I’ve been away for a week (more about this later, maybe), so haven’t had a chance to post anything. It has been a fairly warm week here in the UK, so it’s seem worth mentioning a recent paper about the global risk of deadly heat. It’s been covered already by Carbon Brief, so I don’t need to say much (I’ll probably fail).
A key point is illustrated in the figure on the right, which shows temperature and relative humidity. The black crosses shows temperature and relative humidity during events that were lethal. The blue line shows the likely boundary between lethal and non-lethal events, and the red line is a 95% probability threshold (which, I think, means almost certainly deadly). Our body (in fact, any mammal’s body) generates heat, and the ability to transfer that heat away depends on temperature and relative humidity, and there is a combination of temperature and relative humidity above which it is no longers possible to do so. As the paper says
The fact that temperature and relative humidity best predict times when climatic conditions become deadly is consistent with human thermal physiology, as they are both directly related to body heat exchange. First, the combination of an optimum body core temperature (that is, ~37<supoC), the fact that our metabolism generates heat (~100 W at rest) and that an object cannot dissipate heat to an environment with equal or higher temperature (that is, the second law of thermodynamics), dictates that any ambient temperature above 37oC should result in body heat accumulation and a dangerous exceedance of the optimum body core temperature (hyperthermia). Second, sweating, the main process by which the body dissipates heat, becomes ineffective at high relative humidity (that is, air saturated with water vapour prevents evaporation of sweat); therefore, body heat accumulation can occur at temperatures lower than the optimum body core temperature in environments of high relative humidity.
The Carbon Brief article does highlight some criticisms of this study (the available data did not cover all parts of the world, for example). However, it does seem clear that if we continue to emit CO2 into the atmosphere, we will continue to warm and a larger fraction of the world (and a larger fraction of the world’s population) will experience heatwave conditions (combinations of temperature and relative humidity) that could be deadly.
This is a key point, though. How these conditions will change in the future will depend on what emission (and, hence, concentation) pathway we actually follow. Climate change isn’t guaranteed to lead to a substantial increase in the probability of these conditions occuring; it largely depends on what we choose to do, or not do.