The first part of the latest Assessment Report of the Intergovernmental Report on Climate Change (IPCC) is out, at last.
This 'Working Group 1' report covers the science of climate change. There is a lot of new information contained in the report. We understand better what has happened and why, and what the future will look like, depending on how greenhouse gas emissions play out over the coming century.
Changes in the water cycle are a prominent feature in the new report, with a whole chapter devoted to the topic.
A reliable supply of water is vital to life, and having either too much or too little has very serious consequences, leading to drought and fires at one extreme, and floods at the other.
As the climate warms, the water cycle has to respond. One of the main reasons is that the amount of moisture in the air is a function of temperature – warmer air holds more moisture (water vapour) – about 7 percent more for each 1C of warming. Therefore, when it rains, the chances are that the rainfall amount will be larger, just because there is more water to condense and fall out of the air.
At the same time, in a warmer climate, evaporation works more effectively, so land surfaces dry out faster, allowing droughts to develop more quickly and to last longer.
Overall, we see that the water cycle is becoming more intense – that is, precipitation is increasing over land at the same time as evaporation is too. With global warming, we get an increase at both ends of extremes of precipitation, floods and droughts.
It is clear that all aspects of the water cycle are being affected, including rain and snowfall, glacier mass, groundwater storage, river flows, and the oceans.
One clear signal is that variability and extremes in precipitation are increasing faster than changes in averages. Unless there are rapid reductions in emissions of greenhouse gases, we will see even more substantial changes in the water cycle worldwide, including the loss of glaciers and the river flows they feed, more intense precipitation and more extreme rainfall events and associated river flows, but also more intense droughts and an increase in the danger of wildfires.
As the climate warms, the tracks of storms are moving towards the poles in many regions, notably across the Southern Hemisphere. At the same time, the high-pressure regions in the subtropics are expanding polewards.
The net effects for New Zealand are that the west and south will see increases in precipitation in winter and spring, while the north and east will see reductions. Conversely, in the summer, the east is likely to see increases in rainfall while the west dries out a bit. As is seen across the globe, New Zealand glaciers will keep retreating as the climate warms.
We now have a much better understanding of how aerosols (air pollution) affect the water cycle, especially for the tropical monsoons and tropical rainfall generally.
Increased aerosols have generally offset the effect of the warming climate in recent decades. As the world decarbonises and aerosols decrease in the atmosphere, monsoon rainfall will change in different directions in different places. So, clearing the air is likely to lead to more floods and droughts, affecting very large populations.
Studying both the past and looking to the future, we cannot rule out abrupt human-caused changes to the water cycle. For example, continued Amazon deforestation, combined with a warming climate, means that the Amazon ecosystem could cross a tipping point into a dry state during the 21st Century.
In addition, should the Atlantic Meridional Overturning Circulation (AMOC) collapse, there would be abrupt shifts in the water cycle worldwide. Such an event is, however, not considered likely this century.
One technique for managing climate change is known as 'Solar Radiation Modification' (SRM), for example blocking out sunlight by spraying aerosols into the stratosphere. It is clear that such techniques could drive abrupt changes in the water cycle.
If such SRM approaches are to be used, recent research shows that abrupt water cycle changes will occur during a rapid implementation, or termination.
The impact of SRM varies spatially and it appears that it could affect different regions in potentially disruptive ways.
As we understand more about the water cycle, it is clear that all components are connected, and things are changing across the whole globe.
There are no easy solutions to water availability, but it is clear that the sooner we stop emitting greenhouse gases (especially carbon dioxide), the sooner we stop the climate changing. That will make it easier for everyone to adapt, and to thrive.
Professor James Renwick is Head of Te Kura Tātai Aro Whenua–School of Geography, Environment and Earth Sciences at Te Herenga Waka–Victoria University of Wellington and a co-ordinating lead author for Working Group 1 of the latest IPCC report.
Read the original article on Newsroom.