Weather@home 2015: Western US Drought

Has climate change made the drought in the Western US more likely? That’s what we want to find out!

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The Western US drought has ranged from troublesome to severe. Californians have just experienced a fourth winter of drought, following three years that have marked some of the most severe drought conditions in the past century.

Oregon is in its second year of drought thanks to very low snowpack because of warm, mild winters. Washington is in its first year of drought – a result almost exclusively tied to warmer winter temperatures.

This past winter, Governor Jerry Brown issued water restrictions for the first time in the history of the state. In 2014 alone, the drought cost $2.2 billion and caused over 17,000 farm workers to lose their jobs.

Abby Halperin, Myles Allen and Friederike Otto explain how serious the ongoing drought is in California, which has experienced four years of drought already, and how this experiment will help determine what effect, if any, climate change has had on how likely this drought is.

The drought in the Western US over the period covered by this experiment,
from US Drought Monitor.

Has the chance of such a drought become more likely with climate change? That’s what this experiment will tell us

There is always the possibility that this drought could have happened in a world without climate change. However, the chance of a drought could have increased with climate change just like the chance of lung cancer increases with every cigarette smoked. Alternatively, it is possible that we cannot detect the role of climate change in the likelihood of the drought, or even that climate change made the drought less likely.

Could other weather conditions, like the “blob”, have caused this drought? We’ll look at that too

The “blob” is a giant patch of unusually warm water off the West Coast in the northeast Pacific Ocean. It has been observed since 2013 and has had a strong effect on the weather over the Pacific Northwest region since then. This blob is an unusual phenomenon and could be the main cause of the drought, rather than climate change, or it could be a combination of both acting at the same time.

Climate Explorer plot for DJF (December, January & February) 2013/14 using ERSST v4 dataset, showing the “blob” off the west coast of the USA.

How are we going to find out the link between climate change and the drought?

With your help running simulations on your home computer, we can simulate and compare thousands of possible Western US winter seasons in the world as it might have been without climate change with possible winter weather in the world as we know it. If the chance of a drought in these two worlds is the same, then we can’t blame climate change for this particular event. However, if the chance of a drought is greater in the world with climate change, then we can say that climate change increased the risk of drought.

We will also compare the world as it was observed with another possible world without the blob so that we can work out which had a greater influence on the drought: climate change or the blob, an unusual but natural weather condition.

Over the next few weeks, we will be putting up the results from the many simulations as they come in. Help us figure out if climate change or “the blob” played a role in this damaging drought.

How do we measure the drought?

The simplest measure of drought is the amount of rain and snow in a given wet season. However, looking at just the amount of water that hits the ground doesn’t give us the full story. Higher temperatures cause more water to evaporate, which is a problem for reservoirs and plants. Lake Oroville, the second largest reservoir in the state of California, was already almost half empty in 2014.

The snow pack is also important, and not just for those who like to ski and snowboard. The amount of snow remaining at the end of the wet season (April 1st) needs to last us through the dry California summers as it melts and helps fill the reservoirs and irrigate fields. In this study we look at all three measures of drought: the total amount of water that hits the ground, the average temperatures, and the snow pack at the end of the wet season.

These three indicators are not the only measures of drought. Others include soil moisture, which is important for plants. Ultimately, which drought measure or measures you choose depends on the impacts that you care about. Reservoir mangers, farmers, and homeowners might all have different concerns and choose to look at different sets of indicators.

Read More

Read about the experimental setup.

Read more about the drought in California, Washington and Oregon and references for this experiment.

See our sample results or the latest results for California, Oregon or Washington.

What will the results look like? Find out more about how to interpret Return Time Plots.