Top panel: Minimum monthly labor capacity (%) over the previous decade (2001-2010) from the NCEP climate reanalysis. Bottom panel: Minimum monthly labor capacity (%) in GFDL-ESM2M for 2091-2100 under the highest RCP emissions scenario which gives a global temperature anomaly of 3.4C relative to a 1861-1960 control period.

NOAA study about heat stress and projected labor impact:

John P. Dunne, Ronald J. Stouffer, and Jasmin G. John. Journal: Nature Climate Change. DOI: 10.1038/nclimate1827

Key Findings

Due to projected increases in temperature and water vapor in the atmosphere:

• Labor capacity losses related to heat stress are projected to double globally by 2050 with a warming climate.
• Global warming of more than 6°C (11°F) eliminates all labor capacity in the hottest months in many areas, including the lower Mississippi Valley, and exposes most of the US east of the Rockies to heat stress beyond anything experienced in the world today. In this scenario, heat stress in NYC exceeds present day Bahrain, and Bahrain heat stress would induce hypothermia in even sleeping humans.
• Only by limiting global warming to less than 3°C (5°F) do we retain labor capacity in all areas in even the hottest months.

Summary
The authors use existing occupational health and safety thresholds to establish a new metric to quantify a healthy, acclimated individual’s capacity to safely perform sustained labor under environmental heat stress (labor capacity). Using climate model projections, we apply this metric to quantify the direct impact of global warming on the global human population in the future.

Currently, peak months of heat stress reduce population-normalized labor capacity to about 90%. By mid-century, the labor capacity falls to about 80% during peak months. With global warming relative to pre-industrial conditions of 2°C (4°F), labor capacity is reduced to 74% in peak months. In a high emission scenario of 6.2°C (11°F) warming, the labor capacity is reduced to 39% in peak months. In this much warmer world, many areas would not be able to sustain human labor without environmental control during the warmest months of the year.

All organisms, including humans, have a temperature range over which they can environmentally adapt. Most studies of the direct impact of global warming on humans have focused on mortality under extreme weather events. Unfortunately, these efforts suffer from uncertainties in separating environmental stressors from other factors impacting mortality.

This research establishes a new metric, labor capacity, using existing occupational thresholds. In this study, wet bulb globe temperature (WBGT) is used as a measure of combined thermal and humidity stress on human physiology and activity. The metric is of broad societal utility and quantifiable using WBGT, which is easily computed from climate model output.

We estimate WBGT from both historical observational record (NCEP/NCAR reanalysis) and one of GFDL’s coupled carbon-climate Earth System Models (ESM2M). We bias-correct the model to present historical and projected maximum WBGT values under active mitigation (RCP4.5) and highest emissions (RCP8.5) out to 2200.

This work represents a fundamental step forward in our ability to quantify the direct impact of climate warming on the global human population, finding that even under active mitigation of CO2 emissions, labor capacity reduction due to heat stress doubles by the middle of this century, with severe reduction of labor capacity due to environmental heat stress inferred under continued highest emissions out to 2200.

Figure: Population-weighted individual labor capacity (%) during annual minimum (upper lines) and maximum (lower lines) heat stress months. Shown are the historical period (NCEP reanalysis—black, maximum only; ESM2M historical—green), RCP4.5 (blue), and RCP8.5 (red) derived as in the inset (symbols for heavy, moderate and light labor threshold limit values – TLV) through a continuous representation (labor capacity = 100 – 25 x max(0,WBGT – 25)2/3 with an upper bound of 100; black line in inset).

Related
New NOAA study estimates future loss of labor capacity as climate warms (Feb 2013)
Shanghai to Near Very Dangerous 35 Degree Celsius Wet Bulb Temperatures This Week (August 2013)