Changes In Seasonal Variation Research Articles

Impact of climate change on coastal water quality and its interaction with pollution prevention efforts

The impact of climate change on nearshore coastal water quality and its interaction with pollution prevention efforts (e.g., the development of green and gray water infrastructure) still lack systematic investigation. This study performed a holistic analysis of the impact of climate change on the salinity and concentrations of total nitrogen (TN), total phosphorus (TP) and chlorophyll a (Chl.a) in Shenzhen Bay between Shenzhen and Hong Kong, the two most developed megacities in South China, based on three-dimensional hydrodynamic and water quality modeling. The major study findings were as follows. First, Chl.a was the most sensitive parameter, and its bay-wide average concentration in 2100 was predicted to be approximately 13% and 46% higher than those in 2015 under mild and rapid climate change scenarios, respectively. Second, sea level rise was found to be a major driver of all four water quality parameters, while temperature and radiation mainly influenced Chl.a and precipitation mainly influenced nutrients. Third, water quality responses to climate change were highly heterogeneous over the bay. Even under a mild climate change scenario, the highest location-specific changes (2100 vs. 2015) in salinity and TN, TP and Chl.a concentrations were projected to be approximately 21%, 19%, 25%, and 65%, respectively. Fourth, changes in seasonal variation due to climate change may lead to an enhanced ecological risk of algal blooms. Finally, the effect of reducing TN and TP concentrations by proposed water infrastructure development was found to be significantly weakened (nearly 40% and 20% for TN and TP, respectively, under a mild climate change scenario), while the negative effect (i.e., increase in the Chl.a concentration) was notably accelerated. Regional cooperation is critical for protecting the water quality of the bay, particularly under climate change. The insights obtained in this study are applicable to other coastal water zones around the world with similar socioeconomic backgrounds and climatic conditions.

Impact of the COVID-19 pandemic on the incidence, seasonal distribution, and characteristics of subacute thyroiditis.

PurposeAn increasing number of cases of subacute thyroiditis (SAT) related to the coronavirus disease 2019 (COVID-19) and its vaccines continue to be published. The aim of this study was to investigate any change in the incidence and characteristics of SAT by comparing the pre-pandemic and pandemic periods.MethodsThis retrospective, single-center study included 432 newly-diagnosed SAT patients between January 2018 and December 2021. The annual frequency of SAT was calculated as the number of newly-diagnosed SAT cases divided by the total number of outpatients that year.ResultsThe frequencies of newly-diagnosed SAT were 0.136% in 2018, 0.127% in 2019, 0.157% in 2020, and 0.114% in 2021 (p = 0.19). While SAT patients were clustered in the autumn (35.1%) in 2018 and 2019, it was found that this cluster shifted to the winter (33.0%) in 2020 and 2021, in parallel with COVID-19 case peaks (p = 0.017). The patients were separated into two groups as pre-COVID-19 pandemic SAT (n = 272) and COVID-19 pandemic SAT (n = 160). The mean ages of the groups were similar. There were more male patients in the COVID-19 pandemic SAT group than in the pre-pandemic group (30.6% vs. 18.7%, p = 0.005). Frequencies of overt hyperthyroidism and median free-thyroxine levels were significantly higher in the COVID-19 pandemic SAT group (p = 0.029, p = 0.001). Treatment modalities, recurrence rates, and permanent hypothyroidism were similar in both groups.ConclusionWith the COVID-19 pandemic, although there was a change in seasonal variation of SAT and an increase in the number of male patients, there was no change in the incidence and clinical course of SAT.

Learning from the pandemic: mortality trends and seasonality of deaths in Australia in 2020.

AimTo assess whether the observed numbers and seasonality of deaths in Australia during 2020 differed from expected trends based on 2015–19 data.MethodsWe used provisional death data from the Australian Bureau of Statistics, stratified by state, age, sex and cause of death. We compared 2020 deaths with 2015-19 deaths using interrupted time series adjusted for time trend and seasonality. We measured the following outcomes along with 95% confidence intervals: observed/expected deaths (rate ratio: RR), change in seasonal variation in mortality (amplitude ratio: AR) and change in week of peak seasonal mortality (phase difference: PD).ResultsOverall 4% fewer deaths from all causes were registered in Australia than expected in 2020 [RR 0·96 (0·95-0·98)] with reductions across states, ages and sex strata. There were fewer deaths from respiratory illness [RR 0·79 (0·76-0·83)] and dementia [RR 0·95 (0·93-0·98)] but more from diabetes [RR 1·08 (1·04-1·13)]. Seasonal variation was reduced for deaths overall [AR 0·94 (0·92-0·95)], and for deaths due to respiratory illnesses [AR 0·78 (0·74-0·83)], dementia [AR 0.92 (0.89-0.95)] and ischaemic heart disease [0.95 (0.90-0.97)].ConclusionsThe observed reductions in respiratory and dementia deaths and the reduced seasonality in ischaemic heart disease deaths may reflect reductions in circulating respiratory (non-SARS-CoV-2) pathogens resulting from the public health measures taken in 2020. The observed increase in diabetes deaths is unexplained and merits further study.

Variation in Temperature, Precipitation, and Vegetation Greenness Drive Changes in Seasonal Variation of Avian Diversity in an Urban Desert Landscape

Previous studies in urban desert ecosystems have reported a decline in avian diversity. Herein, we expand and improve these studies by disentangling the effect of land-use and land-cover (LULC) types (desert, riparian desert, urban, riparian urban, agriculture), vegetation greenness (normalized difference vegetation index—NDVI), climate, and their interactions on avian seasonal variation abundance and richness. Avian community data were collected seasonally (winter and spring) from 2001 to 2016. We used generalized linear mixed models (GLMM) and multimodel inference to investigate how environmental predictors explain patterns of avian richness and abundance. Avian abundance and richness oscillated considerably among the years. GLMM indicated that LULC was the most important predictor of avian abundance and richness. Avian abundance was highest in urban riparian and urban LULC types, followed by agriculture. In contrast, avian richness was the highest in riparian environments (urban and desert), followed by agriculture, urban, and desert. NDVI was also strongly related to avian abundance and richness, whereas the effect of temperature and precipitation was moderate. The importance of environmental predictors is, however, dependent on LULC. The importance of LULC, vegetation cover, and climate in influencing the seasonal patterns of avian distribution highlights birds’ sensitivity to changes in land use and cover and temperature.

Water pollution index – A new integrated approach to rank water quality

Several indices have been developed to define water quality for human consumption, but each one has some drawbacks that limit their wide applicability. So there is a need for a new approach to classify water quality that can be accepted universally. Pollution load or water quality status of Birbhum district of West Bengal, India was presented by applying newly formulated water pollution index (WPI) method in 368 groundwater samples (184 in each pre and post monsoon season). This new technique is flexible and easy to calculate compared to the existing methods. According to this new approach water quality is principally based on observed concentration (Ci) and standard permissible concentration (Si); and can be adjusted to the number of total applied variables (n) as user wants. WPI suggested four categories of water quality: WPI < 0.5 signifies an excellent quality, 0.5 > WPI < 0.75 indicates good quality, 0.75 > WPI < 1 is moderately polluted water, whereas if WPI > 1, indicates highly polluted water. In this study area, WPI ranged from 0.38 to 1.94 with a mean value of 0.831, indicating groundwater quality as moderately polluted. As per WPI results, 12% 55%, 16%, 17% samples in PRM and 2%, 34%, 38%, 26% samples in POM were found as excellent, good, moderately polluted and highly polluted water respectively. Empirical bayesian kriging (EBK) interpolation based GIS spatial map clearly reflected the change in seasonal variation of WPI. Normality test of the data sets were checked through Kolmogorov–Smirnov test and significant difference (p < 0.003) was observed. The new WPI was validated through Bland-Altman analysis using limits of agreement (LoA) and bias value (δ). WPI fitted very well indicating its better applicability and output in diverse water pollution studies than the existing water quality indices.

Long-term trends in seasonality of mortality in urban Madagascar: the role of the epidemiological transition

ABSTRACT Background: Seasonal patterns of mortality have been identified in Sub-Saharan Africa but their changes over time are not well documented. Objective: Based on death notification data from Antananarivo, the capital city of Madagascar, this study assesses seasonal patterns of all-cause and cause-specific mortality by age groups and evaluates how these patterns changed over the period 1976–2015. Methods: Monthly numbers of deaths by cause were obtained from death registers maintained by the Municipal Hygiene Office in charge of verifying deaths before the issuance of burial permits. Generalized Additive Mixed regression models (GAMM) were used to test for seasonality in mortality and its changes over the last four decades, controlling for long-term trends in mortality. Results: Among children, risks of dying were the highest during the hot and rainy season, but seasonality in child mortality has significantly declined since the mid-1970s, as a result of declines in the burden of infectious diseases and nutritional deficiencies. In adults aged 60 and above, all-cause mortality rates are the highest in the dry and cold season, due to peaks in cardiovascular diseases, with little change over time. Overall, changes in the seasonality of all-cause mortality have been driven by shifts in the hierarchy of causes of death, while changes in the seasonality within broad categories of causes of death have been modest. Conclusion: Shifts in disease patterns brought about by the epidemiological transition, rather than changes in seasonal variation in cause-specific mortality, are the main drivers of trends in the seasonality of all-cause mortality.

Health effects of seasonal variation in cardiovascular hemodynamics among workers in forest environments.

Seasonal variation in cardiovascular functions (CVFs) associated with climatic changes is an important emerging public health issue. The objectives of this study were to demonstrate seasonal variation in CVFs by comparing intra-individual differences between winter and summer among people working in a forest environment and to discuss the possible mechanisms accounting for the health effects of seasonal variation in cardiovascular hemodynamics. A total of 72 staff members of the Experimental Forest of National Taiwan University were recruited for continuous health monitoring during two seasons to investigate the intra-individual seasonal variation in CVFs, complete blood counts, and biochemical examinations. CVFs were assessed by measuring the arterial pressure waveform by a cuff sphygmomanometer using an oscillometric blood pressure device, and aortic stiffness was measured by brachial-ankle pulse wave velocity (baPWV). The results showed that cholesterol levels, white and red blood cell counts, and platelet counts were higher in winter than in summer. Subjects showed not only higher vascular stress, as indicated by higher levels of brachial systolic and diastolic blood pressure (SBP and DBP), central end-SBP and DBP, systemic vascular resistance (SVR), and baPWV, but also lower cardiac activities, including lower levels of heart rate, left ventricular contractility, and cardiac output in winter than in summer. The central and brachial BP, cardiac output, SVR, and baPWV were significantly associated with temperature changes in seasonal variation after controlling related confounding factors. This study provides evidence of higher vascular stress and susceptibility to atherothrombosis during winter compared with summer.

Decadal Changes in Seasonal Variation of Atmospheric Haze over the Eastern United States: Connections with Anthropogenic Emissions and Implications for Aerosol Composition

The current seasonal summer maximum in surface fine particulate matter (PM2.5) over the eastern United States has been well established. We find that this seasonality has historically changed substantially, based on long-term quality assured inverse visibility (1/Vis) data over 1946–1998. The median summer/winter 1/Vis ratio increased from about 0.8 over both the southeastern and northeastern United States in the late 1940s to 1.24 over the southeastern United States and to 1.04 over the northeastern United States in the mid-1970s. This ratio exhibits weaker changes in both regions afterward. The observed PM2.5 seasonality after the year 2000 has similar spatial distribution as that in 1/Vis over the mid-1990s, with systematically higher summer/winter ratios which rapidly weaken after the mid-2000s. From 1956 to 1975, stronger increases in 1/Vis occurred in summer than in winter in both regions, associated with increases in sulfur dioxide emissions and reductions in anthropogenic carbonaceous emissions. O...

Seasonal variation changes in biosynthesis of Nanoparticles by mangroves

Seasonal variation changes in biosynthesis of Nanoparticles by mangroves

Phenological Sensitivity of Early and Late Flowering Species Under Seasonal Warming and Altered Precipitation in a Seminatural Temperate Grassland Ecosystem

Shifts in flowering phenology of plants are indicators of climate change. The great majority of existing phenological studies refer solely to gradual warming. However, knowledge on how flowering phenology responds to changes in seasonal variation of warming and precipitation regimes is missing. We report the onset of 22 early (flowering before/within May) and 23 late flowering (flowering after May) species in response to manipulated seasonal warming (equal to + 1.2°C; last 100-year summer/winter warming), additional winter rainfall, and modified precipitation variability (including a 1000-year extreme drought event followed by heavy rainfall) over the growing season in two consecutive years for a species-rich temperate grassland ecosystem. The average onset of flowering (over 2 years) was significantly advanced 3.1 days by winter warming and 1.5 days by summer warming compared to control. Early flowering species responded to seasonal warming in both years, while late-flowering species responded in only 1 year to summer warming. The average onset of early flowering species was significantly advanced, 4.9 days by winter warming and 2.3 days by summer warming. Species-specific analysis showed that even within the early flowering community there were divergences. A positive correlation between plant height and shift in flowering onset was detected under winter warming (R2 = 0.20, p = 0.005). The average onsets of early and late flowering community were affected by neither winter rain nor growing season precipitation variability. Seasonal differences in warming, and particularly winter warming, might alter community dynamics among early and late flowering species which can cause shifts in the seasonal performances of temperate ecosystems.

Monitoring and Analysing Land Use/Cover Changes in an Arid Region Based on Multi-Satellite Data: The Kashgar Region, Northwest China

In arid regions, oases ecosystems are fragile and sensitive to climate change, and water is the major limiting factor for environmental and socio-economic developments. Understanding the drivers of land use/cover change (LUCC) in arid regions is important for the development of management strategies to improve or prevent environmental deterioration and loss of natural resources. The Kashgar Region is the key research area in this study; it is a typical mountain-alluvial plain-oasis-desert ecosystem in an arid region, and is one of the largest oases in Xinjiang Uyghur Autonomous Region, China. In addition, the Kashgar Region is an important cotton and grain production area. This study’s main objectives are to quantify predominant LUCCs and identify their driving forces, based on the integration of multiple remote sensors and applications of environmental and socio-economic data. Results showed that LUCCs have been significant in the Kashgar Region during the last 42 years. Cultivated land and urban/built-up lands were the most changed land cover (LC), by 3.6% and 0.4% from 1972 to 10.2% and 3% in 2014, respectively. By contrast, water and forest areas declined. Grassland and snow-covered areas have fluctuated along with climate and human activities. Bare land was changed slightly from 1972 to 2014. According to the land use transfer matrix, cultivated land replaced grass- and forestland. Urban/built-up land mainly expanded over cultivated and bare land. LUCCs were triggered by the interplay of natural and social drivers. Increasing runoff, caused by regional climate changes in seasonal variation, and snow melt water, have provided water resources for LC changes. In the same way, population growth, changes in land tenure, and socio-economic development also induced LUCCs. However, expansion of cultivated land and urban/built-up land led to increased water consumption and stressed fragile water systems during on-going climate changes. Therefore, the selection of adaption strategies relating to climate change and oasis development is very important for sustainable development in the Kashgar Region.

Экологическое состояние реки Урал

The article describes the ecological state of the Ural River at the present stage. Hydrochemical regime of the reservoir is an important indicator to regulating the number of water resources. As the Ural River is the transboundary reservoir, it is prone to contamination from various areas in the upper and middle reaches. Monitoring of the ecological state of the Ural River showed the presence of anthropogenic pollution. The hydrochemical regime of the reservoir is subject to changes in seasonal variations in water composition. In spring, with flood waters, the main pollution enters the river, however in other seasons, the quality of water leaves much to be desired. In autumn, in comparison with the spring, both alkalinity and salinity increased. The reservoir is prone to contamination with nitrites in almost all seasons of the year from 0.05 to 0.20 mg/dm3 (December, 2016). In the lower reaches of the Ural River, the content of readily oxidizable organic compounds ranges from 3.2 mg/dm3 (winter) to 7.36 mg/dm3 (autumn). The content of boron exceeds the maximum permissible concentration 1-3 times. Environmental monitoring conducted in different periods of the year showed a constant source of pollution flow into the reservoir, which was increased in the flood period. Concentrations of nitrite, boron and petroleum products in the Ural River exceed their permitted values at the point in the end of Uralsk city. The results of the research confirm that the source of the contamination of Ural River is its confluent Ilek River originating from Aktobe Oblast.

Seasonal variation in penicillin use in Mexico and Brazil: analysis of the impact of over-the-counter restrictions.

During 2010, Mexico and Brazil implemented policies to enforce existing laws of restricting over-the-counter sales of antibiotics. We determined if the enforcement led to more appropriate antibiotic use by measuring changes in seasonal variation of penicillin use. We used retail quarterly sales data in defined daily doses per 1,000 inhabitant-days (DDD/TID) from IMS Health from the private sector in Mexico and Brazil from the first quarter of 2007 to the first quarter of 2013. This database contains information on volume of antibiotics sold in retail pharmacies using information from wholesalers. We used interrupted time-series models controlling for external factors with the use of antihypertensives with interaction terms to assess changes in trend, level, and variation in use between quarters for total penicillin use and by active substance. The most used penicillin was amoxicillin, followed by amoxicillin-clavulanic acid and ampicillin (minimal use in Brazil). Before the restrictions, the seasonal variation in penicillin use was 1.1 DDD/TID in Mexico and 0.8 DDD/TID in Brazil. In Mexico, we estimated a significant decrease in the seasonal variation of 0.4 DDD/TID after the restriction, mainly due to changes in seasonal variation of amoxicillin and ampicillin. In Brazil, the seasonal variation did not change significantly, overall and in the breakdown by individual active substances. For Mexico, inappropriate penicillin use may have diminished after the restrictions were enforced. For Brazil, increasing use and no change in seasonal variation suggest that further efforts are needed to reduce inappropriate penicillin use.

Groundwater irrigation quality mapping using geostatistical techniques in Amol–Babol Plain, Iran

Groundwater is acknowledged to be a reliable source for agricultural activities in arid and semi-arid regions. An assessment of the suitability of groundwater for agricultural usage was carried out on the Amol–Babol Plain, Iran, where agriculture is the dominant economic activity. Groundwater samples were collected from 154 wells during the wet and dry seasons in 2009. The sodium percentage, sodium adsorption ratio, residual sodium carbonate, magnesium hazard, and Kelly’s ratios were used as indicators for the water quality. Geostatistical technique of ordinary kriging method was used to create spatial distribution maps. The thematic maps of salinity hazard, sodium adsorption ratio, and sodium percentage indicated an increasing trend of concentration from the western and southern areas to the east and north-east of the plain. The maps also show that the groundwater quality decreases gradually from the west and south sides to the north-eastern side. There is also no significant change in seasonal variation of water quality parameters.

Terrestrial Water Storge Changes in the Tangnaihai Basin Measured by GRACE during 2003-2008, China

Abstract.The amount of water storage change in Tangnaihai basin is obtained by using monthly gravity field data, which is derived from GRACE satellite between 2003 and 2008, with Gaussian filter. Combined with the same time period monthly precipitation data of the regional meteorological stations, we analysis spatial-temporal variation trend of water storge change in Tangnaihai basin for nearly 6 years. Results show that the spatial distribution of water storge change in Tangnaihai basin has obvious difference,water storge change is more in west than in east area.Water storge change has obviously seasonal variation change, and it has the same process with precipitation.The trend of water storge change increase in annual in study area. Water storge change increased from 2003 to 2008 in totally in Tangnaihai basin.The average rate is 0.5 mm/month,and water storge change reduced about 6.1*106 m3.

Modeling gradually changing seasonal variation in count data using state space models: a cohort study of hospitalization rates of stroke in atrial fibrillation patients in Denmark from 1977 to 2011.

BackgroundSeasonal variation in the occurrence of cardiovascular diseases has been recognized for decades. In particular, incidence rates of hospitalization with atrial fibrillation (AF) and stroke have shown to exhibit a seasonal variation. Stroke in AF patients is common and often severe. Obtaining a description of a possible seasonal variation in the occurrence of stroke in AF patients is crucial in clarifying risk factors for developing stroke and initiating prophylaxis treatment.MethodsUsing a dynamic generalized linear model we were able to model gradually changing seasonal variation in hospitalization rates of stroke in AF patients from 1977 to 2011. The study population consisted of all Danes registered with a diagnosis of AF comprising 270,017 subjects. During follow-up, 39,632 subjects were hospitalized with stroke. Incidence rates of stroke in AF patients were analyzed assuming the seasonal variation being a sum of two sinusoids and a local linear trend.ResultsThe results showed that the peak-to-trough ratio decreased from 1.25 to 1.16 during the study period, and that the times of year for peak and trough changed slightly.ConclusionThe present study indicates that using dynamic generalized linear models provides a flexible modeling approach for studying changes in seasonal variation of stroke in AF patients and yields plausible results.

Long-term trends in the incidence of type 1 diabetes in Denmark: the seasonal variation changes over time

There is a worldwide increase of type 1 diabetes mellitus (T1DM). In 1996, the Danish population-based registry was initiated including all newly diagnosed children aged 0-15 yr. This is the report of incidence and seasonal variation for the first 10 yr of the registry. The data was analyzed using Poisson's regression analysis. A total of 2166 children with diabetes were diagnosed before the age of 15 yr between 1996 and 2005. In this period, the annual increase in childhood T1DM was 3.43% (95% confidence interval: 1.91-4.97), which was unaffected by age and gender. Seasonal variation in incidence rates varied by year but not by age and gender. In conclusion, there is a steep increase in incidence of childhood T1DM in Denmark; the increase is comparable with the increase seen in other European countries. There is a significant seasonal variation that changes on a year-to-year basis. The observed variations in cadence rates may be associated with viral epidemics, sunshine exposure, or vitamin D levels and suggest further exploration of these relations.

Ecosystem change in the western North Pacific associated with global warming using 3D-NEMURO

We developed a 3D ecosystem-biogeochemical model based on NEMURO (North Pacific Ecosystem Model Used for Regional Oceanography) and applied it to the western North Pacific in order to predict the effects of global warming on ecosystem dynamics and biogeochemical cycles. Using datasets of observed climatology and simulated fields according to a global warming scenario, IS92a (CO-AGCM developed by CCSR/NIES) as boundary conditions for our ecosystem model, we conducted present-day and global warming experiments and compared their results. Model results in the global warming experiment show increases in vertical stratification due to rising temperatures. As a result, the predicted nutrient and chlorophyll- a concentrations in the surface water decrease at the end of the 21st century, and the dominant phytoplankton group shifts from diatoms to other small phytoplankton. The P/B ratio slightly increases from that in the present as a result of favorable temperature conditions, although nutrient conditions become worse. The increase in the P/B ratio causes increases in the NPP and GPP, although new and export productions decrease. Increases in the regeneration rates (i.e., decrease in the e-ratio) also contribute to increases in NPP and GPP through nutrient supplies within the surface water. Changes in seasonal variations of biomass and the dominant phytoplankton group in the subarctic–subtropical transition region associated with the global warming are large in all regions. In the global warming scenario, the onset of the diatom spring bloom is predicted to take place 1.5 month earlier than in the present-day simulation due to strengthened stratification. The maximum biomass in the spring bloom is predicted to decrease drastically compared to the present due to the decreases in nutrient concentration. In contrast, the biomass maximum of the other small phytoplankton at the end of the diatom spring bloom is the same as the present, because they can adapt to the low nutrient conditions due to their small half-saturation constant. Therefore, a change in the dominant phytoplankton group appears notably at the end of spring bloom. Since the present nutrient concentrations and phytoplankton biomass from summer to winter are low compared with those in spring, these changes associated with the global warming are small. That is, it is interesting that the changes do not occur uniformly in all seasons, but occur dramatically at the end of the spring and in the fall bloom.

Recent changes in seasonal variations of climate within the range of northern caribou populations

The Arctic is one region where it is expected that the impacts of a globally changing climate will be readily observed. We present results that indicate that climate derivatives of potential significance to caribou changed during the past 50 years. Many temperature derivatives reflect the increasing overall temperature in the Arctic such as decreases in the number of days with low temperatures, increases in the number of days with thaw, and days with extremely warm temperatures. Other derivatives reflect changes in the precipitation regime such as days with heavy precipitation and number of days when rain fell on snow. Our results indicate that specific caribou herds from across the Arctic were subjected to different variations of these derivatives in different seasons in the recent past. Examination of temperature and precipitation at finer time-steps than annual or monthly means, shows that climatic variations in the region are neither consistent through the seasons nor across space. Decadal changes in seasonal patterns of temperature and precipitation are shown for selected herds. A process for assessing caribou-focused climate derivatives is proposed.

Changes in seasonal deaths from myocardial infarction.

Cardiovascular disease is the major contributor to excess morbidity and mortality in winter. With the rise in temperatures through global warming, and the use of central heating and air conditioning, this seasonal variation may be declining. To study possible changes in seasonal variation in case-fatality rates of myocardial infarction (MI), in men and women, over a 20-year period and compare this with possible environmental influences. Retrospective analysis of death certificate and climatological data. We analysed all monthly death certificate data from Northern Ireland, for death caused by MI from 1979 through 1998 (n=68 683). Mortality data were standardized to a single reference group for the whole period. Seasonal variation in mortality and in environmental variables was described using the cosinor model. A total of 29 875 women and 38 808 men died from MI during the 20-year period. A significant decrease in mortality from MI was observed in both sexes, accompanied by a non-significant decline in the amplitude of the seasonal rhythm over the study period. Low temperature was associated with higher mortality rates from MI. We have documented an overall decline in cardiovascular mortality from 1979 to 1998, together with a small but non-significant decrease in seasonal variation. While improvements in medical care, lifestyle, housing and diet may have contributed to the observed decline in mortality rate, seasonal fluctuations remain a significant problem.