Saturday, October 6, 2007

Misleading headline of the day: groundwater and climate change

From ScienceDaily and the US Department of Agriculture: ”Climate Change Likely To Help With Groundwater Recharge”.

Image source:
I get my water from a well that runs dry every so often. And I’ve heard Jonathan Overpeck talk about how global warming may affect the southwestern US - in particular, how it may affect our pattern of snowfall and snowmelt, and therefore exacerbate our current drought. So I was particularly curious about where, and under what circumstances, increased CO2 would lead to increased groundwater recharge. (The groundwater situation depends, after all, on many things. What’s the nature of the aquifer – is it shallow or deep, spatially restricted or regional? What kind of rocks is it in, and how and where do they connect with the surface? What’s the land use, the vegetation, the rainfall, the snow pack?)

So I checked out the actual paper. It is focused on modeling the effects of increased rainfall on two areas in Australia with different seasonal rainfall patterns. The study’s particular contribution was modeling the effects of plant growth on recharge – previous studies had left out the effects of transpiration.

Probably the most important point left out of the press release had to do with the assumptions: in these two areas, global climate models predict rainfall to increase (14% in one area, and 37% in the other). The study therefore shows that increased evaporation and transpiration shouldn’t use up the increased rainfall, and some of the water should go into the groundwater. And that’s an important result, and the study demonstrates an approach that could be used for other scenarios.

But as the authors themselves say in their conclusion:

The direction and magnitude of the change in local recharge depended on the combination of climate scenarios, vegetation, and soil properties. It is not possible to make generic conclusions about climate change impacts based on this limited sample of climatic zones in Australia.

(Their words; my emphasis.)

Contrast the conclusion of the press release from ScienceDaily:

In both locations, changes caused to soil, precipitation and plant transpiration by simulated climates with twice the existing CO2 led to significant changes to the rate of groundwater recharge. Water recharged from 34 percent slower to 119 percent faster in the Mediterranean climate, and from 74 to 500 percent faster for the subtropical climate.

While the opportunity for decreased recharge rates exists, the general trend is towards increase. Future research will investigate whether those changes would benefit or harm those ecosystems.

The press release never mentions that the simulation included an increase in rainfall... which is probably the single most important factor in increasing groundwater recharge.

And the headline. That headline. “Climate Change Likely to Help With Groundwater Recharge.” The headline does exactly what the authors of the study warn against doing: it makes generic conclusions from a limited study of particular places.

Well. I don't live in Australia, and climate models predict less rainfall for my corner of the world. So regardless of the headline, I'm not going to count on my well recovering any time soon.

1 comment:

Dr. Lemming said...

Hey! Some of us work in places with increasing monsoonal rainfall. And let me tell you- a wet season that starts a month early and ends a month late makes planning interesting. But despite that, there is no doubt that the 100 thousand people living in the rural NT will benefit from global warming. So cut them some slack- it isn't their fault that as the top end rainfall increases, the 17 million residents of subtropical and temerate Australia are going on ever tighter water restrictions. After all, you can't please everyone.