  Earth System Models (ESM) are important tools that allow us to understand and
quantify the physical, chemical & biological mechanisms governing the rates of
change of elements of the Earth System, comprising of the atmosphere, ocean,
land, cryosphere and biosphere (terrestrial and marine) and related components.
ESMs are essentially coupled numerical models which incorporate processes
within and across the different Earth system components and are expressed as
set of mathematical equations. ESMs are useful for enhancing our fundamental
understanding of the climate system, its multi-scale variability, global and
regional climatic phenomena and making projections of future climate change. In
this chapter, we briefly describe the salient aspects of the Indian Institute
of Tropical Meteorology ESM (IITM ESM), that has been developed recently at the
IITM, Pune, India, for investigating long-term climate variability and change
with focus on the South Asian monsoon.
