A network of interdisciplinary scholars studying past climate change
In the June 30, 2013 online issue of Nature Climate Change an article by Li et al. (also discussed here) reconstructs 700 years of ENSO variation from 2,222 tropical tree ring chronologies. The main thrust of the article is that the El Niño Southern Oscillation has been unusually active since the late 20th century, which the authors attribute to global warming. Models still disagree over the impact of warming on ENSO, so this historical approach could provide valuable insights about the system’s sensitivity to radiative forcing.
For climate historians, the study also offers some important finds. The new reconstruction correlates well with past ENSO chronologies based on documentary and coral data, while offering a somewhat better fit with the modern instrumental record (depending on how ENSO is measured). The authors find, “The reconstructed ENSO index exhibits marked variations at interannual to interdecadal timescales over the past seven centuries.” ENSO activity was muted ~1300-1550, growing stronger in the later Little Ice Age and particularly in the late 19th century. However, “the results indicate that the interdecadal modulation of ENSO variance before 1900 may arise stochastically.” Another key find from the reconstruction is that large tropical volcanoes (but not small) have had a major impact on ENSO, producing significant cooling in the Eastern Pacific the year of the eruption and then significant warming (an El Niño) the year after. The effect is so pronounced the authors conclude that precipitation anomalies usually ascribed to tropical eruptions may be better understood as resulting from ENSO effects rather than the effects of the eruption itself. (“The close resemblance of the moisture pattern at yeart = 0 (t+1) to that of La Niña (El Niño) provides strong evidence that tropical eruptions affect global moisture largely through the influence on ENSO.”)