Gravity waves (GWs) are key drivers of atmospheric dynamics, with major impacts on climate and weather processes. However, they are challenging to measure in observational data, and as a result no large-area multi-decadal GW time series yet exist. This has prevented us from quantifying the interactions between GWs and long-timescale climate processes. Here, we exploit temperatures measured by commercial aircraft since 1994 as part of the IAGOS atmospheric chemistry research programme to produce a novel 26-year time series of upper troposphere/lower stratosphere (UTLS) GW measurements across most of the northern hemisphere. We analyse 90\,342 flight-hours (76.2 million flight-kilometres) of data, typically at a temporal resolution of seconds and with high temperature precision. We show that GW activity in the northern-hemisphere UTLS is consistently strongest north of and above the upper tropospheric jet. We also show that GW sources not typically observed in stratospheric data but assumed in model schemes, such as the Rocky Mountains, are visible at these altitudes, suggesting that wave momentum from these sources is deposited specifically between $\sim$200–50\,hPa. Our data shows no significant impact of the Quasi Biennial Oscillation, the Northern Annular Mode, or climate change. However, we do see strong evidence of links with the El Ni\ no-Southern Oscillation, which modulates the measured GW signal by $\sim$25\%, and weak evidence of links with the 11-year solar cycle. These results have important implications for atmospheric process modelling and for understanding large-scale climate teleconnections.