PLOS-One: Climate change in the Himalayas, a biodiversity hotspot, home of many sacred landscapes, and the source of eight largest rivers of Asia, is likely to impact the well-being of ~20% of humanity. However, despite the extraordinary environmental, cultural, and socio-economic importance of the Himalayas, and despite their rapidly increasing ecological degradation, not much is known about actual changes in the two most critical climatic variables: temperature and rainfall. Nor do we know how changes in these parameters might impact the ecosystems including vegetation phenology.
By analyzing temperature and rainfall data, and NDVI (Normalized Difference Vegetation Index) values from remotely sensed imagery, we report significant changes in temperature, rainfall, and vegetation phenology across the Himalayas between 1982 and 2006. The average annual mean temperature during the 25 year period has increased by 1.5°C with an average increase of 0.06°C yr−1. The average annual precipitation has increased by 163 mm or 6.52 mmyr−1. Since changes in temperature and precipitation are immediately manifested as changes in phenology of local ecosystems, we examined phenological changes in all major ecoregions. The average start of the growing season (SOS) seems to have advanced by 4.7 days or 0.19 days yr−1 and the length of growing season (LOS) appears to have advanced by 4.7 days or 0.19 days yr−1, but there has been no change in the end of the growing season (EOS). There is considerable spatial and seasonal variation in changes in climate and phenological parameters.
Conclusions/Significance: This is the first time that large scale climatic and phenological changes at the landscape level have been documented for the Himalayas. The rate of warming in the Himalayas is greater than the global average, confirming that the Himalayas are among the regions most vulnerable to climate change.
The Himalayas, which represents the major parts of the Greater Hindu-Kush Himalayan mountain system, extends in an arc about 3000 kilometers in length and covers ~750,000 km2 of northern Pakistan, Nepal, Bhutan, and the northwestern and northeastern states of India (Fig. S1) . Climatic, topographic, geological, and altitudinal variations have generated unique landscapes, ecosystems, and biota in the Himalayas. Of the 825 ecoregions in the world, 13 are represented in the Himalayas . This immense biological diversity is matched by cultural and ethnic diversity. Himalayas is also the source of the 8 largest rivers of Asia and is known as “water tower of Asia” ; the rivers and their tributaries sustain about 1.4 billion people . Thus climate change in the region is a matter of global concern.
Much of the recent discussion about climate change in the Himalayas has been dominated by the extent of glacial melting , ; however, glaciers have not been systematically monitored . The IPCC report predicts large scale changes in temperature and precipitation over the Asian land mass . Limited studies on temperature or precipitation for a few localized places show that warming in the Himalayas is 3 times greater than the global average . However, changes at the regional level remain to be documented. Furthermore, the impacts of climate change on phenological patterns are not well understood due to lack of historical ground-based observations on phenology. Thus despite the fact that the Himalayas are among the regions most vulnerable to climate change , have unique biodiversity, and are undergoing rapid environmental change , there is no systematic analysis of climate change and its effects on ecosystems and biodiversity, nor on hydrology, agriculture, and livelihoods in this important and extraordinary region of the world.
We used global mean monthly surface air temperature , precipitation , and GIMMS-NDVI dataset  from the year 1982 to 2006 for 13 ecoregions of the Himalayas. The global mean monthly surface air temperature dataset of 0.5-degree grid is produced from ground-based weather station data collected from the Global Historical Climatology Network version 2 and the Climate Anomaly Monitoring System using interpolation methods. The quality of this dataset is found to be reasonably good in comparison with several other ground-based land surface temperature datasets, and it captures most of the common temporal-spatial features observed in climatology both at regional and global scale . Similarly, we used precipitation data produced by Climate Prediction Center Merged Analysis of Precipitation (CMAP), the 2.5-degree gridded global monthly precipitation data were produced by the combination of different sources: gauge observations and estimates inferred from a variety of satellite observations . The GIMMS-NDVI dataset is corrected for different sources of non-vegetation error introduced by inter-sensor calibration, orbital drift, cloud cover, solar angle differences, volcanic eruptions, and other atmospheric contaminations –. Long term temporal and large spatial coverage of this data make it possible to detect trends in vegetation phenology –. The extracted data were analyzed in a GIS framework to detect spatio-temporal changes in climate. Since changes in temperature and precipitation are directly manifested as changes in phenology of local ecosystems, we examined the impact of such changes on the onset of growing season and senescence in the 13 different ecoregions of the Himalayas. Furthermore, we correlated changes in phenological parameters with changes in climatic variables.
IPCC predicts that average annual mean temperature over the Asian land mass, including the Himalayas, will increase by about 3°C by the 2050 s and about 5°C by the 2080 s , . Similarly, average annual precipitation in this region will increase by 10–30% by 2080 . During the last few decades, the Himalayas have experienced increasing temperature , , . However, data on precipitation are not consistent; the precipitation has increased in some areas but decreased in other areas , . Changes in temperature and precipitation have influenced phenology of plants –. Although basic research on ecological responses, including phenology, to climatic change is substantially lacking in the Himalayas, it is generally anticipated that climate change in this region may alter phenology at both individual species and community levels . Our results demonstrate significant changes in temperature and precipitation of the Himalayas–greater than the upper bounds predicted by IPCC and the recent Indian assessments . Furthermore, we demonstrate that landscape-level changes in the phenology of local ecosystems all across the Himalayas are well correlated with changes in climate.
Authors: Uttam Babu Shrestha1,2*, Shiva Gautam3, Kamaljit S. Bawa1,4,5 [ 1 University of Massachusetts, Boston, Massachusetts, United States of America, 2 Harvard University Herbarium, Harvard University, Cambridge, Massachusetts, United States of America, 3 Department of Biostatistics, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, United States of America, 4 Sustainability Science Program, Harvard University, Cambridge, Massachusetts, United States of America, 5 Ashoka Trust for Research in Ecology and Environment (ATREE), Bangalore, India ]
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