The
IPCC 2014 data gives the following CO2equivalent emissions by energy source for electricity generation (gCO2eq/kWh):
820 Coal
650 Oil
490 Gas
230 Biomass
235 Solar + Storage*
188 Wind + Storage*
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45 Solar when available*
38 Geothermal
24 Hydro
12 Nuclear
11 Wind when available*
Obviously the lower down this list your choice of electricity source appears, the better. Equally obviously, third place is better than first or second, but hardly qualifies as "good". Coal is 68x as bad as Nuclear power, Oil is 54x as bad, while Gas is "only" 41x as bad.
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If all current human energy consumption was to fall below that approximately 100gCO2eq/kWh level, the atmospheric greenhouse gas level would start to decline, rather than increase, due to natural processes that remove carbon dioxide from the air (mostly the activity of photosynthetic organisms). Obviously if global energy consumption doubles, we would need to target <50g, or if it quadruples, <25g, etc.