AGU Advances最新文献

Temperate regions around the world are experiencing longer fire weather seasons, yet trends in burned area have been inconsistent between regions. Reasons for differences in fire patterns can be difficult to determine due to variable vegetation types, land use patterns, fuel conditions, and human influences on fire ignition and suppression. This study compares burned areas to climate and fuel conditions in three temperate regions: the desert, shrub, and forest ecoregions of western North America, west-central Europe, and southwestern South America. In each region the mean annual aridity index (AI, precipitation over potential evapotranspiration) spans arid to humid climates. We examined how the fraction of area burned from 2001 to 2021 varied with mean annual AI, mean aboveground biomass, and land cover type distributions. All three regions had low fractions of area burned for the driest climate zones (AI < 0.5), a sign of fuel limitation to burned area. Fraction of area burned increased with mean aboveground biomass for these dry zones. Fraction of area burned peaked at intermediate AI (0.7–1.5) for all regions and declined again in the wettest climate zones (AI > 1.5), a sign of climate limitation to burned area. Of the three regions, western North America had the highest burned area, fraction of area burned, and fire sizes. Fragmentation of vegetation patches by the high Andes Mountains in southwestern South America and by intensive land use changes in west-central Europe likely limited fire sizes. All three regions are at risk for future wildfires, particularly in areas where fire is currently climate limited.

Anthropogenic greenhouse gas emissions significantly impact the middle and upper atmosphere. They cause cooling and thermal shrinking and affect the atmospheric structure. Atmospheric contraction results in changes in key atmospheric features, such as the stratopause height or the peak ionospheric electron density, and also results in reduced thermosphere density. These changes can impact, among others, the lifespan of objects in low Earth orbit, refraction of radio communication and GPS signals, and the peak altitudes of meteoroids entering the Earth's atmosphere. Given this, there is a critical need for observational capabilities to monitor the middle and upper atmosphere. Equally important is the commitment to maintaining and improving long-term, homogeneous data collection. However, capabilities to observe the middle and upper atmosphere are decreasing rather than improving.

Faults can slip at vastly different rates, generating both high-stress-drop regular earthquakes and low-stress-drop slow slip events (SSEs). Here, we document a transitional mode of high-stress-drop but “silent” slip with two M_w $��\ge �$ 5 events that were triggered by the 2023 Turkey Kahramanmaraş earthquake sequence. Specifically, we identify eight fault slip events from radar interferometry, none of which are reported in seismic catalogs. Except for four typical aseismic creep events, two events represent earthquakes whose waveforms were obscured within the coda waves of the mainshocks. Notably, we find two silent events that did not radiate discernible high-frequency seismic energy nor produce local aftershocks but did exhibit high stress drops of 4–6 MPa, similar to regular earthquakes. These two “silent” events likely represent a new fault slip mode positioned between regular earthquakes and SSEs. The identification of these diverse shallow slip events, which are missed in seismic catalogs, calls for reevaluation of faulting mechanisms, the nature of static/dynamic triggering effects, and seismic hazard assessments.

The stress field perturbation caused by magmatic intrusions within volcanic systems induces strain in the surrounding region. This effect results in the opening and closing of microcracks in the vicinity of the intrusion, which can affect regional seismic velocities. In late November 2023, we deployed a distributed acoustic sensing interrogator to convert an existing 100-km telecommunication fiber-optic cable along the coast of Iceland's Reykjanes peninsula into a dense seismic array, which has run continuously. Measuring changes in surface wave moveout with ambient noise cross-correlation, we observe up to 2% changes in Rayleigh wave phase velocity $��\left(��d�v�/�v��\right)�$ following eruptions in the peninsula's 2023–2024 sequence that are likely associated with magmatic intrusions into the eruption-feeding dike. We apply a Bayesian inversion to compute the posterior distribution of potential dike opening models for each eruption by considering $��d�v�/�v�$ measurements for varying channel pairs and frequency bands, and assuming this velocity change is tied to volumetric strain associated with dike-opening. Our results are in agreement with those based on geodetic measurement and provide independent constraints on the depth of the dike, demonstrating the viability of this novel inversion and new volcano monitoring directions through fiber sensing.

Erosion can remain active and changing in landscapes long after tectonic drivers have ceased, potentially due to local-geologic controls, climate changes, or geodynamics. We present new fluvial incision-rate histories and terrain analyses of the Colorado River system through the central Colorado Plateau to understand what has caused the variable erosion across this post-orogenic landscape. Results from new cosmogenic and luminescence dating of fluvial terrace and upland gravel deposits in Glen and Meander Canyons establish incision-rate histories that are marked by an Early-Middle Pleistocene erosion hiatus, followed by ∼200 m of rapid incision over the last ∼350 kyr. Projection of fluvial topography from above knickzones of the Colorado River drainage system roughly agree with the observed magnitude of recent incision and reflect a common baselevel fall from Pliocene river integration through Grand Canyon, which is still propagating through the drainage. A response-time model indicates that baselevel fall from integration likely took 2–4 Myr to reach the central Colorado Plateau and 100s kyr to travel across the study area, potentially accounting for incision rate changes in the fluvial terrace records of Meander and Glen Canyons. The upstream-migrating incision has likely been partitioned into multiple waves across the landscape due to the local geologic controls of lava damming, salt tectonics, and heterogenous bedrock. As baselevel fall from Pliocene Colorado River integration diffuses upstream, it can only account for perhaps a quarter of the total ∼2 km of exhumation in the central Colorado Plateau, demanding an unknown driver for significant erosion in the Pliocene.

Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system. However, fundamental questions regarding their formation and prevalence remain unanswered. Here, we endeavor to answer how congestus cloud tops form by detraining preferentially at altitudes between 5 and 6 km and why this detraining outflow is invigorated by drier mid-tropospheric conditions. We construct a clear-sky radiative-convective framework of congestus cloud-top formation that is grounded in the discovery of an important spectroscopic property of water vapor. In this mass- and energy-conserving framework, convective detrainment maximizes at a height of 5 and 6 km due to a swift decline in radiative cooling in clear-sky regions. This decline is, in turn, a consequence of water vapor spectroscopy: more specifically, a drop in the number of strong absorption lines in the water vapor rotation band. In a simple spectral model, we link this spectroscopic property to the shape of the rotation band, which can be approximated as the product of a power law and a sine wave representing the band's deviation from statistical log-linearity. The characteristic “C”-shaped relative humidity profile in the tropics further strengthens the outflow in drier mid-level conditions by amplifying vertical decreases in the clear-sky cooling rate. Essential to this process are strong RH gradients, which are most pronounced under the driest conditions and induce a vertical decrease in the optical depth lapse rate across the mid-troposphere.

While rising global temperatures have altered global drought risk and are projected to continue to change large-scale hydroclimate, it has proved difficult to detect the influence of external factors on drought-relevant variables at regional scales. In addition to the inherent difficulty in identifying signals in noisy data, detection and attribution studies generally rely on general circulation models, which may fail to accurately capture the characteristics of naturally forced and internal hydroclimate variability. Here, we use a long tree-ring based paleoclimate record of drought to estimate pre-industrial variability in the Palmer Drought Severity Index (PDSI), a commonly used metric of drought risk. Using a Bayesian framework, we estimate the temporal and spatial characteristics of hydroclimate variability prior to 1850. We assess whether observed twenty-first century PDSI is compatible with this pre-industrial variability or is better explained by a forced response that scales with the global mean temperature. Our results suggest that global warming likely contributed to dry PDSI in Eastern Europe, the Mediterranean, and Arctic Russia and to wet PDSI in Northern Europe, East-central Asia, and Tibet.

The strongest geomagnetic storm in the preceding two decades occurred in May 2024. Over these years, ground-based observational capabilities have been significantly enhanced to monitor the ionospheric weather. Notably, the newly established Sanya incoherent scatter radar (SYISR) (Yue, Wan, Ning, & Jin, 2022, https://doi.org/10.1038/s41550-022-01684-1), one of the critical infrastructures of the Chinese “Meridian Project,” provides multiple parameter measurements in the upper atmosphere at low latitudes over Asian longitudies. Unique ionospheric changes on superstorm day 11 May were first recorded by the SYISR experiments and the geostationary satellite (GEO) total electron content (TEC) network over the Asian sector. The electron density or TEC displayed wavelike structures rather than a regular diurnal pattern. Surprisingly, two humps, a common feature in the daytime equatorial ionization anomaly structure, disappeared. The SYISR observations revealed that multiple wind surges accompanied the downward phase propagation caused by atmospheric gravity waves (AGWs) originating from auroral zones. Meanwhile, strong upward and large downward drifts were respectively observed in the daytime and around sunset. The Thermosphere-Ionosphere Electrodynamics Global Circulation Model (TIEGCM) simulations demonstrated that abnormal ionospheric changes were attributed to meridional wind disturbances associated with AGWs and recurrent penetration electric fields corresponding to larger B_z southward excursions and disturbance dynamo. The complicated interplay between AGWs and disturbance electric fields contributed to this unique ionospheric variation.

Prediction of equatorial plasma bubbles (EPBs) is a need of the hour for many modern navigation/communication applications. In this paper, we demonstrate an ionosonde based technique for the prediction of EPB formation overhead and its robustness using a large ionosonde data set, covering diverse solar flux and geomagnetic conditions, from three low-latitude Indian stations, namely, Trivandrum, Sriharikota and Gadanki. The technique relies on localized upwelling at the bottomside F layer, characterized by the second time derivative of the base height of the F layer observed by ionosonde, as the prime criterion deciding whether EPB will be formed overhead or not. Results show that prediction for the formation of EPB over an ionosonde station can be made with an accuracy of 99.86%. The accuracy of prediction of EPB formation over a station using data from a nearby station separated by 3.2° in longitude, however, is found to be only 83.87%, underlining the crucial role of longitudinally localized background ionospheric conditions at the bottom of the F region. We discuss the prospective of the present technique and propose a cost effective approach for developing an effective EPB prediction strategy.