Human emissions drive recent trends in North Pacific climate variations

Nature Portfolio (Springer Nature) | August 14th, 2025

Click here for a plain language discussion of the article by co-authors Pedro N. DiNezio and Timothy M. Shanahan

Click here for a plain language discussion of the article by co-authors Pedro N. DiNezio and Timothy M. Shanahan

Human-Induced Changes in the Hydrology of the Western United States

American Association for the Advancement of Science (AAAS) | February 22nd, 2008

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regi

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climate change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

Hydroclimate Report Water Year 2015

California Department of Water Resources (DWR) | December 23rd, 2016

Water year 2015 added a fourth year to the ongoing drought in California, with observations indicative of a changing climate, including record warmth. The Water year ende

Water year 2015 added a fourth year to the ongoing drought in California, with observations indicative of a changing climate, including record warmth. The Water year ended with record high temperatures, and preceded a period of historically low precipitation that started in 2012. April snowpack measurements in 2014 tied the historic record low of 1977. Expectations of a developing El Niño event in the eastern tropical Pacific fueled notions that water year 2015 would be better. However, during the first two months of the water year, warm temperatures persisted and precipitation continued to fall short of expectations.  The developing El Niño event stalled as California headed into the heart of its wet season. In 2014, the snowpack level was 25% of its average on April 1. That mark was shattered on April 1, 2015 when snowpack amounted to a meager 5% of average. Satellite-imagery compares the Sierra Nevada snowpack near the end of March 2015 to the average conditions in water year 2011. This extreme low in snowpack exceeds end-of-century climate projections. When accompanied by the record warm temperatures experienced in the 2015 water year, there is conversation of California having shifted to a new climate “normal”.

Hydroclimate Report Water Year 2016

California Department of Water Resources (DWR) | September 5th, 2017

Water year 2016 ended up being different than expected, from a climate standpoint. Water year 2015 marked the 4th year of drought in California, ending with record high t

Water year 2016 ended up being different than expected, from a climate standpoint. Water year 2015 marked the 4th year of drought in California, ending with record high temperatures, record low precipitation, and a record low snowpack. Forecasts were for a strong, wet El Nino during water year 2016, with the potential to rival large flood producing events of 1983 and 1998, higher than average sea levels, increased wave action and potential damage along the coast, warmer than average temperatures, and above-average precipitation producing storms. As a result of the cooling of the tropical Pacific sea surface temperatures, and the decay of El Nino, only some of the anticipated impacts were realized, above-average precipitation and greater than average snowpack did not come to pass. While statewide air temperatures averaged lower than the record setting 2015 water year they were still well above the long-term record, ranking water year 2016 warmest 117 of 121 since 1895. Precipitation was above normal in the Northern Sierra with the majority of precipitation falling in January and March. Precipitation in the Southern Sierra was on average with the long-term trend. Differences in Northern and Southern Sierra precipitation were due to the majority of Atmospheric River landfalls occurring in the north including the two strongest occurring in the first two weeks of March while only two Atmospheric Rivers made landfall south of Monterey Bay. Statewide snowpack was 15 percent below average and impacted the April-July snowmelt with streamflow on the Sacramento and San Joaquin Rivers by 32 percent and 22 percent below average respectively. Water year type was classified as “Below Normal” for the Sacramento River system and “Dry” for the San Joaquin. Overall, water year 2016 was an improvement to the previous four years of drought conditions with enough precipitation to offset some of the large deficits in water storage reservoirs.

Hydroclimate volatility on a warming Earth

Nature Portfolio (Springer Nature) | January 9th, 2025

Hydrological Intensification Will Increase the Complexity of Water Resource Management

American Geophysical Union (AGU) | March 9th, 2022

Climate change is intensifying the hydrologic cycle, resulting in an increase in floods and droughts. These changes increase the complexity of water resource management t

Climate change is intensifying the hydrologic cycle, resulting in an increase in floods and droughts. These changes increase the complexity of water resource management that must balance between releasing water to reduce flood risk and storing water for long periods without precipitation. Using updated climate model projections, we show that the amount of precipitation during events will become larger combined with longer dry periods where the daily evaporative demand exceeds precipitation for much of the Earth's surface. Additionally, for the late 21st century, a large portion of the years will be hydrologically intense in major river basins, resulting in conditions that deviate from those for which infrastructure and management policies were developed.

Identifying Major Hydrologic Change Drivers in a Highly Managed Transboundary Endorheic Basin: Integrating Hydro-ecological Models and Time-Series Data Mining Techniques

American Geophysical Union (AGU) | August 23rd, 2022

The fragile balance of endorheic lakes in highly managed semi-arid basins with transboundary water issues has been altered by the intertwined effects of global warming an

The fragile balance of endorheic lakes in highly managed semi-arid basins with transboundary water issues has been altered by the intertwined effects of global warming and long-term water mismanagement to support agricultural and industrial demand. The alarming rate of global endorheic lakes’ depletion in recent decades necessitates formulating mitigation strategies for ecosystem restoration. However, detecting and quantifying the relative contribution of causal factors (climate variability and anthropogenic stressors) is challenging. This study developed a diagnostic multivariate framework to identify major hydrologic drivers of lake depletion in a highly managed endorheic basin with a complex water distribution system. The framework integrates the Soil and Water Assessment Tool (SWAT) simulations with time-series decomposition and clustering methods to identify the major drivers of change. This diagnostic framework was applied to the Salton Sea Transboundary Basin (SSTB), the host of the world's most impaired inland lake. The results showed signs of depletion across the SSTB since late 1998 with no significant changes in climate conditions. The time-series data mining of the SSTB water balance components indicated that decreases in lake tributary inflows (-16.4 Mm3 yr-2) in response to decline in Colorado River inflows, associated with state water transfer agreements, are causing the Salton Sea to shrink, not changes in the irrigation operation as commonly believed. The developed multivariate detection and attribution framework is useful for identifying major drivers of change in coupled natural-human systems.

Impacts of California’s Five-Year (2012-2016) Drought on Hydroelectricity Generation

Pacific Institute | April 13th, 2017

The severe hydrological drought afflicting California between 2012 and 2016 has ended, with an extremely wet winter beginning in October 2016. That five-year per

The severe hydrological drought afflicting California between 2012 and 2016 has ended, with an extremely wet winter beginning in October 2016. That five-year period was the driest and hottest in the instrumental record going back to the late 1800s. Impacts on communities, ecosystems, farmers, and the economy were widespread but have been unevenly distributed. The Pacific Institute has regularly analyzed the consequences of California droughts, beginning with comprehensive assessments of the 1987-1992 drought (Gleick and Nash 1991, Nash 1993), the 2007-2009 drought (Christian-Smith et al. 2011), and this most recent drought (Cooley et al. 2015, Gleick 2015, Cooley et al. 2016, Feinstein et al. 2017). This analysis is the third assessment of the impacts of the drought on hydropower production and greenhouse gas emissions from California’s energy sector (Gleick 2015, Gleick 2016). This analysis finds that during the five years ending September 30, 2016 (the end of the 2016 “water year”), hydropower generation was substantially below average, and the associated economic cost to California ratepayers was approximately $2.45 billion dollars. The additional combustion of fossil fuels for electricity generation also led to a 10 percent increase in carbon dioxide emissions from California’s in-state power plants (CARB 2015) along with increased emissions of other pollutants.

Impacts of California’s Ongoing Drought: Agriculture

Pacific Institute | August 26th, 2015

California’s agriculture sector has exceeded expectations during the most severe drought in recorded history at the cost of massive but unsustainable groundwater pumpin

California’s agriculture sector has exceeded expectations during the most severe drought in recorded history at the cost of massive but unsustainable groundwater pumping. Continued groundwater overdraft, while reducing the economic impacts of the drought for the agricultural sector now, has shifted the burden to others, including current and future generations forced to dig deeper wells, find alternative drinking water sources, and repair infrastructure damaged by subsidence. That is the conclusion of the new study, Impacts of California’s Ongoing Drought: Agriculture, released today by the Pacific Institute, an independent global water think tank. This new study is the first comprehensive analysis of the actual impacts of the drought on California agricultural revenue and employment through 2014 – the last year for which data are available. The study, drawing on data from the USDA National Agricultural Statistics Survey and the California Employment Development Department, analyzes acreage, revenue, and employment figures from 2000-2014. Production costs, impacts on animal or nursery products, or regional impacts are not examined because these data are not yet available. The study’s results provide critical insight into how the state can maintain a healthy agriculture sector in a future likely to see less water, more extreme weather, and greater uncertainty. California remains the largest agricultural producer in the U.S. in total output and in exports. During the drought, California’s agriculture sector has experienced record-high crop revenue and employment. Last year farmers harvested 640,000 fewer acres, which was 9 percent below pre-drought levels, yet crop revenue remained strong. Indeed, crop revenue peaked in 2013 at $34 billion – the highest level in California history. In 2014, crop revenue declined by $480 million, representing a 1.4 percent reduction from 2013 levels. All economic estimates have been corrected for inflation. Statewide agriculture-related jobs also reached a record-high of 417,000 people in 2014, highlighting the sector’s ability to withstand the reduction of available water.

Impacts of California’s Ongoing Drought: Hydroelectricity Generation

Pacific Institute | March 17th, 2015

California’s hottest and driest drought in recorded history is shifting the sources of energy for electricity with adverse economic and environmental consequences. The

California’s hottest and driest drought in recorded history is shifting the sources of energy for electricity with adverse economic and environmental consequences. The Pacific Institute, an internationally-renowned independent think tank focused on water issues, released a report that reveals diminished river flows have resulted in less hydroelectricity, more expensive electricity, and increased production of greenhouse gas emissions. “This severe drought has many negative consequences. One of them that receives little attention is how the drought has fundamentally changed the way our electricity is produced,” said the report’s author, Pacific Institute President Peter Gleick. “We hope this report prompts a lively debate on how to factor in a changing climate when we plan for electricity generation,” he added. Under normal conditions, electricity for the state’s millions of users is produced from a blend of many sources, with natural gas and hydropower being the top two. Since the drought has reduced the state’s river flows that power hundreds of hydropower stations, natural gas has become a more prominent player in the mix. This is an expensive change. According to the Institute’s report, between October 2011 and October 2014, California’s ratepayers spent $1.4 billion more for electricity than in average years because of the drought-induced shift from hydropower to natural gas. In an average year, hydropower provides 18 percent of the electricity needed for agriculture, industry, and our homes. Comparatively, in this three-year drought period, hydropower comprised less than 12 percent of total California electricity generation. A longer view reveals an even more startling economic impact: factoring in the dry years from 2007-2009, the total additional energy cost to the state’s electricity users during the six years of recent drought was $2.4 billion. This further reliance on natural gas for the state’s electricity production also has environmental costs. Hydropower produces few or no air contaminants, whereas burning natural gas emits many pollutants, including climate-changing greenhouse gases. During the 2011- 2014 drought period, burning more natural gas to compensate for limited hydropower led to an eight-percent increase in emissions of carbon dioxide and other pollutants from California power plants. If the current drought persists, water flowing to drive hydroelectric turbines will continue to shrink and expensive and polluting natural gas will become even more of a factor in the electricity production game.