Peter Andreas Thejll (born 1956) is a Danish astrophysicist and climate researcher. His research in solar variation helped provide evidence of the greenhouse effect on the Earth's climate in the late 20th century. In particular, his study with Knud Lassen on Northern Hemisphere land air temperature showed that the rise of 0.4 degrees Celsius since 1980 could not be accounted for by the solar cycle, solely. Climatologists have pointed to this finding as a " piece of evidence for greenhouse warming".