Faster increases in apparent than air temperature under climate warming

1. Faster increases in apparent than air temperatureunder climate warming World Conference on Climate Change, 25 Oct 2016, Valencia, Spain Yongqin David Chen Department of Geography and Resource Management Institute of Environment, Energy and Sustainability The Chinese University of Hong Kong Email: ydavidchen@cuhk.edu.hk Investigators: J.F. Li (HKBU), Y.D. Chen, T.Y. Gan (UoAlberta), and G.N.C. Lau (CUHK)

2. Global warming hiatus/pause/slowdown (1998 – 2013)

3. Consequences of Global Warming: Three biggest challenges Enhance greenhouse effect Rising temperature Thermal expansion of sea water & melting of snow on land Sea level rise Regional differences in precipitation and increase in occurrence of extreme weather and climate events Change in atmospheric circulation and enhance the water cycle

4. Source: IPCC AR5

5. Future near-surface air temperature changes Source: IPCC AR5

7. Is Hong Kong climate ready and resilient in the 21st century?

8. How human perceives temperature change under climate warming?Air Temp (AT) vs Apparent Temp (AP) • Apparent temperature, as human perceives, is determined by not only air temperature, but also other meteorological variables, particularly relative humidity and wind speed. • When AT is high, atmospheric humidity causes surplus heat stress to human body, AP is represented by a Heat Index (HI). • When AT is low, strong wind causes chilling and cooling effect, AP is represented by wind-chill equivalent temperature (WCT). • Extreme AP events (e.g. heat waves and cold surges) associated with abnormally high or low AT combined with abnormal weather conditions can potentially lead to reduced labor capacity, temperature-related discomfort, stress, morbidity, and even mortality. • Even though non-extreme AP with normal AT and other climatic factors is generally not harmful to human health, it can influence how human perceives the effect of global warming and very often affect human comfort.

9. Source: NWS, NOAA

10. Research Motivation and Objective • Although past studies have demonstrated that global warming raises AP more than AT under extremely hot conditions, possible changes in long-term AP under both extreme and normal conditions throughout the year have not been well studied. • A better understanding of the long-term physiological impacts of global warming on human being represented by AP under not only extreme but also non-extreme weather conditions is needed for climate change adaptation. • This study aims to project an overall picture of the possible changes in AP under both extreme and normal weather conditions throughout the year, and compare the temporal evolution and spatial patterns of future AP with those of AT.

11. Input Variables and Data Sources • Three input variables: Near-surface air temperature, specific humidity at 2m, and wind speed at 10m • Data sources: (1) ERA-Interim reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) as observation, and (2) 3-hour and daily outputs from the following seven GCMs in Coupled Model Intercomparison Project Phase 5 (CMIP5) for the historical period (1981-2000), as well as three future (2081-2100 and 2006-2100) climate change scenarios, i.e. RCP2.6, RCP4.5 and RCP8.5.

12. Methodology: AP Calculations and Statistical Analysis • Heat Index for hot condition (AT>26.67 oC), using Rothfusz regression (1990) HI=-8.7847+1.6114×T-0.012308×T2+RH×(2.3385-0.14612×T+2.2117×10-3×T2)+RH2 ×(-0.016425+7.2546×10-4×T-3.582×10-6×T2) • Wind-chill equivalent temperature for cold and windy condition (AT<10 oC and wind speed > 4.8 km/h), adopted from US NWS and Meteorological Service of Canada WCT=13.12+0.6215×T-11.37×v0.16+0.3965×T×v0.16 • Universal AP under normal weather conditions, proposed by Steadman (1984) UAP=-2.7+1.04×T+2×P-0.65×v • AP is first estimated from raw ERA-Interim reanalysis and GCMs data at the original spatial resolutions, and then re-gridded to 2.5º×2.5º resolution for multimodel ensemble. We use the geographic local time of each grid cell to calculate the daytime (06:00–18:00) and nighttime (18:00–06:00) climatic variables. • Modified Mann-Kendall trend test for AP and AT, and Two-sample t test for the differences of the means of climatic variables in two different periods

13. Comparison of GCM simulations against ERA-Interim reanalysis data Daytime apparent temperature (AP; oC), air temperature (AT; oC), relative humidity (RH; %) and wind speed (Wind; km/h) in GCMs under the historical scenario and the ERA-Interim reanalysis during 1981-2000. a, c, e and g are multimodel ensemble means of AP, AT, RH and Wind, respectively, while b, d, f, and h are those of the ERA-Interim reanalysis.

14. Comparison of GCM simulations against ERA-Interim reanalysis data (cont’d) This comparison demonstrates the capability of GCMs in simulating AP and its contributing factors.

15. Temporal evolution of daytime continental mean AP and AT • During 1960-2005, AP was about 1.5 oC lower than the AT. • Under RCP2.6, AP-AT is projected to remain almost the same as in the historical past. • Under RCP4.5, AP is projected to increase faster than AT and will be about 1 oC lower than AT by the end of the 21st century. • Under RCP8.5, both AP and AT will increase substantially, but AP is projected to increase faster than AT, and exceed AT by the end of the 21st century.

16. Temporal evolution of the continental mean of AP-AT • AP–AT is well captured by GCM simulations, as indicated by the comparison between AP–AT derived from GCM simulations and ERA-Interim reanalysis data. • AP-AT from the ERA-Interim reanalysis data (1980-2015) increases by 0.04 oC/decade. This rate increases to 0.06 oC/decade and 0.2 oC/decade under RCP4.5 and RCP8.5, respectively. All trends are significant at the 5% significance level in the Modified Mann-Kendall trend test. • Under RCP4.5 and RCP8.5, increasing trends of AP–AT are found, indicating that AP increases faster than AT.

17. Spatial distribution of daytime AP and AP–AT in the historical and climate change scenarios • During 1981-2000, AP is higher than AT along the tropics and the subtropics, but lower in the mid-latitudes. • Historically, AP in the tropics is 2-4 oC higher than AT due to higher relative humidity and lower wind speed in this region. AP-AT is projected to be 3-6 oC in 2081-2100 under RCP8.5. • Study results also show the distinctive spatial patterns of AP and AP-AT in different regions around the world, such as the subtropical deserts, Tibetan Plateau, mid-latitudes and Amazon River basin.

18. Changes in AT, specific humidity, relative humidity and wind speed in 2081-2100 under RCP4.5 relative to 1981-2000. Stippling indicates the change is insignificant. Changes in relative humidity and wind speed will be very small. However, as AT increases, stronger heat stress but weaker cooling effect will lead to larger increases in AP. This can be explained by the relationships among heat index, AT and relative humidity, and among wind-chill equivalent temperature, AT and wind speed, as shown in the next slide.

19. Wind-chill eq. temp = f(AT, WS) Heat index = f(AT, RH)

20. Summary of Major Findings and Conclusions • Overall, we anticipate faster increases in AP than AT under climate warming, which means that human beings will sense a larger increase in temperature than the actual increase in air temperature. • The faster increase in AP is especially significant in the tropics and subtropics under high climate change scenario. During 1981-2000, AP in the tropics is 0-4 oC higher than AT, but is projected to be 3-6 oC higher in 2081-2100 under RCP8.5. The mean annual AP in the tropics is projected to exceed 35 oC which can cause severe health impacts. • The global land average of apparent temperature was 1.5 oC lower than air temperature in 2000, but is projected to be 0.25 oC higher by the end of the 21st century under RCP8.5. • The larger increase in apparent temperature compared to air temperature is due to the composite effects of stronger heat stress and weaker cooling effect caused by increasing air temperature with negligible projected changes in relative humidity and wind speed.

21. THANK YOU 谢谢! 香港中文大学 The Chinese University of Hong Kong