An igloo is made of ice, but it traps heat inside so you can stay warm.
Imagine now, a thin layer of ice all around the Earth that lets the sun’s rays pass through it but insulates the globe so it heats up the surface, keeping that heat trapped inside the ice layer.
A hundred years ago, aircraft did not add trillions of cubic feet of water vapor into the atmosphere. At 20,000 to 40,000 feet, water is ice, so jet exhaust from millions of airplanes constantly replenishes our new ice layer. Where is the most ice (jet exhaust) added? In the northern hemisphere, where most of the air traffic is. The southern poles are still freezing up just fine (despite the high levels of CO2 mixed around pretty well all over the globe).
When ice crystal saturation is high and/or artificial condensation nuclei are added to the atmosphere, visible clouds form as the moisture condenses around the crystalline nuclei of ice, salt or metal.
Moisture can condense into cloud cover that creates shade, pressure differentials and wind. When clouds thicken enough in the troposphere, they produce precipitation, but the high cirrus clouds created by aircraft don’t provide rain. However, they can facilitate rain by creating shade and ultimately, as the ice crystals increase in size, they are drawn to the Earth by gravity and contribute to the rain- and snow-inducing lower tropospheric condensation nuclei.
In many arid parts of the world, including here in California, artificial clouds from air traffic are more common than natural tropospheric clouds. That is, most days that you see clouds they were formed by contrails produced from jet air traffic.(That’s not including the western coastal low clouds and fog. After the fog clears, you can often see artificial clouds much higher, most commonly from September through June; July and August typically have the least contrail clouds since the weather is dry and no cloud seeding is usually taking place… weather modification programs for precipitation enhancement only work when there is a chance of rain from incoming moisture.)
So, sometimes artificial clouds affect the weather in the daytime, but the trillions of cubic feet of water vapor added to the upper atmosphere are now a constant influence on nighttime temperatures, even when they’re thin or invisible.
A hundred years ago they weren’t.