by Judith Curry
‘Nature is about to carry out a very interesting experiment’
The quote is from the Daily Mail article ‘Forget global warming: its Cycle 25 we need to worry about.’ On the Week in Review thread, I made the following statement in reference to the article:
We don’t know what the climate will be for the next several decades, there are a number of reasons to expect the continue flat trend for the next several decades. In terms of when global warming will come “roaring back”, it is possible that this may not happen for the first half of the 21st century.
I am being asked what is meant by “possible”. More than 50%? Well the whole situation is too uncertain to put probabilities on, IMO. Hence my previous suggestions for looking at a “possibility distribution” rather than trying to guess at probabilities. And for generating scenarios via scenario falsification.
The nominal IPCC AR4 scenario for the period 2015-2050 has two elements: (1) creation of a suite of emissions scenarios and (2) climate model simulations forced by the emission scenarios. A range is given for the scenario based on different emission scenarios (which aren’t all that different prior to 2050) and simulations by different models with different sensitivities. CMIP5 for AR5 has decadal simulations (10 year and 30 year), and the UK Met Office is making ongoing 10 year predictions. Initialized decadal projections are in their infancy, but these do provide some new model generated scenarios out to 2035.
An alternative method for future scenario generation was described in the previous post:
Modal falsificationism further permits creatively constructed scenarios to be accepted as long as the scenarios cannot be falsified by being incompatible with background knowledge. Developing a suite of scenarios by modal falsification is a two step process: the first step is coming up with as many potential future scenarios as possible (using inductive as well as other more creative methods) and then submitting these future scenarios to tests in order to see which ones can be discarded as impossible.
So lets generate some scenarios for the period 2015-2050, based upon possible combinations of external forcing and natural internal variability:
Solar forcing (S):
- I) same as average for 1980-2000;
- II) higher than average
- III) lower than average
Volcanic forcing (V):
- I) same as average for 1980-2000;
- II) higher than average
- III) lower than average
Anthropogenic forcing (A):
- I) same as Representative Concentration Pathways RCP 6 for AR5
- II) higher than RCP 6
- III) lower than RCP 6
Natural internal variability (N):
- I) same as for 1950-2000
- II) warmer than 1950-2000
- III) cooler than 1950-2000
For 21st century projections (implicitly including the period of interest here 2015-2050), the AR4 looked only NI-VI-SI with variations in A. It is obvious from consideration of the list above that there are dozens of possible combinations for the period 2015-2050.
The key scenarios of interest here are those that could counteract the anthropogenic forcing to result in minimal warming, a flat temperature curve, or even cooling. SIII, VII, and NIII would all contribute to cooling. There are then two key questions:
- To what extent is cooling from S, V, N more likely than not for 2015-2050?
- How would the cooling effects interact with anthropogenic forcing to determine the magnitude of the future climate change?
With regards to solar forcing, the balance of evidence that we currently have points to lower solar forcing for ~90 years.
With regards to volcanic forcing, as far as I know, we have absolutely no idea, and I, II, III are equally possible.
With regards to natural internal variability, we are currently in the cool phase of the PDO. Based upon the recent historical record, we would anticipate several decades in the cool phase, although these oscillations aren’t predictable. We are currently in the warm phase of the AMO, and based upon the recent historical record, we might expect another decade in the warm phase, although these oscillations aren’t predictable.
—-quick retake of missing text
Previous warm AMO/cool PDO occurred 1946-1964, and cool AMO/cool PDO 1964-1976, both of which were cool periods.
It is a plausible scenario that we will continue to see relatively flat trend in temperature for the coming decade. The most recent climate shift has been argued (Tsonis et al) to have occurred 2001/2002. A climate shift would probably occur sometime before 2050, although whether the shift trends warmer or cooler remains to be seen. At some point a warming trend would likely resume. To me this seems like a more plausible scenario than the 0.2C/decade projection to 2050 from AR4.
So the crux of the debate is whether S, V, N are minor noise on a substantial trend from AGW, or whether S,V,N will dominate for the next several decades. We don’t know. Personally I am not convinced by the AR4 argument that S, N are minor noise for the 20th century attribution. Until the warming from 1910-1940 and the cool period from 1940’s to 1970’s are convincingly explained, I find it difficult to think than S, N aren’t important in explaining the variability earlier in the century.
So with a change in N and S underway, nature is conducting an interesting experience for us.