As we approach the targets given in Paris, the amount of precision we need on
these allowable carbon budgets — to meet the temperature changes — is going to get sharper and sharper, and so we're going to need better climate models to address those carbon budget issues.
Research efforts on concepts like «tolerable windows» and «pathways» and «
allowable carbon budgets» and the such are all rooted in this notion.
The figure below shows various estimates of
the allowable carbon budget to keep temperatures well below 1.5 C, where «well below» implies a 66 % chance of keep temperatures below 1.5 C warming (and a corresponding 33 % chance of exceeding the target).
What had started out as a simple communication tool has become quite complicated, with different studies getting very different results as to
the allowable carbon budget for very low emission pathways like 1.5 C.
Limiting warming to 1.5 °C requires staying within
an allowable carbon budget.
I selected ~ 1 C as the target, but even if I had selected high chance 2 C, there would still be
no allowable carbon budget remaining.
Today's paper compares
this allowable carbon budget with scientists» best estimate of how much oil, gas and coal exist worldwide in economically recoverable form, known as «reserves».