Although the oceanic records are continuous, they are proxy to intensification of southwesterly winds. Terrestrial records on the other hand are direct measure of reconstructing monsoon rainfall. Also, unlike oceanic records, they have annual to decadal resolution and can be precisely dated. Two such proxies will be discussed in detail below.
Information on climate during the last 1000 years, which is the period of man’s greatest impact on the planet, is vital to understand the various external and internal forcing on the climate and thereby make reliable future projections. Significant contributions to climate science within the last decade have firmly established tree rings as valuable sources of proxy data for evaluating long-term climate variability/trends and as useful tools for developing long-term records of extreme climatic events (Mann et al. 1999).
New Tree-Ring Chronologies: Western Himalaya
Kinnor and Gangotri regions of the higher ranges of the Western Himalaya (altitudes varying between 2850 and 3200 m above the mean sea level (msl)) were
Fig. 3 a Occurrence of large-scale droughts over India during 900-1991 CE. Historical records were sporadic and isolated up to 1600 CE and continuous later. b Frequency of occurrence of drought events were relatively higher and randomly distributed after 1600 CE. Source Pant et al. (1993) explored. The northern part of this region is classified as alpine, glacial climatic zone. Dry cold weather with severe and prolonged winter prevails here. The region behaves as upper tree line boundary; it is under snow cover during winter and spring. In five chronologies of some conifers (e.g., Picea smithiana Boiss and Cedrus deodara D. Don), a higher growth was seen in the last few decades. A strong positive relationship with the mean annual and winter (December- February) temperatures of the concurrent year is seen in these chronologies. Using these five chronologies, a 553-year-long master chronology was prepared (Borgaonkar et al. 2010a, b). This shows a few decadal and longer epochs of Little Ice Age (LIA) cooling during CE 1453-1590 and CE 1780-1930 (Fig. 4). These are relatable to the other proxy records (e.g., glacial fluctuations) (Mayewski et al. 1980; Thompson et al. 1995; Yao et al. 1995; Duan and Yao 2003).
Millennial-scale reconstruction of precipitation from tree-ring oxygen isotope records from high mountain ranges of north Pakistan indicates wet condition during recent centuries and dry conditions at the beginning of the past millennium and also during eighteenth and early nineteenth centuries (Treydte et al. 2006). However, isotope analysis of Dasuopu glacier from the central Himalayas indicated a major precipitation trend in the central Himalayas since last 300 years (Duan and Yao 2003). Monsoon rainfall in the central Himalayas has decreased over the past decades in the condition of global warming. This was also shown by Sano et al. through 618O measurements on tree rings from Nepal (Sano et al. 2010, 2011).
Fig. 4 A 553 years (1452-2004 CE) long tree-ring index chronology of high-altitude Himalayan conifers from Western Himalaya. Smooth gray line is 30-year cubic spline filter. Suppressed (cooling) and released (warming) growth patterns in tree-ring chronology have also been observed to be well related to the past glacial fluctuation records of the region. After Borgaonkar et al. (2010a) (color figure online)