Acclimatization and its potential limitation due to climate change

Written by: Stefanie Dumberger

Photo credit

Acclimatization is the ability of the human body to adapt its physiological and morphological functions to different climatic conditions 1, 2, 3. Basically it´s the ambient temperature which impacts us, but also humidity, wind velocity 4 and the progressive increasing lack of oxygen at high altitudes challenge our vital systems 5, 6.

Thermoregulation aims at keeping our body within a stable temperature range of 37°C+/-0.2°C, the optimum for smooth working of our metabolism 6. Therefore we perceive 21°C-26°C as a comfortable ambient temperature wearing clothes and 26°C-33°C when naked 7. Mainly we control the heat exchange with our environment through the skin by dilating and constricting our outer veins 8, 9, 10, 11. Other mechanisms involve shivering 12, 13 and non-shivering 14, 15, 16 heat production as well as insulation by our fat layer in cold conditions 17, 18, whereas in hot climates heat is lost by sweating 8, 19 and production diminished by lower core temperature and improved metabolic activity 20, 21, 22, 23. In addition, adapting our behaviour (e.g. seeking shelter, air conditioning, heating, clothing level) and habituating to specific conditions over a longer term are important instruments for human acclimatization 2.

With increasing temperatures due to climate change humans may reach their limitations of adaptation 24. Extreme events like heat and cold waves are likely to occur more often 24, 25, 26. During these, excess rates in morbidity and mortality are observed (figure 1) as the body cannot acclimatize rapidly enough 27, 28. It needs at least 4-6 days of daily exposure in combination with physical activity or 2-6 weeks without exercising to adapt properly 24, 29. Additionally acclimatization is only a transient state with the benefits decaying twice as fast as they are acquired 29. Therefore temperatures lying widely outside our comfortable zone are highly challenging for our vital systems. Humans can usually bear core temperatures of up to 41°C 30, 31 or down to 30°C 8, 32, but only with unlimited access to water and food as well as appropriate clothing. Beyond these temperatures heat and cold illnesses appear which finally lead to exhaustion of the defending mechanisms and thereby death 24.

Overall mortality in association with the perceived temperature in the state Baden-Württemberg, Germany; the x-axis shows the median over three days of perceived outdoor temperature at 12 am in °C and the y-axis the rate of mortality per 100000 inhabitants; the lowest rate of mortality appears around 20°C-25°C, the slope for heat mortality is steeper (1.1% per K) than the slope for cold mortality (0.6% per K), but higher rates of mortality occur in very cold conditions (around -10°C) (Credit).


Challenging factors in this context especially for vulnerable subgroups (e.g. the elderly, children) may be the habituation to overall warmer conditions which leads to impaired reactions to cold waves 24, 27. In addition augmentation of humidity as a result of rising ambient temperature will impede cooling through sweating 33. Since evaporation is the only possible mechanism of heat loss if ambient temperature is higher than the skin temperature 8, 19, heat waves or even hot summer months may overtax our thermoregulation.

Moreover indoor work performance diminishes with each degree after 26°C and outdoor working capacity will be limited or even getting impossible 34. Permanent air conditioning as it is now used in desert regions like the United Arab Emirates will be necessary to enable comfortable working and living resulting in a higher energy demand and thereby increasing energy consumption. Actually these technical adaptations delay the acclimatization process and enhance impaired wellbeing during outdoor activities. 24

We cannot predict if there may happen genetic adaptations to push the current temperature limitations of the body upwards. Probably adapting our behaviour and habituation processes will get more important 24. Furthermore migration towards colder areas of higher altitude or latitude are also likely, though space and resources are limited in these regions.

As it is an interesting field, more research should be done on the effects of acclimatization and the results taken into consideration when investigating the overall impact of climate change on human beings.



  1. de Freitas, C.R. and E.A. Grigorieva, The impact of acclimatization on thermophysiological strain for contrasting regional climates. Int J Biometeor, 2014. 58: p. 2129-2137. doi:10.1007/s00484-014-0813-9
  2. Sawka, M.N., J.W. Castellani, K.B. Pandolf and A.J. Young, Human Adaptations to Heat and Cold Stress.RTO HFM Symposium on “Blowing Hot and Cold:Protecting Against Climatic Extremes” in Dresden, Germany, 2002. RTO-MP-076. doi:10.14339/RTO-MP-076
  3. Mäkinen, T.M., Different types of cold adaptation in humans.Front Biosci, 2010. 2: p.1047-1067. doi:10.2741/S117
  4. Karlsen, A., S. Racinais, M.V. Jensen, S.J. Nørgaard, T. Bonne and L. Nybo, Heat acclimatization does not improve VO2max or cycling performance in a cool climate in trained cyclists. Scand J Med Sci Sports, 2015. 25: p. 269-276. doi:10.1111/sms.12409
  5. Beall, C.M.(2006) Andean, Tibetan, and Ethiopian patterns of adaptation to high-altitude hypoxia. Integr Comp Biol, 2006. 46:p. 18-24. doi:10.1093/ich/icj004
  6. Lorente-Aznar, T., G. Perez-Aguilar, A. García-Espot, S. Benabarre-Ciria, J.L. Mendia-Gorostidi, D. Dols-Alonso and J. Blas-co-Romero, Estimation of arterial oxygen saturation in relation to altitude.Med Clin (Barc), 2016. 147: p. 435-440
  7. Kingma, B.R.M., A.J.H. Frijns, L. Schellen and W.D. van Marken Lichtenbelt, Beyond the classic thermoneutral zone. Temperature, 2014. 1: p. 142-149.
  8. Tansey, E.A. and C.D. Johnson, Recent advances in thermoregulation. Adv. Physiol Educ, 2015. 39: p. 139-148. doi:10.1152/advan.00126.2014.
  9. Charkoudian, N., Mechanisms and modifiers of reflex cutaneous vasodilation and vasoconstriction in humans. J Appl Physiol, 2010. 109: p. 1221-1228. doi:10.1152/japplphysiol.00298.2010
  10. Castellani, J.W. and A.J. Young, Human physiological responses to cold exposure: Acute responses and acclimatization to prolonged exposure. Auton Neurosci, 2016. 196: p. 63-74.
  11. Kellogg, D.L.Jr., In vivo mechanisms of cutaneous vasodilation and vasoconstriction in humans during thermoregulatory challenges. J Appl Physiol, 2006. 100: p. 1709-1718. doi:10.1152/japplphysiol.01071.2005
  12. Haman, F., O.L. Mantha, S.S. Cheung, M.B. DuCharme, M. Taber, D.P. Blondin, G.W. McGarr, G.L. Hartley, Z. Hynes and F.A. Basset, Oxidative fuel selection and shivering thermogenesis during a 12- and 24-h cold-survival simulation. J Appl Physiol, 2016. 120: p. 640-648. doi:10.1152/japplphysiol.00540.2015
  13. Haman, F., Shivering in the cold: from mechanisms of fuel selection to survival. J Appl Physiol, 2006. 100: p. 1702-1708. doi:10.1152/japplphysiol.01088.2005.
  14. Ouellet V., S.M. Labbé, D.P. Blondin, S. Phoenix, B. Guérin, F. Haman, E.E. Turcotte, D. Richard and A.C. Carpentier, Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans. Clin Invest, 2012. 122: p. 545–552. doi:10.1172/JCI60433
  15. van Marken Lichtenbelt, W.D. and P. Schrauwen, Implications of nonshivering thermogenesis for energy balance regulation in humans. Am J Physiol Regul Integr Comp Physiol, 2011. 301: p. 285-296. doi:10.1152/ajpregu.00652.2010
  16. Chondronikola, M., E. Volpi, E. Børsheim, T. Chao, C. Porter, P. Annamalai, C. Yfanti, S.M. Labbe, N.M. Hurren, I. Malagaris, F. Cesani and L.S. Sidossis, Brown Adipose Tissue Is Linked to a Distinct Thermoregulatory Response to Mild Cold in People. Front Physiol, 2016. 7: p.129. doi:10.3389/fphys.2016.00129
  17. Kingma, B.R.M., A.J.H. Frijns and W.D. van Marken Lichtenbelt, The thermoneutral zone: implications for metabolic studies.Front Biosci, 2012. 4: p. 1975-1985. doi: 10.2741/518
  18. Nahon, K.J., M.R. Boon, I.M. Jazet, P.C.N. Rensen and G. Abreu-Vieira G, Lower critical temperature and cold-induced thermogenesis of lean and overweight humans are inversely related to body mass and basal metabolic rate. J Therm Biol, 2017. 69: p. 238-248.
  19. Cramer, M.N. and O. Jay, Biophysical aspects of human thermoregulation during heat stress. Auton Neurosci, 2016. 196: p. 3-13.
  20. Rivas, E., M. Rao, T. Castleberry and V. Ben-Ezra V., The change in metabolic heat production is a primary mediator of heat acclimation in adults.J Therm Biol, 2017. 70: p. 69-79.
  21. Charlot, K., P-E. Tardo-Dino, J-F. Buchet, N. Koulmann, S. Bourdon, B. Lepetit, M. Roslonski, L. Jousseaume and A. Malgoyre, Short-Term, Low-Volume Training Improves Heat Acclimatization in an Operational Context. Front Physiol, 2017. 8: p. 419. doi:10.3389/fphys.2017.00419
  22. Garrett, A.T., N.G. Goosens, N.G. Rehrer, M.J. Patterson and J.D. Cotter J.D., Induction and decay of short-term heat acclimation. Eur J Appl Physiol, 2009. 107: p. 659–670. doi:10.1007/s00421-009-1182-7
  23. Périard, J.D., S. Racinais and M.N. Sawka, Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scand J Med Sci Sports, 2015. 25: p. 20-38. doi: 10.1111/sms.12408
  24. Hanna E.G. and P.W. Tait, Limitations to Thermoregulation and Acclimatization Challenge Human Adaptation to Global Warming. Int J Environ Res Public Health, 2015. 12: p. 8034-8074. doi: 10.3390/ijerph120708034
  25. IPCC, Climate Change 2013: The Physical Science Basis: Summary for Policymakers. Intergovermental Panel on Climate Change, Working Group I Contribution to the IPCC Fifth Assessment Report, IPCC: Geneva, Switzerland, 2013
  26. Fischer, E.M. and R. Knutti, Anthropogenic contribution to global occurrence of heavy precipitation and high-temperature extremes. Nat Clim Chang, 2015. 5: 560-564
  27. Huynen, M.M.T.E. and P. Martens, Climate Change Effects on Heat- and Cold-Related Mortality in the Netherlands: A Scenario-Based Integrated Environmental Health Impact Assessment. Int J Environ Res Public Health, 2015. 12: 13295-13320. doi: 10.3390/ijerph121013295
  28. Huang, C., A.G. Barnett, X. Wang, P. Vaneckova, G. FitzGerald and S. Tong, Projecting Future Heat-Related Mortality under Climate Change Scenarios: A Systematic Review. Environ Health Perspect, 2011. 119: 1681-1690.
  29. Daanen, H.A.M., S. Racinais and J.D. Périard, Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports Medicine, 2018. 48: 409-430
  30. Kenney, W.L., D.W. DeGroot and L.C. Holowatz L.C., Extremes of human heat tolerance: life at the precipice of thermoregulatory failure. J Therm Biol, 2004. 29: p. 479-485. doi:10.1016/j.jtherbio.2004.08.017
  31. Piantadosi, C.A., The biology of Human Survival: Life and Death in Extreme Environments.Oxford University Press. New York, 2003
  32. Eyolfson, D.A., P. Tikuisis, X. Xu, G. Weseen and G.G. Giesbrecht, Measurement and prediction of peak shivering intensity in humans. Eur J Appl Physiol, 2001. 84: p. 100-106
  33. Matthews T.K.R., R.L. Wilby and C.Murphy, Communicating the deadly consequences of global warming for human heat stress. PNAS, 2017. 114: 3861-3866. doi: 10.1073/pnas.1617526114
  34. Lundgren, K., K. Kuklane, C. Gao and I. Holmer, Effects of Heat Stress on Working Populations when Facing Climate Change. Industrial Health, 2013. 51: 3-15