Mann is a Climategate protege of James Hansen, and a 2013 paper by Hansen et al provides his conservative projections for the next 118 years:
If we assume that fossil fuel emissions increase by 3% per year, typical of the past decade and of the entire period since 1950, cumulative fossil fuel emissions will reach 10 000 Gt C in 118 years. [Fossil fuel emissions were 7.8 GtC in 2005 per IPCC estimates.] Are there sufficient fossil fuel reserves to yield 5000–10 000 Gt C? Recent updates of potential reserves, including unconventional fossil fuels (such as tar sands, tar shale and hydrofracking-derived shale gas) in addition to conventional oil, gas and coal, suggest that 5×CO2 (1400 ppm) is indeed feasible. Our calculated global warming in this case [1400 ppm] is 16°C, with warming at the poles approximately 30°C. Calculated warming over land areas averages approximately 20°C. Such temperatures would eliminate grain production in almost all agricultural regions in the world. Increased stratospheric water vapour would diminish the stratospheric ozone layer. More ominously, global warming of that magnitude would make most of the planet uninhabitable by humans.Holy Hansen's grandchildren!
Never mind the following showing Hansen exaggerates warming by at least 2 orders of magnitude:
- CO2 levels have increased an average of 1.46 ppm per year since 1959, i.e. 0.3% of current 400 ppm levels, not 10 times higher as Hansen claims of 3% per year. Thus, using the same linear extrapolation of Hansen et al, in 118 years * 1.46 ppm/year = 172 ppm added CO2 in the atmosphere. Thus, 400 ppm currently + 172 added = 572 ppm in 118 years, not 1400 ppm as Hansen claims using the same type of linear extrapolation.
- The 'greenhouse effect' of CO2 is logarithmically declining, not linear
- Using the IPCC formula and (false) assumptions, an increase of 172 ppm over the next 118 years would cause 5.35*ln(572/400)*(3C/3.7Wm-2) = 1.5C warming, not 20C - 30C as Hansen claims
- Using 15 peer-reviewed, published, more realistic estimates of climate sensitivity based upon observations, an increase of 172 ppm over the next 118 years could cause 5.35*ln(572/400)*(0.5C/3.7Wm-2) = 0.26C warming, not 20C - 30C as Hansen claims [2 orders of magnitude higher]
- Even this tiny 0.26C warming based upon realistic climate sensitivities would be "short-circuited" by convection in the troposphere, and thus actual warming close to zero, not 20C - 30C as Hansen claims.
- Hansen debunks himself, finding the man-made airborne fraction of CO2 has significantly decreased over the past 40+ years, offers bogus excuses for the 18+ year "pause" of global warming, and finds that climate models exaggerate warming
It's a pity Hansen's grandchildren are used as pawns for his shoddy and unscientific alarmism.
h/t to Kenneth Richard
Related: Time for Meds? NASA scientist James Hansen endorses book which calls for ‘ridding the world of Industrial Civilization’ – Hansen declares author ‘has it right…the system is the problem’ Book proposes 'razing cities to the ground, blowing up dams and switching off the greenhouse gas emissions machine'