Using economic tools such as the Lorenz Curve and the Gini Index with adjustments to per capita historical cumulative emissions, this paper constructs a Carbon Gini Index to measure inequality in climate change discussions. The analysis using the Carbon Gini Index shows that 70 percent of carbon space in the atmosphere has been used for unequal distribution, which is almost the same as that of income in the country with the biggest gap between rich and poor in the world. The paper uses historical cumulative emissions per capita to quantitatively analyse the impact of the starting year on a party’s historical responsibility – a significant stumbling block in climate negotiations. The authors assert that different starting years have had a substantial influence on the Gini index value; the later the starting year, the lower the Gini index. It is noted that in terms of global level carbon, the Gini index does not change substantially when comparing the 1850 starting year and the 1900 starting year. However, the 1950 starting year and the 1990 starting year have been largely underestimated in terms of carbon inequality. The paper concludes that irrespective of the starting year, there exists inequality of carbon space. Therefore, future allocation needs are a key point of contention.