The Impact of Climate Change on the Hydrological Cycle

The Impact of Climate Change on the Hydrological Cycle

Kevin Trenberth

Former Coordinating Lead Author of the IPCC

Water is irreplaceable, non-substitutable and absolutely essential for life. The global water cycle is also changing. Join Kevin Trenberth in this video where he outlines what precipitation is, the hydrological cycle and how this is changing due to the climate crisis.

Water is irreplaceable, non-substitutable and absolutely essential for life. The global water cycle is also changing. Join Kevin Trenberth in this video where he outlines what precipitation is, the hydrological cycle and how this is changing due to the climate crisis.

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The Impact of Climate Change on the Hydrological Cycle

16 mins 35 secs

Overview

Precipitation results from the condensation of water vapour in the atmosphere that falls from clouds as a result of gravity. This includes drizzle rain and snow. The hydrological cycle is the process of evaporation from the surface moisture of the land and its vegetation, and ocean surface into the atmosphere and then back again as precipitation. Higher air temperatures mean there is more atmospheric demand for moisture. The increased heat in the climate is causing more evaporation, leading to prospects of more storms of increasing size, intensity and length.

Key learning objectives:

  • Understand what precipitation is

  • Understand how the hydrological cycle works

  • Outline the effect of the climate crisis on the hydrological cycle

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Summary

What is precipitation?

Precipitation results from the condensation of water vapour in the atmosphere that falls from clouds as a result of gravity. This includes drizzle, rain, sleet, snow, ice pellets and hail. The type of precipitation depends on the temperature and location. 

How does humidity affect precipitation?

Precipitation is hugely affected by surface temperatures and humidity. Humidity is the measure of moisture in the atmosphere. Specific humidity is measured as the mass of water vapour relative to the mass of air (in grams of vapour per kg of air). Relative humidity is a measure of the water vapour as a percentage of the total moisture-holding capacity of the air. This means that air with 100% humidity will form rain or clouds with ease, unlike air with 20% humidity found in desert regions. Warmer air will also hold 7% more moisture per degree Celsius rise in temperature (Clausius–Clapeyron equation). 

What is the relationship between precipitation and evaporation?

Rates of precipitation on average are 9x the evaporation rates. In other words, the area required to feed a precipitating weather system is 9x the precipitation area through the convergence of moisture. This means when the moisture goes up (as happens with a warmer climate due to climate change) it will rain harder, increasing the risk of flooding. 

What is the hydrological cycle?

The hydrological cycle is the process of evaporation from the surface moisture of the land and its vegetation, and ocean surface into the atmosphere and then back again as precipitation. On land the precipitation seeps into soils, streams and rivers and flows to lakes and the ocean. It involves the continuous circulation of water, driven by exchanges of energy. Typical evaporation rates are 1-5mm per day. 

How do the warmer temperatures caused by the climate crisis affect the hydrological cycle? 

Higher air temperatures mean there is more atmospheric demand for moisture - the air is "thirstier". Increased evaporation in dry regions can also intensify droughts. Droughts, defined as a prolonged or marked deficiency of precipitation that results in water shortages, occur naturally. With climate change however, they are likely to develop sooner, become stronger and last longer, leading to an increased risk of wildfires and oppressive heat waves. 

Increased precipitation in wetter areas increases risk of flooding. In previous snowy areas in winter, the warmer climate will mean that it will likely rain instead.

Hurricanes rely on atmospheric moisture to fuel their strong thunderstorms and upward currents of air that come from warm tropical oceans. As hurricanes spin up their winds, evaporation increases and with increased ocean heat content (OHC), sea levels and sea surface temperatures from climate warming, storms occur in a hotter, moister environment. The hurricanes become more intense, longer lasting and bigger, and makes them more powerful. 

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Kevin Trenberth

Kevin Trenberth

Dr. Kevin Trenberth is a Distinguished Scholar at the National Center for Atmospheric Research (NCAR). He was a Coordinating Lead Author of the 1995, 2001, and 2007 Scientific Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). Kevin also shared the 2007 Nobel Peace Prize which went to the IPCC and Al Gore. Between 1999 to 2006, Kevin served on the Joint Scientific committee of the World Climate Research Programme (WCRP). Kevin then went on to chair the WCRP Observation and Assimilation Panel from 2004 to 2010 and the Global Energy and Water Exchanges (GEWEX) Scientific Steering Group from 2010 to 2013. He has also served on many US national committees and is a Fellow of the American Meteorological Society (AMS), the American Association for the Advancement of Science (AAAS), the American Geophysical Union (AGU), and an honorary fellow of the Royal Society of New Zealand Te Apārangi. Kevin has received many awards throughout his career. In 2000, he received the Jule G. Charney award from the AMS; in 2003, he was given the NCAR Distinguished Achievement Award. In 2013 he was awarded the Prince Sultan Bin Abdulaziz International Prize for Water, and he received the Climate Communication Prize from AGU and in 2017 he was honoured with the Roger Revelle medal by the AGU.

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