Climate commitment in an uncertain world

Several studies have looked at what would happen if human emissions of carbon dioxide were stopped, but these studies have overlooked the role of aerosols and other greenhouse gases such as methane and nitrous oxide, which Armour and Roe include in a new study. If human emissions stopped, atmospheric aerosols, which cool the planet by blocking light from the surface, would fall to preindustrial levels within weeks. However, other greenhouse gases would remain in the atmosphere for decades to centuries, and elevated levels of carbon dioxide would persist for millennia.

The authors find that the sudden loss of cooling from aerosols would cause abrupt warming of as much as 0.9 degrees Celsius (2 degrees Fahrenheit) above current temperatures in the decades following the cessation of greenhouse gas emissions. This could be followed by slight cooling or by further increased warming. Because there are large uncertainties in current observations of aerosol radiative forcing, there are large uncertainties in determining the amount of climate changes to which the Earth is already committed.

The authors note that Intergovernmental Panel on Climate Change's models produce a narrower range of climate commitment than is allowed by constraints based on current observations.

Although there is large uncertainty, the authors find that fundamental constraints provide lower and upper bounds on climate commitment. Given these bounds, if all human emission stopped immediately, Earth would still not return to pre-industrial temperatures in the indefinite future, and we could already be committed to dangerous levels of warming.