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THIS POST IS A CRITICAL REVIEW OF A PNAS RESEARCH FINDING [LINK] THAT AGW CLIMATE CHANGE WILL FORCE 1 TO 3 BILLION PEOPLE TO LIVE IN A CLIMATE THAT IS HOTTER THAN OPTIMAL FOR HUMANS: Chi Xu etal, “Future of the human climate niche”, PNAS, May 4, 2020, ttps://doi.org/10.1073/pnas.1910114117
ABSTRACT: All species have an environmental niche, and despite technological advances, humans are unlikely to be an exception. Here, we demonstrate that for millennia, human populations have resided in the same narrow part of the climatic envelope available on the globe, characterized by a major mode around ∼11 °C to 15 °C mean annual temperature. Supporting the fundamental nature of this temperature niche, current production of crops and livestock is largely limited to the same conditions, and the same optimum has been found for agricultural and nonagricultural economic output of countries through analyses of year-to-year variation. We show that in a business-as-usual climate change scenario, the geographical position of this temperature niche is projected to shift more in the next 50 years than it has done in the last 6,000 years. Populations will not simply track the shifting climate, as adaptation in situ may address some of the challenges, and many other factors affect decisions to migrate. Nevertheless, in the absence of migration, one third of the global population is projected to experience a mean average temperature >29C currently found in only 0.8% of the Earth’s land surface, mostly concentrated in the Sahara. As the potentially most affected regions are among the poorest in the world, where adaptive capacity is low, enhancing human development in those areas should be a priority alongside climate mitigation.

REVIEW OF THIS PAPER PART-1: WHAT THE PAPER SAYS

The essence of the paper is contained in the chart below. The data in the chart are from the HYDE database [LINK] in Holland and the Archaeo-Globe Dataverse [LINK] at Harvard. In a grid of x=mean annual precipitation (map)and y=mean annual temperature (mat), human success in terms of population and well being is mapped with redness of colors. This color graphic analysis shows that in terms of welfare, survival, and success, we humans do best in a narrow band of map and mat. These are about 1000mm of map and somewhere between 10C and 20C in mat.

These guidelines of climatic measures of human welfare are compared with AGW climate change forecasts of future climate 50 years from now using the “business as usual” measure of warming (RCP8.5) and the projected population growth. The study uses a temperature rise of 7.5C since pre-industrial or 6.5C in the next 50 years (2020-2070) and takes into consideration that population growth is fastest in warmer climates. In terms of the narrow comfort zone of 10C to 20C, regions closer to 20C will warm to above the comfort zone and will thus be driven off the  MAT comfort zone by a warming of 6.5C. This is the core finding of the paper from which many conclusions are drawn.

REVIEW OF THIS PAPER PART-2: CRITICAL COMMENTARY

It is noted that the paper uses a warming of 7.5C from pre-industrial to the year 2070. It is generally recognized that the earth has already warmed about 1C since pre-industrial leaving 6.5C of the projected  warming to occur in the next 50 years from 2020 to 2070. The average warming rate required to reach that temperature forecast is 6.5/5 or 1.3C per decade or 0.13C per year.

By way of comparison we present land surface temperature for the Tropics, the Northern extent, and the Southern extent in the 40-year period 1979-2019 provided by UAH. They show overall annual warming rates of 0.0159C, 0.0198C, and 0.0151C per year respectively. These warming rates are lower than the projected warming rate 2020-2070 by factors of 8.2, 6.6, and 8.6 respectively and thus, on the face of it, the projected warming for 2020-2070 of 0.65C appears to be an unrealistic forecast.

However, since RCP8,5 is based on rising fossil fuel emissions, the relevant warming rate for the period 2020 to 2070 is likely to be higher. Accordingly we look for evidence of rising decadal warming rates in the same data and we find some evidence of rising decadal warming rates in a moving 10-year window from 2009-2019 as seen in the “decadal warming rate” charts below. The observed rate of increase in the decadal warming rate in a moving 10-year window for land surfaces in the three regions are found to be 0.00361, 0.00450, and 0.00507 degC per year per year. If that growth rate sustains, the average decadal warming rate in the period 2020-2070 is expected to be 0.12626,  0.14762, and  0.18281 degC/per decade respectively. It appears that the warming rate used in the reference paper of 1.3C per decade is an order of magnitude larger than the warming rates estimated in this work on the basis of historical observations. The extremely high warming rate used in the reference paper requires more support than just a reference to the RCP8.5. The authors should provide the the forecast for emissions, atmospheric CO2 and climate sensitivity as support for the extremely high rate of warming that led them to the conclusion that the earth will become too hot for human comfort by the year 2070.

UAH LOWER TROPOSPHERE MAT 1979-2019

UAH LOWER TROPOSPHERE MAT: DECADAL WARMING RATES