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Concentration of CO2 in the Atmosphere

Climate Change and the Water Cycle

E-book by Climate Reality Project

Cyclone Catarina, from the International Space Station on March 26, 2004. This was the first hurricane ever observed on the South Atlantic Ocean. The climate is changing. NASA image

Cyclone Catarina, from the International Space Station on March 26, 2004. This was the first hurricane ever observed on the South Atlantic Ocean. The climate is changing. NASA image

By Rick Wackernagel

The Climate Reality Project recently released an e-book “Climate Change and the Water Cycle: Four Big Questions Answered.” In writing this e-book, the author shows good instincts but falls short in executing some of them.

The e-book proposes to explain the sometimes counter-intuitive impacts of global warming on Earth’s water cycle. This is a worthy goal. We don’t have a brief, simple and accurate explanation.

The four questions posed are good, seeking explanations for increases in rainfall, drought, extreme weather and wildfires.

The e-book appropriately ends with a call to action, and recognition that we can manage climate change and ease its effects.

While the overall organization is good, some of the individual sections are poorly organized. They digress, and sometimes are hard to follow.

Answering the question, “What does climate change have to do with hurricanes and typhoons?”, the e-book jumps from storms picking up more energy, from warmer oceans, to storms having intense winds and very heavy rainfall, without saying that more energy produces stronger winds. After identifying effects of stronger storms and expectations for the future, the book explains that hurricanes and typhoons are both strong tropical cyclones fed by heat from the ocean. This explanation would have helped readers more if it had preceded the statement about storms picking up more energy from warmer oceans. The book then jumps into sea-level rise and its potential impacts on coastal areas, in which the world’s largest cities are concentrated. It doesn’t mention that more water in the oceans and less in glaciers represents a change in Earth’s water cycle.

A good infographic summarizes some effects of climate change on the water cycle. It is not explained, however, so does less to improve readers’ understanding than it could.

The e-book more often describes what changes occur than how or why the changes occur. For example, it does not explain why, with increased moisture in Earth’s atmosphere, rains become less frequent but heavier.

The primary explanation given for drought is that rain will be heavier and less frequent. In heavy-rainfall events, rain will fall faster than it can infiltrate into the soil, so more will run off rather than recharging soil moisture. The impact of global warming on the distribution of rainfall is not mentioned, however. Atmospheric circulation has been transporting moisture from the subtropics into middle-latitude regions. Global warming will displace this circulation pattern poleward, expanding the subtropical area from which moisture is drawn. Warmer temperatures will increase evaporation in these subtropical areas, making them more droughty.

Other sources provide better, though more technical, explanations of changes in the water cycle. “Will the Wet Get Wetter and the Dry Drier?”, from the Geophysical Fluid Dynamics Laboratory, outlines projected changes in precipitation through the rest of the century, as well as explaining why the changes occur. “Water Cycles and Climate Change,” by Kevin Trenberth says that global warming will increase the moisture-holding capacity of the atmosphere more than it increases evaporation. This imbalance leads to less frequent and heavier rain and snowfall events.

Climate change will indeed have important impacts on Earth’s water cycle. Heavy rainfall events and sea-level rise will cause more flooding, destroying crops, other property and infrastructure. Drier droughts over larger areas will wreak their havoc with agriculture. People will respond by migrating to moister areas. All these impacts can be lessened by reducing greenhouse-gas emissions. Expanding understanding of the water cycle will help us make this a goal and achieve it.

Rick Wachemagel is an itinerant climate activist and former extension farm-management specialist from Burlington, Vermont.

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