Widening of the tropical belt in a changing climate

Nature Portfolio (Springer Nature) | December 2nd, 2007

Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic. But recent studies

Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic. But recent studies are showing that the tropics are also changing. Several lines of evidence show that over the past few decades the tropical belt has expanded. This expansion has potentially important implications for subtropical societies and may lead to profound changes in the global climate system. Most importantly, poleward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture, and water resources. The implications of the expansion for stratospheric circulation and the distribution of ozone in the atmosphere are as yet poorly understood. The observed recent rate of expansion is greater than climate model projections of expansion over the twenty-first century, which suggests that there is still much to be learned about this aspect of global climate change.

Widespread deoxygenation in warming rivers

Nature Portfolio (Springer Nature) | September 14th, 2023

Deoxygenation is commonly observed in oceans and lakes but less expected in shallower, flowing rivers. Here we reconstructed daily water temperature and dissolved oxygen

Deoxygenation is commonly observed in oceans and lakes but less expected in shallower, flowing rivers. Here we reconstructed daily water temperature and dissolved oxygen in 580 rivers across the United States and 216 rivers in Central Europe by training a deep learning model using temporal weather and water quality data and static watershed attributes (for example, hydro-climate, topography, land use, soil). Results revealed persistent warming in 87% and deoxygenation in 70% of the rivers. Urban rivers demonstrated the most rapid warming, whereas agricultural rivers experienced the slowest warming but fastest deoxygenation. Mean deoxygenation rates (−0.038 ± 0.026 mg l−1 decade−1) were higher than those in oceans but lower than those in temperate lakes. These rates, however, may be underestimated, as training data are from grab samples collected during the day when photosynthesis peaks. Projected future rates are between 1.6 and 2.5 times higher than historical rates, indicating significant ramifications for water quality and aquatic ecosystems.

Widespread potential loss of streamflow into underlying aquifers across the USA

Nature Portfolio (Springer Nature) | March 17th, 2021

Most rivers exchange water with surrounding aquifers. Where groundwater levels lie below nearby streams, streamwater can infiltrate through the streambed, reducing stream

Most rivers exchange water with surrounding aquifers. Where groundwater levels lie below nearby streams, streamwater can infiltrate through the streambed, reducing streamflow and recharging the aquifer. These ‘losing’ streams have important implications for water availability, riparian ecosystems and environmental flows, but the prevalence of losing streams remains poorly constrained by continent-wide in situ observations. Here we analyse water levels in 4.2 million wells across the contiguous USA and show that nearly two-thirds (64 per cent) of them lie below nearby stream surfaces, implying that these streamwaters will seep into the subsurface if it is sufficiently permeable. A lack of adequate permeability data prevents us from quantifying the magnitudes of these subsurface flows, but our analysis nonetheless demonstrates widespread potential for streamwater losses into underlying aquifers. These potentially losing rivers are more common in drier climates, flatter landscapes and regions with extensive groundwater pumping. Our results thus imply that climatic factors, geological conditions and historic groundwater pumping jointly contribute to the widespread risk of streams losing flow into surrounding aquifers instead of gaining flow from them. Recent modelling studies have suggested that losing streams could become common in future decades, but our direct observations show that many rivers across the USA are already potentially losing flow, highlighting the importance of coordinating groundwater and surface water policy.

Wildfire Induces Changes in Receiving Waters: A Review With Considerations for Water Quality Management

American Geophysical Union (AGU) | September 15th, 2022

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and ur

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and urbanization also raise the prospect of fires burning into urban areas, mobilizing pollutants few have considered to date. As a result, water quality managers lack information to anticipate, respond to and potentially mitigate wildfire impacts. Here, we reviewed the scientific literature to assess wildfire effects on response endpoints of a conceptual model linking fire to water quality, quantifying response directionality, magnitude and duration. Physically, water yield, sediments, and temperature all increased post-fire. Chemically, nutrients, ions, organic chemicals, and metals increased in burned watersheds, sometimes by orders of magnitude over pre-fire or reference conditions. In select cases, post-fire concentrations exceeded aquatic life criteria or drinking water standards, at times even in the finished drinking water. Biological assemblages commonly declined after post-fire runoff events. The duration of effects was less than 5 yr for most endpoints (e.g., metals) on average following fire, although effects did extend 15 yr or more in some individual cases. We found only a few studies on pollutants mobilized from wildfire impacted urban areas with benzene contamination in drinking water and high metal concentrations in ash prominent exceptions. Overall, this review provides a resource for understanding wildfire impacts on water quality endpoints, with the goal of informing the response of managers and other decision makers to this growing problem.

Wildfire response to changing daily temperature extremes in California’s Sierra Nevada

American Association for the Advancement of Science (AAAS) | November 17th, 2021

Burned area has increased across California, especially in the Sierra Nevada range. Recent fires there have had devasting social, economic, and ecosystem impacts. To unde

Burned area has increased across California, especially in the Sierra Nevada range. Recent fires there have had devasting social, economic, and ecosystem impacts. To understand the consequences of new extremes in fire weather, here we quantify the sensitivity of wildfire occurrence and burned area in the Sierra Nevada to daily meteorological variables during 2001–2020. We find that the likelihood of fire occurrence increases nonlinearly with daily temperature during summer, with a 1°C increase yielding a 19 to 22% increase in risk. Area burned has a similar, nonlinear sensitivity, with 1°C of warming yielding a 22 to 25% increase in risk. Solely considering changes in summer daily temperatures from climate model projections, we estimate that by the 2040s, fire number will increase by 51 ± 32%, and burned area will increase by 59 ± 33%. These trends highlight the threat posed to fire management by hotter and drier summers.

Wildfire smoke reduces lake ecosystem metabolic rates unequally across a trophic gradient

Nature Portfolio (Springer Nature) | May 22nd, 2024

Wildfires increasingly impact western US fluvial networks

Nature Portfolio (Springer Nature) | April 30th, 2021

Wildfires are increasing globally in frequency, severity, and extent, but their impact on fluvial networks, and the resources they provide, remains unclear. We

Wildfires are increasing globally in frequency, severity, and extent, but their impact on fluvial networks, and the resources they provide, remains unclear. We combine remote sensing of burn perimeter and severity, in-situ water quality monitoring, and longitudinal modeling to create the first large-scale, long-term estimates of stream+river length impacted by wildfire for the western US. We find that wildfires directly impact ~6% of the total stream+river length between 1984 and 2014, increasing at a rate of 342 km/year. When longitudinal propagation of water quality impacts is included, we estimate that wildfires affect ~11% of the total stream+river length. Our results indicate that wildfire activity is one of the largest drivers of aquatic impairment, though it is not routinely reported by regulatory agencies, as wildfire impacts on fluvial networks remain unconstrained. We identify key actions to address this knowledge gap and better understand the growing threat to fluvial networks, water security, and public health risks.

Wildfires, Floods, and Climate Variability

University of California, San Diego (UCSD) | May 10th, 2018

The first chapter surveys fire and fuels managers at local, regional, and national levels. Survey results in the form of fire managers’ decision calendars show how clim

The first chapter surveys fire and fuels managers at local, regional, and national levels. Survey results in the form of fire managers’ decision calendars show how climate information needs vary seasonally, over space, and through the organizational network. The study identifies opportunities to use climate information in fire management, including seasonal to inter-annual climate forecasts at all organizational levels, to improve the targeting of fuels treatments and prescribed burns, the positioning and movement of initial attack resources, and staffing and budgeting decisions. The second chapter analyzes National Flood Insurance Program (NFIP) data to quantify the economic impacts of flooding across the western United States from 1978 to 2007. The study compares National Flood Insurance Program data to National Weather Service measures of total damages, and presents a spatial and temporal analysis of daily claims and loss data over this period. The NFIP data reveals that a small number winter-season extreme hydrologic events, covering wide spatial areas, are responsible for a large proportion of total losses. In coastal southern California and across the southwest, El Niño conditions have had a strong effect in producing more frequent and higher magnitudes of insured losses while La Niña conditions significantly reduce both the frequency and magnitude losses. In the Pacific Northwest, the opposite pattern appears, though the effect is somewhat weaker, and less spatially coherent. The third chapter quantifies the economic impacts of flooding due to atmospheric river (AR) events in the western United States from 1978 to 2007, using NFIP claims and loss data. The study confirms that AR-related flood events cause significant economic damages and form the primary source of insurance claims and insured flood losses in the western coastal states. It provides spatial and temporal characterizations of damages as a function of integrated vapor transport (IVT) and antecedent hydrologic conditions. As the magnitude and frequency of wildfire and flood events change in response to anthropogenic climate change, and as economic and demographic contributions to vulnerability increase over time, public policy must adapt to respond. The results in these papers may be used to inform policies to mitigate losses and respond to future disaster scenarios, and may be of interest to policy makers and applied climate researchers alike.

Winter melt trends portend widespread declines in snow water resources

Nature Portfolio (Springer Nature) | April 5th, 2021

In many mountainous regions, winter precipitation accumulates as snow that melts in the spring and summer, which provides water to one billion people globally. Climate wa

In many mountainous regions, winter precipitation accumulates as snow that melts in the spring and summer, which provides water to one billion people globally. Climate warming and earlier snowmelt compromise this natural water storage. Although snowpack trend analyses commonly focus on the snow water equivalent (SWE), we propose that trends in the accumulation season snowmelt serve as a critical indicator of hydrological change. Here we compare long-term changes in the snowmelt and SWE from snow monitoring stations in western North America and find 34% of stations exhibit increasing winter snowmelt trends (P < 0.05), a factor of three larger than the 11% showing SWE declines (P < 0.05). Snowmelt trends are highly sensitive to temperature and an underlying warming signal, whereas SWE trends are more sensitive to precipitation variability. Thus, continental-scale snow water resources are in steeper decline than inferred from SWE trends alone. More winter snowmelt will complicate future water resource planning and management.

Winter wet–dry weather patterns driving atmospheric rivers and Santa Ana winds provide evidence for increasing wildfire hazard in California

Climate Dynamics (Springer) | August 12th, 2022

Floods caused by atmospheric rivers and wildfires fanned by Santa Ana winds are common occurrences in California with devastating societal impacts. In this work

Floods caused by atmospheric rivers and wildfires fanned by Santa Ana winds are common occurrences in California with devastating societal impacts. In this work, we show that winter weather variability in California, including the occurrence of extreme and impactful events, is linked to four atmospheric circulation regimes over the North Pacific Ocean previously named and identified as the “NP4 modes”. These modes come in and out of phase with each other during the season, result- ing in distinct weather patterns that recur throughout the historical record. Some phase combinations favor atmospheric river landfalls and extreme daily or multi-day precipitation, while other phase combinations favor anomalously hot weather and drying Santa Ana wind conditions over Southern California. This historical perspective of atmospheric circulation and impacts over 70 years reveals that weather patterns are changing in a way that enhances wildfire hazard in California, while the frequency of weather patterns linked to historical floods is not diminishing. These changes highlight the rising hazards of cascading weather extremes in California’s present and future.