CONSTRAIN’s new Knowledge Gains: Summary and Implication Report (KGSIR) details the latest scientific knowledge on rapid adjustments – the rapid changes to the atmosphere when greenhouse gases, aerosols or other “forcing agents” are introduced.
This is the latest in a series of updates covering topics ranging from effective radiative forcing and climate sensitivity to carbon budgets and climate projections. For each of these, we will outline key findings and assessments from the CONSTRAIN project and their implications for climate science.
Radiative forcing happens when external influences on the climate system, such as greenhouse gases and aerosol particles, lead to a change in the Earth’s energy balance. Positive radiative forcing means a shift in the energy balance so that the Earth gains heat and the climate warms, whilst negative forcing has a cooling effect.
There are different aspects to radiative forcing: instantaneous radiative forcing (IRF) is the sudden change in energy balance at the tropopause (around 13km up in the atmosphere) when greenhouse gases, aerosols or other forcing agents are introduced.
Forcing agents can also cause additional changes to the atmosphere, in terms of heat, clouds, or humidity. These changes are called rapid adjustments: they act within a relatively short time and can either increase or reduce the overall radiative forcing.
Combining rapid adjustments with instantaneous radiative forcing gives us a measure called effective radiative forcing, which is a better measure of how global mean temperatures will eventually respond than IRF alone.
However, rapid adjustments behave differently for each forcing agent and their overall patterns can be complex. For greenhouse gases, rapid adjustments can increase temperatures in the stratosphere (higher up in the atmosphere, above the tropopause), but reduce temperatures in the troposphere (nearer to the Earth’s surface).
Aerosol rapid adjustments meanwhile tend to affect clouds. Increasing aerosol concentrations lead to increased cloud cover, almost doubling the negative (cooling) radiative forcing from cloud-aerosol interactions; but this is offset somewhat by a decrease in the amount of water mass they contain.
Gaining a better understanding of these rapid adjustments will improve our knowledge of how different forcing agents contribute to climate change, feeding into global climate models and assessments.
We will provide further updates on rapid adjustments as the project progresses.
Rapid adjustment processes: Atmospheric perturbations can cause (A) horizontal and (B) vertical gradients in radiative heating (dotted orange) or cooling (dotted blue) that drive circulation. Similarly, land and ocean surfaces may respond to changes in atmospheric composition on different time scales, with heating gradients and circulation changes (C). Changes in direct vs. diffuse solar radiation, and in CO2 concentrations, induce changes in plant growth and water use efficiency, with impacts on water vapour fluxes and cloudiness (D). Aerosol particles cause specific cloud modifications (unpolluted, E, vs. polluted cloud, F).