Predictable and unpredictable behaviour

Other examples include distinct oscillatory models of variability with decadal and longer timescales, related also to oceanic Rossby wave propagation and gyre spinup processes, or timescales associated with the AMOC.

It is possible to get enhanced variability on those timescales as a result of dynamical mechanisms without needing to appeal to higher climate sensitivity.

Nevertheless, the bottom line is that Curry must prove that the feedbacks involved in the natural variations are different to those affecting the climate sensitivity before she can conclude that natural variability dominates over a warming due to increasing greenhouse gases.

It’s not the sun

When Curry believes that the changes in earth’s temperature are due changes in the sun, it is important to keep in mind that the variations in the sun only affect as a small fraction of earth’s energy input. Amplifying feedback processes are needed to explain the magnitude of the observed changes.

Curry makes a point of the temperature increase before the 1940s, and that the CO2 concentrations were low then. But she seems to have forgotten that the forcing is proportional to the logarithm of the concentration: the effect of an increase is initially higher with lower concentrations.

The changes in the climate before 1940 were a result a combination of factors when there was an increase in the number of sunspots that coincided with increasing CO2-concentrations.

It is well-known that the sunspot record suggests an increase up to the 1950, but various solar indicators indicate no long-term trend in the sun since the 1950.

Only the increase in the greenhouse gases can explain a forced warming since the 1950s because no other physical forcings exhibit long-term trends since then.

Problematic statistics

Another issue is that early temperature record does not give as complete global data coverage as more recent measurements. The global temperature analysis is based on smaller sample in the early part, for which we expect to see stronger random sampling fluctuations.

This is consistent with what Figure 4 in Curry’s report shows. However, she misinterpreted this as being strong natural variability in the early part of the record.

Curry also makes the same mistake as John Christy by using the ensemble mean as a yardstick for the models (here): model evaluations must be based on the individual simulations taking into account the spread of the ensemble run.

It’s not just the temperature

The climate sensitivity is one indicator for the consequences of a global warming which only accounts for the change in temperature, but it is important not to ignore that changes in the global hydrological cycle may also have a severe impact on society.

It is possible that a weaker temperature increase is associated with a larger shift in the convective activity and more pronounced changes in the rainfall patterns (Benestad, 2016).

The comprehensive picture and consistency

I often find it useful to look at the comprehensive picture in science and look for consistencies, both when it comes to physics and the logic.

A curious twist in Curry’s report is (a) her claim that climate models have exaggerated climate sensitivity because they did not reproduce the observed warming over the 2000-2015 period and then (b) her emphasis on natural variations having scales of “weeks, years, decades, centuries and millennia”.

If the claims hypothetically were correct, then how would she know that the temperature variations over brief intervals are not just a result of the natural variations that she emphasised?

We should expect some brief periods with both rapid as well as slow warming (Easterling and Wehner, 2009), and some of the model simulations have indicated a weak warming over the same period. This is explained in the IPCC AR5 (Box 9.2).

Another question is whether the warming rate reported by the AR5 was correct, and more recent studies suggest artificially weak warming connected to changing observational networks (Karl et al, 2015). This has been discussed here. Hence, Curry’s claim about slower warming rates has lost substance.


There is a curious remark in Curry’s report about the climate models’ inability to match the phase and timing of the natural variations. Yes, it is true, but it is also a well-known fact.

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  1. R.E. Benestad, "A mental picture of the greenhouse effect", Theoretical and Applied Climatology, 2016.
  2. D.R. Easterling, and M.F. Wehner, "Is the climate warming or cooling?", Geophysical Research Letters, vol. 36, 2009.
  3. T.R. Karl, A. Arguez, B. Huang, J.H. Lawrimore, J.R. McMahon, M.J. Menne, T.C. Peterson, R.S. Vose, and H. Zhang, "Possible artifacts of data biases in the recent global surface warming hiatus", Science, vol. 348, pp. 1469-1472, 2015.