08 August 2008

Indian Ocean climate impacts

In the recent post regarding the continuing impacts of Cyclone Nargis, I noted some unusual behaviour in the Indian Ocean SST. In particular, I noted that we are quite possibly headed into an apparently unprecedented third consecutive occurrence of the so-called Indian Ocean Dipole.

This behaviour of the IOD has not only been observed during the last few years, but is also a part of a longer trend of the past decades. The years 1994 and 1997 saw unusually strong positive IODs (e.g. this time series to 1990); 2006 was a moderate to strong event and this year remains to be determined...

In recent days, several instances of the impact of the Indian ocean have been noted. Both short- and long(er)-term impacts are noted.

The first item notes that tuna catches across the Indian Ocean have fallen sharply in the last two years and early indicators for this year show catches to be markedly below recent averages. Conservationists blame years of unchecked exploitation while processors say climatic conditions may be driving the fish deeper away from their nets.

As before, this illustrates the complexity of environmental problems and the difficulty of assigning ultimate cause and effect (and hence legal blame punishable by lawsuits). There are merits to both sides of the argument, and the reality of the situation is some nebulous gray between the two extremes. Years of overfishing have undoubtedly occurred and weakened the population, making it more susceptible to an unusual climatic event.

The recent IOD behaviour and The timing of the fishing loss event provides strong circumstantial evidence in favor of a climate effect. Normally a negative dipole follows a positive event. Fish populations are affected by changes in ocean conditions (El Nino was known for years by Peruvian fisherman for its impact of the anchovy haul, and the first(?) 'ENSO' paper explicitly linking ocean and atmosphere appeared in a tuna fishery journal...) and breaking the 'regular cycle' with the recent double IOD may have disrupted the fish's recovery. A triple positive IOD event could prove a disaster for Indian Ocean fisheries, hastening the inevitable end from overfishing.

The second item notes a recent study which identifies a link between a warming Indian Ocean and less rainfall in Africa. Rainfall in eastern Africa during the rainy season, which runs from March through May, has declined about 15 percent since the 1980s. Declines in rainfall in Ethiopia, Kenya, Tanzania, Zambia, Malawi and Zimbabwe were linked to increases in rainfall over the ocean.

Using both diagnostic (to identify how the observed decline work) and predictive modeling, the authors suggest "We can be quite certain that the decline in rainfall has been substantial and will continue to be. This 15 percent decrease every 20-25 years is likely to continue."

The study also notes that with current trends in emissions and local agricultural capacity the number of undernourished people could increase by more than 50 percent in eastern Africa by 2030. Coupled with the fishing problems noted above, the region looks set for continued troubles for years as a result of climate change.

A few final thoughts. As I've hinted at before (see also this post at Atmoz), these seemingly natural (but unusual) events are not caused by climate change, rather they are the climate change. This is how climate change manifests itself: increasing climate instability and an 'exaggeration' of normal behaviour.

The atmosphere and ocean radiate and stir, flux and transport, edging towards equilibrium, while never fully achieving it. This is in response to myriad of climate forcings on a variety of time scales. The forcing from a slowly-varying orbit and a reasonably well-behaved Sun has brought a certain dynamic equilibrium to the climate in recent millennia. Unfortunately, the relentless radiative forcing from the Great CO2 Enhancement Experiment is particularly large at this moment in Earth's long history and is driving the Earth's climate system in new ways (releasing the stored solar energy of millions of years in a century will do that...).

We are witnessing but the beginning of a long process of change; there are no stopping points in the immediate future. The CO2 already in the atmosphere will remain* for 100 years or so, committing us to a lengthy period of climate instability. Continuing the emissions of CO2 just adds to the forcing and drives the Earth systems harder, resulting in more chaos. These events in the Indian Ocean are just a foretaste of things to come.

*barring the unlikely event of successful geo-engineering project or some such...

Image: Extracted from Africa: Atlas of our Changing Environment

1 comment:

Anonymous said...

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