Carbon free energy by 2100 would be nice, but much too late to solve the crisis of the biosphere.
This is a radar image of the surface of Venus taken by the Magellan NASA orbiter. Something similar awaits the Earth if we don't do something drastic now. By 2100, the year that the G7 has agreed to be carbon free, the Earth will not look anything like this, but we humans will have set it on an inexorable course with this end. The problem is that it takes about 200 years for CO2 to work its way out of the atmosphere. Consequently, the greenhouse gases, especially CO2, are cumulative. Even if we stopped burning fossil fuels tomorrow, the earth's atmosphere and the hydrosphere will continue to heat up, thus setting in motion dreadful feed-back cycles that are already in motion - think melting methane clathrates. Under these circumstances, it is difficult to see how nature could restore balance and life will pass from the Earth forever in a horrifyingly short time