Warming of a few tenths of a degree is probably all that is needed to make global warming noticeable to the perceptive lay person, per James Hansen et al., 1998. In 1999, Hansen said an increase of .5C for 2-3 yrs. would represent a "smoking gun," would settle global warming issue in average person's mind

August 1999, "GISS analysis of surface temperature change," James Hansen et al, NASA, GISS
 
p. 22, "Warming of a few tenths of a degree is probably all that is needed to make global warming noticeable to the perceptive lay person. [Hansen et al., 1998c]...We expect the global temperature to average about 0.5°C or higher, relative to 1951-1980, within the next 2 or 3 years [early 2000s]....Maintenance of a .5 degree higher "global temperature over 2 or 3 years will represent a "smoking gun" providing both scientific and practical confirmation that global warming in the industrial era is unnatural." ....

 





"(Received March 29, 1999; revised July 28, 1999;

accepted August 4, 1999)" Published in 1999 by AGU 

===========================

More from p. 22 of Hansen 1999 report:

p. 22, "The past two years," subhead 11.1.3

parag. 2: "We expect the global temperature to average
about 0.5°C or higher relative to 1951-1980, within

the next 2 or 3 years [early 2000s]. Confirmation of such a

high temperature level will have significance

that extends beyond the question of short-term

temperature records. Such a high temperature level

should be sufficient to settle the contentious issue of 

whether global warming is occurring during the 

satellite era [post 1979], regardless of measurement

problems. More generally, maintenance of such a high

global temperature over 2 or 3 years will

represent a "smoking gun" providing both

scientific and practical confirmation that global warming in the industrial era is unnatural. Scientifically, such

continued warmth will make the present climate clearly

the warmest of the millennium, with the greatest warming 

rate, exceeding previous epochs such as the

“Medieval Climatic Optimum” [Mann et al., 1999].

Practically, warming of a few tenths of a degree is

probably all that is needed to make global 

warming noticeable to the perceptive lay person. [Hansen et al., 1998c]."...

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