2010 Onwards
A 2,000 year extratropical Northern Hemisphere reconstruction by Ljungqvist published by Geografiska Annaler in September 2010 drew on additional proxy evidence to show both a Roman Warm Period and a Medieval Warm Period with decadal mean temperatures reaching or exceeding the reference 1961–1990 mean temperature level. Instrumental records of the period 1990–2010 were possibly above any temperature in the reconstruction period, though this did not appear in the proxy records. They concluded that their "reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. ad 300–800, despite significant differences in both data coverage and methodology."
A 2010 opinion piece by David Frank, Jan Esper, Eduardo Zorita and Rob Wilson (Frank et al. 2010) noted that by then over two dozen large-scale climate reconstructions had been published, showing a broad consensus that there had been exceptional 20th century warming after earlier climatic phases, notably the Medieval Warm Period and Little Ice Age. There were still issues of large-scale natural variability to be resolved, especially for the lowest frequency variations, and they called for further research to improve expert assessment of proxies and to develop reconstruction methods explicitly allowing for structural uncertainties in the process.
As several studies had noted, regression-based reconstruction methods tended to underestimate low-frequency variability. Bo Christiansen designed a new method (LOC) to overcome this problem, and with Ljungqvist used LOC to produce a 1,000 year reconstruction published in 2011. This showed more low frequency variability and a colder Little Ice Age than previous studies. They then extended the LOC reconstruction back using selected proxies which had a documented relation to temperature and passed a screening procedure. This 2,000 year reconstruction, published in 2012, again showed more variability than earlier reconstructions. It found a homogenous Little Ice Age from 1580–1720 showing colder conditions in all areas, and a well defined but possibly less homogenous Medieval Warm Period peak around 950–1050, reaching or slightly exceeding mid 20th century temperatures as indicated by previous studies including Mann et al. 2008 and 2009.
Ljungqvist et al. 2012 used a larger network of proxies than previous studies, including use low-resolution proxy data with as few as two data points per century, to produce a reconstruction showing centennial patterns of temperature variability in space and time for northern hemisphere land areas over the last 1,200 years. At this broad scale, they found widespread warmth from the 9th to 11th centuries approximating to the 20th century mean, with dominant cooling from the 16th to 18th centuries. The greatest warming occurred from the 19th to the 20th centuries, and they noted that instrumental records of recent decades were much warmer than the 20th century mean. Their spatial reconstruction showed similarities to the Mann et al. 2009 climate field reconstruction, though the different resolution meant these were not directly comparable. The results were robust, even when significant numbers of proxies were removed.
Read more about this topic: Hockey Stick Controversy