40% of US electricity is now emissions-free...

There's a nice pie chart at the link showing the 2023 mix.

44% natural gas
18% nuclear
16% coal
10% wind
6% hydro
6% solar

Good news, right?

Maybe not.

Let's look at nuclear power. If we build more nuclear power plants we can displace every other energy source on the chart, starting with fossil fuels. It's been done before. France closed its last coal mines a little over two decades ago.

Can we do the same with wind and solar power? No. For every gigawatt of solar or wind power installed there must be another gigawatt of gas power to back it up whenever the wind isn't blowing and the sun isn't shining. In actual practice gas becomes a primary power source. We can reduce our gas consumption, but we cannot displace it entirely. The damage we'd do by extracting and burning gas exclusively in a gigawatt power plant in one year is simply spread out over a couple of years when we install an additional gigawatt of solar and wind capacity. In the long run we are doing nothing to reduce the total amount of greenhouse gasses humans eventually dump into the atmosphere.

In popular imagination batteries are often proposed as a solution to the dilemma of fossil fuel backup power. Simply install more solar panels and wind turbines and store whatever excess energy they generate for use later. Unfortunately the amount of batteries that would be required to carry an electric grid through weeks when the sun isn't shining and the wind isn't blowing is entirely ludicrous.

Two frequently cited success stories of wind and solar power are Uruguay and South Australia.

Uruguay's primary power source is hydroelectricity. Facing power shortages Uruguay upgraded their grid with wind turbines. Wind power and hydroelectricity work very well together. When the wind isn't blowing hydroelectric plants take up the slack. When the wind is blowing less water can be released by the dams. The hydroelectric plants also play an important role in grid stability. Unfortunately this hybrid system does not reduce Uruguay's vulnerability to extreme drought.

Venezuela is another nation that is almost entirely dependent upon hydroelectricity for electric power. An extreme drought and shortages of electricity were aggravating factors in the collapse of their national economy.

The success of renewable energy in South Australia is somewhat of an accounting trick. Their state is connected to an electric grid that covers the entire east coast of Australia, but they attribute all the positive features of their renewable electric supply to themselves even though they depend upon their neighbors, who have much dirtier electric supplies, for the stability and economic viability of their grid. Excess renewable energy can be exported which pays for wind turbines that would be otherwise spinning uselessly when supply exceeded demand.

In the past year the carbon intensity of South Australia has been 147 gCO₂eq/kWh. Compare that to France at 52gCO₂eq/kWh. (Lower numbers are better.)

https://app.electricitymaps.com/zone/AU-SA