Warmer Nights Could Lead To A 60% Increase In Global Mortality, Finds Study

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Increasing night-time heat could have high toll on human health, finds a new research. According to models, by the year 2100, the world could see an up to 60 percent spike in mortality due to rising temperatures at night.

Published in The Lancet Planetary Health, data from Japan, South Korea and China show hotter night-time temperatures could lead to a 60 percent spike in mortality rates around the world by the end of the century.

Global climate has already been changed by human activities. As an immediate consequence of global climate change, extreme heat conditions could increase the risks of mortality and morbidity from all causes or specific diseases, such as acute cardiovascular events, kidney disorders, and psychiatric illnesses. Based on well documented epidemiological findings on daily high temperatures or heatwaves, most studies to date have projected an increase in heat-related disease burden under various climate change scenarios.

Previous research on the deadly effects of rising heat has typically focused on excessive day-time temperatures, while “the risks of increasing temperature at night were frequently neglected,” explained study co-author Yuqiang Zhang of the University of North Carolina in a statement.

Night-time ambient heat can interrupt the normal physiology of sleep and subsequently lead to a host of complications including immune system damage, chronic illness, and systemic inflammation, authors said. In urban areas, these outcomes might be exacerbated due to the urban heat island effect.

Furthermore, data show that by the 2100s, “total populations exposed to night-time heat are projected to increase four to eight times compared with the 2010s across the northern hemisphere.”

Researchers measured hot night excess (HNE) in the three regions between 1980 and 2015 and modeled projections for the years 2016 through 2100. Different climate change scenarios were employed and measurements were controlled for the effects of daily mean temperatures.

A total of 28 cities with varying climates were included in the models along with daily death records from local health agencies.

By 2090, the average intensity of hot nights in these cities will nearly double from around 69 degrees Fahrenheit to 103.5 degrees Fahrenheit. Severity of night-time heat worsened even under a scenario where the goals of the Paris Agreement were met. Under the models, regions with the lowest average temperatures had the largest potential for warming.

“The occurrences of HNE are projected to occur more rapidly than the daily mean temperature changes,” Zhang continued. “The frequency and mean intensity of hot nights would increase more than 30% and 60% by the 2100s, respectively, compared with less than 20% increase for the daily mean temperature.”

Co-author Haidong Kan of Fudan University in China added that governments and local policymakers should take the findings into account to better prepare for the impending consequences of climate change.

However, because the data were collected from three countries, researchers urged caution when generalizing findings to wider populations. They are currently working to develop a broader, global dataset.

In the meantime, “Locally, heat during the night should be taken into account when designing the future heatwave warning system, especially for vulnerable populations and low-income communities who may not be able to afford the additional expense of air conditioning,”  Zhang said. “Stronger mitigation strategies, including global collaborations, should be considered to reduce future impacts of warming.”

The study report (The effects of night-time warming on mortality burden under future climate change scenarios: a modelling study, The Lancet Planetary Health, Volume 6, Issue 8, August 2022, https://doi.org/10.1016/S2542-5196(22)00139-5Get rights and content) said:

The health impacts of climate warming are usually quantified based on daily average temperatures. However, extra health risks might result from hot nights. We project the future mortality burden due to hot nights.

The scientists selected the hot night excess (HNE) to represent the intensity of night-time heat, which was calculated as the excess sum of high temperature during night time. They collected historical mortality data in 28 cities from three east Asian countries, from 1981 to 2010. The associations between HNE and mortality in each city were firstly examined using a generalised additive model in combination with a distributed lag non-linear model over lag 0–10 days. They then pooled the cumulative associations using a univariate meta-regression model at the national or regional levels. Historical and future hourly temperature series were projected under two scenarios of greenhouse-gas emissions from 1980–2099, with ten general circulation models. We then projected the attributable fraction of mortality due to HNE under each scenario.

Their dataset comprised 28 cities across three countries (Japan, South Korea, and China), including 9 185 598 deaths. The time-series analyses showed the HNE was significantly associated with increased mortality risks, the relative mortality risk on days with hot nights could be 50% higher than on days with non-hot nights. Compared with the rise in daily mean temperature (lower than 20%), the frequency of hot nights would increase more than 30% and the intensity of hot night would increase by 50% by 2100s.

The study provides evidence for significant mortality risks and burden in association with night-time warming across Japan, South Korea, and China. Our findings suggest a growing role of night-time warming in heat-related health effects in a changing climate.

The study report said:

In the future, the intensity of night-time warming is projected to surpass daytime warming in many regions across the world. By the 2100s, total populations exposed to night-time heat are projected to increase four to eight times compared with the 2010s across the northern hemisphere. Therefore, it could be reasonably hypothesised that the health effects and disease burden due to hot nights will worsen in a changing climate. However, almost all studies on projecting the health impacts of future warming focused on daily mean or maximum temperature, whereas the potentially independent health impacts of night-time warming have not been taken into account.

The study report said:

As indicated by the results from some Japanese cities, such as Osaka, Tokyo, and Nagoya, a relatively lower warming level will still bring higher hot night frequency and more intensive HNE. The projected night-time warming was higher than was the daytime warming in these locations, which is consistent with a study that found a higher increase of daily minimum temperature than daily maximum temperature in many areas across Japan. According to a previous research, as these Japanese cities are surrounded by the sea, stronger night-time warming or higher decrease in diurnal temperature range in these areas are mainly caused by higher mean cloud coverage. By throwing back sunlight into space during daytime and trapping outgoing longwave radiation at night, higher coverage of clouds tend to increase night-time temperature when the average temperature increases.

It said:

Recently, there is much evidence of increasing tolerance to ambient heat due to many aspects of potential adaptations, including physiological change, improved health services, and behavioural changes (eg, the use of air conditioning).

The report said:

Several policy implications could be derived from our results. First, in assessment of disease burden due to non-optimum temperature, one should consider the extra health impacts of disproportional intra-day temperature variations. A more complete health-risk assessment of future climate change can help policy makers improve resource allocation and set priorities. Second, heat during night time should be taken into account in designing heatwave warning systems, especially for vulnerable populations. Third, our results highlight the importance of the night-time use of air conditioning in reducing heat-related mortality risk and burden, although this could also elevate the cost of air conditioning use, increase the heat in the built environment, and might further aggravate the heat-related problems such as urban heat island effects. Fourth, night-time heat could be more severe for residents in low-income communities if they cannot afford the additional expense of air conditioning and tend to close windows during the night time especially in locations where there is a high crime rate. These factors highlight the importance of future public health policies that can ensure the indoor comfort for lower-income groups and older adults to accelerate the achievement of equity in the future development. Fifth, our results highlight the need of stronger mitigation strategies to reduce future warming and adaptation interventions to protect populations from the impacts of warming we cannot avoid. Some interventions, such as high-intensity implementation of cool and evaporative roofs and urban landscaping, have been shown to decrease mean and extreme heat effectively. Nowadays, green roofs have been adopted in some cities across China. Our results might be helpful to promote these interventions in urban planning.

The scientists noted some limitations of this study:

First, we included only 28 cities from three East Asian countries, and thus extrapolation of our results to the whole East Asian region or other regions should be cautious.

Second, the study periods and socioeconomic status varied among these cities. The relatively shorter periods for Chinese cities might attenuate the statistical power for risk estimation and the comparability of results from Japan and Korea, but could capture more recent changes in heat adaptation.

Third, we did not collect data on accidental deaths, which could also be affected by non-optimum temperatures.

Fourth, there are still some potential uncertainties existing in downscaling temperature series because our downscaling method did not add local details (eg, the built environment and demographic conditions) to the projected temperature dataset. For example, the potential urban land-use expansion after future urbanization might amplify night-time heat exposure. However, general circulation models used in this study cannot capture these influences in projecting future temperatures.

Fifth, we only evaluated two most probable and common climate scenarios, and some other climate and socioeconomic scenarios, such as SSP119, SSP460, and SSP370, were not taken into account; consequently, our projections do not capture all possible future scenarios.

In conclusion, the report said:

This study provides novel evidence for the significant mortality risks and burden in association with night-time warming across three countries in east Asia. We project at least a doubling intensity of hot night with higher increase in mortality burden due to hot nights, suggesting a growing role of night-time warming in heat-related health effects in a changing climate.

Small Temperature Changes Will Significantly Affect North American Forests

New research demonstrates the extent of damage resulting from slight temperature increases on North American tree species.

Results show that in addition to increased mortality, warming would significantly restrict growth for some species and effects will be compounded by reduced rainfall.

An analysis of more than 4,500 seedlings of nine North American tree species revealed just a slight temperature increase of 1.6 degrees Celsius (about 2.9 degrees Fahrenheit) alone, or combined with reduced rainfall, would increase mortality among the trees and significantly restrict growth.

The research, carried out by a team at The University of Michigan, was published in the journal Nature and underscores the precarious situation of North American boreal forests seen throughout Alaska, Canada and parts of Michigan and Minnesota.

These areas are one of Earth’s largest nearly intact forested ecosystems and play a significant role in decreasing human-made carbon emissions; they are located below tundra regions but above more temperate forests.

Over five years, researchers used infrared lamps and soil heating cables to study near-term impacts of warming on the seedlings.

Several common northern conifer species including balsam fir, white spruce, and white pine exhibited severely reduced growth under experimental conditions, while modest warming did enhance growth for some species more commonly found in southern temperate forests.

However, the enhanced growth of these species is not enough to offset the effects of the vanishing conifers, researchers warned.

In addition to testing the effects of a 1.6 degree Celsius warming, researchers also tested outcomes of a 3.1 degrees Celsius (about 5.6 degrees Fahrenheit) increase above ambient temperatures. Rainwater tarps were used to test drier conditions and outcomes were compared with control trees grown at ambient temperatures and under normal moisture conditions.

Data showed reduced rainfall exacerbated the effects of slight temperature increases.

“Our results spell problems for the health and diversity of future regional forests,” said study co-author Peter Reich of the University of Michigan in a statement.

“Present-day southern boreal forest may reach a tipping point with even modest climate warming, resulting in a major compositional shift with potential adverse impacts on the health and diversity of regional forests,” he added.

The resulting consequences could have sweeping impacts on forests’ ability to produce timber, host other plant and animal biodiversity and reduce flooding and carbon in the air.

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