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The winters in California have become more dry over the past 40 years. Farmers rely on winter precipitation to irrigate their crops, which is a problem for the region's agricultural operations. If the trend will reverse or if California will continue getting drier, it has implications for millions of residents.
Climate models that account for changes in greenhouse gases and other human activities have had trouble reproducing California's observed drying trends. Climate models can't agree on long-term precipitation trends. The mixed results aren't very useful for future water resource planning, so researchers at the Pacific Northwest National Laboratory want to know why.
"When we see large uncertainties in model simulations and projections, we have to ask whether or not the models are up for the task," said Ruby Leung, a Battelle Fellow and atmospheric scientist at PNNL. Long-term natural cycles affect precipitation in California.
The Interdecadal Pacific Oscillation is one of the cycles that range from years long to decades long. They show the variability of sea surface temperature in the Pacific Ocean and affect winter precipitation in California.
How much of a role do they play in California's precipitation projections? It turns out to be a big one. Natural cycles are responsible for 70% of the uncertainty in model simulations of precipitation trends over the past 40 years. By isolating the effects of the natural cycles, scientists can focus on improving models to reduce the uncertainty related to how greenhouse gases and other human activities affect climate.
The impact of groups.
Researchers can now run large ensemble simulations with more computing power. Climate models are run from 40 to 100 times. The ensembles give a unique representation of natural variability because everything is the same. Simulations that contribute toward multi-model ensembles are run by modeling centers around the world. The total uncertainty is due to both natural variability and model uncertainty.
Three ensemble simulations were generated by three different climate models and two multi-model ensembles of two recent climate model generations. They wanted to know the sources of uncertainty in the projections. They were surprised by what they found.
Natural climate cycles were responsible for 70% of the total uncertainty in simulations of California precipitation trends over the past 40 years. 30 percent of the uncertainty is about how models represent human influence on climate.
"We know that natural cycles have major impacts on California's climate, but we didn't think that they would dominate the total uncertainty in climate simulations to this extent." This result shows the importance of large ensemble simulations for isolating human influence on climate, which may be small compared to natural cycles in some regions.
Human impacts and natural cycles.
The IPO is one of the most important natural cycles in California's climate. The decades-long phases help determine if California is in a drying trend. The team's results show that it has a large role in the drying of California over the past 40 years.
Climate models are limited in their ability to predict the transition between the IPO phases. Due to IPO cycles, future projections of California precipitation have large uncertainty.
Warming and increasing greenhouse gases are human-caused changes. They still have a big role to play in shaping the future. Warming will become more pronounced as greenhouse gases accumulate in the atmosphere and the ocean's large heat capacity catches up with increasing temperatures.
"Natural variability, such as the IPO, is like background noise," said Leung. The climate response to rising concentrations of greenhouse gases is a signal that grows over time. Reducing model disagreement for this signal is important, particularly when looking to the far future.
Understanding the extent to which natural and external factors affect California precipitation helps researchers better understand their projections. This knowledge helps modelers explain why their models are missing. Scientists can communicate more nuanced results to people planning California's water future.
There is uncertainty in El Nio-like warming and California precipitation changes linked by the Interdecadal Pacific Oscillation. There is a DOI: 10.1038/s41467-021-26797-5.
Nature Communications is a journal.
Climate cycles create uncertainty in California precipitation.
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