Um, climate scientists who spend their whole lives studying this?
It's really pretty simple. CO2 stays in the atmosphere for centuries to millennia. Every molecule of CO2 in the atmosphere warms the planet by trapping some of the heat escaping from the earth's surface. The more CO2, the warmer it gets. Same for other greenhouse gases. Stop adding these gases to the atmosphere and temps level off, no longer increasing but not decreasing either.
Yes, there are other factors besides net zero - there are feed-back loops and tipping points that can cause more melting of permafrost, thus releasing more methane (more potent but shorter lived than CO2), which leads to more warming, in a disastrous loop, or when changing ocean temps disrupt major ocean currents which lead to radical changes in weather patterns. But stopping adding greenhouse gases to the atmosphere is key to limiting warming.
Sadly we have not chosen to prioritize this yet. All of the disasters happening now are happening at 1.1-1.2C of warming. Every additional tenth of a degree of warming will make these worse and more frequent. Right now we are in a path to around 3.2C according to the last IPCC report. That is a dramatically different planet than the one we live on today. We will most likely surpass 1.5C around 2033, and 2C by mid century. This is our problem, not just our grandkids problem.
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