Back in the day, the IPCC published its 3rd assessment report and declared a high confidence in the science of doom – humans have caused the unprecedented modern warming and the future is very bleak.
In their view the science was well enough understood that climate models could be relied upon – especially as they predicted the past so nicely.
As well as their Summary for Policymakers they also published a Complete Report. It’s a very informative document and runs to just under 800 pages in 14 chapters, not including appendices. Worth a read for the serious student.
Chapter 5 examines the role of aerosols, which they introduced in Chapter 1 (p.93):
The effect of the increasing amount of aerosols on the radiative forcing is complex and not yet well known. The direct effect is the scattering of part of the incoming solar radiation back into space. This causes a negative radiative forcing which may partly, and locally even completely, offset the enhanced greenhouse effect. However, due to their short atmospheric lifetime, the radiative forcing is very inhomogeneous in space and in time.
This complicates their effect on the highly non-linear climate system. Some aerosols, such as soot, absorb solar radiation directly, leading to local heating of the atmosphere, or absorb and emit infrared radiation, adding to the enhanced greenhouse effect.
Aerosols may also affect the number, density and size of cloud droplets. This may change the amount and optical properties of clouds, and hence their reflection and absorption. It may also have an impact on the formation of precipitation. As discussed in Chapter 5, these are potentially important indirect effects of aerosols, resulting probably in a negative radiative forcing of as yet very uncertain magnitude.
(My emphasis added). A few snippets of interest from chapter 5, which runs to 60 pages:
p. 304, Under Summary of Main Uncertainties Associated with Aerosol Sources and Properties
Perhaps the most important uncertainty in aerosol properties is the production of cloud condensation nuclei (Section 5.3.3).
Earlier they lead up to cloud condensation nuclei being the tricky part – with the other bits being easier? p291-2:
.. An analysis of the contributions of the uncertainties in the different factors needed to estimate direct forcing to the overall uncertainty in the direct forcing estimates can be made. This analysis leads to an overall uncertainty estimate for fossil fuel aerosols of 89% (or a range from –0.1 to –1.0 Wm–2) while that for biomass aerosols is 85% (or a range from –0.1 to –0.5 Wm–2 ).
.. An analysis of the contributions of the uncertainties in the different factors needed to estimate indirect forcing of the first kind can be made. This analysis leads to an overall uncertainty estimate for indirect forcing over Northern Hemisphere marine regions by fossil fuel aerosols of 100% (or a range from 0 to −2.8 Wm–2).
.. The indirect radiative effect of aerosols also includes effects on ice and mixed phase clouds, but the magnitude of any indirect effect associated with the ice phase is not known. It is not possible to estimate the number of anthropogenic ice nuclei at the present time. Except at very low temperatures (<−40°C), the mechanism of ice formation in clouds is not understood. Anthropogenic ice nuclei may have a large (probably positive) impact on forcing.
So clearly the science was settled back in 2001.
(By the way, I think the scientists who put chapter 5 together did a great job and have clearly been hard at work trying to understand an extremely complex subject).
Of course, science moves forward so let’s take a great Six Year Leap to 2007 and the 4th Assessment Report.
What do we find? First let’s note that the radiative equivalence of the anthropogenic increase in CO2 is around 1.7 Wm–2 (the radiative equivalence can view the CO2 increase as if it was an equivalent amount of radiation from the sun).
Direct aerosol radiative forcing is now considerably better quantified than previously and represents a major advance in understanding since the time of the TAR, when several components had a very low level of scientific understanding. A total direct aerosol radiative forcing combined across all aerosol types can now be given for the first time as –0.5 ± 0.4 Wm–2, with a medium-low level of scientific understanding.
(My italics). Note that one aspect of aerosols is somewhere between no effect and cancelling out half the anthropogenic CO2 warming.
Anthropogenic aerosols effects on water clouds cause an indirect cloud albedo effect (referred to as the first indirect effect in the TAR), which has a best estimate for the first time of –0.7 [–0.3 to –1.8] Wm–2
And the other key climate impact of aerosols is somewhere between not much and cancelling out all of the anthropogenic CO2 warming.
Thank goodness we now have a much better understanding of aerosols from 2001 when we knew for sure that humans had caused – and would continue to cause – significant warming.
Now we can quantify the aerosol effect, we know that they either do nothing or completely cancel out anthropogenic global warming. Back in 2001, we didn’t have to let that dent our confidence!
Just as a little footnote, those IPCC skeptics had to try and calculate error bars, but at such a low level of scientific understanding the error bars themselves might be in question.
And another footnote, I used the “Report accepted by Working Group I of the Intergovernmental Panel on Climate Change but not approved in detail” because I downloaded this when it was first released in 2007.