Unprecedented 21st century drought risk in the American Southwest and Central Plains

In the Southwest and Central Plains of Western North America, climate change is expected to increase drought severity in the coming decades. These regions nevertheless experienced extended Medieval-era droughts that were more persistent than any historical event, providing crucial targets in the paleoclimate record for benchmarking the severity of future drought risks. We use an empirical drought reconstruction and three soil moisture metrics from 17 state-of-the-art general circulation models to show that these models project significantly drier conditions in the later half of the 21st century compared to the 20th century and earlier paleoclimatic intervals. This desiccation is consistent across most of the models and moisture balance variables, indicating a coherent and robust drying response to warming despite the diversity of models and metrics analyzed. Notably, future drought risk will likely exceed even the driest centuries of the Medieval Climate Anomaly (1100–1300 CE) in both moderate (RCP 4.5) and high (RCP 8.5) future emissions scenarios, leading to unprecedented drought conditions during the last millennium

Model-dependent spatial skill in pseudoproxy experiments testing climate field reconstruction methods for the Common Era

The spatial skill of four climate field reconstruction (CFR) methods is investigated using pseudoproxy experiments (PPEs) based on five last millennium and historical simulations from the Coupled and Paleo Model Intercomparison Projects Phases 5 and 3 (CMIP5/PMIP3) data archives. These simulations are used for the first time in a PPE context, the frameworks of which are constructed to test a recently assembled multiproxy network and multiple CFR techniques. The experiments confirm earlier findings demonstrating consistent methodological performance across the employed methods and spatially dependent reconstruction errors in all of the derived CFRs. Spectral biases in the reconstructed fields demonstrate that CFR methods can alone alter the ratio of spectral power at all locations in the field, independent of whether there are any spectral biases inherent in the underlying pseudoproxy series. The patterns of spectral biases are model dependent and indicate the potential for regions in the derived CFRs to be biased by changes in either low or high-frequency spectral power. CFR methods are also shown to alter the pattern of mean differences in the tropical Pacific during the Medieval Climate Anomaly and the Little Ice Age, with some model experiments indicating that CFR methodologies enhance the statistical likelihood of achieving larger mean differences between independent 300-year periods in the region. All of the characteristics of CFR performance are model dependent, indicating that CFR methods must be evaluated across multiple models and that conclusions from PPEs should be carefully connected to the spatial statistics of real-world climatic fields

Development and analysis of spring plant phenology products: 36 years of 1-km grids over the conterminous US

Time series of phenological products provide information on the timings of recurrent biological events and on their temporal trends. This information is key to studying the impacts of climate change on our planet as well as for managing natural resources and agricultural production. Here we develop and analyze new long term phenological products: 1 km grids of the Extended Spring Indices (SI-x) over the conterminous United States from 1980 to 2015. These new products (based on Daymet daily temperature grids and created by using cloud computing) allow the analysis of two primary variables (first leaf and first bloom) and two derivative products (Damage Index and Last Freeze Day) at a much finer spatial resolution than previous gridded or interpolated products. Furthermore, our products provide enough temporal depth to reliably analyze trends and changes in the timing of spring arrival at continental scales. Validation results confirm that our products largely agree with lilac and honeysuckle leaf and flowering onset observations. The spatial analysis shows a significantly delayed spring onset in the northern US whereas in the western and the Great Lakes region, spring onset advances. The mean temporal variabilities of the indices were analyzed for the nine major climatic regions of the US and results showed a clear division into three main groups: early, average and late spring onset. Finally, the region belonging to each group was mapped. These examples show the potential of our four phenological products to improve understanding of the responses of ecosystems to a changing climat

Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest

Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration. A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate. We find that changes in the mean hydroclimate state, rather than its variability, determine megadrought risk in the American Southwest. Estimates of megadrought probabilities based on precipitation alone tend to underestimate risk. Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties. We find that regional temperature increases alone push megadrought risk above 70, 90, or 99% by the end of the century, even if precipitation increases moderately, does not change, or decreases, respectively. Although each possibility is supported by some climate model simulations, the latter is the most common outcome for the American Southwest in Coupled Model Intercomparison 5 generation models. An aggressive reduction in global greenhouse gas emissions cuts megadrought risks nearly in half

Insights from earth system model initial-condition large ensembles and future prospects

Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed

Megadroughts in the Common Era and the Anthropocene

Exceptional drought events, known as megadroughts, have occurred on every continent outside Antarctica over the past ~2,000 years, causing major ecological and societal disturbances. In this Review, we discuss shared causes and features of Common Era (Year 1–present) and future megadroughts. Decadal variations in sea surface temperatures are the primary driver of megadroughts, with secondary contributions from radiative forcing and land–atmosphere interactions. Anthropogenic climate change has intensified ongoing megadroughts in south-western North America and across Chile and Argentina. Future megadroughts will be substantially warmer than past events, with this warming driving projected increases in megadrought risk and severity across many regions, including western North America, Central America, Europe and the Mediterranean, extratropical South America, and Australia. However, several knowledge gaps currently undermine confidence in understanding past and future megadroughts. These gaps include a paucity of high-resolution palaeoclimate information over Africa, tropical South America and other regions; incomplete representations of internal variability and land surface processes in climate models; and the undetermined capacity of water-resource management systems to mitigate megadrought impacts. Addressing these deficiencies will be crucial for increasing confidence in projections of future megadrought risk and for resiliency planning

THE CONTINUUM OF DROUGHT IN WESTERN NORTH AMERICA

The continuum of western North American hydroclimate during the last millennium is analyzed here using instrumental records, proxy data, and global climate model (GCM) simulations. We find that variance at long timescales (low frequencies) is generally more substantial than variance at short timescales (high frequencies). We find that local sources of autocorrelation (e.g., soil moisture storage) likely explain the tendency for variance to increase from monthly to interannual timescales, but that variance at longer timescales requires remote climate sources of variability. Our analysis of global climate model data indicates that at least one fully coupled GCM can reproduce the characteristics of the continuum on short (interannual) and long (multicentury) timescales, but that proxy spectra and GCM spectra disagree about the amount of variance present on intermediate (decadal to centennial) timescales. Since instrumental records, as well as multiple independent types of paleoclimate records, provide evidence that variance increases with timescale at these frequencies, and because numerical experiments indicate that local autocorrelation is not a likely source of variance at these timescales, we argue that climate model simulations underestimate the full range of low-frequency drought variability. Moreover, the models may also underestimate the risk of future megadroughts, which we attempt to quantify using a new method that combines frequency information from observational data with projections of 21st century hydroclimate. Our results indicate that the risk of a severe, decadal-scale drought during the coming century is at least 1-in-10 for most of the US Southwest, and may be as high as 1-in-3. These findings should be incorporated into adaptation and mitigation strategies to cope with regional climate variability and climate change.Embargo: Release after 5/12/201

Global trends in the frequency and duration of temperature extremes

Anthropogenic climate change has affected the frequency and duration of extreme climate events, including extreme heat events (EHE) and extreme cold events (ECE). How the frequency and duration of both EHE and ECE have changed over time within both terrestrial and marine environments globally has not been fully explored. Here, we use detrended daily estimates of minimum and maximum temperature from the ERA5 reanalysis over a 70-year period (1950-2019) to estimate the daily occurrence of EHE and ECE across the globe. We measure the frequency and duration of EHE and ECE by season across years and estimate how these measures have changed over time. Frequency and duration for both EHE and ECE presented similar patterns characterized by low spatial heterogeneity and strong seasonal variation. High EHE frequency and duration occurred within the Antarctic during the austral summer and winter and within the Arctic Ocean during the boreal winter. High ECE frequency and duration occurred within the Nearctic and Palearctic during the boreal winter and the Arctic Ocean during the boreal summer. The trend analysis presented pronounced differences between frequency and duration, high spatial heterogeneity, especially within terrestrial environments, and strong seasonal variation. Positive EHE trends, primarily in duration within marine environments, occurred during the boreal summer within the mid-latitudes of the Northern Hemisphere and during the austral summer within the mid-latitudes of the Southern Hemisphere. The eastern tropical Pacific contained positive EHE and ECE trends, primary in duration during the boreal winter. Our findings emphasize the many near-term challenges that extreme temperature events are likely to pose for human and natural systems within terrestrial and marine environments, and the need to advance our understanding of the developing long-term implications of these changing dynamics as climate change progresses.</p

A Matlab© toolbox for calculating spring indices from daily meteorological data

AbstractMetrics to track seasonal transitions are needed for a wide variety of ecological and climatological applications. Here a MATLAB©toolkit for calculating spring indices is documented. The spring indices have been widely used in earlier studies to model phenological variability and change through time across a wide range of spatial scales. These indices require only daily minimum and maximum temperature observations (e.g., from meteorological records) as input along with latitude, and produce a day of year value corresponding to the simulated average timing of first leaf and first bloom events among three plant cultivars. Core functions to calculate the spring indices require no external dependencies, and data for running several illustrative test cases are included. Instructions and routines for conducing more sophisticated monitoring and modeling studies using the spring indices are also supplied and documented

Global trends in the frequency and duration of temperature extremes

Anthropogenic climate change has affected the frequency and duration of extreme climate events, including extreme heat events (EHE) and extreme cold events (ECE). How the frequency and duration of both EHE and ECE have changed over time within both terrestrial and marine environments globally has not been fully explored. Here, we use detrended daily estimates of minimum and maximum temperature from the ERA5 reanalysis over a 70-year period (1950-2019) to estimate the daily occurrence of EHE and ECE across the globe. We measure the frequency and duration of EHE and ECE by season across years and estimate how these measures have changed over time. Frequency and duration for both EHE and ECE presented similar patterns characterized by low spatial heterogeneity and strong seasonal variation. High EHE frequency and duration occurred within the Antarctic during the austral summer and winter and within the Arctic Ocean during the boreal winter. High ECE frequency and duration occurred within the Nearctic and Palearctic during the boreal winter and the Arctic Ocean during the boreal summer. The trend analysis presented pronounced differences between frequency and duration, high spatial heterogeneity, especially within terrestrial environments, and strong seasonal variation. Positive EHE trends, primarily in duration within marine environments, occurred during the boreal summer within the mid-latitudes of the Northern Hemisphere and during the austral summer within the mid-latitudes of the Southern Hemisphere. The eastern tropical Pacific contained positive EHE and ECE trends, primary in duration during the boreal winter. Our findings emphasize the many near-term challenges that extreme temperature events are likely to pose for human and natural systems within terrestrial and marine environments, and the need to advance our understanding of the developing long-term implications of these changing dynamics as climate change progresses.</p