Transcript PowerPoint 演示文稿 - World Conference on Climate Change

World Conference on Climate Change, 25 Oct 2016, Valencia, Spain
Faster increases in apparent than air temperature
under climate warming
Yongqin David Chen
Department of Geography and Resource Management
Institute of Environment, Energy and Sustainability
The Chinese University of Hong Kong
Email: [email protected]
Investigators: J.F. Li (HKBU), Y.D. Chen, T.Y. Gan (UoAlberta), and G.N.C. Lau (CUHK)
Global warming hiatus/pause/slowdown (1998 – 2013)
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
Change in atmospheric
circulation and enhance the
water cycle
Regional differences in
precipitation and
increase in occurrence of
extreme weather and
climate events
Source: IPCC AR5
Future near-surface air temperature changes
Source: IPCC AR5
Is Hong Kong climate ready and resilient in the 21st century?
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.
Source: NWS, NOAA
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.
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.
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
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.
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.
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.
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.
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.
Changes in AT, specific humidity, relative humidity and wind speed in 20812100 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.
Wind-chill eq. temp = f(AT, WS)
Heat index = f(AT, RH)
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.