The role of climate and population change in global flood exposure and vulnerability

Nature Portfolio (Springer Nature) | February 3rd, 2025

Flooding is a pervasive natural hazard with wide-ranging impacts on society. Using a high-resolution global flood model considering coastal, fluvial, and pluvial hazards,

Flooding is a pervasive natural hazard with wide-ranging impacts on society. Using a high-resolution global flood model considering coastal, fluvial, and pluvial hazards, we clarify the role of climate effects versus population growth effects in changing flood exposure. Between 2020 and 2100, the population likely exposed to 1% annual risk (100-year) flood hazard will increase from 1.6 to 1.9 billion people. Of this change from the 2020 exposure, we attribute 21.1% to climate change, 76.8% to population change, and 2.1% to both climate and population change. The largest driver of uncertainty in exposure is population change, while climate change remains a smaller but still important driver. The global increase in exposure between 2020 and 2100 is primarily driven by low-GDP regions, and by 2100 the lowest GDP areas will make up 63% of the exposure both overall and in urban areas. Urban areas are especially vulnerable in nearly all global regions, and urban areas sensitive to extreme events are expected to see a 33% increase in population exposure. This study highlights the vast inequities in flood exposure, and future work should direct resources and strategies toward sustainable risk mitigation in these areas.

The season for large fires in Southern California is projected to lengthen in a changing climate

Nature Portfolio (Springer Nature) | February 17th, 2022

Southern California is a biodiversity hotspot and home to over 23 million people. Over recent decades the annual wildfire area in the coastal southern California region h

Southern California is a biodiversity hotspot and home to over 23 million people. Over recent decades the annual wildfire area in the coastal southern California region has not significantly changed. Yet how fire regime will respond to future anthropogenic climate change remains an important question. Here, we estimate wildfire probability in southern California at station scale and daily resolution using random forest algorithms and downscaled earth system model simulations. We project that large fire days will increase from 36 days/year during 1970–1999 to 58 days/year under moderate greenhouse gas emission scenario (RCP4.5) and 71 days/year by 2070–2099 under a high emission scenario (RCP8.5). The large fire season will be more intense and have an earlier onset and delayed end. Our findings suggest that despite the lack of a contemporary trend in fire regime, projected greenhouse gas emissions will substantially increase the fire danger in southern California by 2099.

The shrinking Great Salt Lake contributes to record high dust-on-snow deposition in the Wasatch Mountains during the 2022 snowmelt season

Environmental Research Letters (IOP) | June 15th, 2023

The Stakes are Rising: Lessons on Engaging Coastal Communities on Climate Adaptation in Southern California

Cities and the Environment | November 16th, 2017

Preparing for climate change is rising as a priority for many public policy agendas, driving a demand for information that allows communities to identify both cur

Preparing for climate change is rising as a priority for many public policy agendas, driving a demand for information that allows communities to identify both current and projected vulnerabilities to climate change at local and regional levels. In response, a developing climate change adaptation service sector is bringing science and technical training to policy-makers. Approaching adaptation planning through a regional lens is critical, due to the large number of stakeholders and the intensely interconnected nature of geographies, communities, and economies. Decisions made in one jurisdiction will undoubtedly affect its neighbors. In this emerging field, boundary organizations play a unique role in building capacity across jurisdictions and bridging the gaps among various community, science and government stakeholders. The University of Southern California (USC) Sea Grant Program, located in Los Angeles, has developed a robust stakeholder engagement process to help communities plan for the impacts of climate change along the urbanized coastline. In 2016, USC Sea Grant analyzed its climate change adaptation outreach program to gain insights about its effectiveness. Drawing from this analysis, this paper explores: 1) stakeholder processes; 2) communications methods, particularly the challenges of communicating scientific information; 3) barriers to planning and implementation; 4) how to identify community needs; and, 5) what kinds of investments have been made to meet those needs. Four primary lessons are identified: 1) place-based boundary organizations can be an effective broker in establishing trust among stakeholders; 2) the ever-evolving and complex nature of climate science can overwhelm stakeholders and stall progress, so it is important to emphasize key messages provided by the scientific information, rather than dive deep into technical details and methods; 3) adaptive management is a promising approach to help communities move forward; and, 4) lack of significant and sustained funding for adaptation will continue to limit progress, however, even modest investments made at the right time can be impactful. Finally, the paper discusses the challenges USC Sea Grant faced in the first six years of its climate adaptation outreach program, and provides thoughts on how to help communities continue to advance their adaptation planning goals in the years to come.

The Status of Rainbow Trout (Oncorhynchus mykiss) in the Stanislaus River Summary report of 2015 snorkel surveys

Delta Stewardship Council (Delta Council) | August 1st, 2016

The twenty-first century Colorado River hot drought and implications for the future

American Geophysical Union (AGU) | February 17th, 2017

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906–1999 average, the worst 15-year drought on record. At least one-sixth to one-half (averag

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906–1999 average, the worst 15-year drought on record. At least one-sixth to one-half (average at one-third) of this loss is due to unprecedented temperatures (0.9°C above the 1906–1999 average), confirming model-based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend toward greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model-based temperature projections indicate that continued business-as-usual warming will drive temperature-induced declines in river flow, conservatively −20% by midcentury and −35% by end-century, with support for losses exceeding −30% at midcentury and −55% at end-century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur.

The twenty‐first century Colorado River hot drought and implications for the future

American Geophysical Union (AGU) | February 17th, 2017

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906–1999 average, the worst 15‐year drought on record. At least one‐sixth to one‐half (

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906–1999 average, the worst 15‐year drought on record. At least one‐sixth to one‐half (average at one‐third) of this loss is due to unprecedented temperatures (0.9°C above the 1906–1999 average), confirming model‐based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend toward greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model‐based temperature projections indicate that continued business‐as‐usual warming will drive temperature‐induced declines in river flow, conservatively −20% by midcentury and −35% by end‐century, with support for losses exceeding −30% at midcentury and −55% at end‐century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur.

The U.S. Infrastructure Investment and Jobs Act: Water Components

Pacific Institute | November 12th, 2021

On November 5, 2021, the U.S. Congress passed President Biden’s major infrastructure bill, HR 3684, the $1.2 trillion Infrastructure Investment and Jobs Act. The Presid

On November 5, 2021, the U.S. Congress passed President Biden’s major infrastructure bill, HR 3684, the $1.2 trillion Infrastructure Investment and Jobs Act. The President is expected to sign the bill into law. The bill is the largest single federal investment in infrastructure in a generation, with the funds to be expended over five years. It aims to rebuild and replace failing, aging, and outdated water, energy, transportation, and communications systems. As the first significant federal investment in climate resilience, it also begins to address the growing consequences of climate change, including intensifying extreme weather events, increasing temperatures, and rising sea levels, on communities throughout the United States. One key component of the Act is the set of proposals to address the wide range of water-related challenges facing the United States. This Pacific Institute analysis provides an overview of how the Infrastructure Act addresses these challenges.

The Uneven Nature of Daily Precipitation and Its Change

American Geophysical Union (AGU) | October 19th, 2018

Rain falls unevenly in time, which can lead to floods and droughts. It is widely known that precipitation is uneven, but it is difficult to quantify. Here we develop a me

Rain falls unevenly in time, which can lead to floods and droughts. It is widely known that precipitation is uneven, but it is difficult to quantify. Here we develop a measure for the unevenness of precipitation: the number of the wettest days each year in which half of the annual rain falls. We apply this to rain observed by gauges around the world. At all gauges combined, it takes only 12 days each year for half of the rain to fall. We also apply the measure to climate model simulations, with projections for the rest of the century. In the climate model simulations, the change in future rainfall is even more uneven than rainfall today: In a scenario with high greenhouse‐gas emissions, half of the increase in rainfall happens in the wettest 6 days each year. Rather than assuming more rain in general, society needs to take measures to deal with little change most of the time and a handful of events with much more rain.

The uneven response of different snow measures to human-induced climate warming

American Meteorological Society (AMS) | June 25th, 2013

The effect of human-induced climate warming on different snow measures in the western United States is compared by calculating the time required to achieve a stat

The effect of human-induced climate warming on different snow measures in the western United States is compared by calculating the time required to achieve a statistically significant linear trend in the different measures, using time series derived from regionally downscaled global climate models. The measures examined include the water content of the spring snowpack, total cold-season snowfall, fraction of winter precipitation that falls as snow, length of the snow season, and fraction of cold-season precipitation retained in the spring snowpack, as well as temperature and precipitation. Various stakeholders may be interested in different sets of these variables. It is found that temperature and the fraction of winter precipitation that falls as snow exhibit significant trends first, followed in 5–10 years by the fraction of cold-season precipitation retained in the spring snowpack, and later still by the water content of the spring snowpack. Change in total cold-season snowfall is least detectable of all the measures, since it is strongly linked to precipitation, which has large natural variability and only a weak anthropogenic trend in the western United States. Averaging over increasingly wider areas monotonically increases the signal-to-noise ratio of the 1950–2025 linear trend from 0.15 to 0.37, depending on the snow measure.