A Tale of Two Cities: Evaporation Rates and Flood Risk
Within the time-frame of a month, the regular news has served us two helpings of "natural" disasters that set me thinking. Those disasters are the June 20th flooding in Duluth, MN, and the weekend flooding of the Black Sea town of Krymsk in the Krasnodar region of Russia. While the Duluth flood thankfully caused no loss of human life, the Krymsk flood killed over 170 people. Flood mitigation aside, the situation of both towns had peculiar similarities to one another: Both were coastal towns of a large inland water body that had not seen flooding of this scale in modern history. While I have no doubt that some may dismiss the idea that these events were linked to climate change, I'm more inclined to remember a saying from a famous fictional engineer from decades past: "Fool me once, shame on you. Fool me twice, shame on me."
It's well known that the evaporation of water increases with temperature. This is not climate science, but basic physics and chemistry. It's called the Clausius–Clapeyron relation, and it relates temperature to liquid vapor pressure. Liquid vapor pressure is the measured air pressure above a liquid at which the constantly-moving molecules of the liquid are sped-up enough (through heating) to overcome the surface tension, thus releasing the liquid molecules into the gas phase. The liquid vapor pressure of water at 100 degrees Celsius (212 degrees Fahrenheit) is 29.92 inches of mercury, also known as sea level pressure. You will also note that 100 degrees is the temperature at which water boils.
When water boils, it releases water from a liquid state to a gas state (steam). However, it's not an on/off relationship. In fact, at lower temperatures, water is still releasing molecules to the atmosphere, albeit at a lower rate than if it were boiling. Even at a more moderate temperature of 20 degrees Celsius (68 degrees Fahrenheit) the vapor pressure of water is 0.68 inches of mercury, which means that although the water is not boiling, it's still flinging off water molecules into the air as the water evaporates.
The relationship is non-linear, but suffice it to say, when the temperature of water increases, the vapor pressure rises too. As the vapor pressure of a given volume of water gets closer to that of sea level (the boiling point), water is evaporating from the liquid state into the air at faster rates and in greater volumes (that's why you still see steam while a pot of water heats up but doesn't boil). So, when the temperature of a body of water such as Lake Superior or the Black Sea increases, the air above it gains more water.
So, here's the 800-pound gorilla in the room: Lake Superior is warming. So is the Black Sea. The connection between warming surface waters and the significance of extreme rainfall is still being correlated, but that hasn't stopped educated dialog about how a warmer and wetter atmosphere might effect overall rainfall totals. Both the Duluth event and the Krymsk event had additional circumstances that caused the deluge of roughly a foot of rain within 48 hours and 24 hours, respectively. Both fell into an unusual weather pattern with winds that effectively pushed water onto shore like a storm surge, inundating storm sewers that had not seen water in such volumes in over 100 years (at least for the Duluth event - Dr. Masters' blog entry on the subject suggests a 1-in-20 year event for Krymsk). However, I'll leave it to the real meteorologists to provide a more accurate description.
I think the moral of the "Tale of Two Cities" is that it doesn't matter if you live on the ocean-side coasts of the world or not; your flood risks are increasing. With a warming atmosphere comes a warming hydrosphere, and along with it, an enhanced ability for nature to transport vast amounts of water to different parts of the planet in quantities not seen in recorded history. The implication being that no place on Earth is immune from our changing climate. I close with pictures from my graduate study area in the Midwestern United States, where I analyzed precipitation runoff from an agricultural watershed. The study area is a hydrological confluence of several square miles of farmland, and shows how even a moderate rainfall event can turn a lazy, Mayberry-like country creek into a raging torrent literally overnight: