This story is part of CNBC’s “Transmission Troubles” series, an inside look at why the aging electrical grid in the U.S. is struggling to keep up, how it’s being improved, and why it’s so vital to fighting climate change.

The network of transmission lines that carry electricity across the U.S. is old and not set up to meet the anticipated demand for clean energy sources like wind and solar.

Currently, electricity generation results in 32% of carbon dioxide emissions in the United States, mostly from burning fossil fuels like oil, coal, and natural gas. Those fuels are transported and burned where electricity is needed.

But inexpensive emissions-free sources of energy, like solar and wind, are only abundant in places where the sun shines or wind blows, and that’s not necessarily close to homes and businesses. Moreover, demand for electricity is going to rise as fossil fuels are gradually replaced for a whole host of other uses, such as electric vehicles and heat pumps.

Keeping the lights on and the air clean will require a lot of new transmission.

Most of the U.S. electric grid was built in the 1960s and 1970s. Currently, over 70% of the U.S. electricity grid is more than 25 years old, according to the White House.

That creates “vulnerability,” the U.S. Department of Energy said in an announcement of an initiative included in President Biden’s Bipartisan Infrastructure Law to catalyze investment in the nation’s grid.

In 2021, the most recent year for which data is available, U.S. electricity customers were without power for slightly longer than seven hours on average, according to data from the U.S. Energy Information Administration. More than five of those seven hours were during what the EIA calls “major events,” including snowstorms, hurricanes, and wildfires. That’s a significant rise from the three-to-four-hour average for outages between 2013 (the first year the data is available) and 2016, and the main culprit is extreme weather.

“Extreme weather events like the Dixie Wildfire, Hurricane Ida, and the 2021 Texas Freeze have made it clear that America’s existing energy infrastructure will not endure the continuing impacts of extreme weather events spurred by climate change,” the U.S. Department of Energy said.

Transmission infrastructure lasts between 50 and 80 years, according to a 2021 presentation from the advisory firm, the Brattle Group. Replacing transmission infrastructure that’s reaching its age limit is likely to costing an estimated $10 billion a year, according to the Brattle Group analysis.

American Electric Power, an energy company that owns 40,000 miles of transmission miles, has said 30% of its transmission lines will need replacement over the next 10 years, as highlighted by a 2022 report from the transmission policy group, Grid Strategies.

In addition to the increasing age, the location of the existing transmission lines is a problem.

Fossil fuels like oil, coal and natural gas are typically transported by railroads or pipelines, then burned in power plants near cities.

The electricity industry in the U.S. grew up through a patchwork of local utility companies meeting local demand, Rob Gramlich, the founder of Grid Strategies, told CNBC. The system of transmission lines in the U.S. was built to serve that model of energy generation.

Clean energy sources, like wind and solar, do not release greenhouse gas emissions, but the energy generated must be moved from where the wind and sun are strongest to where the electricity is actually used.

That’s especially true for tapping into the highest quality of wind energy, explained Princeton professor Jesse Jenkins, a macro-scale energy systems engineer.

“Wind turbine power scales with the wind speed cubed. That means the best wind power sites are eight times more productive than the worst ones, versus just twice as productive for solar,” Jenkins said.

“That greater degree of variation in wind power potential means we need to build wind farms where it’s really windy, and that tends to not be where too many people live! So wind power development is a big driver of expanded transmission needs,” Jenkins told CNBC.

It’s easier to build solar panels close to where they are needed, but “not so for wind farms,” Jenkins said.

The combination of an aging infrastructure that needs costly upgrades and an energy grid doesn’t go where clean — and cheap — forms of renewable energy are located is “unfortunately a double whammy for consumers,” Gramlich told CNBC.

“But consumers benefit from the cheap generation that transmission enables,” Gramlich said. He advocates for replacing old infrastructure with advanced technology that can handle next generation transmission needs.

“It would be such a waste to replace old assets with replacements of the same capacity and quality,” Gramlich said.

In the 1960s and 1970s, electricity construction boomed in both the United States and in Europe, said Konstantin Staschus, who has been focusing on the issue of transmission for his entire career, both in California and Europe.

“Those were the times when California was planning to have a nuclear power plant every 100 miles or so up and down the coast, many more than they ended up building in reality, because they kept projecting 7% annual electricity demand increases, which they used to have in the 60s, into the indefinite future,” Staschus told CNBC. “And they thought they would need generation and transmission coming out of the ears to cover future demands.”

But during and after the oil shocks of the 1970’s, the U.S. dramatically reduced its own energy demand. “Demand growth essentially dropped to 1 or 2% rather than seven and more or less stayed there,” he told CNBC.

From the late 1970’s through the early 2000’s, the U.S. transmission grid expanded at about 2% per year, Jenkins told CNBC.

Now, demand for electricity is going to increase rapidly as efforts to respond to global warming and mitigate the effects of climate change ramp up.

Demand for electricity in 2030 will be 14% to 19% higher than 2021 levels, according to an analysis from REPEAT(Rapid Energy Policy Evaluation and Analysis Toolkit), an energy policy project Jenkins is part of leading, and 27% to 39% higher by 2035, Jenkins said.

“A 21st century grid has to accommodate steadily rising electricity demand to power electric vehicles, heat pumps, industrial electrification and hydrogen electrolysis, and it needs to extend to new parts of the country to harness the best wind and solar resources. Both factors mean we simply need a bigger grid with more long-distance transmission,” Jenkins told CNBC.

“Throw in resiliency benefits of stronger inter-regional grid connections so a region that’s struggling with a extreme event can call on its neighbors for help, and you’ve got even more reason to build a stronger, bigger grid,” Jenkins said.