Powering the grid with 100 percent green energy may sound like a nice idea, but it would actually be extremely difficult to do, a electric grid expert told Greentech Media in an interview Wednesday.

“Let’s say we have a 100% [renewables] system, hypothetically,” Christopher Clack, a National Oceanic and Atmospheric Administration (NOAA) mathematician, said in the interview.

“Now you have got think about working out forecasting of load and of the weather, because that’s your fuel source now, seasons or years ahead of time with really good accuracy so that you know how much energy to store, how much to shed, how much to transmit, how much to consume, and you need to do that all the time, predicting far enough ahead that you will never run out of power, because you have got nothing there as backup,” Clack said.

Clack made waves with a recent study that challenged a widely-cited 2015 study claiming the U.S. could run on 100 percent green energy. Clack and 20 colleagues argued the 2015 research “used invalid modeling tools, contained modeling errors, and made implausible and inadequately supported assumptions.”

“Policy makers should treat with caution any visions of a rapid, reliable, and low-cost transition to entire energy systems that relies almost exclusively on wind, solar, and hydroelectric power,” wrote the 21 experts, led by Clack. Clack was worried politicians took the 2015 study too seriously, and 2016 Democratic presidential contender Bernie Sanders touted the study in his promise to move the U.S. off fossil fuels.

Using 100 percent green energy would require a total restructuring of the world’s economy that’s “unnecessarily daunting” compared to simply adapting to global warming or reducing emissions via other methods, according to Clack.

Clack said low-emissions technologies, like nuclear power and natural gas, would be more cost effective for reducing carbon dioxide (CO2) emissions.

To function, power grids require demand to exactly match supply, which is an enormous problem for variable wind and solar power.

Wind and solar can also burn out the grid if they produce too much, or not enough, electricity, leading to brownouts or blackouts. Such damage has already occurred in power grids relying too much on solar and wind power — for example, in California and Germany.

When the islands of Tasmania and El Hierro tried to power their economies with 100 percent green energy, both islands quickly switched back to diesel generators after suffering from reliability problems and soaring costs. The analysis suggests it would have taken 84 years for El Hierro’s wind and hydropower systems to simply pay back their capital costs.

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This article originally appeared in The Daily Caller

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