We need zero carbon dioxide emissions (and greatly reduced methane emissions) because of the planet's carbon cycle.

The various systems on Earth can remove carbon dioxide only at a very slow rate. At the same time, our fossil fuel economy is dumping CO₂ into the atmosphere at a very fast rate. The amount in, minus the amount out, is what's left behind in the atmosphere.

Earth's slow systems involve converting carbon into rock (for example, creating limestone, which is calcium carbonate), which is an ocean since process, and converting carbon into fossil carbon (coal, natural gas, oil), which is a land sink process.

But both of these processes take thousands, if not millions, of years. As a result, 20% of all carbon dioxide emissions remain in the atmosphere for 1,000 years. A millennium. At least 50 generations of humans.

To give the planet the chance to remove some of the CO₂ that has been around for the past 900+ years, we need to move immediately to zero carbon emissions. Even better would be to develop processes that allow us to remove carbon from the atmosphere, but first things first.

We have to get to zero carbon dioxide emissions as soon as possible!

Why? Because of the very long delay between the reduction and any atmospheric response, including reduced CO₂ and lower global temperatures. Even if we stop all emissions now, atmospheric carbon dioxide levels will continue to increase for decades, although at a much slower rate. Once CO₂ concentrations have stabilized, then they will begin to drop naturally. But that will not occur for a century or more.

Temperature changes respond even more slowly. Temperatures continue to climb, slowing down over the centuries, for over 1,000 years after the CO₂ was cut.

As it is, we do not know if an immediate cut to zero carbon emissions will avoid the impending catastrophe. But not cutting to zero ensures that we will face the climate catastrophe.

Fossil fuels are a cornerstone of every major economy. Unfortunately, as long as we continue to burn them, we will continue to add carbon dioxide to the atmosphere. But CO₂ is not the only greenhouse gas.

Fossil fuels also emit nitrous oxide and methane, and indirectly result in ground level ozone (while ozone protects us from dangerous radiation at high altitudes, at ground level it's a pollutant and a greenhouse gas).

The planet's carbon sinks, the oceans and plant life, have taken up more than half of the CO₂ emissions since we started using fossil fuels.

The oceans are the ultra long carbon sink. Plankton convert carbon into calcium carbonate. As plankton die, they fall to the ocean bottom. Pressure and time combine to create limestone and dolomite, effectively removing carbon indefinitely.

Plant life is the short-term, temporary carbon sink. As plants grow, they remove the carbon from carbon dioxide and use it to make cellulose, the building block of plant structures. But as these plants decompose, or are burned, the carbon is released back into the atmosphere. This process takes as little as a few months, and as much as a few hundred years. Not enough time to be an effective tool for cleaning the atmosphere of carbon dioxide.