Climate models to study the dynamics of climate system:

• Global climate models (GCMs) are mathematical formulations of the processesthat comprise the climate system including, radiation, energy transfer by winds, cloud formation, evaporation and precipitation of water, and transport of heat by ocean currents. Climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. Climate models can be used to make projections about future climate and the knowledge gained can contribute to policy decisions regarding climate change. An advantage of GCMs is their ability to perform multiple simulation experiments using different greenhouse gas emissions scenarios.
• These climate models have indicated that climate change will lead to an increase in extreme rainfall events. According to the Intergovernmental Panel on Climate Change (IPCC) Special Report on Extreme Events, global warming leads to “changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events”.
• For India, examining daily rainfall data between 1951 and 2000 showed that there has been a significant increase in the magnitude and frequency of extreme rainfall events along with a decrease in the number of moderate events over central India. These changesinteracting with land-use patterns are contributing to floods and droughts simultaneously in several parts of the country.

The use of understanding Extreme events:

Extreme weather and climate events(e.g., heat waves, droughts, heavy rainfall, hurricanes) have always posed risks to human society. Given that climate change affects the climate system globally, it is impossible to rule out some contribution from climate change to any extreme event. A matter of growing interest is the degree to which humans
are changing these risks through anthropogenic climate change. This concern has been driven by the growing impacts on ecosystems, communities, and infrastructure of recent extreme events across the world. Extreme weather is one way that people experience climate change. Extreme events are abrupt, occur in the present, and are highly visible, as opposed to long-term climate change trends that seem abstract, distant, gradual, and complicated.  There are several motivations for investigating the causes of individual extreme events. There is an element of scientific curiosity, but the primary motivation for event attribution goes beyond science.

• Extreme events are directly traceable to loss of life, rising food and energy prices, increasing costs of disaster relief and insurance, fluctuations in property values, and concerns about national security.
• To help policymakers, emergency responders and local communities to plan and prepare for them.
• In addition, it is important to assess what is known about climate and non-climate causes of such events in order to evaluate whether they are likely to pose increasing risksto life and property in particular regions in the future.
• Cities could be laid out to reduce flooding by following natural contours, drainage and tank systems.
• Emergency responders could be well prepared to transport and care for people who may become stranded during disasters.
• Some of the anticipated impacts can be reduced, however, through such management strategies as land use planning if the connections between climate change and extreme events like intense precipitation are better understood.
• Such planning would ideally be based on a broad risk assessment, including projections of future trends in extreme events.
• Once an extreme event such as a heat wave or heavy rain occurs, people want to know to what extent a single event has been caused by climate change, that is, by greenhouse gases released through human activities.

‘Attribution’ studies more reliable when based on sound physical principles
Research that tries to understand this relationship between anthropogenic climate change and extreme events in particular locations is called “attribution”.

• Attribution studies could able to answer the following questions:
• Is an extreme event, such as torrential rainfallor record storm surges, part of a natural cycle of variability or due to human-induced climate change?
• To what extent do poor preparedness and ecologically insensitive land-use worsen the impacts?
• Attribution studies can be a tool for informing choices about assessing and managing risk and guiding adaptation strategies.
• Such information may be critical to multiple decision makers, among them insurers, elected officials and policy makers, local and regional land and resource managers, zoning and infrastructure planners and engineers, litigators, and emergency managers who focus on disaster risk reduction.

Event attribution is more reliable when based on sound physical principles, consistent evidence from observations, and numerical models that can replicate the event. Conversely, for rainfall simulation, climate models cannot mimic orsimulate extreme rainfall such as the kind Chennai experienced in 2015. The 494 mm rain in Chennai was a rare event, with less than a 0.2% chance of occurring in any given year. The Chennai flood of 2015 did not have a clear climate signature to show that it was due to warming of the earth. On the other hand, with regard to Hurricane Harvey, climate change made the impact much worse, because of higher sea surface temperatures and a blocking region of high pressure that kept the rain clouds over Houston for a long period.

Urbanisation and hydrology
The actual patterns of flooding in Chennai, Mumbai and Houston, however, were due to several human-induced activities:

• Rampant increase in built-up area across natural drainage channels,
• The diversion or damming of rivers upstream leading to sediment transport and siltation,
• Coastal subsidence and other effects of development.

Any rain that falls on soil or vegetation is mostly absorbed into the earth’s surface. Some of it slowly trickles into shallow or deep protected aquifers that make up what we call groundwater. The rest usually flows downhill along surface orsubsurface stream channels. The spread of infrastructure such as roads, highways, buildings, and residential complexes, tiled or asphalt-covered land obstructs rainwater from percolating into the soil. Often there are further barriers that block movement of water and increase flooding.
In India, urban growth over the past few decades has casually ignored the hydrology of the land. In Chennai, for example, systematic intrusion into the Pallikaranai marsh and other wetlands by housing complexes and commercial buildings; slums along Adyar riverand large-scale construction along the coast are just examples of the open encroachment of the built environment that obstructs rivulets and absorption of rainwater into the earth. When it rains heavily, exceeding the capacity of the soil to absorb it and regular stream flows are blocked from proceeding into the sea, these heavily built-up areas get inundated. Satellite images from 15 or more years back show the existence of hundreds of lakes and tanks, and several waterways within the city’s boundaries.

What is to be done?

• Development needs to be climate-smart, but also avoid social and institutional challenges such as moral hazard.
• No investments should made in places where severe impacts are likely,
• Construction on existing lake beds and other water bodies needs to be removed or redesigned to allow flood drainage along natural water channels.
• Construction in cities or in urbanising areas should take into consideration the existing topography,surface water bodies, stream flows or other parts of terrestrial ecosystems.
• Based on the traditional hydrological work, more monitoring networks with high density and precision, automatic water quality analysis and evaluation systems, flood protection and forecasting models and emergency response plans should be established. All of these urban hydrological strategies provide strong support for urban places.
• Water Resources monitoring like routine measurements of river flow
• Water quality monitoring of surface water, groundwater and drinking water.
•  Research on the Urban Hydrological Issues
• Improve the Capacity of Hydrological Forecasting Systems