For scientists who have dismissed the 1.5 degree limit as impossible, the updating process might seem pointless. But Rogelj stresses that his team looked only at geophysical limitations, not political ones. Their report assumes that countries will agree to a zero emissions commitment—a much more ambitious scenario than other researchers have considered.
There is a misconception, Rogelj says, that the report claims to have found an inaccuracy in the Earth system models (ESMs) that are used to estimate human-driven warming. “We are using precisely those models to estimate the carbon budget from today onward,” Rogelj explains.
The problem is not the models, but rather the data fed into them. These simulations are often run using inexact projections of CO2 emissions. Over time, small discrepancies accumulate and are reflected in the warming predictions that the models make.
Given information about current CO2 emissions, however, ESMs make temperature predictions that are “quite accurate.” And when they are provided with an ambitious future scenario for emissions reduction, the models indicate that it is possible for global temperature increases to remain below 1.5 degrees.
So what would such a scenario look like? First off, emissions have to fall to zero. At the same time, the carbon budget needs to be continually reevaluated, and strategy changes must be based on the updated budget. For example, if emissions fall to zero but we’ve surpassed our carbon budget, then we’ll need to focus on making our emissions negative—in other words, on carbon dioxide removal.