In 2022 we have already increased the global average temperature by 1.3 °C (compared to preindustrial levels). Reducing all emissions to zero would make the natural carbon capturing processes dominate and CO2 concentrations decrease for some decades. In this scenario, however, the global average temperature is predicted to stay roughly constant, the adverse effects of climate change we are experiencing today not alleviated. To restore the average temperature to preindustrial levels, large-scale carbon capturing methods will be necessary.
In the game, it is necessary to research and build these carbon sequestering solutions to reduce the carbon bar on the right to zero. To me, this seems intentional.