Using a variety of proxy records (tree-ring, coral, ice-core and documentary) to reconstruct the "most comprehensive La Niña event record compiled to date", the authors found evidence that the ENSO (El Niño Southern Oscillation) cycle has indeed increased in intensity during the most recent century:
Despite their limitations, reconstructions of past climate are unique in their ability to provide a long-term context for evaluating 20th century climate change. The unusual nature of late 20th century ENSO is evident. The height of La Niña activity occurred during the 16th and 19th centuries, while the 20th century is identified as the peak period of El Niño activity. Overall, 55% of extreme El Niño event years reconstructed since A.D. 1525 occur within the 20th century. Although extreme ENSO events are seen throughout the 478-year ENSO reconstruction, approximately 43% of extreme and 28% of all protracted ENSO events reconstructed occur in the 20th century. Of particular note, the post-1940 period alone accounts for 30% of extreme ENSO years noted since A.D. 1525.The image (click to enlarge), from the paper by Gergis and Fowler (Climatic Change (2009) 92:343–387, DOI 10.1007/s10584-008-9476-z), shows the distribution of ENSO events by century, from the 1500s through the 1900s. The intensity is classified by percentile: extreme (>90th percentile), very strong (70th–90th percentile), strong (50th–70th), moderate (50th–30th) and weak (<30th).