SIMPLE: Heat in minus heat out equals change of heat. Just as a pot of water warms up when more heat is put into the bottom than escapes the top, so the Earth warms when more sunlight is absorbed than outgoing heat emitted. Earth floats in a vacuum so the only heat exchange with the rest of the universe is through radiation. The air is selectively transparent: incoming sunlight passes through but outgoing infrared is absorbed by trace greenhouse gases and then re-emitted. Adding CO2 by burning carbon increases the absorption and re-emission of IR.
SERIOUS: Every bit of coal, oil, and gas we burn increases the amount of IR-absorbing CO2 and thereby warms the Earth. For all practical purposes the warming is permanent. Warming will get worse and worse without bound until we stop making it worse. Global warming is guaranteed to produce extreme heat waves, torrential rainfall, rising seas, increasing water demand, and wildfires. The consequences of unmitigated warming will become totally unacceptable to people, ecosystems, and the global economy. Without strong policy, global warming will quickly become the worst problem in the world.
SOLVABLE: The only way to prevent global catastrophe is to stop setting carbon on fire! More than 90% of the carbon burned is coal, oil, and gas. Two key components of the solution are (1) to live well with less energy and (2) to make energy without combustion. These are big asks especially since 1 billion people are energy poor, but solutions are at hand. Energy efficiency costs negative money, and we can use savings from efficiency to finance clean energy production & distribution. Economists estimate the cost of saving the world to be more than we spend on mobile phones but less than we spend on cars. Technically and economically this is a solved problem. The only impediment is policy.