  Global warming arises from 'temperature forcing', a net imbalance between
energy fluxes entering and leaving the climate system and arising within it.
Humanity introduces temperature forcing through greenhouse gas emissions,
agriculture, and thermal emissions from fuel burning. Up to now climate
projections, neglecting thermal emissions, typically foresee maximum forcing
around the year 2050, followed by a decline. In this paper we show that, if
humanity's energy use grows at 1%/year, slower than in recent history, and if
thermal emissions are not controlled through novel energy technology,
temperature forcing will increase indefinitely unless combated by
geoengineering. Alternatively, and more elegantly, humanity may use renewable
sources such as wind, wave, tidal, ocean thermal, and solar energy that exploit
energy flows already present in the climate system, or act as effective sinks
for thermal energy.
