To understand what this means, realize that climate change is fundamentally an energy balance problem.

Energy in for the Earth comes from the Sun, energy out comes from heat (IR radiation) back to space.
These are out of balance — energy from the Sun exceeds heat radiated to space. So the Earth accumulates energy.

Temperature can be thought of as a measure of energy, so as the Earth accumulates energy, temperature rises. This is the fundamental physics behind climate change.
If you took physics in college, you might remember that objects radiate more infrared (IR) energy as they warm up, so as the Earth warms, energy radiated to space increases.

Eventually, the Earth will warm enough that the energy imbalance will be wiped out.
So energy imbalance is presently +0.87 W/m2/K. How much more does the Earth have to warm to wipe out that imbalance? That connection between energy imbalance and temperature is known as the climate sensitivity.
If climate sensitivity is ~0.75 K/(W/m2) (corresponding to ~3°C for doubled CO2), then that tells us that we need to warm the climate about 1°C *more* to reach energy balance, at which point the Earth will be in energy balance.
Except it won't be. We're still emitting carbon dioxide, so by the time the planet has warmed enough to wipe out the +0.87 W/m2 of energy imbalance that we measure today, we'll have emitted a whole lotta CO2 that will have increased the energy imbalance.
As long as we emit CO2, the planet will be out of energy balance and warming.

Only if we reach net zero CO2 emissions will we be able to stabilize the climate.
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