Scientists outline health limits of heat stress with climate change

New scientific research on the livable limits with regard to temperature mean over half the earth may be too hot for habitation in 300 years, if there is no appreciable reduction in greenhouse emissions from business as usual. The scientific research paper published published by the National Academy of Sciences (PNAS) in the US says that while central estimates of business-as-usual warming by 2100 are 3–4 °C, eventual warmings of 10 °C are quite feasible and even 20 °C is theoretically possible. Temperatures will continue to increase past 2100 unless strong mitigation action is taken now. Human health will suffer due to heat stress once the wet-bulb temperature reaches 35 °C for extended periods.

The research focuses on the limits of human health in coping with temperature extremes. It is often assumed that humans would be able to adapt to any possible warming so the researchers focused on what the limits of livable conditions and heat stress are. They calculated the highest tolerable "wet-bulb" temperature and found that this temperature could be exceeded for the first time in human history in future climate scenarios if greenhouse gas emissions continue unabated.

The research was conducted by Professor Steven Sherwood from the University of NSW and Professor Matthew Huber from Purdue University and published as 'An Adaptability Limit to Climate Change Due to Heat Stress' in the Proceedings of the National Academy of Sciences (PNAS).

Wet-bulb temperature is used as it includes temperature and atmospheric humidity and is equivalent to what is felt when wet skin is exposed to moving air. Humans and most mammals maintain a core body temperature around 37 °C that may vary slightly among individuals but does not adapt to the local climate. To allow transfer and regulation of metabolic heat human skin is strongly regulated at 35 °C or below, a couple of degrees colder than core body temperature. This allows the body to dissipate heat through the skin at wet-bulb temperatures below 35 °C.

According to the research paper abstract "Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning."

At wet-bulb temperatures above 35 °C heat stress and hyperthermia starts - the inability of the body to dissipate excess heat. Heat stroke may occurr which is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat and/or humidity. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably. At wet bulb temperatures of 37–38 °C lethal conditions are reached, even for acclimated and fit individuals.

At the moment the highest instantaneous wet-bulb temperature anywhere on Earth is about 30 °C, with the most common range at 26–27 °C. While maximum temperature may reach 50 °C in tropical desert areas, these areas mostly have very low humidity, thus allowing perspiration to work in cooling the skin and thus core body temperature. Thus, peak potential heat stress is surprisingly similar across many regions on Earth.

"Although areas of the world regularly see temperatures above 100 degrees (F), really high wet-bulb temperatures are rare," Huber said. "This is because the hottest areas normally have low humidity, like the 'dry heat' referred to in Arizona. When it is dry, we are able to cool our bodies through perspiration and can remain fairly comfortable. The highest wet-bulb temperatures ever recorded were in places like Saudi Arabia near the coast where winds occasionally bring extremely hot, humid ocean air over hot land leading to unbearably stifling conditions, which fortunately are short-lived today."

Matthew Huber did the necessary climate modelling on supercomputers operated at Purdue University while Steven Sherwood completed the wet-bulb calculations. In comparing the peak wet-bulb temperatures to the global temperatures for various climate simulations they found that the peak wet-bulb temperature rises approximately 1 degree Centigrade for every degree Centigrade increase in tropical mean temperature.

The research paper says "The simplest prediction of global warming’s effect on TwMax is to assume a uniform upward shift of the Tw distribution. A 4 °C increase in T(wet) would then subject over half the world’s population
annually to unprecedented values and cut the “safety buffer” that now exists between the highest maximum T(wet) and 35 °C to roughly a quarter. A shift of 5 °C would allow maximum T(wet) to exceed 35 °C in
some locations, and a shift of 8.5 °C would bring the most-common value to 35 °C. It has been similarly pointed out that a few degrees of warming will produce unprecedented temperature and agricultural stresses in the tropics"

"These temperatures haven't been seen during the existence of hominids, but they did occur about 50 million years ago, and it is a legitimate possibility that the Earth could see such temperatures again," Huber said. "If we consider these worst-case scenarios early enough, perhaps we can do something to address the risk through mitigation or new technological advancements that will allow us to adapt."

In an accompanying comentary article in PNAS Professor Tony McMichael and Dr Keith Dear from the National Centre for Epidemiology and Population Health at ANU provided additional comments on the health risks and the research.

"Much of the climate change debate has been about whether the world will succeed in keeping global warming to the relatively safe level of only two degrees Celcius by 2100", Professor McMichael said. "But climate change will not stop in 2100, and under realistic scenarios out to 2300, we may be faced with temperature increases of 12 degrees or even more. If this happens, our current worries about sea level rise, occasional heatwaves and bushfires, biodiversity loss and agricultural difficulties will pale into insignificance beside a major threat - as much as half the currently inhabited globe may simply become too hot for people to live there."

Dr Dear warned the UN's Intergovernmental Panel on Climate Change (IPCC) have tended to strike a cautious tone in forecasting future temperature rise and associated impacts. "In presenting its warnings about the future, the IPCC is very careful to be conservative, using mild language and low estimates of impacts," Associate Professor Dear said. "This is appropriate for a scientific body, but world governments - including our own - should be honest with us about the full range of potential dangers posed by uncontrolled emissions and the extremes of climate change that would inevitably result."

Both Professor McMichael and Dr Dear warned that projected environmental impacts of climate change will ultimately impact on human health and wellbeing, posing a considerable threat to the survival of our species.

Can someone please tell our Prime Minister, Kevin Rudd, that tackling climate change and setting a carbon price are important health reforms for our species (let alone other species we share the earth with) that needs his urgent attention?

Sources:

• Purdue University Media Release, May 4, 2010 - Researchers find future temperatures could exceed livable limits
• Australian Health Directory News, May 11, 2010 - Climate change predictions put focus on health impacts
• Image courtesy Purdue University - "A high-CO2 model run that produces a global-mean T 12 °C warmer; accounting for GCM bias, the maximum T(wet) distributions are roughly what would be expected with 10 °C of global-mean warming relative to the last decade."