Nathan Myhrvold, founder of Intellectual Ventures, US, and Ken Caldeira of the Carnegie Institution for Science, US, estimated the life-cycle emissions and global warming expected to occur when adopting seven low-emission energy options: natural gas, carbon capture and storage (CCS), hydroelectric, solar thermal, nuclear, solar photovoltaic and wind.

They show that many decades pass before the transition from coal-based power to the alternative technology yields notable temperature benefits (Environmental Research Letters). There are two main reasons it takes so long.

First, the ocean has to release its heat. “Because of the thermal inertia of the ocean, it takes around four decades to feel the full benefit of an emission reduction,” explained Caldeira. Changing over to renewable energy systems will also take many years. During those years (while nuclear power stations are built, and wind and solar farms are erected) coal power stations will continue to emit vast amounts of greenhouse gas.

Second, making solar panels, constructing wind farms and building nuclear power plants all use energy and produce emissions. So, a rapid transition to low greenhouse-gas-emission technology would actually increase emissions in the short term.

Myhrvold and Caldeira calculate that solar thermal, nuclear, solar photovoltaic and wind will take around 40 years to pay back their so-called "emissions debt" and yield significant temperature benefits (assuming they replace 1 TWe of coal-based electricity, which is the current global electrical output from coal).

Meanwhile, hydropower would produce such large emissions that it could add to global warming more than coal power alone for a good 60 years or so. “Hydropower is often associated with high methane emissions [due to] the decay of organic matter in the flooded land surface,” explained Caldeira. "There may be some niche locations where additional hydropower could be environmentally desirable, but more often environmental considerations weigh in favour of removing dams, not building them."

And natural gas will just delay the problem. "Our calculations show that with natural gas it will take a little longer to get to the same amount of cumulative emissions and warming, so it would delay a bad outcome, but not avoid it," Caldeira told environmentalresearchweb.

Carbon capture and storage (CCS) is still somewhat of an unknown – the low estimates for life-cycle emissions make CCS appear promising, but the high estimates make the technology similar to natural gas.

It is clear that the transition to ‘green’ energy is going to be difficult and, for most of us, the benefits will not be felt in our lifetime. And it won’t just be a case of switching from fossil to green fuels. "Improved conservation and efficiency is necessary and will play a role on the same scale as new technologies. It is not either/or, it is … and … and … and …," said Caldeira. “This is a massive undertaking and there is no time to waste. We have delayed too long already and it is time to get started.”