Scientists have reported that the surface temperature of our planet has increased by 0.9°C since the onset of the Industrial Revolution. They have also warned that life on Earth will face unforeseeable adverse consequences and be seriously threatened if the increase in surface temperature exceeds 2°C; thus, the scientific community set an ultimate target to limit the rise in global temperature by this amount (2°C). Scientists have estimated that a maximum of 450 ppm (parts per million molecules) of CO2 should be released into the atmosphere to maintain this target. The atmosphere was estimated to hold 220 ppm of CO2 at the time of the Industrial Revolution.

The total allowable CO2 emission to contain the temperature rise at 2°C was calculated to be in the order of 2900 Gt (1 Gt = 1 billion tons). This was referred to as the global carbon budget, and approximately 1900 Gt of this allowance (65%) has already been spent. This leaves approximately 1000 Gt of allowable CO2 emissions to limit global warming to 2°C. To meet this challenge, various nations came together in Paris, 2015, to share their intended nationally determined contributions with the international community. From that point onward, nations were free to exercise their “own” commitments to contribute to the global reduction of emissions to meet the 2°C (or more technically the 450 ppm) target.

科学家报告说，自工业革命开始以来，我们星球的表面温度上升了0.9℃。他们还警告说，如果地表温度上升超过2摄氏度，地球上的生命将面临无法预见的不利后果，并受到严重威胁；因此，科学界制定了一个最终目标，将全球气温的上升幅度限制在这一水平（2摄氏度）。科学家们估计，要维持这个目标，最多应向大气释放450 ppm（百万分之几）的二氧化碳。据估计，在工业革命时期，大气中含有220 ppm的二氧化碳。

控制2℃温升的总允许二氧化碳排放量计算为2900 Gt（1 Gt=10亿吨）。这被称为全球碳预算，大约1900吨这一津贴（65%）已经被使用。这就留下了大约1000吨二氧化碳的允许排放量，以将全球变暖限制在2°C。为应对这一挑战，各国于2015年在巴黎汇聚一堂，与国际社会分享其由国家决定的贡献。从那时起，各国可以自由地履行其“自己”的承诺，为全球减排做出贡献，以达到2℃（或更严格地说是450 ppm）的目标。