"At a physiological level, too little exercise can produce costly health outcomes like obesity, cardiovascular disease and diabetes, and insufficient physical activity is a leading cause of death in the United States [it is the fourth-leading cause of death worldwide and the second biggest contributor to cancer in Australia]," say the authors of a new study, published in Nature Human Behaviour. "Sedentary lifestyles are also associated with psychological concerns such as impaired cognitive performance and a greater risk of clinical depression and anxiety. Human well-being clearly suffers from insufficient physical activity. Yet even in spite of its substantial benefits, people in many countries participate in below-recommended levels of physical activity and are becoming increasingly sedentary." About 70 per cent of Australian adults are sedentary. Our modern work lifestyles and the mechanisation of our lives have been engineered to minimise movement, and the environment we live in also plays its part in shaping our behaviour. "Of the environmental factors affecting physical activity rates, temperature plays a noteworthy role," say authors Nick Obradovich and James Fowler from Harvard. "When it is too cold or too hot, adults perform less physical activity, resulting in more sedentary lifestyles. This reduction is mostly due to the nature of adult physical activity: the vast majority of exercise-related physical activity occurs outdoors. When it is too cold or to hot to go outdoors – for a walk, a jog, or to garden – many simply forgo physical activity entirely." With the world's leading scientists anticipating increasingly extreme fluctuations in the weather as a result of climate change, Obradovich and Fowler wanted to understand the impact this might have on the way we move.

They took data showing the recreational physical activity from more than 1.9 million Americans between 2002 and 2012, measured it against daily weather data over the same period and then made forecasts based on climate change projections over the next 30 to 80 years. They found that peak physical activity was typically around 28 degrees Celsius and that 80 per cent of the temperature ranges between 2002 and 2012 fell below that. They also found that hot weather had the most negative effect on the obese and the elderly who were respectively seven times and four times less likely to be physically active. As climate change leads to warmer weather in years to come, they said "our results suggest that the physical activity rates of both obese and elderly individuals may be most susceptible to higher ambient temperatures". In general, however, they anticipate that warmer weather across the year means "in both our city-level and geographic forecasts, we predict that much of the United States will experience increased physical activity due to future climatic changes". It is likely to be a different story for Australians. In America, the average temperatures range between about 8 and 18.2 degrees.

In NSW, the average temperatures range between about 16 and 26 degrees, while in Victoria the maximum average ranges between 14.2 and 25.3. Leading scientists advise climate change will cause increases to the frequency and intensity of extreme weather events in Australia, with many more extreme heat days of over 35 degrees Celsius by 2070. Obradovich and Fowler took this into account, noting that "our data are restricted to observations from one country with a temperate climate". "It is critical to repeat this analysis where possible in countries with warmer average climates and lower prevalence of airconditioning, as they may see net reductions in physical activity due to climate change," they add. "Ultimately, most of the social impacts of climate change are likely to be negative," Obradovich and Fowler conclude.

"Climate change may reduce economic output, amplify rates of conflict, produce psychological distress, increase exposure to the social effects of drought and increase heat-related mortality and morbidity, among other illnesses. However, climatic changes are unlikely to be uniformly costly to society, and it is important to investigate both costs and benefits. Here we uncover a possible beneficial effect of climate change for the United States... "The more we know about the full range of potential climate impacts, the better we will be able to prepare for what is likely to be humanity's greatest challenge in the 21st century."