More people die in winter than summer, but climate change may see this reverse


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Ivan Charles Hanigan, University of Sydney; Alistair Woodward, University of Auckland, and Keith DearClimate change not only poses enormous dangers to the planet, but also harms human health. In our study published today, we show some of the first evidence climate change has had observable impacts on Australians’ health between 1968 and 2018.

We found long-term heating is associated with changed seasonal balance of deaths in Australia, with relatively more deaths in summer months and relatively fewer deaths in winter months over recent decades.

Our findings can be explained by the gradual global warming associated with climate change. Over the 51 years of our study, annual average temperatures increased by more than 1°C in Australia. The last decade (2011 to 2020) was the hottest in the country’s recorded history.

If we continue on this trajectory, we’re likely to see many more climate-related deaths in the years to come.

What we did and found

Using the Australian Institute of Health and Welfare, the Australian Bureau of Statistics and other sources, we gathered mortality data for people aged 55 and over between 1968 and 2018. We then looked at deaths in summer compared to winter in each year.

We found that in 1968 there were approximately 73 deaths in summer for every 100 deaths in winter. By 2018, this had risen to roughly 83 deaths in summer for every 100 deaths in winter.

The same trend, albeit of varying strength, was evident in all states of Australia, among all age groups over 55, in females and males, and in the three broad causes of death we looked at (respiratory, heart and renal diseases).

Elderly woman coughing with blanket over her
Historically, winter death rates have tended to be higher than in summer. But this is changing as our planet warms.
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Hot and cold weather can have a variety of direct and indirect effects on our health. Winter death rates generally exceed those in summer months because infectious diseases, like influenza, tend to circulate more in winter. Meanwhile, heat stress can exacerbate chronic health conditions including heart disease and kidney disease, particularly for older adults.

But the gap between cold-related deaths and heat-related deaths appears to be narrowing. And when we compared deaths in the hottest summers with the coldest winters, we found particularly warm years increase the likelihood of seasonal mortality ratios approaching 1 to 1 (meaning equal deaths in summer and winter).

With summers expected to become hotter, we believe this is an early indication of the effects of climate change in the future.




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Our research is unique

Globally, our study is one of very few that directly shows the health impacts of climate change. Most other studies examine the effects of past weather or climate conditions on health and extrapolate these into the future based on projected climate change scenarios, with associated uncertainties. For example, demographic characteristics of the population are likely to change over time.

Climate change occurs slowly, so typically, we need at least 30–50 years of records to accurately show how climate change is affecting health. Suitable health information is seldom available for such periods due to a variety of challenges in collecting electronic health data (especially in low- and middle-income countries).

Further, long-term health trends can be influenced by numerous non-climate related factors, such as improvements in health care.

In our study, we used Australian mortality records that have been collected with remarkable consistency of detail and quality over the last half century. And by focusing on the ratio of summer to winter deaths within each year, we avoid possible confounding associated with, say, improvements to health care.




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However, we were unable to consider some issues such as the different climate trends in small areas within each state/territory, or the effects of changing temperatures on different occupation groups, such as construction workers.

Our data also don’t allow us to account for the possible effects of people’s adaptation to warmer temperatures in the future.

Dry, cracked riverbed
Summer deaths will almost certainly increase in the years to come.
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Looking ahead

The changing ratio of summer to winter deaths has previously been identified as a possible warning sign of the impact of climate change on human health.

In one study on the topic, the authors found Australia may initially experience a net reduction in temperature-related deaths. That is, increased deaths from heat during summer would be offset by fewer deaths in winter, as winters become more mild.

However, they predict this pattern would reverse by mid-century under the business-as-usual emissions scenario, with increases in heat-related deaths outweighing decreases in cold-related deaths over the long term.

Our findings support these worrying predictions. If warming trends continue, it’s almost certain summer deaths will increase, and come to dominate the burden of temperature-related deaths in Australia.

We found the speed of change in the ratio of summer to winter deaths was fastest in the hottest years within each decade. This strengthens our conclusion we’re observing an effect of long-term climate change.




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Besides helping to answer the question, “does climate change affect human health?”, we believe our findings should inform planning for climate change mitigation and adaptation. The implications are considerable for the planning of hospital services and provision of health care, as well as for emergency services, housing, energy supply, holiday periods and bushfire disaster preparedness.The Conversation

Ivan Charles Hanigan, Data Scientist (Epidemiology), University of Sydney; Alistair Woodward, Professor, School of Population Health, University of Auckland, and Keith Dear, Adjunct Professor of Public Health

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Slowing climate change could reverse drying in the subtropics


Kale Sniderman, University of Melbourne; Andrew King, University of Melbourne; Jon Woodhead, and Josephine Brown, Australian Bureau of Meteorology

As the planet warms, subtropical regions of the Southern Hemisphere, including parts of southern Australia and southern Africa, are drying. These trends include major drought events such as Cape Town’s “Day Zero” in 2018.




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Climate projections suggest this subtropical drying will continue throughout the 21st century. Further drying in these regions will place great stress on ecosystems, agriculture and urban water supplies.

Our new study, published today in Nature Climate Change, suggests the subtropical Southern Hemisphere drying trend may reverse, if global temperatures stabilise in a future world with zero net greenhouse gas emissions.

Dry places get drier, wet places get wetter

As global temperatures increase, some regions get wetter while others get drier. Climate models indicate that many parts of the tropics, where it is already very wet, will become wetter. The subtropics, which sit between the wet tropics and the wet mid-latitudes, are expected to get drier.

Spatial plot of global rainfall projections for 2100 from IPCC AR5, showing percent change in annual rainfall for each °C of global warming, for the last two decades of the 21st century relative to 1986-2005. Subtropical regions, like the Mediterranean and southern Australia are projected to dry.
Modified from IPCC AR5 Ch. 12 Fig 12.10

Over southern Australia, rainfall is expected to decline, particularly in the cool season (which is currently the rainy time of year). This has already happened in Perth and the surrounding southwest of Western Australia.

The drying trend in South-west Western Australia over the last century is significant.
BoM

What will happen when warming slows or stops?

Climate models are typically used to explore future climate under transient or rising temperatures, at least until the end of the 21st century. International efforts to reduce greenhouse gas emissions are aimed at slowing and eventually stopping temperature rises so that the climate is stabilised. For example, the Paris Agreement aims to stabilise global warming within 1.5℃ or 2℃ above pre-industrial levels.

But if temperatures stop rising, how will rainfall patterns respond? To investigate, we used pre-existing climate model runs created by the international scientific community to project different conditions extending from the present to the year 2300.

The chart below shows two different scenarios: one in which greenhouse gases and temperatures level off around 2100 (this referred to as Extended Representative Concentration Pathway 4.5), and the one next to it (Extended Representative Concentration Pathway 8.5) in which greenhouse gases don’t level off until around 2250, creating a much warmer climate.

Smoothed global temperature and subtropical (25°S-35°S) winter (June through August) rainfall in Extended Representative Concentration Pathway (ECP) 4.5 and ECP8.5, from 2006 to 2300.
Author provided

We found that rainfall in the Southern Hemisphere subtropics decreases while temperatures are rising rapidly, with most of the rainfall reduction occurring in the winter months. When temperatures begin to stabilise, subtropical rainfall starts to recover.

How rainfall reversal works

The subtropics are relatively dry right now because they are the region where dry air descends from the upper atmosphere to the surface, suppressing rainfall. Studies have shown that the subtropics may be expanding or shifting southward in the Southern Hemisphere as the global climate warms.

Our study found a link between the trend in Southern Hemisphere subtropical rainfall and the temperature gradient between the tropics and subtropical regions. This temperature gradient gets steeper during periods of rapid warming because the tropics warm faster. Once warming stops, the regions further from the Equator catch up and the temperature gradient gets weaker.

The pattern of temperature warming drives the shifts in rainfall: when the tropics are warming faster, the subtropics become drier as more moisture is exported to the tropics.




Read more:
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A wetter or drier future?

Our results suggest that stabilising global temperatures may lead to a reversal in the drying trend in the subtropics.

The path to stabilising global temperatures will be a long journey from the current trajectory of rising emissions, but this research is potentially good news for the future generations who will live in subtropical regions.


The authors would like to acknowledge Nathan P. Gillett, Katarzyna B. Tokarska, Katja Lorbacher, John Hellstrom, Russell N. Drysdale and Malte Meinshausen, who contributed to this study.The Conversation

Kale Sniderman, Senior Research Fellow, School of Earth Sciences, University of Melbourne; Andrew King, ARC DECRA fellow, University of Melbourne; Jon Woodhead, Research Scientist, and Josephine Brown, Senior research scientist, Australian Bureau of Meteorology

This article is republished from The Conversation under a Creative Commons license. Read the original article.