If I understand how the study worked, the 80 % reflects the comparison of likelihood with actual trend, relative to likelihood with stationary (no trend) temperatures. (realclimate.org)
This inherent trend in the number of record - breaking events confounds the interpretation of actual trends in the presence of any underlying climate change. (realclimate.org)
It appears that Lister's group crunched some data on Chinese urban temperature monitoring and found that only about 60 % of measured temperature increase was actual trend as opposed to urban heat island effect. (scholarsandrogues.com)