Climate sensitivity, the long-term warming from doubled CO2 levels, is estimated between 1.5 °C to 4.5 °C. Regardless of the exact value, future emissions must be limited. The Earth's response to increased CO2 has been studied extensively, revealing insights into climate system timescales and limitations in observations and models. Estimates show that to limit warming to 2 °C, CO2 emissions must be strongly restricted, regardless of climate sensitivity.