Trends and oscillations in the Indian summer monsoon rainfall over the last two millennia

Jayant Biswas,R. L. Edwards,Sebastian F. M. Breitenbach,Manfred Mudelsee,Gayatri Kathayat,Ashish Sinha,Max Berkelhammer,Hai Cheng

doi:10.1038/ncomms7309

Abstract

Observations show that summer rainfall over large parts of South Asia has declined over the past five to six decades. It remains unclear, however, whether this trend is due to natural variability or increased anthropogenic aerosol loading over South Asia. Here we use stable oxygen isotopes in speleothems from northern India to reconstruct variations in Indian monsoon rainfall over the last two millennia. We find that within the long-term context of our record, the current drying trend is not outside the envelope of monsoon's oscillatory variability, albeit at the lower edge of this variance. Furthermore, the magnitude of multi-decadal oscillatory variability in monsoon rainfall inferred from our proxy record is comparable to model estimates of anthropogenic-forced trends of mean monsoon rainfall in the 21st century under various emission scenarios. Our results suggest that anthropogenic-forced changes in monsoon rainfall will remain difficult to detect against a backdrop of large natural variability.

Highlights

Observations show that summer rainfall over large parts of South Asia has declined over the past five to six decades
The observed summertime drying trend has been attributed to increased anthropogenic aerosol loading over South Asia[8,9,12], but considerable uncertainty remains in modelling the role of aerosols on monsoon rainfall[13]
Existing proxy reconstructions from South Asia suggest that the Indian summer monsoon (ISM) exhibits complex spatio-temporal variations, the finer details of which are highlighted by speleothem oxygen isotope (d18O) records from central India (CI) and northeastern India[14,15,16,17,18]

Summary

Introduction

Observations show that summer rainfall over large parts of South Asia has declined over the past five to six decades It remains unclear, whether this trend is due to natural variability or increased anthropogenic aerosol loading over South Asia. Within the long-term context of our record, the observed decline in ISM rainfall since the middle of the 20th century is not outside the envelope of natural variability, albeit at the lower edge of this variance. The magnitude of ISM multi-decadal oscillatory variability inferred from our record is comparable to model estimates of anthropogenic-forced trends of mean monsoon rainfall in the 21st century under various representative concentration pathway (RCP) scenarios[11]. Our results underscore the challenge of detecting anthropogenic-forced changes in ISM against a background of large internal variability

Methods

Results

Conclusion