Climate Econometrics and the Carbon Budget

Though I recently abandoned a follow up paper on our Journal of Econometrics climate modeling paper, we are now working on a different one. I'm scheduled to give a presentation (remotely) on it at the American Geophysical Union conference in December. In the course of our research, I ran some simple simulations on our Journal of Econometrics model. This model is a two equation vector autoregression of global surface temperature and radiative forcing with energy balance restrictions imposed. This is done using the concept of multicointegration. But it is still a simple time series model once the complicated estimation is complete.

I ran three scenarios that all have the same peak level of radiative forcing equivalent to doubling CO2:

Single Shock: Forcing is doubled in one year and then the system is allowed to move to equilibrium.

Shock and Maintain: Forcing is doubled suddenly and then that level of forcing is maintained forever. This is the scenario in our published paper and is used to estimate the equilibrium climate sensitivity in general circulation models.

Transient: Forcing is increased linearly for 70 years until the CO2 equivalent would be doubled. Then emissions are cut to zero.

This is what happens to temperature in the three scenarios:

Under the Shock and Maintain scenario, we reach the equilibrium climate sensitivity of 2.78ºC. Under the Transient scenario, the temperature increases by 1.85ºC when emissions are cut to zero and then continues to increase by about 0.3ºC before flatlining. Under the Single Shock scenario, temperature increases quite rapidly, reaching equilibrium in around 40 years with only a 0.98ºC increase.

This is what happens to radiative forcing in the three scenarios:

Under the Single Shock scenario there is a steep fall in forcing after the single pulse of greenhouse gases. A new equilibrium concentration and temperature is reached. Under the Transient scenario the equilibrium level of forcing is much higher even though in both cases emissions are cut to zero. Of course, much more carbon would need to be pumped into the atmosphere to achieve the Transient path as all the time carbon is also being absorbed. This shows the importance of the carbon budget. The total amount of emissions, not just the peak concentration matters. It is interesting that our very simple model seems to pick this up from the data without imposing any information about the carbon budget on the model.