Climate change impacts on human health can come in many forms. One of the most obvious is through an increase in likelihood of the warmest temperatures, and particularly the highest temperature in a region. These extremes of temperature are what have some of the highest impacts on people and infrastructure that could ameliorate these peaks. Although alone not sufficient to quantify heat impacts in general, record high temperature estimates are a quantity that people can easily relate to, more so than for example the successive number of warm nights, where the organisms can't cool down and recover from the temperature stress during the day. Overall, the projected changes are one measure in a portfolio of factors, and interestingly the expected changes in peak temperatures are often larger than the changes in mean temperatures.

The map shows projected change in Maxima of Daily Maximum Temperature by 2050 compared to the reference period (1986-2005) under RCP 8.5 of CIMP5 ensemble modeling.

The annual distribution of days with a high heat index provides insight into the health hazard of heat. Computed by combining temperature and relative humidity, the heat index provides a measure of apparent temperature, the temperature that reflects comfort or discomfort. Often, high temperature alone can be compensated for by evaporative cooling such as from transpiration. But if the air is nearly saturated with moisture, then that cooling potential is reduced and the apparent temperature increases. Here a standard heat index is used where 35 degrees is a high threshold beyond which humans not only feel uncomfortable but where health dangers increase rapidly.

The graph shows projected change in Number of Heat Days (Tmax > 35°C) per month by 2050 compared to the reference period (1986-2005) under all RCPs of CIMP5 ensemble modeling. Positive values indicate that number of heat days will likely to increase compared to the baseline, and vice versa. The shaded area represents the range between 10th and 90th percentile of the projection.

The calculation of the heat index requires both temperature and moisture output from climate models. Particularly the moisture field is not always as well reproduced by models as one might desire, and thus the uncertainty in the calculated heat index for any model is fairly high. Additionally, moisture was reported by fewer models than temperature, and therefore the total number of heat index projections is more limited. Together, the ranges of projected changes can be quite wide for any of the RCPs. However, comparing the medians generally provides the expected picture of more pronounced increases under the higher emission RCPs compared to lower emission levels. Not only are the expected counts of days higher, but also the duration of what could be called the heat season, the period of year within which days with excessively high heat index, is generally extended.

The counterpart to daily peak temperatures is the nighttime cooling. Many organisms can cope with high temperatures during the day if there is sufficient cooling for recovery at night. If the daily minimum temperatures don't drop below the 20°C threshold, then the night is called a "tropical" (or hot) night. The increase in health threats can be monitored through the frequency of tropical nights. In the projection shown in the time series graph, the difference in expected numbers of tropical nights can be seen across the different RCPs. Generally, a drastic increase is found for the high-emission related scenario of RCP8.5 and significantly less for the lower scenarios.

The graph shows the recorded number of Tropical Nights (Tmin > 20°C) per year for 1986-2005, and projected values for 2020-2100 under all RCPs of CIMP5 ensemble modeling. Note, the shaded ranges illustrate the inter-model differences, here using the +/- one standard deviation. The reason for using a narrower metric compared to the 10th and 90th percentile is that the inter-model difference is large for precipitation, and in particular for the count of days with rainfall.

As with other temperature fields, the confidence in the projection of changes in the likelihood for tropical nights has fairly high. As the nighttime temperatures are often increasing faster than daytime highs, the change in the number of tropical nights can be large. The number of tropical nights can be particularly high in oceanic regions of the tropics where day to day variations are very small. Depending on how close to the 20-degree threshold the country is, the more dramatic are the projected changes with high-emission scenario numbers increasing rapidly compared to lower emission RCPs.

The hottest places in low latitudes don't necessarily have longer warm spells, because they generally exhibit a large variability over the course of a few days. In the tropics, however, where day-to-day variability and even month-to-month variability is small, a phase of warming can immediately generate large numbers. The cumulative nature of a sequence of multiple days with high temperatures can raise the impact on the human body and lead to health issues in broad segments of the population. Many studies have reported associations between temperature extremes and mortality rates of circulatory diseases (WHO, 2009). In addition, Heat amplified levels of some urban-industrial air pollutants could cause respiratory disorders and exacerbate heart and blood vessel disease (Turn Down the Heat, 2012).

The boxplot shows recorded Warm Spell Duration Index (WSDI) for 1986-2005 and projected WSDI by 2050 under all RCPs of CIMP5 ensemble modeling. The WSDI is a measure of such an uninterrupted sequence of at least 6 days that surpass the currently observed 95th percentile of temperature. It therefore not only reflects increases in temperature but looks at the likelihood of sequences of conditions that today are considered the warmest conditions in the year.

The median change for RCP8.5 is almost uniformly surpassing a doubling by 2050 compared to present day (reference 1986-2005), with somewhat lesser changes projected for the lower RCPs. This follows quite closely the magnitude of radiative forcing as represented by the different scenarios. Therefore, the slightly reduced changes for RCP6.0 compared to what would be expected between RCP4.5 and RCP8.5 are likely more tied to the limited number of reporting models than a different trajectory of change.