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Temperature forecasting given that countries meet their NDCs

Procedure

We start from the probabilistic forecasts of the CO2 emissions for all countries and regions given the current trend, with the same procedure as in the previous section. We also summarized the NDCs and INDCs submitted to the Paris Agreement and calculated the target year emissions or intensity. Then for each trajectory and each country, if the forecasted emissions or intensity is higher than their promises, we set an additional decline rate on intensity to move that trajectory, so that it matches with their promises.

Specifically, for countries promising cuts in emissions intensity such as China and India, suppose the promised intensity is JT in year 2015+T, and the forecasted intensity for that trajectory is IT (> JT). Then we need an extra annual intensity cut of $a=1−(JTIT)1/T$ from 2016 to the target year. This leads to an expected annual change in the GDP per capita gap by a factor of $(1−a)ρσGDP/στ$, where ρ is the correlation between carbon intensity and GDP per capita.

For countries promising emissions cuts directly, such as European countries, suppose the promised emissions are FT in year 2015 + T, and the forecasted intensity for that trajectory is ET. We denote the required extra intensity cut by a per year. Then at the target year 2015 + T, the intensity will be multiplied by a factor of (1 − a)T , while the frontier gap of GDP per capita will be multiplied by a factor of $(1−a)TρσGDP/στ$. Since our goal is to increase the emissions cuts in line with commitments or targets, we get the required intensity cut a by solving equation (6) as follows:

For years from 2015 + T to 2100, we forecast each country’s intensity and GDP per capita under different scenarios. Under the “Adjusted” scenario, the intensity and emissions forecast for each trajectory between 2015 + T and 2100 is multiplied by the ratio of promised and forecasted intensity and GDP per capita at the target year 2015+T. Under the “Continued” scenario, for each year 2015 + t from 2015 + T to 2100, assuming the extra annual intensity cut is a factor a, then the intensity forecast is multiplied by the factor of (1 − a)t, and the forecast frontier gap of GDP per capita is multiplied by the factor $(1−a)tρσGDP/στ$.

Lastly, we reconstruct the CO2 emissions forecast for all countries and regions, and follow the same procedure with the modified CO2 emissions forecast for global mean temperature forecast for different scenarios.

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