Cold Climate

We used data on all 85,854,176 deaths in the USA from 1980 to 2016 from the National Center for Health Statistics (NCHS). Age, sex, state of residence, month of death, and underlying cause of death were available for each record. The underlying cause of death was coded according to the international classification of diseases (ICD) system (9^th revision of ICD from 1980 to 1998 and 10^th revision of ICD thereafter). Yearly population counts were available from NCHS for 1990 to 2016 and from the US Census Bureau prior to 1990 (Ingram et al., 2003 (link)). We calculated monthly population counts through linear interpolation, assigning each yearly count to July.
We also subdivided the national data geographically into nine climate regions used by the National Oceanic and Atmospheric Administration (Figure 18 and Table 2) (Karl and Koss, 1984 ). On average, the Southeast and South are the hottest climate regions with average annual temperatures of 18.4°C and 18°C respectively; the South also possesses the highest average maximum monthly temperature (27.9°C in July). The lowest variation in temperature throughout the year is that of the Southeast (an average range of 17.5°C). The three coldest climate regions are West North Central, East North Central and the Northwest (7.6°C, 8.0°C, 8.2°C respectively). Mirroring the characteristics of the hottest climate regions, the largest variation in temperature throughout the year is that of the coldest region, West North Central (an average range of 30.5°C), which also has the lowest average minimum monthly temperature (−6.5°C in January). The other climate regions, Northeast, Southwest, and Central, possess similar average temperatures (10°C to 14°C) and variation within the year of (23°C to 26°C), with the Northeast being the most populous region in the United States (with 19.8% total population in 2016).
Data were divided by sex and age in the following 10 age groups: 0–4, 5–14, 15-24, 25–34, 35–44, 45–54, 55–64, 65–74, 75–84, 85+ years. We calculated monthly death rates for each age and sex group, both nationally and for sub-national climate regions. Death rate calculations accounted for varying length of months, by multiplying each month’s death count by a factor that would make it equivalent to a 31 day month.
For analysis of seasonality by cause of death, we mapped each ICD-9 and ICD-10 codes to four main disease categories (Table 1) and to a number of subcategories which are presented in the Supplementary Note. Cardiorespiratory diseases and cancers accounted for 56.4% and 21.2% of all deaths in the USA, respectively, in 1980, and 40.3% and 22.4%, respectively, in 2016. Deaths from cardiorespiratory diseases have been associated with cold and warm temperatures (Basu, 2009 (link); Basu and Samet, 2002 (link); Bennett et al., 2014 (link); Braga et al., 2002 (link); Gasparrini et al., 2015 (link)). Injuries, which accounted for 8% of all deaths in the USA in 1980 and 7.3% in 2016, may have seasonality that is distinct from so-called natural causes. We did not further divide other causes because the number of deaths could become too small to allow stable estimates when divided by age group, sex and climate region.
We obtained data on temperature from ERA-Interim, which combines predictions from a physical model with ground-based and satellite measurements (Dee et al., 2011 (link)). We used gridded four-times-daily estimates at a resolution of 80 km to generate monthly population-weighted temperature by climate region throughout the analysis period.

This study was carried out from April 2012 to March 2014 in the Eerguna National Natural Reserve Area (120°00′26″to 120°58′02″E, 51°29′25″ to 52°06′00″N), which is situated on the China-Russia border in Northern Inner Mongolia, China. This area links China to the Russian Far East and has a cold temperate continental climate. The lowest temperature in winter and the highest temperature in summer were −45 °C and 33 °C, respectively. Monthly minimum, maximum and mean air temperatures and relative humidity from April 2012 to March 2014 were obtained by hygrothermographs (Qingsheng Electronic Technology Ltd., China) positioned about 50 cm above the ground (Fig. 1).Fig. 1

Monthly minimum (T_min), maximum (T_max) and mean (T_mean) air temperature (°C) and minimum (RH_min), maximum (RH_max) and mean (RH_mean) relative humidity (%) from April 2012 to March 2014, in Inner Mongolia, northeast China

The White River Basin (33°00′–33°30′N, 102°00′–103°00′E) is located within the Zoige wetland on the northeastern Tibetan Plateau of China, with a mean elevation >3,500 m a.s.l (Figure 1). The study area lies within the continental cold temperate climate zone, having a mean annual air temperature of 0.7–1.1°C and a mean annual precipitation of 600–700 mm. Approximately 90% of precipitation is concentrated in the flood period (May to late August) (Wang et al., 2020 (link)). The hydrological conditions here differ markedly between non-flood and flood periods: mean monthly runoff ranged from 75.79 m³/s in September 2019 to 166.96 m³/s in June 2020 (based on data from the Tangke Hydrological Station located at the downstream of the White River Supplementary Figure 1).
The White River (270 km) contributes 10–20% of the annual runoff entering the Yellow River (Xiang et al., 2009 (link)). As the White River meanders through the Zoige wetland, oxbow lakes commonly form due to frequent meander cutoffs caused by wide valleys in the wetland (Wang et al., 2020 (link)). Because of its harsh alpine climate, population density in White River Basin is considerably low, at 5.8 persons/km². Consequently, the aquatic ecosystem is barely impacted by human activities, leaving natural evolution trends intact (Wang et al., 2020 (link)). Given its unique environmental and biological conditions, the White River Basin provides a perfect opportunity for examining the spatiotemporal patterns and drivers of phytoplankton metacommunity assembly in a highland river-oxbow lake system.

The copper-nickel mine tailings pond was built in 1999 and is located in northern Xinjiang Province and in the eastern part of Fuyun County (Fig 1). The design level of the pond capacity is grade Ⅴ and the floor area is about 1.5 × 10⁵ m². The slag waste is disposed of by the traditional wet discharge method and the discharge volume is about 1,000 t/d. The altitude of the area ranges from 500–900 m and the soil types are mainly light brown calcium soil and typical brown calcium soil. The area is subjected to a continental cold temperate climate. It is dry and windy in spring, but cold in winter. There are also large temperature differences between day and night. The annual average temperature is 3.0°C, the annual precipitation is 186.4 mm, the annual evaporation is 1,829.7 mm, the annual extreme maximum temperature is 42.2°C, and the extreme minimum temperature is –51.5°C. It is one of the high, cold regions in China. In September, 2021, the natural plants around the tailings pond were investigated and sampled. During this survey, 10 native plants belonging to eight families were collected and recorded (Table 1). The plant samples were then divided into underground and aboveground parts and each plant was replicated three times. Soil samples from around the plant roots were collected at the same time as the plant samples. The soil sampling depth was 0–20 cm and a total of 60 plant samples and 30 soil samples were collected.