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California May See More Rainfall Over the Next Century

New research by Robert Allen, a climatologist at the University of California, Riverside, suggests that climate change may actually result in more precipitation – and more floods and storms.

Written by Ian Evans Published on Read time Approx. 6 minutes
A bicyclist rides along a flooded street as a powerful storm moves across Southern California on February 17, 2017 in Sun Valley, California. New research shows parts of California could expect more rain (and more floods) in the next century.David McNew/Getty Images/AFP

After undergoing five years of drought, many Californians may fear a future with less rainfall thanks to climate change. However, research carried out at the University of California, Riverside, suggests precipitation across the state may actually increase over the next hundred years.

The paper, published in the journal Nature Communications, looked at several different California climate models and aggregated the results. However, not every model is good at predicting factors like precipitation. So, rather than simply aggregating the findings, Robert Allen, a climatologist at the University of California, Riverside, and the lead author on the paper, was picky. To get the most accurate look at what California’s rainfall might look like in the future, he chose only those models that most accurately measured precipitation across the state.

Under “business as usual” scenarios – where humans continue to burn fossil fuels at the same rate we do now – Allen found that there could be a 12 percent increase in precipitation across the state by the end of the century. If his results hold up, they suggest that California will not suffer as much from climate change as previous models suggest.

But Allen says that even if there is more rain over a century, his research does not indicate how often that rain will come. California may still have years of drought followed by years of floods. To better understand the research, Water Deeply talked with him about the paper, the future of precipitation in the state and the use of climate models.

Water Deeply: This is a little bit counter to what several other papers before had found. Were you surprised by the results?

Robert Allen: I was surprised, yes. I mean, these other papers, they were either using the older models or they were using the newer models but a very limited subset. They weren’t able to perform an analysis similar to the one I did where I was trying to assess, or rank, the models basically.

Water Deeply: You mention in the paper that California is just very difficult to model because it is in what you call a transition zone. What does that mean and why does that makes it difficult to model?

Allen: If you zoom out to larger spacial scales like, say, hemispheric or global, climate models project some robust precipitation changes. These include a wettening of the tropical area, a drying of the sub-tropics and then a wettening of the mid-latitudes and high latitudes. California is a state that’s oriented north-south. It covers several different latitudes. It falls within these various zones. Southern California includes part of the sub-tropics. Then Central California’s more mid-latitude, including Northern California. California is difficult to model precipitation changes because it falls in the middle of these two zones where climate models project differing precipitation tracks.

Water Deeply: Just to build on that, you did not find that precipitation would increase under business-as-usual conditions across the entire state – there were differences. Can you say a little bit more about that?

Allen: Yes, based on an annual average, if you look at the entire state, the models that I looked at showed a 12 percent increase in precipitation by the end of the century. Then if you looked at regions like Northern California versus Central California versus Southern California, most of that increase in annual mean precipitation is in the central and northern parts of the state. Southern California actually is projected to have a relatively weak decrease. I think it was about 3 percent of the decrease for the annual. If you look at the winter season – December, January and February – the entire state is projected to get wetter. This includes the southern portions of the state. That’s where the bulk of this wettening trend comes from – from the winter season.

Water Deeply: What are the implications of this?

Allen: Well, in the broad sense, one of the implications is that climate change may be less detrimental to California than previously thought, particularly in the context of freshwater resources, agriculture – you know, all the things that rely on freshwater.

Ultimately, this is just one study. I think additional follow-up studies are necessary by other groups to confirm my findings. Also, the models are continually being developed and improved. The newer set of models are expected to be released sometime this next year. Obviously, continuing to look at these newer models to see if they continue to project wettening trends is vital.

Water Deeply: You also show in the paper that there will be an increase in storms in California.

Allen: Yes, exactly. That’s ultimately what I attribute the increase in precipitation to. That’s kind of like another test of the climate signal. I mean ultimately modeling precipitation is difficult, particularly when you get down to regional-type scales like California. Precipitation is driven by certain atmospheric patterns and circulations. Though if you can relate the precipitation response to a change in the atmospheric circulation, then that gives you improved confidence on the climate response. I spent a lot of time in the paper trying to not only explain or lay out the climate response, but then to try to understand why that response occurs in the models. An increase in storm track activity off the coast of California was the primary driving mechanism.

Water Deeply: Have you heard any reaction from either other researchers or people involved in, say, California agriculture since your results came out?

Allen: I’ve had some feedback. I mean, ultimately the journal that I published in was a peer-reviewed journal. I had a lot of feedback through the whole review process from experts in the field. Obviously, they eventually approved or accepted the final manuscript, so they must find some value in the work. I haven’t been contacted specifically by, say, farmers, say in the Central Valley, but I’m wondering if that’s going to happen.

Water Deeply: Was that something you were thinking about even as you were writing and publishing this paper?

Allen: Well, I definitely was aware that my results were against prior results and that mine, as I mentioned earlier, suggest that climate change may be less detrimental in terms of freshwater resources for the state. I knew maybe it was going to be a little controversial. There are additional things that are important for assessing changes in precipitation besides long-term trends. That was the focus of the paper. I was just looking at, say, a shift in the climatology.

Climate change is also associated with other changes in the hydrological cycle besides changes in the climatology. This includes changes in the intensity of precipitation or in the number of consecutive dry days. This gets into things like floods and droughts. Those are also very important, particularly for farmers and agriculture. That’s something that I didn’t really address in the paper but something I hope to follow up with very soon.

Water Deeply: Do you have any idea of how your results might change if it’s not a business-as-usual scenario?

Allen: There are currently four different scenarios that all these models are driven by. There’s the business-as-usual scenario, which is the one that assumes that we do nothing over the century to reduce our greenhouse gas emissions. It has about two-and-a-half times the level of atmospheric greenhouse gases by the end of the century. It’s the scenario that has the most warmth.

Then, I looked at less aggressive scenarios that have less warming, so they have less of an increase in atmospheric greenhouse gases through the century. I get similar results as what I did based on business as usual, but the increase in precipitation is smaller.

Warming leads to an increase [in precipitation]. More warming leads to a larger increase. None of these future emission scenarios really include a situation where there’s a lack of warming.

Water Deeply: What would you want readers to take away from this paper?

Allen: Well, I guess in terms of fellow climate scientists, I would want them to take away the notion that often in climate science, we have a bunch of models and we analyze climate projections by averaging all of the models together. We call that the multi-model mean. The idea is that this gives you the best future climate projection; based on my results, that may not be true.

Ultimately, if you’re interested in a specific climate-change signal, it’s worthwhile to try to assess each individual model to see if they can simulate the observations for that specific climate-change signal. Not all models can. Some models perform better and it makes sense to focus on the models that better simulate the observed climatology for your specific climate-change phenomenon and use them as the models that you assess for future projects.

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