Forecasting daily total pollen concentrations on a global scale.

BACKGROUND: There is evidence that global anthropogenic climate change may be impacting floral phenology and the temporal and spatial characteristics of aero-allergenic pollen. Given the extent of current and future climate uncertainty, there is a need to strengthen predictive pollen forecasts. METHODS: The study aims to use CatBoost (CB) and deep learning (DL) models for predicting the daily total pollen concentration up to 14 days in advance for 23 cities, covering all five continents. The model includes the projected environmental parameters, recent concentrations (1, 2 and 4 weeks), and the past environmental explanatory variables, and their future values. RESULTS: The best pollen forecasts include Mexico City (R2(DL_7) ≈ .7), and Santiago (R2(DL_7) ≈ .8) for the 7th forecast day, respectively; while the weakest pollen forecasts are made for Brisbane (R2(DL_7) ≈ .4) and Seoul (R2(DL_7) ≈ .1) for the 7th forecast day. The global order of the five most important environmental variables in determining the daily total pollen concentrations is, in decreasing order: the past daily total pollen concentration, future 2 m temperature, past 2 m temperature, past soil temperature in 28-100 cm depth, and past soil temperature in 0-7 cm depth. City-related clusters of the most similar distribution of feature importance values of the environmental variables only slightly change on consecutive forecast days for Caxias do Sul, Cape Town, Brisbane, and Mexico City, while they often change for Sydney, Santiago, and Busan. CONCLUSIONS: This new knowledge of the ecological relationships of the most remarkable variables importance for pollen forecast models according to clusters, cities and forecast days is important for developing and improving the accuracy of airborne pollen forecasts.

Elevated atmospheric CO2 has small, species-specific effects on pollen chemistry and plant growth across flowering plant species.

Elevated atmospheric carbon dioxide (eCO2) can affect plant growth and physiology, which can, in turn, impact herbivorous insects, including by altering pollen or plant tissue nutrition. Previous research suggests that eCO2 can reduce pollen nutrition in some species, but it is unknown whether this effect is consistent across flowering plant species. We experimentally quantified the effects of eCO2 across multiple flowering plant species on plant growth in 9 species and pollen chemistry (%N an estimate for protein content and nutrition in 12 species; secondary chemistry in 5 species) in greenhouses. For pollen nutrition, only buckwheat significantly responded to eCO2, with %N increasing in eCO2; CO2 treatment did not affect pollen amino acid composition but altered secondary metabolites in buckwheat and sunflower. Plant growth under eCO2 exhibited two trends across species: plant height was taller in 44% of species and flower number was affected for 63% of species (3 species with fewer and 2 species with more flowers). The remaining growth metrics (leaf number, above-ground biomass, flower size, and flowering initiation) showed divergent, species-specific responses, if any. Our results indicate that future eCO2 is unlikely to uniformly change pollen chemistry or plant growth across flowering species but may have the potential to alter ecological interactions, or have particularly important effects on specialized pollinators.

A temporally and spatially explicit, data-driven estimation of airborne ragweed pollen concentrations across Europe.

Ongoing and future climate change driven expansion of aeroallergen-producing plant species comprise a major human health problem across Europe and elsewhere. There is an urgent need to produce accurate, temporally dynamic maps at the continental level, especially in the context of climate uncertainty. This study aimed to restore missing daily ragweed pollen data sets for Europe, to produce phenological maps of ragweed pollen, resulting in the most complete and detailed high-resolution ragweed pollen concentration maps to date. To achieve this, we have developed two statistical procedures, a Gaussian method (GM) and deep learning (DL) for restoring missing daily ragweed pollen data sets, based on the plant's reproductive and growth (phenological, pollen production and frost-related) characteristics. DL model performances were consistently better for estimating seasonal pollen integrals than those of the GM approach. These are the first published modelled maps using altitude correction and flowering phenology to recover missing pollen information. We created a web page (http://euragweedpollen.gmf.u-szeged.hu/), including daily ragweed pollen concentration data sets of the stations examined and their restored daily data, allowing one to upload newly measured or recovered daily data. Generation of these maps provides a means to track pollen impacts in the context of climatic shifts, identify geographical regions with high pollen exposure, determine areas of future vulnerability, apply spatially-explicit mitigation measures and prioritize management interventions.

Mechanisms underlying food insecurity in the aftermath of climate-related shocks: a systematic review.

Food insecurity is prevalent, affecting 1·2 billion people globally in 2021. However, the effects of food insecurity are unequally distributed across populations and climate-related shocks threaten to exacerbate food insecurity and associated health consequences. The mechanisms underlying this exacerbation at the household level are largely unknown. We aimed to synthesise the available evidence on the mechanisms connecting extreme climate events to household-level food insecurity and highlight the research gaps that must be addressed to inform better food security and health policy. For this systematic review, a comprehensive literature search was done by a medical librarian in February, 2021 for articles about food security and climate-related shocks. Relevant publications were identified by searching the following databases with a combination of standardised index terms and keywords: MEDLINE, Embase, CINAHL, GreenFILE, Environment Complete, Web of Science Core Collection, and Global Health. Searches were limited to human studies published in English. Included studies measured food security outcomes using indicators developed by the UN Food and Agricultural Organization (ie, consumption patterns, livelihood change, malnutrition, and mortality) and explained the mechanism behind the household-level or population-level food insecurity. Purely theoretical, modelling, and review studies were excluded. Quality assessment was conducted using the appropriate Joanna Briggs Institute Critical Appraisal Tool. Data were analysed using thematic analysis of the categories of mechanism (interpreted using internationally accepted frameworks), risk and resilience factors, and author policy recommendations. We found a paucity of data with only 18 studies meeting criteria for inclusion out of 337 studies identified for full-text review. All the studies that were included in our analysis showed worse food security outcomes after climate-related shocks. Food availability was the most common mechanism cited (17 studies), although most studies addressed at least one additional mechanism (15 studies). Studies were of mixed methodologies with nuanced discussions of risk and resilience factors, and of policy recommendations. This systematic review shows that there is an incomplete assessment of food security at the household and community level after climate-related shocks in the literature and finds that food availability is the primary mechanism studied. The low number of studies on this topic limits subgroup analysis and generalisability; however, the good quality of the studies allows for important policy recommendations around improving resilience to climate shocks and suggestions for future research including the need for a more granular understanding of mechanisms and feasible adaptation solutions.

Accelerated sea-level rise is suppressing CO2 stimulation of tidal marsh productivity: A 33-year study.

Accelerating relative sea-level rise (RSLR) is threatening coastal wetlands. However, rising CO2 concentrations may also stimulate carbon sequestration and vertical accretion, counterbalancing RSLR. A coastal wetland dominated by a C3 plant species was exposed to ambient and elevated levels of CO2 in situ from 1987 to 2019 during which time ambient CO2 concentration increased 18% and sea level rose 23 cm. Plant production did not increase in response to gradually rising ambient CO2 concentration during this period. Elevated CO2 increased shoot production relative to ambient CO2 for the first two decades, but from 2005 to 2019, elevated CO2 stimulation of production was diminished. The decline coincided with increases in relative sea level above a threshold that hindered root productivity. While elevated CO2 stimulation of elevation gain has the potential to moderate the negative impacts of RSLR on tidal wetland productivity, benefits for coastal wetland resilience will diminish in the long term as rates of RSLR accelerate.

Rising Carbon Dioxide and Global Nutrition: Evidence and Action Needed.

While the role of CO2 as a greenhouse gas in the context of global warming is widely acknowledged, additional data from multiple sources is demonstrating that rising CO2 of and by itself will have a tremendous effect on plant biology. This effect is widely recognized for its role in stimulating photosynthesis and growth for multiple plant species, including crops. However, CO2 is also likely to alter plant chemistry in ways that will denigrate plant nutrition. That role is also of tremendous importance, not only from a human health viewpoint, but also from a global food-web perspective. Here, the goal is to review the current evidence, propose potential mechanistic explanations, provide an overview of critical unknowns and to elucidate a series of next steps that can address what is, overall, a critical but unappreciated aspect of anthropogenic climate change.

Higher airborne pollen concentrations correlated with increased SARS-CoV-2 infection rates, as evidenced from 31 countries across the globe.

Pollen exposure weakens the immunity against certain seasonal respiratory viruses by diminishing the antiviral interferon response. Here we investigate whether the same applies to the pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is sensitive to antiviral interferons, if infection waves coincide with high airborne pollen concentrations. Our original hypothesis was that more airborne pollen would lead to increases in infection rates. To examine this, we performed a cross-sectional and longitudinal data analysis on SARS-CoV-2 infection, airborne pollen, and meteorological factors. Our dataset is the most comprehensive, largest possible worldwide from 130 stations, across 31 countries and five continents. To explicitly investigate the effects of social contact, we additionally considered population density of each study area, as well as lockdown effects, in all possible combinations: without any lockdown, with mixed lockdown-no lockdown regime, and under complete lockdown. We found that airborne pollen, sometimes in synergy with humidity and temperature, explained, on average, 44% of the infection rate variability. Infection rates increased after higher pollen concentrations most frequently during the four previous days. Without lockdown, an increase of pollen abundance by 100 pollen/m3 resulted in a 4% average increase of infection rates. Lockdown halved infection rates under similar pollen concentrations. As there can be no preventive measures against airborne pollen exposure, we suggest wide dissemination of pollen-virus coexposure dire effect information to encourage high-risk individuals to wear particle filter masks during high springtime pollen concentrations.

Crop Adaptation: Weedy and Crop Wild Relatives as an Untapped Resource to Utilize Recent Increases in Atmospheric CO2.

Adaptation measures are necessary to ensure the stability and performance of the food supply relative to anthropogenic climate change. Although a wide range of measures have been proposed (e.g., planting dates, crop choices, drought resistance), there may be a ubiquitous means to increase productivity relatively quickly. Numerous studies have shown that the projected increase in atmospheric CO2 can stimulate crop growth and seed yield with noted intra-specific differences within crop cultivars, suggesting potential differences to CO2 that could be exploited to enhance seed yield in the future. However, it is worth emphasizing that atmospheric CO2 has already risen substantially (≈27% since 1970) and that, at present, no active effort by breeders has been made to select for the CO2 increase that has already occurred. In contrast, for weedy or crop wild relatives (CWR), there are indications of evolutionary adaptation to these recent increases. While additional steps are needed, the identification and introgression of these CO2-sensitive traits into modern crop cultivars may be a simple and direct means to increase crop growth and seed yield.

Responses of rice qualitative characteristics to elevated carbon dioxide and higher temperature: implications for global nutrition.

BACKGROUND: Protein and some minerals of rice seed are negatively affected by projected carbon dioxide (CO2 ) levels. However, an in-depth assessment of rice quality that encompasses both CO2 and temperature for a wide range of nutritional parameters is not available. Using a free-air CO2 enrichment facility with temperature control, we conducted a field experiment with two levels of CO2 (ambient; ambient + 200 ppm) and two levels of temperature (ambient; ambient + 1.5 °C). An in-depth examination of qualitative factors indicated a variable nutritional response. RESULTS: For total protein, albumin, glutelin, and prolamin, elevated CO2 reduced seed concentrations irrespective of temperature. Similarly, several amino acids declined further as a function of higher temperature and elevated CO2 relative to elevated CO2 alone. Higher temperature increased the lipid percentage of seed; however, elevated CO2 reduced the overall lipid content. At the nutrient elements level, whereas elevated CO2 reduced certain elements, a combination of CO2 and temperature could compensate for CO2 reductions but was element dependent. CONCLUSION: Overall, these data are, at present, the most detailed analysis of rising CO2 /temperature on the qualitative characteristics of rice. They indicate that climate change is likely to significantly impact the nutritional integrity of rice, with subsequent changes in human health on a global basis. © 2020 Society of Chemical Industry.

Climate, Carbon Dioxide, and Plant-Based Aero-Allergens: A Deeper Botanical Perspective.

There is global evidence of a general increase in the incidence and prevalence of respiratory diseases including allergic rhinitis and associated asthma. This increase in turn, has been related, in part, to concurrent increases in carbon dioxide (CO2) and temperature on pollen production and allergic disease generated from plant-based sources of pollen. Such links to anthropogenic climate change has suggested three significant and interrelated consequences associated with respiratory allergies or disease. First, warmer temperatures and a longer frost-free growing season can influence pollen season length and temporal exposure to airborne aeroallergens. Second, both warmer temperatures and additional CO2 can increase the amount of pollen, the seasonal intensity, from spring through fall. Thirdly, there is evidence from oak and ragweed that rising levels of CO2 could increase the allergen concentration of the pollen and symptom severity. However, while these outcomes are of obvious consequence, they do not fully encompass all of the plant derived changes that could, directly or indirectly, influence aeroallergen production, exposure, and consequences for public health. In this overview, I will delve deeper into other plant-based links to climate/CO2 that are consequential either directly or indirectly to allergic rhinitis and associated disease. Such interactions range from pollen morphology to fire occurrence, from volatile organic compounds to potential changes in pesticide usage. The goal in doing so is to provide a broader context and appreciation for the interactions between plant biology and climate that can also affect allergen production and human impact but which, to date, have received little recognition or research.

Global Climate Change and Pollen Aeroallergens: A Southern Hemisphere Perspective.

Climatic change will have an impact on production and release of pollen, with consequences for the duration and magnitude of aeroallergen seasonal exposure and allergic diseases. Evaluations of pollen aerobiology in the southern hemisphere have been limited by resourcing and the density of monitoring sites. This review emphasizes inconsistencies in pollen monitoring methods and metrics used globally. Research should consider unique southern hemisphere biodiversity, climate, plant distributions, standardization of pollen aerobiology, automation, and environmental integration. For both hemispheres, there is a clear need for better understanding of likely influences of climate change and comprehending their impact on pollen-related health outcomes.

Leaf characteristics of rice cultivars with a stronger yield response to projected increases in CO2 concentration.

Rising levels of atmospheric carbon dioxide (CO2 ) could, potentially, be exploited as a means to increase seed yield and maintain food security, especially for cereal grains. Although there have been multiple cultivar trials indicating that significant yield variation occurs, the basis for these differences has not been entirely elucidated. Here, we focus on two rice cultivars that differed in field trials to their yield sensitivity to elevated CO2 : Yangdao6hao (YD6), and Wuyunjing23 (W23) to assess whether observed yield differences (YD6 > W23) were associated with concurrent changes in leaf-level characteristics. At ambient levels of CO2 , leaf net photosynthesis (A) of YD6 was compatible with that of W23. However, at elevated CO2 , A was higher for YD6 relative to W23. The stability of leaf Rubisco content, biochemical characteristics (Vc,max, and Jmax ), nitrogen enzymatic activity, and chlorophyll concentration differed significantly, with greater values observed for YD6 relative to W23 at elevated CO2 . While such results are consistent with other studies, we also demonstrate that a higher ratio of carbon sinks (seed) to carbon sources (leaf), were linked to increases in cytokinins, and slower flag leaf senescence for the YD6 relative to the W23 cultivar at elevated CO2 . While additional data for a broader genetic selection are needed, the current study suggests a link between source/sink carbon assimilation, maintenance of photosynthetic biochemistry, and slower leaf senescence for rice cultivars that show a stronger yield response to projected CO2 levels. This information, in turn, may provide suitable metrics for future CO2 selection among rice cultivars.

An Overview of Rising CO2 and Climatic Change on Aeroallergens and Allergic Diseases.

There are a number of implications of climate change in regard to human health. Among these, the role of rising carbon dioxide (CO2) and temperature in aeroallergen exposure and associated changes in the start, duration and intensity of the pollen season, and associated consequences in aeroallergens and allergic disease are a primary concern. This review is intended to provide a synopsis of CO2 and climate factors associated with likely changes in aeroallergen biology (indoor and outdoor), including changes in the demography of flowering plants, pollen seasonality, aeroallergen production, and potential biotic and abiotic interactions. These factors, in turn, are compared to clinical trials that have linked aeroallergens to allergic disease and associated health impacts. Finally, suggestions to address unmet needs and critical knowledge gaps are offered. Such recommendations are not meant to be inclusive, but to serve as a spur for the additional research and resources that will be necessary to acquire a better understanding of climate change, CO2, aeroallergens and associated allergic diseases. Such resources will be critical to derive time-relevant scientific and policy solutions that will minimize public health consequences in a changing climate.

Early growth phase and caffeine content response to recent and projected increases in atmospheric carbon dioxide in coffee (Coffea arabica and C. canephora).

While [CO2] effects on growth and secondary chemistry are well characterized for annual plant species, little is known about perennials. Among perennials, production of Coffea arabica and C. canephora (robusta) have enormous economic importance worldwide. Three Arabica cultivars (Bourbon, Catimor, Typica) and robusta coffee were grown from germination to ca. 12 months at four CO2 concentrations: 300, 400, 500 or 600 ppm. There were significant increases in all leaf area and biomass markers in response to [CO2] with significant [CO2] by taxa differences beginning at 122-124 days after sowing (DAS). At 366-368 DAS, CO2 by cultivar variation in growth and biomass response among Arabica cultivars was not significant; however, significant trends in leaf area, branch number and total above-ground biomass were observed between Arabica and robusta. For caffeine concentration, there were significant differences in [CO2] response between Arabica and robusta. A reduction in caffeine in coffee leaves and seeds might result in decreased ability against deterrence, and consequently, an increase in pest pressure. We suggest that the interspecific differences observed (robusta vs. Arabica) may be due to differences in ploidy level (2n = 22 vs. 2n = 4x = 44). Differential quantitative and qualitative responses during early growth and development of Arabica and robusta may have already occurred with recent [CO2] increases, and such differences may be exacerbated, with production and quality consequences, as [CO2] continues to increase.

The potential role of sucrose transport gene expression in the photosynthetic and yield response of rice cultivars to future CO2 concentration.

The metabolic basis for observed differences in the yield response of rice to projected carbon dioxide concentrations (CO2 ) is unclear. In this study, three rice cultivars, differing in their yield response to elevated CO2 , were grown under ambient and elevated CO2 conditions, using the free-air CO2 enrichment technology. Flag leaves of rice were used to determine (1) if manipulative increases in sink strength decreased the soluble sucrose concentration for the 'weak' responders and (2), whether the genetic expression of sucrose transporters OsSUT1 and OsSUT2 was associated with an accumulation of soluble sugars and the maintenance of photosynthetic capacity. For the cultivars that showed a weak response to additional CO2 , photosynthetic capacity declined under elevated CO2 and was associated with an accumulation of soluble sugars. For these cultivars, increasing sink relative to source strength did not increase photosynthesis and no change in OsSUT1 or OsSUT2 expression was observed. In contrast, the 'strong' response cultivar did not show an increase in soluble sugars or a decline in photosynthesis but demonstrated significant increases in OsSUT1 and OsSUT2 expression at elevated CO2 . Overall, these data suggest that the expression of the sucrose transport genes OsSUT1 and OsSUT2 may be associated with the maintenance of photosynthetic capacity of the flag leaf during grain fill; and, potentially, greater yield response of rice as atmospheric CO2 increases.

Rice: Importance for Global Nutrition.

Rice, a staple food for more than half of the world's population, is grown in >100 countries with 90% of the total global production from Asia. Although there are more than 110,000 cultivated varieties of rice that vary in quality and nutritional content, after post-harvest processing, rice can be categorized as either white or brown. Regional and cultural preferences as well as need for stability during storage and transport are the final determinants of market availability and final consumption. In addition to calories, rice is a good source of magnesium, phosphorus, manganese, selenium, iron, folic acid, thiamin and niacin; but it is low in fiber and fat. Although brown rice is promoted as being "healthier" because of bioactive compounds, including minerals and vitamins not present in white rice after polishing, white rice is more widely consumed than brown. This is for several reasons, including cooking ease, palatability, and shelf life. Polished rice has a higher glycemic load and may impact glucose homeostasis but when combined with other foods, it can be considered part of a "healthy" plate. With the projected increase in the global population, rice will remain a staple. However, it will be important to encourage intake of the whole grain (brown rice) and to identify ways to harness the phytonutrients that are lost during milling. Furthermore, as the world faces environmental challenges, changing demographics and consumer demands, farmers, healthcare providers, food manufacturers and nutritionists must work collaboratively to assure adequate supply, nutritional integrity and sustainability of rice production systems globally.

Combining the effects of increased atmospheric carbon dioxide on protein, iron, and zinc availability and projected climate change on global diets: a modelling study.

BACKGROUND: Increasing atmospheric concentrations of carbon dioxide (CO2) affect global nutrition via effects on agricultural productivity and nutrient content of food crops. We combined these effects with economic projections to estimate net changes in nutrient availability between 2010 and 2050. METHODS: In this modelling study, we used the International Model for Policy Analysis of Agricultural Commodities and Trade to project per capita availability of protein, iron, and zinc in 2050. We used estimated changes in productivity of individual agricultural commodities to model effects on production, trade, prices, and consumption under moderate and high greenhouse gas emission scenarios. Two independent sources of data, which used different methodologies to determine the effect of increased atmospheric CO2 on different key crops, were combined with the modelled food supply results to estimate future nutrient availability. FINDINGS: Although technological change, market responses, and the effects of CO2 fertilisation on yield are projected to increase global availability of dietary protein, iron, and zinc, these increases are moderated by negative effects of climate change affecting productivity and carbon penalties on nutrient content. The carbon nutrient penalty results in decreases in the global availability of dietary protein of 4·1%, iron of 2·8%, and zinc of 2·5% as calculated using one dataset, and decreases in global availability of dietary protein of 2·9%, iron of 3·9%, and zinc of 3·4% using the other dataset. The combined effects of projected increases in atmospheric CO2 (ie, carbon nutrient penalty, CO2 fertilisation, and climate effects on productivity) will decrease growth in the global availability of nutrients by 19·5% for protein, 14·4% for iron, and 14·6% for zinc relative to expected technology and market gains by 2050. The many countries that currently have high levels of nutrient deficiency would continue to be disproportionately affected. INTERPRETATION: This approach is an improvement in estimating future global food security by simultaneously projecting climate change effects on crop productivity and changes in nutrient content under increased concentrations of CO2, which accounts for a much larger effect on nutrient availability than CO2 fertilisation. Regardless of the scenario used to project future consumption patterns, the net effect of increasing concentrations of atmospheric CO2 will slow progress in decreasing global nutrient deficiencies. FUNDING: US Environmental Protection Agency, Consultative Group on International Agricultural Research (CIGAR) Research Program on Policies, Institutions and Markets (PIM), and the CGIAR Research Program on Climate Change and Food Security (CCAFS).

Rising Atmospheric CO2 Lowers Concentrations of Plant Carotenoids Essential to Human Health: A Meta-Analysis.

Plant and human tissues (e.g., leaves, retina) share the need for carotenoids to protect against light-induced and other oxidative stresses. While plants synthesize carotenoids de novo, humans must obtain them primarily through plant-based foods. In plants, elevated levels of atmospheric carbon dioxide (eCO2 ) decrease the concentrations of essential minerals, including magnesium and zinc (essential for brain and eye health), but the overall effect of globally rising CO2 levels on carotenoids is unknown. Here, investigation is sought on how eCO2 affects carotenoids in plants. A meta-analysis of 1026 experimental observations from 37 studies shows that eCO2 decreases plant carotenoid concentrations by 15% (95% CI: -26% to -6%). The meta-analysis of available gene expression data for Arabidopsis thaliana points to a potential CO2 -induced downregulation of carotenoid biosynthesis (Log2 fold-change -13%, 95% CI: -17% to -9%). Some other stoichiometric and biochemical mechanisms related to CO2 -induced changes in carotenoids are also highlighted. While overall eCO2 decreases carotenoid concentrations, individual CO2 studies report variable responses, including increases in carotenoid levels, especially in abiotically stressed plants. The initial assessment raises a novel question about the potential effects of rising CO2 on human health through its global effect on plant carotenoids.

Temperature-related changes in airborne allergenic pollen abundance and seasonality across the northern hemisphere: a retrospective data analysis.

BACKGROUND: Ongoing climate change might, through rising temperatures, alter allergenic pollen biology across the northern hemisphere. We aimed to analyse trends in pollen seasonality and pollen load and to establish whether there are specific climate-related links to any observed changes. METHODS: For this retrospective data analysis, we did an extensive search for global datasets with 20 years or more of airborne pollen data that consistently recorded pollen season indices (eg, duration and intensity). 17 locations across three continents with long-term (approximately 26 years on average) quantitative records of seasonal concentrations of multiple pollen (aeroallergen) taxa met the selection criteria. These datasets were analysed in the context of recent annual changes in maximum temperature (Tmax) and minimum temperature (Tmin) associated with anthropogenic climate change. Seasonal regressions (slopes) of variation in pollen load and pollen season duration over time were compared to Tmax, cumulative degree day Tmax, Tmin, cumulative degree day Tmin, and frost-free days among all 17 locations to ascertain significant correlations. FINDINGS: 12 (71%) of the 17 locations showed significant increases in seasonal cumulative pollen or annual pollen load. Similarly, 11 (65%) of the 17 locations showed a significant increase in pollen season duration over time, increasing, on average, 0·9 days per year. Across the northern hemisphere locations analysed, annual cumulative increases in Tmax over time were significantly associated with percentage increases in seasonal pollen load (r=0·52, p=0·034) as were annual cumulative increases in Tmin (r=0·61, p=0·010). Similar results were observed for pollen season duration, but only for cumulative degree days (higher than the freezing point [0°C or 32°F]) for Tmax (r=0·53, p=0·030) and Tmin (r=0·48, p=0·05). Additionally, temporal increases in frost-free days per year were significantly correlated with increases in both pollen load (r=0·62, p=0·008) and pollen season duration (r=0·68, p=0·003) when averaged for all 17 locations. INTERPRETATION: Our findings reveal that the ongoing increase in temperature extremes (Tmin and Tmax) might already be contributing to extended seasonal duration and increased pollen load for multiple aeroallergenic pollen taxa in diverse locations across the northern hemisphere. This study, done across multiple continents, highlights an important link between ongoing global warming and public health-one that could be exacerbated as temperatures continue to increase. FUNDING: None.