What is aerosol radiative forcing?

There is a lot of research about the climate impact of aerosols. One of the fundamental measures of the climate impact is the "radiative forcing" associated with aerosols. It's not obvious what exactly aerosol radiative forcing is, however, so here we begin our examination of this question.

The IPCC AR4 provides a nearly useless description: [LINK]

We can discern two important facets of aerosol radiative forcing from that description:

1. It is measured based on top-of-atmosphere (TOA) radiative fluxes. 
2. It includes the impact of aerosol on clouds. 

It is also useful to look at other parts of AR4, where better text describes aerosol effects. I started at the link above because when I search for "aerosol radiative forcing" that is one of the top hits I get. That's an unfortunate hit because the text surrounding that small section is much more informative.  

The first thing that can be clarified is that #1 above is part of the definition of radiative forcing. As far as IPCC reports go, radiative forcing is the impact that a forcing agent has on the net TOA fluxes. The concept is useful because it is derived from the basic physics of conservation of energy and thermodynamics. In equilibrium the net TOA flux is zero (averaged over a year, or many years). When a forcing agent is applied to they system, such as anthropogenic aerosol, the energetic consequence may be a change in that TOA balance (i.e., a radiative forcing), and having a TOA imbalance causes the system to respond. We deduce that if the forcing is negative the system will cool to achieve a new balance, but if the forcing is positive (i.e., more energy is entering the system than leaving) the system will warm to achieve a new balance. Aerosols typically fall into the negative forcing category, and so cause a cooling, but the story is not really so simple.

In particular, it is helpful to split aerosol effects into two pieces:

1. direct effects of aerosol particles on radiative transfer through the atmosphere (scattering and absorption) (aka, aerosol-radiation interaction, ari)
2. indirect effects of aerosol that change the radiative properties of clouds, or change the lifetime of clouds (aka, aerosol-cloud interaction, aci)

The IPCC AR5 [LINK] includes a lot of treatment of aerosol radiative forcing. Since it's newer, perhaps we should focus there for some clarity on this issue. An important distinction is drawn in AR5 between radiative forcing (RF) and effective radiative forcing (ERF). While RF is just what we were describing, namely the change in the TOA net flux (allowing adjustment of the stratosphere), but ERF allows the troposphere to also adjust to the forcing agent. To establish ERF is tricky because it does not allow the global average surface temperature to adjust; the idea is that ERF includes tropospheric "rapid adjustments" to occur, while RF only allows for the rapid stratospheric adjustment to occur. Confused yet? 

We will return to this distinction in another post. For now, we need to consider that both direct and indirect effects have a RF but also an ERF. This complicates the picture because it further muddies the water with respect to how we describe how aerosols effect the climate system. Mostly AR5 seems to deal with RF associated direct aerosol effects and ERF for indirect effects.  For now, though, let's return to our basic question of what is aerosol radiative forcing.

Based on IPCC AR4 and AR5, along with a lot of literature reviewed therein, and also my own literature review that spans from the 1980s to today, the easiest way to express the meaning of aerosol radiative forcing is:

Aerosol radiative forcing is the change in TOA radiative fluxes between the preindustrial period and the present day. The aerosol radiative forcing can be divided into direct effects in which aerosol effects radiative transfer and indirect effects in which aerosol interacts with clouds. 

Estimates of the total direct aerosol radiative forcing is around -0.35 (-0.85 to +0.15) W m^-2. Including indirect effects switches to using the ERF concept, which we will examine in another post, but the AR5 bottom line is that the total aerosol effect is a negative forcing of about -1 W m^-2, but that is basically plus or minus 1 W m^-2. 

What I want to point out before closing is that I described RF in the beginning as fundamental, but the definition that I've just provided seems far from fundamental. When we use this definition of aerosol radiative forcing, we need to define what pre-industrial means and what present day means. We know intuitively what both are supposed to mean, but quantitatively this is ambiguous. Particularly troublesome is that we do not have adequate observations from pre-industrial times to really know what the aerosol concentrations or emissions were. This provides an irreducible uncertainty for aerosol radiative forcing using this definition.  We will revisit some of these concepts in future posts, and we will return to the difficulties associated with this definition of aerosol radiative forcing.