Once upon a time, Londoners coined the term "pea souper" for the thick, yellow-greenish combination of fog and smog that filled their city and gave it the distinctive look Charles Dickens described. That famous London fog was partly composed of particulate matter and sulfur dioxide from coal fires, and it killed untold numbers of children, the elderly, and people with respiratory problems.

Times have changed. On Friday,  Britain's National Grid produced 24 hours worth of electricity without burning a single lump of coal. It was the first time Britain's power was coal-free in 135 years. The National Grid tweeted its accomplishment to the world around 3 pm local time.

This accomplishment might seem timed to gain publicity on Earth Day, but it may just be coincidence. Britain's National Grid has been gradually replacing its coal-based electricity plants with other sources of power. The country has already had coal-free periods several hours long, including one 19-hour stretch last May. And the country will be going entirely coal-free over the next several years as its last remaining coal-powered plant is scheduled to close in 2025 in accordance with the nation's climate change commitments. Coal accounted for only 23 percent of Britain's electric power in 2015, further reduced to 9 percent last year.

If not coal, then what?

So what are the Brits using instead? According to the BBC, around half of Britain's power during the coal-free day came from natural gas. Another 25 percent came from nuclear energy, with a combination of solar, wind, and biomass such as wood pellets providing the rest.

Most of these power sources still aren't perfect. Nuclear energy comes with well-known risks. Natural gas is still a fossil fuel and contributes to climate change, particularly if it leaks. Wood pellets obviously aren't a fossil fuel, but they do produce emissions when burned. Still, most scientists and environmentalists agree that these options are better than coal, and that Britain's milestone is a great thing.

"The direction of travel is that both in the UK and globally we are already moving towards a low carbon economy," Hannah Martin, head of energy at Greenpeace UK told The Guardian. And, she added, it was a clear message to future UK leaders that they should prioritize making the nation a "world leader in clean, green, technology."

Thomas Edison created the first coal-powered electricity plant in the world in London in 1882. So closing down the nation's last coal-powered plant, temporarily for now and permanently in eight years, marks the end of an era. Like the "pea soupers" of old, it probably won't be missed.