ScienceWise - Spring 2011

Panning out

What does evaporation data mean for Australia’s climate?

As it’s name suggests, an evaporation pan is a device scientists use to collect data on the rate of evaporation at a given location, usually in association with other weather monitoring instruments. Essentially, the pan records an upper limit on how much evaporation can occur at the site. You can think of this as the evaporative demand of the atmosphere. The pan measurements need to be multiplied by a factor (~0.7) to account for the fact that the pan is raised off the ground and intercepts additional sunlight. When that is done, the pan is a good measure of the evaporation and transpiration from the surrounding soil and plants if the soil were saturated. In practice, the moisture level in the soil is rarely that high so corrections factors have to be applied to calculations.

Looking at the data from such pans over the past 30 years, a strange pattern emerges. In spite of rising average temperatures, the rate of evaporation has been steadily declining, not just in Australia, but across the world from the US to Europe to China. One of the leading scientists in the field of evaporation is Dr Mike Roderick from ANU.
In 2011, Dr Roderick, together with other scientists around the country, have been successful in attracting over 20 million dollars in funding for a new ARC Centre of Excellence in Climate System Science. The centre will study potential changes to the climate of Australia with a special focus on water availability.

“A lot of people think that in a warming world, evaporation rates will increase and therefore soils will become dryer,” Dr Roderick says, “but the reality is a lot more complicated than that. Evaporation depends on a number of factors, not just temperature. Humidity, wind speed and sunlight are just as important. For example Darwin is hotter than Alice Springs but the average pan evaporation in Darwin is lower because it’s more humid.”

Back in 2007, Dr Roderick’s team were the first to effectively explain the falling evaporation rate anomaly. After an extensive study of vast quantities of data, they concluded that the reduced pan evaporation rates were the result of falling wind speeds - and coined the term, global stilling. Over the last 30 years the average wind speed has dropped by 0.01 ms-1 each year resulting in average wind speeds 0.3 ms-1 lower than they were 30 years ago. Given that the average wind speed is around 2 ms-1, this represents a very significant change.

At the same time the average wind speed over the oceans is actually increasing by a similar amount. “The tricky thing with wind speeds is that the data over the ocean is collected by satellites, whilst that on the land comes from anemometer measurements. So you have to be careful in combining the two.” Dr Roderick says.

Wind speed can’t be measured directly by satellite so the data is based on the roughness of the ocean. A radar beam is bounced off a square of ocean eight by eight kilometres. If the sea is rough the back scatter will be greater and a rougher sea means a higher wind speed at the surface. The land measurements on the other hand are directly based on standard anemometers all set at two meters off the ground.

“In spite of the different measurement techniques, we are pretty sure that this is a real effect because recent research from land stations right on the coast in Canada shows that the average wind speed is rising when the wind blows from the sea and falling when it blows from the land.”

There are a number of theories that aim to explain this phenomena one of the most popular being that increasing CO2 levels are leading to increased plant size. This is not so much an increase in the size of a given species as a gradual change in vegetation types. Woody plants are starting to encroach more and more into what were traditionally grasslands and of course shrubs and trees are far taller than grass. The net result being that wind speeds near the ground are decreasing because of the increased drag and shielding effects of these plants.

“No one is really sure as yet if plant changes are the mechanism behind the reducing wind speeds,” Dr Roderick says, “but what we are sure of is that plants respond directly to even modest changes in CO2 levels and there’s no doubt that those are rising.”

But what does all this mean for Australia? “I think the message here is that climate science is complicated. You can’t just say it’s hotter so it will be drier.” Dr Roderick explains. “Increasing temperatures and wind speeds over the oceans will almost certainly lead to increased evaporation. However where that moisture falls as precipitation will depend very much on the large-scale atmospheric currents which may themselves change. Decreasing evaporative demand, as shown by the decline in pan evaporation, and an increase in the efficiency of plant water use that is accompanied by high CO2 may lead to less water being required by agriculture. Whether this will be outweighed by decreasing rainfall or indeed if rainfall will actually increase is of course one of the things what we’ll be trying to figure out at the new Centre of Excellence.”
 
 

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