; but instead found severe turbulence passing FL370. The crew then requested FL340 and descended.