Biography & Net Worth: Climate Cycle Creates California Precipitation Uncertainty

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Over the past 40 years, California’s winters have been drier. This is a problem for agricultural businesses in the region, as farmers depend on winter rains to irrigate their crops. Determining whether California will continue to dry up or reverse the trend will affect millions of people.

But yet climate models explain changes in greenhouse gas and other human activities struggled to recreate the arid trends seen in California. When climate models predict the future or simulate the past, it is not possible to agree on long-term rainfall trends. Researchers at Pacific Northwest National Laboratory (PNNL) want to know why these mixed results are not very useful for future water resource planning.

“If you see these large uncertainties in your model’s simulations and predictions, you need to make sure your model is task-aware,” said Ruby Leung, a Betterfellow. Atmospheric scientist at PNNL. “One challenge with California modeling is its long-term problems. This has a great impact on the amount of natural cycle precipitation.”

These cycles range from years, such as El Nio and La Nia, to decades, such as the Pacific Decadal Oscillation (IPO). They represent natural fluctuations associated with Pacific sea surface temperature patterns and influence California’s winter precipitation.

But how much do they play in creating uncertainty in California’s rainfall forecasts? Turned out big Results from Leung and the PNNL teams show that more than 70% of the uncertainty in model simulations of rainfall trends over the past 40 years is attributable to the natural cycle. By isolating the effects of the natural cycle, scientists can focus on improving models and reduce the remaining uncertainty about the impact of greenhouse gases and other human activities on the climate.

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With more computing power, researchers are now able to perform larger sets of simulations called large-scale ensemble simulations. To create them, the researchers ran climate models 40-100 times, with slight differences in starting conditions. These ensembles provide a unique representation of natural variability, as everything remains the same except the starting conditions. Modeling centers around the world are also running simulations that contribute to the multi-model ensemble. These represent the overall uncertainty due to both natural variation and model uncertainty.

Leung and his team analyzed three ensemble simulations generated by two multi-model ensembles from three different climate models and two recent climate model generations. They wanted to identify the reason for the uncertainty in California’s rainfall forecasts. What they found surprised them.

The team has a natural climate cycle, modeling simulation trends in California precipitation over the past 40 years. This leaves 30 percent of the uncertainty in how the model represents human influence about the climate.

“We know that the natural cycle has a significant impact on California’s climate, but we did not think to the extent it would dominate the overall uncertainty of climate simulations,” Leung says. “This result shows the importance of large-scale ensemble simulations for isolating human impacts on climate, which in some regions may be smaller than the natural cycle.”

Natural cycles and human influence

IPOs are one of the most important of the natural cycles affecting California’s climate. Its decades of phases help determine whether California is moist or dry. The team’s results show its important role in California’s drought over the past 40 years.

Currently, climate models have limited skill in predicting the transition between IPO phases, especially decades later. Therefore, there is a lot of uncertainty in future forecasts of California’s rainfall due to the IPO cycle.

So where are human-caused changes, such as global warming and an increase in greenhouse gases? They still play an important role in shaping future climate and weather. Warming and its effects become more pronounced as greenhouse gases continue to accumulate in the atmosphere and the ocean’s large heat capacity keeps up with rising temperatures.

“Natural fluctuations such as IPOs are like background noise,” Leung says. “Although the noise is considerable, the climate response to increases in greenhouse gas concentrations is a signal that increases over time. The focus on reducing model anomalies in this signal is particularly for the distant future. If you do, it will affect you.”

Understanding how natural and external factors affect California’s rainfall helps researchers better understand their predictions. This knowledge helps the modeler to explain why the model misses points when simulating trends observed in the past. Scientists can then tell more nuanced results to those planning the future of California’s waters.


Even if global warming is stopped, the climate of the region will change. New experiments are needed to understand how to do this.


for more information:
Warming uncertainties such as Lu Dong et al and La Nia and changes in California precipitation are associated with decades of Pacific tremor. nature communication (2021). DOI: 10.1038/s41467-021-26797-5

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Pacific Northwest National Laboratory

Citation: Climate cycle creates precipitation uncertainty in California (December 11, 2021) December 11, 2021 Retrieved from https://phys.org/news/2021-12-climate-california-precipitation-uncertainty.html

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