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Global warming is caused by the rising level of greenhouse gases in the atmosphere. The science behind this is well understood. Because the level of these gases in the atmosphere is cumulative and only slowly returns to equilibrium naturally, all additional CO2 (and other greenhouse gases) directly results in increased heating effects. Human activity based on fossil fuels is releasing additional carbon which was stored in the ground over millions of years.
The resulting effect on the ecosphere is highly complex, since individual subsystems react in different, complicated and interconnected ways. Some of the resulting changes also result in feedback loops which reinforce the warming effect. These tipping points can result in catastrophic changes which are potentially irreversible. And, because the level of greenhouse gases in the atmosphere reaches global equilibrium very quickly, every local emission affects the whole world globally in short order.
For an easy-to-read summary of the science: myclimate Climate Booklet 2020.