There has been much discussion recently about clouds and feedback. The Spencer and Dessler debate, and the blog hosted science being done around the issue has captured a lot of attention.
Let’s take a look at the primary cause of the change in temperature over the last few decades. This graph is put together from two graphs which appeared on Skeptical Science, with a correction to the Y axis, which John Cook has two orders of magnitude too small. It’s been that way for over a year, despite two separate efforts on my part to get him to correct it. The data comes from two sources; the ISCCP international cloud project which uses weather satellites and integrates the data to produce a global series, plus data from the Earthshine project, which measures the amount of light being reflected from the Earth and bouncing back off the new Moon.
According to the Palle et al graph of Earthshine from 1999.5 to 2007.5, a percentage point change in albedo is equivalent to around 3.5W/m^2 entering the atmosphere from the Sun. As you can see, from the early ’80’s to around 1997.5 this means a big change of around 10W/m^2 at the top of the atmosphere. Allowing for the Sun only illuminating one side of Earth and for obliquity towards the limbs and poles by dividing by 4, this means an average additional insolation forcing of around 1.25W/m^2 on the atmosphere, land and most importantly, the oceans over the period 1984.5-1997.5. Clearly, it was a very significant contributor to the surface warming seen over the period, which amounted to approximately 0.2C globally. My calculations on sea level change indicate this forcing would account for most if not all the increase in the steric sea level measured by TOPEX/JASON. Something had to be responsible, because as we know from previous lengthy debates here and on WUWT, back radiationdoesn’t heat the ocean, and the time to equilibrium from a lowered air/ocean temperature differential would be too long to account for the accelerated sea level rise.
Because it is such a strong factor in the surface warming, we naturally want to know what caused the changed in cloud albedo. This is a contentious area, the IPCC barely discusses the change, let alone the cause of the change. The Svensmark effect is still a long way from confirmation although it looks promising. Another possible explanation is the drop in specific humidity at the 300 millibar level up near the tropopause, noted in a previous post on this blog. The specific humidity level corresponds closely to the variation in solar activity, according to the NCEP re-analysis data.
If as the IPCC claims, increased temperature results in more evaporation and so more cloud, this is clearly a second order effect, since cloud amount reduced as temperature rose over the 1980’s and 1990’s. But temperature is only a measured symptom of physical causes, not a real physical force in itself. If as the IPCC claims, the cloud feedback is positive, then logically, this is due to a reduction in cloud amount as empirically measured. This means the cause of this ‘positive cloud feedback’ is the extra energy from the Sun entering the oceans, not ‘temperature’. The increase in temperature is therefore largely a consequence of the reduction in cloud amount related to changes in solar activity, not the cause.