Climate data... in circles

Understanding El Nino

The only way to better understand El Niño in unknown conditions is to study the record over centuries, when world temperatures and sea levels have moved up and down. Prof Jinbao Li of the International Pacific Research Centre, University of Hawai’i at Manoa and associates, report in Nature Climate Change, important advances in getting such information for the last 1,100 years from the record in tree trunks in the US South-west.

The El Niño effect, which brings periodic luxuriance to vegetation of the West coast of South America is a major climate phenomenon that affects the weather. But it has become customary to say, “maybe it’s El Niño” when things go wrong. Maybe not without reason! This weather phenomenon has been studied in recent times and scientists have learnt to anticipate El Niño events pretty accurately. But with the progress of global warming, these predictions may get uncertain, and could create problems for a world already facing warming-related trouble.

The weather in the Peruvian coast is cool and dry, which makes for good fishing, but not much vegetation. In contrast, the other side of the Pacific, the Eastern side of Australia, is warm and humid. With warm water rising and pushed by winds from the East, water level around Australia is a whole meter higher than along South America. The piling up of water on the Australia side causes deep, cool water to well up on the Peru side, which brings nutrients to the surface and makes for great fishing!

Usually in December, when it is warmest in the Southern hemisphere, warm water invades the Peruvian coast and ruins the fishing, for some time at least. But every once in a way, the warm inflow is large indeed and there is torrential, tropical rain. Once in a way could be every other year, or sometimes as far as ten years apart. The fishing is wrecked, no doubt, but the vegetation flourishes. Because this happens around the time of Christmas, they say it is the coming of baby Christ, El Niño, the little one. The other extreme, of very dry weather is named in the opposite gender, La Niña.

Such major changes in large masses of air and water, naturally affect winds and currents at the global level. The onset of El Niño is a signal to weathermen the world over to watch for local effects. In recent times, as El Niño is carefully monitored and can be predicted accurately, precautions are taken. It is only when it strikes unexpectedly or with great severity that the little one needs to be blamed.

Global warming may change the picture. With melting of glaciers, the Greenland ice mass, changes in ocean currents and salinity patterns, El Niño may become unpredictable. This would then impact global weather, which may, in turn, drive El Niño to grow more erratic. While there have been times in the past when global temperatures were high and comparable to what we may face in a few decades, to help us prepare, we have no record of the timing of past El Niño events.

Tree records

This is where the work of the group at Manoa may prove to be the answer. Trees that grow over many years mark each year by a visible ring of growth around their girth. Apart from growth upwards and at the stems and leaves, the tree also grows outwards. The growth is the fastest during the warm and damp months but slows down when it is cool and dry. The tissue that forms during fast growth is sparse, while that which forms slowly is dense. The difference in tissue shows in the cross-section of the tree trunk as rings, marking the fast and slow growth in each annual cycle.

Trees thus mark each year with a separate growth ring, a record that reflects the climatic conditions during that year. A good growing season would produce a wide ring, while a drought year would be marked by scant growth and a narrow ring. Sometimes, fluctuating weather in the same year can result in more than one ring and very, very rarely, a poor year can result in no ring. And in this way, sampling of trunks of trees in an area can reveal the climatic conditions over a long span of time, even centuries, or with some trees, a millennium!

The field of work is called dendrochronology and involves understanding the growth patterns of different trees, principles of statistical sampling and also related dating technology, to confirm and verify what the tree trunks reveal.

Tree records match actual data

The work of Jinbao Li and his international team have found that the record of tree trunks in South-western US match well with the actual climate records available for the last 150 years. Even going back a few hundreds of years, the data is in agreement with the results of carbon dating of organic matter in coral reefs.

“Our work revealed that the towering trees on the mountain slopes of the US Southwest and the colourful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings,” explains Li. “The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1,100 years.”

The rings reveal that El Niño behaviour has been highly variable over the centuries. The interesting thing is that peaks and lows agree with evidence of varying Pacific climate, as revealed by the structure of lake-bed sediments. The weakest El Niño period, the Medieval Climate Anomaly, of the 11th century, for instance, corresponds to a period when the eastern tropical Pacific was cool.

This correspondence of El Niño events to specific ocean temperature markers, during periods of high and low variability promises better predictions from ocean and other data. “Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” says co-author Shang-Ping Xie.

“Current models diverge in their projections of its future behaviour, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming.”