Frank Wentz: Will global warming bring more rain?

As our climate warms, we might be in for a wetter world. EarthSky spoke with climate scientist Frank Wentz, director of Remote Sensing Systems in Santa Rosa, CA. Predicting how our warming climate will change rainfall patterns around the world is extremely complicated, said Wentz. He uses satellites to measure changes in water in Earth’s atmosphere over decades – not just the rain and clouds you can see – but the invisible vapor that comprised 99% of the water in our atmosphere. Wentz spoke with EarthSky’s Jorge Salazar about the complex puzzle of how climate change affects rainfall.

Do scientists think that global warming will bring more rain?

The one piece of the puzzle that I think we do understand well is that as the atmosphere warms, it can hold more water and the water in the atmosphere increases with warming. Now, the more difficult question is how much of this additional water is going to turn into rain?

If there were no changes in the motion of the air and atmospheric circulation, then the rain would increase at the same rate as the total water, which we think we know fairly well to be about seven percent per degree warming.

However, most climate models predict that the atmospheric circulation slows down and the overturn of the atmosphere doesn’t occur as quickly, and thus what you end up with is a more wet but stagnant atmosphere. And according to the client models, the rain is going to increase at a considerably slower rate, about one-third of what the total water in the atmosphere increases by. But that said, the climate models are trying to model a very complicated process. There are some very difficult things to model such as the radiative forcing by clouds and in my opinion, that’s a real challenge for climate models.

And I certainly wouldn’t claim that we have any definitive answers. I think, though, we’ve been able to leave behind the upper and lower rates of the expected increase in rain. But to determine what the exact increase is going to be and where the increased rain is going to fall, those are questions that still need to be answered.

What did you mean by ‘overturn’ of the atmosphere, and how does that influence rainfall patterns?

We all agree, the scientists and the measurements all agree in a sense that we really see increased moisture in the atmosphere as the air warms. But just because there’s more water in the atmosphere doesn’t necessarily mean there’s going to be that much more rain because the rainfall is also determined by the air circulation.

It’s sort of like a conveyor belt. And even though the packets on the conveyor belt contain more water, if the conveyor belt slows down then there’ll be less rain.

There are two things going on – the total water going into the atmosphere and also the circulation. The total water is increasing, but the circulation, the speed of the conveyor belt is decreasing. And so the combined effect of that is why the climate models are predicting not very much increase in rainfall.

You use Earth-orbiting satellites to gather data about how water in our air is changing over time.

That’s correct. We use about ten different satellites and we piece them all together, starting back in the 1980’s until today to generate these 25 and 30 year time series of water vapor and rainfall.

For example, NASA’s Aqua satellite flies a microwave radiometer called AMSR-E. And the way this radiometer works is it actually listens to the natural emission of microwaves from the Earth and by measuring these microwaves you can very accurately measure water vapor in the atmosphere.

And that’s important because almost all the water in the atmosphere, 99 percent of the water in the atmosphere is water vapor. It’s invisible.

You don’t see it when you look up at the sky. You look up in the sky, you see clouds and rain but the clouds and rain are only about one percent of the total water. Almost all the water is in terms of this vapor. And the microwaves can measure the vapor very, very precisely. In addition, they also can measure directly the rainfall. So you get a measurement of both things, both of the relative parameters, the water vapor and rainfall.

Aqua, because it’s flying a microwave sensor, can directly measure rain and the water in the atmosphere. And there’s a close connection between the two. What we’re trying to understand better is as the water in the atmosphere increases, what effect that has on rain. Because Aqua can measure both at the same time, we can study that relationship better.

You’ve written that changes in rainfall across the globe may be one of the most serious risks from climate change. What do you mean?

Our world food production depends critically on rainfall. We need to know where it will rain and how much rain there’s going to be. If these rain patterns begin to change and intensify then we need to adapt quickly. We need to adapt our agriculture and crop selections to match with the new rain patterns.

There are some very heavily populated regions on the planet, such as Bangladesh, that are already plagued with flooding. A significant increase in rainfall in these areas would really cause some problems.

To farmers, the thought of more rain in the future sounds pretty good at first.

Yes, it does and certainly there’s some positive aspects. And they should be pointed out. But there’s also some possible negative scenarios and some of the client modelers, they have an expression; the wet areas are going to get wetter and the dry areas are going to get drier. That’s sort of a worst case scenario and certainly hope that doesn’t happen.

But that is one negative scenario. Even though in general, over the whole planet you get more rain, what happens is in the wet areas where they really don’t need more rain, they get a lot more rain and meanwhile the dry areas become drier. Now whether or not that will actually happen, I don’t know. But that is in the realm of possibilities. The other possibility is that there will just generally be more rain and it will be beneficial.

What’s the most important thing you want people today to know about global warming and rainfall?

Global warming is real. It’s not a hoax. However, one should not expect a perfectly steady increase in temperature. We’ve already seen in the 1990’s that there was a relatively large amount of warming and moistening while this last decade, 2000 – 2010, the warming trends have decreased some. But this type of variability is to be expected and it doesn’t mean that global warming isn’t occurring.

Some decades will warm more and some decades it will warm less. But having said that, I think we also have to acknowledge that the Earth is an extremely complex system to predict and, as I said, particularly with respect to rainfall as opposed to simply predicting overall warming. Our study on how much more rain global warming will bring was intended to put an upper and lower bound on future rainfall intensity increasing. I think most scientists are confident that we will see more rain the future but exactly how much more and where it will occur are still unknowns.

Our thanks today to NASA’s Aqua Mission, improving our knowledge of our home planet through satellite observations.

November 17, 2011

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