July 2, 2013, by Tim Radford
What every dendrochronologist needs: Tree rings tell El Niño’s story
FOR IMMEDIATE RELEASE New scientific findings support the evidence which links more frequent El Niño weather disruptions in the eastern Pacific to the build-up in greenhouse gas emissions. London, 2 July – Climate change means just that: an unpredictable, moody weather system and volatile temperatures that vary on large and small scales, with unpredictable consequences for humans, their crops and their forests, according to three new studies. Jinbao Li of the International Pacific Research Centre in Hawaii and colleagues report in Nature Climate Change that they looked at the evidence from a seven-century “almanac” inscribed in the growth rings of 2,222 trees from the tropics and mid-latitudes in both hemispheres. Tree rings are records of annual change, of periods of drought, healthy rainfall, high and low temperatures and – once accurately dated and calibrated with each other – are accurate testimony of local weather long before humans started taking temperature and rainfall data. The researchers used these dendrochronologies – the formal name for the science – to measure the behaviour of the Pacific Ocean’s wild card, the El Niño-Southern Oscillation (ENSO) for the past seven centuries. El Niño – christened ‘The Child’ by Peruvian fishermen because it often arrives at Chjristmastide – tends to trigger droughts, floods and other weather disturbances worldwide: hurricanes ease in the north Atlantic during an El Niño year and in the Pacific winter storms shift southward, while California floods.
High-risk regions identified
By observing the pattern of events since about 1400 AD, the scientists confirmed that El Niño has been unusually active in the 20th century, a century in which carbon dioxide levels in the atmosphere began to rise and, with this, average planetary temperatures. “This supports the idea that unusually high ENSO activity in the late 20th century is a footprint of global warming”, says Jinbao Li. His colleague and co-author Shan-Ping Xie, a meteorologist at the Centre, warned: “If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts.” In a separate study, an international team of researchers led by the Potsdam Institute for Climate Impact Research report in the Proceedings of the National Academy of Sciences (PNAS) that the Amazon, the Mediterranean and East Africa might all expect severe change in a warming world – change that could affect the livelihoods of people in those regions. Since about one in ten of the world’s population lives in such hotspots, this is an awful lot of possible disruption to crop yields, water availability, ecosystems and human health. The research began with an examination of the story of climate so far, and a more detailed look at the predictions that follow from various climate models in specific sectors. These included, for example, precipitation patterns and the implications of drought for the Mediterranean.
The last straw
“What today is considered extreme could become the new normal”, said Qiuhong Tang of the Chinese Academy of Sciences. And Alex Ruane of the NASA Goddard Institute of Space Studies said: “In the hotspot parts of Africa, for instance, even small temperature rises can lead to additional losses that many small farmers simply cannot afford.” Disequilibrium for farmers will be echoed in the planet’s forests, and this state of woodland disturbance could become the norm, according to researchers from Aarhus University in Denmark. That is because plant communities react very slowly, they argue, in the American Journal of Botany, creating problems for long-term forest management. “Our forests take an extremely long time to adapt”, says Jens-Christian Svenning. “We still have a small amount of small-leaved lime in Denmark which has held on since the warm period in the Bronze Age, i.e. about 3,000 years. “Perhaps it will get another chance to spread when the summers once more get much warmer. However, such expansion would take a long time, as lime is not a particularly fast-growing tree, or particularly good at dispersing, even under optimum conditions. “The climate will change considerably in the course of a single tree generation so we should not assume the forest we are looking at in a given place is suitable for the climate. Future climate will constantly shift, which will increasingly result in these strange situations of disequilibrium.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.