Emissions and lags in the climate system

The climate system responds to human influences on a range of different time-scales. Changes in the net amount of the sun’s energy absorbed by the Earth, known as the ‘radiative forcing’, happen quickly in response to emissions of greenhouse gases. For many other climate metrics there is a slower response to a change in radiative forcing, of the order of decades or more for temperature and centuries or more for sea-level rise. Realising the full effect can take more than a thousand years. Therefore, it is prudent to ask:

• When human emissions of greenhouse gases or aerosols lead to a change in radiative forcing, how long will the system continue to experience warming or sea-level rise?
• If we change our emissions, with a resulting change in radiative forcing, how long before the difference in warming response becomes detectable in the real climate system?
• Are time lags the same regardless of whether we are increasing emissions or decreasing them, even to the point of removing more greenhouse gases than we are emitting (known as net negative emissions)?

Because there is a limit on how fast emissions might be reduced, and further there is a time-lag in the response of the climate system to the emission changes, there is also a level of warming we are committed to. Thus, it is also useful to ask:

• How much future warming are we already committed to, given the constraints on future emission scenarios?
• What does this mean for adaptation planning under different mitigation scenarios?

When discussing commitment, the early literature focused on constant concentration (or constant radiative forcing) commitment. This involved looking at warming and other climate changes if stabilisation of atmospheric concentrations of greenhouse gases or aerosols suddenly takes place. Today there is more focus on the commitment if emissions of greenhouse gases or aerosols are suddenly zeroed.

In this Met Office briefing note on lags in the climate system, we consider evidence around the changes in climate associated with adjustment of the emissions for both theoretical cases of suddenly zeroing emissions, and the more policy relevant commitment to emission scenarios with potentially feasible emission reductions.