The natural 60 year heat cycle recently observed in the Atlantic implies that the underlying trend of anthropogenic warming since 1942 has been only ~0.45C. This value results in a derived transient climate response (TCR) of ~1.5C. The conclusions of the AR5 attribution study now look questionable because they ignore any natural warming component post 1970.
A new paper Varying planetary heat sink led to global-warming slowdown and acceleration challenges the AR5 attribution statement that all observed warming can be explained by anthropogenic forcing alone because it shows clear evidence of a natural 60 year ocean heat cycle. This cycle is also evident in the global temperature data – see A 60 year oscillation. A long discussion on this paper can also be seen at Judith Curry’s blog.
Figure 10.5 in the AR5 attribution chapter is based on model comparisons from 1951 to 2010 is now looking rather unlikely because this result leaves no room for any natural component to warming, as shown below.
The new evidence of a significant oceanic warming and cooling cycle means that between 1950 and 2010 anything up to 50% of the rise in observed temperature was actually due to the warming phase of the 60y cycle. The error bars on the ‘ANT’ component in figure 10.5 are just too small to allow for this. If this is the case how can we best estimate the underlying anthropogenic component?
The fit to the H4 data in Figure 1 is based on the assumption of a logarithmic dependence on CO2 forcing and temperature response. The (transient) temperature response includes climate feedbacks and is to be measured. The CO2 forcing is given by which is derived in Radiative Forcing of CO2 and includes a 60y harmonic cycle which was previously identified as described in the post “A 60 year oscillation in Global Temperature data and possible explanations” .
The true anthropogenic component of warming can be identified by subtracting off the natural warming/cooling cycle. The peaks of the oscillation occur both in 1942 and 2008 so the rise in temperature between these two dates should measure the underlying human induced CO2 warming.
|Date||Anomaly (degC)||CO2 level (ppm)|
|1942||-0.01 ± 0.01||308|
|2008||0.44 ± 0.01||378|
Now assuming that this now represents the underlying anthropogenic warming between 1942 and 2008, we can measure a value for TCR as follows
with an error of about 0.1 C
Conclusion: The new paper by Chen and Tung provides independent evidence for a global 60 year warming and cooling cycle due to natural variations in AMOC. The IPCC attribution statement is based on model fingerprint evidence that all warming since 1950 can be explained by AGW. However this measured warming must contain a component from the AMOC warming cycle from 1970 to 2000. Only by comparing dates at similar positions in the cycle can the true anthropogenic component be identified. We choose 1942 and 2008 as the peaks in this cycle to show a net warming of 0.45±0.2 C. CO2 levels rose by 70 ppm between those two dates. This is then used to derive from observations a value for TCR = 1.5 ± 0.1 C.