1Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes

Observations indicate that the average height of the tropopause – the transition zone between the stratosphere and troposphere – has increased by several hundred meters since 1979. Comparable increases are evident in climate model experiments. The latter show that human-induced changes in ozone and well-mixed greenhouse gases account for over 80 % of the simulated rise in tropopause height over 1979-1999. Their primary contributions are through cooling of the stratosphere (ozone) and warming of the troposphere (well-mixed greenhouse gases). Tropopause height changes simulated over 1900-1949 are smaller than in recent decades, and are driven largely by variations in volcanic aerosols and solar irradiance. A model-predicted fingerprint of tropopause height changes is statistically detectable in two different observational (“reanalysis”) datasets. This positive detection result allows us to attribute overall tropopause height changes to a combination of anthropogenic and natural forcings. Our study shows that the increase in tropopause height over the second half of the 20th century was predominantly due to human activity, and provides independent support fo