Part I introduced the challenges of climate prediction on decadal scales, specifically in the context of global climate model simulations. On the Part I thread, Paul_K writes:
While a small effort on upgrading numerical methods in GCMs might well be justified, the sort of large-scale attack on regional-scale decadal prediction using dynamic modeling which you describe here seems to be far too early, given the level of understanding of the climate system and the maturity of the GCMs – even if one were to accept the argument for the utility of such predictions. It is worth examining the history of development of dynamic simulators in other disciplines. In every instance that I am familiar with, the application of improved numerical methods – dynamic local grid refinement, flexible gridding, local options on type of solution routine and boundary condition characterizations – were all introduced to allow improved local characterization and reduced numerical error only AFTER certainty had been gained in the completeness and aptness of the governing equations to be applied. The [current] models cannot obtain even a coarse match of key observational data in hindcast, and therefore cannot be expected to found sensible boundary conditions to support a local grid refinement scheme in any useful way.
The anticipated 0.2C/per decade increase in surface temperature in the coming decades may be only a small part of the climate picture relative to natural variability for the period 2010-2030. So given the current challenges for climate models, how can we make more useful projections of natural climate variability, both global and regional? Empirical, statistical modeling is one way to approach this.
Lets assume that the anthropogenic forcing from long lived greenhouse gases and aerosols can be specified reasonably well out to 2030. That leaves us to consider solar variability, volcanic variability, the multidecadal ocean oscillations, and possibly other terrestrial and extraterrestrial factors.
Multidecadal ocean oscillations
One of the key skeptical talking points has been that climate change over the past century can mostly be explained by the multidecadal ocean oscillations, particularly the PDO and AMO. This essay by Roy Spencer typifies this argument.
A new paper by DelSole et al (h/t Craig Loehle) identifies a significant component of unforced multidecadal variability in the recent acceleration of global warming, which they identify as the AMO. This is interesting because it comes from a mainstream climate modeling group. The mainstream is increasingly recognized the unforced multidecadal ocean oscillations as key elements in 20th century attribution and 21st century projections.
The two main ocean oscillations that get discussed are the AMO and PDO, although the NAO and NPGO also get mentioned and deserve consideration in this context. If you are unfamiliar with the multidecadal ocean oscillations, I just spotted a really interesting online textbook “Oceans and 21st Century Climate”, see this section.
We are currently in the warm phase of the AMO (since 1995) and the cool phase of the PDO (since about 2008), with an expectation of remaining in this regime for the next 1-2 decades. The last period that we saw this particular AMO-PDO combination was 1946-1964, a period that was characterized in the U.S. by abundant landfalling major hurricanes and drought in the southwest.
The issue of future projection then becomes to estimate the duration of the current regimes (e.g. warm AMO, cool PDO), particularly the changepoints. The changepoints can be estimated statistically, or decadal climate simulations could be used.
What’s going on with the sun?
I am a novice in this area, and have no idea which are the key references on this topic, but here are some current papers and web posts that caught my eye, I would appreciate some clarification/synthesis of all this from those of you that follow this more closely. Note, on establishment climate blogs like Realclimate, not much mention of the sun, what appears to be their main article on this was written in 2005. Also, i just spotted the blog Heliogenic Climate Change, which is about the sun (but also climate change politics).
The CMIP5 simulations recommend the following for solar forcing. Given the uncertainties and problems with forecasting the current solar cycle, I’m surprised they aren’t exploring the impact of solar uncertainties on future climate. Especially given the controversy regarding the PMOD vs ACRIM total solar irradiance (see section 7 in Scafetta’s paper).
David Archibald has guest post on WUWT that argues another Dalton minimum is shaping up. See also this article by Livingston and Penn. http://www.leif.org/EOS/2009EO300001.pdf
Akasofu has a recent paper entitled “On the recovery of the Little Ice Age.” He argues that “It is suggested . . . that the Earth is still in the process of recovery from the LIA; there is no sign to indicate the end of the recovery before 1900. Cosmic-ray intensity data show that solar activity was related to both the LIA and its recovery. The multi-decadal oscillation of a period of 50 to 60 years was superposed on the linear change; it peaked in 1940 and 2000, causing the halting of warming temporarily after 2000.”
A recent paper by Scafetta examines the solar contribution to recent climate change. Future climate projections are actually provided in Scafetta’s longer document.
Climate Etc. Denizen Rocket Scientist has a long essay on this topic, that extends Scafetta’s arguments.
I would appreciate other good references on this topic. I note here that the University of Colorado LASP has established with NASA a Sun-Climate Research Center, which I view as a very positive thing.
Other terrestrial and extraterrestrial factors
Scafetta has a recent paper entitled “Empirical evidence for a celestial origin of the climate oscillations and its implications.” Text from the abstract:
We investigate whether or not the decadal and multi-decadal climate oscillations have an astronomical origin. Several global surface temperature records since 1850 and records deduced from the orbits of the planets present very similar power spectra. Eleven frequencies with period between 5 and 100 years closely correspond in the two records. Among them, large climate oscillations with peak-to-trough amplitude of about 0.1 and 0.251C, and periods of about 20 and 60 years, respectively, are synchronized to the orbital periods of Jupiter and Saturn. Schwabe and Hale solar cycles are also visible in the temperature records. A 9.1-year cycle is synchronized to the Moon’s orbital cycles. A phenomenological model based on these astronomical cycles can be used to well reconstruct the temperature oscillations since 1850 and to make partial forecasts for the 21st century. It is found that at least 60% of the global warming observed since 1970 has been induced by the combined effect of the above natural climate oscillations. The partial forecast indicates that climate may stabilize or cool until 2030–2040. Possible physical mechanisms are qualitatively discussed with an emphasis on the phenomenon of collective synchronization of coupled oscillators.
Climate Etc. Denizen Vukcevic argues that the Atlantic Multidecadal Oscillation is ultimately driven by the Geomagnetic Field.
Denizen Richard Holle writes of the influence of the Saros cycle (sun, inner planets and moon) on the climate, with an 18 year cycle.
So what to make of these ideas? They are interesting, and if correct, they would certainly be useful for decadal-scale predictions. But that are at the frontier border with ignorance. How can we test these ideas?
Volcanic eruptions
Oops almost forgot volcanoes. Seems like there is ~1 big one per decade?
Regional climate predictions of rainfall
Once you have made your decadal projection in terms of decadal regimes, then you can use historical data to estimate the distribution of El Nino, La Nina, and Modoki events, and hence to estimate distributions of hurricanes, floods and droughts, heat waves. I suspect that these decadal oscillations have more of an impact on severe weather on a timescale out to 2030 than solar variability or greenhouse warming.
The impact of PDO/AMO on U.S. drought is described in this paper. The combination of warm AMO and cool PDO is bad news for the southwest U.S. What has changed since the previous regime in the 1950’s? Warmer overall temperatures and decreasing incidence of drought apparently associated with global warming are cited in the paper. Another interesting aspect is the La Ninas. In cool PDO, you expect a predominance of La Ninas, which means drought in the southwest U.S. How to explain the recent deluge in California during La Nina? Well, the La Nina seems to be associated with a central Pacific cooling (rather than eastern Pacific), the so-called Modoki pattern. The impacts of the changing nature of El Nino/La Nina are just beginning to be studied; whether the increase in the Modoki pattern is related to global warming or to something like the NPGO remains uncertain.
Another region that I have been looking at lately is the Arctic. An article by Denizen describes the dominant role of the AMO on Arctic temperatures. This graph of Alaska temperatures show a marked jump ca. 1976, the time of the switch to the PDO warm phase.
Clearly these ocean oscillations can have a large influence on regional climates.
JC note: I’ve run out of time that I can spend on thispost; but with over 60 comments before the post is completed, I can rest assured that my posts are becoming increasingly irrelevant on the blog :) I know that these issues have been discussed by a number of you on other threads but I haven’t been able to keep track of them. If you you would like to provide a pointer to one of your posts, please do so and I will consider adding to the links to the main post.