The thunderstorms of the Intertropical Convergence Zone form a line across the eastern Pacific Ocean.
It’s called the Pacific Intertropical Convergence Zone (PICZ) and its activity brings roughly 4 meters of rainfall per year to the Pacific equatorial region. Tropical rainfall patterns greatly impact the livelihoods of more than a billion people. Historically, this zone appears to shift in tandem with cooling and warming trends in more northern latitudes. And, it may be on the move again.
This possibility is born out in the results from a recent, oceanographic research project detailing the southward movement of this zone in the past (Southward movement of the Pacific intertropical convergence zone AD 1400–1850, Sachs et al, Dept. of Oceanography/Atmospheric Sciences, Univ. of Wash., June 2009, Nature GeoScience), but which also suggest that, in the present era, a potential, northward movement of this important, climate-impacting zone may be underway.
However, the results of this study are not conclusive or definitive. The best that these scientists are able to do is extrapolate their data findings into the future and present their most informed estimations. In general, better data means better climate models, and better climate models mean better forecasting. But the very complexity of climate science often leads to over-simplification of the data (in the popular press) and frequently, differing ideological camps make use of the same research to buttress their arguments and vested world-views. Such is the case with Dr. Sachs’ research.
After encountering (on-line) differing interpretations of the same research (Sachs et al), this author decided to contact Dr. Julian P. Sachs of the University of Washington in order to clarify his results and other issues regarding climate impacts. Our email interview follows:
Interview with Dr. Julian P. Sachs (lead researcher, Univ. of Wash. Dept. of Oceanography)
(M.R.):
First, your work concerning a shifting in the PICZ (note: also referred to here as the ITCZ) has been used by others to justify both the acceptance of anthropogenic climate change, and also, a refutation of said climate change* (Note: see both listed links following this interview). What assertions, if any, do you make regarding the issue of climate change (for the equatorial land regions, and for northern regions), based upon your research?
(Dr. Sachs responds)
Neither got it just right, but both got aspects of it correct. Our work shows that the ITCZ was much closer to the equator during the Little Ice Age, about 1400-1800 A.D., than it was during the Medieval Warm Period (~800-1300 A.D.) or than it is today. That conclusion is drawn from indicators of rainfall we measured in lake sediments on three Islands in the tropical North Pacific Ocean–Palau in the west, Washington Island in the central, and Galapagos in the east). Anthropogenic activity likely contributed little or nothing to the change in the tropical Pacific rainfall and climate regime at the onset of the Little Ice Age, some 600 years ago. The ITCZ moved close to the equator for reasons that were very likely to have been natural. A lower solar output (i.e., just a fraction of 1% lower) *may* have been the cause, but this is quite speculative. The shift northward that began in the 18th century and continued into the 20th century may have been influenced by anthropogenic influences in the climate system, such as the addition of large amounts of greenhouse gases into the atmosphere (from tailpipe and smokestack emissions, and deforestation). But there is no way to prove the link from my work.
In recent climate modeling experiments (Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing, Meehl et al, Science Magazine, 28 Aug. 2009), two mechanisms–ocean-air coupling (“bottom up” forcing) and solar wind – stratospheric ozone coupling (“top down” forcing) have been shown to be drivers of climate change (in the Pacific)…How do these effect mesh with, or contribute to, your findings regarding past or current shifting in the PICZ? What do you see as the main drivers of this shift?
The mechanisms they propose seem quite reasonable to me. It’s (i.e., determining the drivers of climate change) not simple, and cannot be solved in a simple thought experiment.
“Small changes in tropical Pacific rainfall influence climate around the globe. But the teleconnections are not well worked out yet, and it is an area I am working on with my colleague Dr. David Battisti (a climate dynamicist and climate modeler) here in the UW Atmos Sci department to understand.”
Infra-red image from GOES 14 showing the intertropical convergence zone (note: white features indicate cooler temperatures).
(interview continued)
To what extent does human-caused CO2 build-up play a part in these natural processes and cycles?
CO2 from human activities has accumulated in the atmosphere, the ocean and the terrestrial biosphere (30-40% in each roughly). The CO2 that has accumulated in the atmosphere (and the methane) have altered the radiative balance of the planet. In the simplest case, the surface of the Earth must warm up in order to bring the planet back into radiative balance. The increase in atmospheric CO2 and methane since about 1850 are unprecedented in the last 800,000 years in terms of the amount and rate. High levels of these gases in the atmosphere in the geologic past (last 800 kyr there is excellent data from ice cores) have always been associated with warm temperatures, and vice versa. It is not a stretch to argue that the markedly higher levels of these gases in the atmosphere that have built up in the last 150 yr or so
have, and will continue, to warm the planet. There are natural variations in the climate on all time-scales (from diurnal to billion-yr time scales). But that does not mean that the climate system cannot be perturbed by non-natural processes. Indeed, there is every reason to believe that it can.
Whether the cause of this shift is natural (cyclical), human-induced, or both… What do you think can be done, if anything, to mitigate, compensate for, or slow this northward shift in the ‘tropical rain band’?
We do not know that it is actually continuing to shift north. That is merely an extrapolation from the last 150 years or so of our data. If it were to continue to move north, and that movement was caused by higher levels of greenhouse gases in the atmosphere causing warming of global surface temperatures–which would be consistent with several climate models– then one obvious solution would be to stop or slow the warming. This can be done by capturing and sequestering atmospheric CO2 (and methane). Or it could be accomplished by other means, such as cooling the planet by some other geoengineering solution (e.g., sunshades–reflective mirrors in space, etc; reflective sulphate aerosol injections to the upper atmosphere,
etc) [note: see my previous blog: Emergency Climate Control - Geoengineering Risks]. Beyond that, if the movement of the ITCZ is linked to global surface temperatures, and those continue to rise, then I don’t see a way to stop the movement.
Additional links of interest for this article (source: Prof. John M. Wallace, Atmospheric Science Dept., Univ. of Washington):
* (Referenced, popular articles using Dr. Sachs’ research):
Equatorial rain band moving north due to global warming
Tropical Rains Dampen Alarmist Agenda
photo credits: (top) NOAA, (middle) NOAA (GOES – 11). Note: top image is a combination of cloud data from NOAA’s newest Geostationary Operational Environmental Satellite (GOES-11) and color land cover classification data. The ITCZ is the band of bright white clouds that cuts across the center of the image.
About Michael Ricciardi
