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Attribution of the United States "warming hole": aerosol indirect effect and precipitable water vapor.

Yu S, Alapaty K, Mathur R, Pleim J, Zhang Y, Nolte C, Eder B, Foley K, Nagashima T - Sci Rep (2014)

Bottom Line: We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor.These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth.Our results reaffirm that LWCF can warm both winter Tmax and Tmin.

View Article: PubMed Central - PubMed

Affiliation: Research Center for Air Pollution and Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, P.R. China.

ABSTRACT
Aerosols can influence the climate indirectly by acting as cloud condensation nuclei and/or ice nuclei, thereby modifying cloud optical properties. In contrast to the widespread global warming, the central and south central United States display a noteworthy overall cooling trend during the 20(th) century, with an especially striking cooling trend in summertime daily maximum temperature (Tmax) (termed the U.S. "warming hole"). Here we used observations of temperature, shortwave cloud forcing (SWCF), longwave cloud forcing (LWCF), aerosol optical depth and precipitable water vapor as well as global coupled climate models to explore the attribution of the "warming hole". We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor. A global coupled climate model reveals that the observed "warming hole" can be produced only when the aerosol fields are simulated with a reasonable degree of accuracy as this is necessary for accurate simulation of SWCF over the region. These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth. Our results reaffirm that LWCF can warm both winter Tmax and Tmin.

No MeSH data available.


Related in: MedlinePlus

The trends in summer monthly mean Tmax, SWCF, AOD and Q for the period 1950–2011 (A, B, C, D, right column) and 1950–1985 (E, F, G, H, left column) on the basis of GHCNM (Tmax), MIROC-ESM-CHEM (AOD at 550 nm, SWCF) and NCEP/NCAR reanalysis (Q) data sets. The units for trends of Tmax, SWCF, AOD and Q are °C/100 yrs, (W/m2)/yr, /yr and cm/yr, respectively. The maps were created by NCAR Command Language (NCL) (http://www.ncl.ucar.edu/).
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f4: The trends in summer monthly mean Tmax, SWCF, AOD and Q for the period 1950–2011 (A, B, C, D, right column) and 1950–1985 (E, F, G, H, left column) on the basis of GHCNM (Tmax), MIROC-ESM-CHEM (AOD at 550 nm, SWCF) and NCEP/NCAR reanalysis (Q) data sets. The units for trends of Tmax, SWCF, AOD and Q are °C/100 yrs, (W/m2)/yr, /yr and cm/yr, respectively. The maps were created by NCAR Command Language (NCL) (http://www.ncl.ucar.edu/).

Mentions: Nationally, SO2 emissions grew from 1950 to about 1980 and then decreased by more than 60% between 1980 and 201030 and there is a linear relationship between decreasing aerosol sulfate concentrations and SO2 emissions30. This is in agreement with the observations that for summer Tmax in EUS, there are almost uniformly negative trends during 1950–1985 (Fig. 4E) in contrast to almost uniformly positive trends during 1985–2011 (Supplementary Fig. S41I) and 2000 to 2011 (Fig. 2A). This is supported by the fact that over the United States cloud cover has increased from 1949 to 2001 in summer and annual means with all of this increase occurring prior to the early 1980s35. The trend analyses for the global coupled models from CMIP5 (Supplementary Note 12) indicate that only MIROC-ESM-CHEM model successfully shows negative trends in summer Tmax (i.e., the U.S. “warming hole”) (Figs. 4A for the observations, and S39A for the models) and SWCF (Fig. 4B) over the central U.S. during 1950–2011. Detailed analysis (Supplementary Note 13) shows that our MIROC-ESM-CHEM model successfully and consistently reproduced the observed summer features for the long-term (i.e., “warming hole” over the central U.S. for the 1901–2011 (Supplementary Fig. S38) and 1950–2011 periods (Supplementary Fig. S39) and negative trends in Tmax over the EUS for the 1950–1985 period with positive trends in AOD and negative trends in SWCF (Supplementary Fig. S40) and for the short-term of 2000–2011 for Tmax (positive trends), AOD (negative trends), SWCF (positive trends) and Q (positive trends in southeast and negative trends in northeast) over the EUS (Supplementary Fig. S43, Supplementary Note 13). MIROC-ESM-CHEM missed the “warming hole” over the south central U.S. during the 1950–2011 period because MIROC-ESM-CHEM did not include the AIE on subgrid convective clouds and this effect is dominantly important over the south central U.S.363738. On contrary, MIROC-ESM did not capture the “warming hole” for the 1950–2011 (Supplementary Fig. S39) and other observed features such as Q for the 2000–2011 period (Supplementary Fig. S43) because of different distributions of AOD and SWCF from the MIROC-ESM simulations (Supplementary Figs. S39G and S39F) relative to those from the MIROC-ESM-CHEM (Supplementary Figs. S39C and S39B). Comparisons of distribution patterns from the MIROC-ESM-CHEM and MIROC-ESM simulations (Supplementary Note 13), especially for AOD and SWCF, indicate that the results of AOD and SWCF from the MIROC-ESM simulations are not in right locations as shown in Supplementary Figs. S39G and S39F relative to those from the MIROC-ESM-CHEM (see Supplementary Figs. S39C and S39B) for the 1950–2011 period. The results from the MIROC-ESM-CHEM showed the positive trends in AOD over the central U.S. while MIROC-ESM-CHEM showed the negative trends in AOD over the central U.S. for the 1950–2011 period. This difference causes the different results for the SWCF and Tmax as shown in Supplementary Fig. S39. Since the only difference between MIROC-ESM-CHEM and MIROC-ESM is that the MIROC-ESM simulations used prescribed monthly mean 3-D chemical fields while the MIROC-ESM-CHEM simulations used chemical fields calculated by the online photochemical module (Supplementary Note 13). Since good chemical fields will affect greenhouse gases such as H2O and aerosol (AOD) fields, the much better performance of MIROC-ESM-CHEM relative to MIROC-ESM suggests the attribution of the “warming hole” to aerosol indirect effect (Supplementary Note 13). The U.S. “warming hole” (i.e. the decrease of summer Tmax) over the central and south central U.S. regions in Fig. 4A is caused by the increase of clouds (Fig. 4B) due to increase of aerosols (Fig. 4C) with offset from the greenhouse effect of Q (increase of Q) (Fig. 4D) during 1950 to 2011 (Supplementary Notes 11, 13). The consistent cooling trends in summer Tmax in EUS during 1950–1985 (Fig. 4E) are because of both increase of clouds (Fig. 4F) due to increase of aerosols (Fig. 4G) and decrease of Q (Fig. 4H) (Supplementary Notes 11, 13).


Attribution of the United States "warming hole": aerosol indirect effect and precipitable water vapor.

Yu S, Alapaty K, Mathur R, Pleim J, Zhang Y, Nolte C, Eder B, Foley K, Nagashima T - Sci Rep (2014)

The trends in summer monthly mean Tmax, SWCF, AOD and Q for the period 1950–2011 (A, B, C, D, right column) and 1950–1985 (E, F, G, H, left column) on the basis of GHCNM (Tmax), MIROC-ESM-CHEM (AOD at 550 nm, SWCF) and NCEP/NCAR reanalysis (Q) data sets. The units for trends of Tmax, SWCF, AOD and Q are °C/100 yrs, (W/m2)/yr, /yr and cm/yr, respectively. The maps were created by NCAR Command Language (NCL) (http://www.ncl.ucar.edu/).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4221782&req=5

f4: The trends in summer monthly mean Tmax, SWCF, AOD and Q for the period 1950–2011 (A, B, C, D, right column) and 1950–1985 (E, F, G, H, left column) on the basis of GHCNM (Tmax), MIROC-ESM-CHEM (AOD at 550 nm, SWCF) and NCEP/NCAR reanalysis (Q) data sets. The units for trends of Tmax, SWCF, AOD and Q are °C/100 yrs, (W/m2)/yr, /yr and cm/yr, respectively. The maps were created by NCAR Command Language (NCL) (http://www.ncl.ucar.edu/).
Mentions: Nationally, SO2 emissions grew from 1950 to about 1980 and then decreased by more than 60% between 1980 and 201030 and there is a linear relationship between decreasing aerosol sulfate concentrations and SO2 emissions30. This is in agreement with the observations that for summer Tmax in EUS, there are almost uniformly negative trends during 1950–1985 (Fig. 4E) in contrast to almost uniformly positive trends during 1985–2011 (Supplementary Fig. S41I) and 2000 to 2011 (Fig. 2A). This is supported by the fact that over the United States cloud cover has increased from 1949 to 2001 in summer and annual means with all of this increase occurring prior to the early 1980s35. The trend analyses for the global coupled models from CMIP5 (Supplementary Note 12) indicate that only MIROC-ESM-CHEM model successfully shows negative trends in summer Tmax (i.e., the U.S. “warming hole”) (Figs. 4A for the observations, and S39A for the models) and SWCF (Fig. 4B) over the central U.S. during 1950–2011. Detailed analysis (Supplementary Note 13) shows that our MIROC-ESM-CHEM model successfully and consistently reproduced the observed summer features for the long-term (i.e., “warming hole” over the central U.S. for the 1901–2011 (Supplementary Fig. S38) and 1950–2011 periods (Supplementary Fig. S39) and negative trends in Tmax over the EUS for the 1950–1985 period with positive trends in AOD and negative trends in SWCF (Supplementary Fig. S40) and for the short-term of 2000–2011 for Tmax (positive trends), AOD (negative trends), SWCF (positive trends) and Q (positive trends in southeast and negative trends in northeast) over the EUS (Supplementary Fig. S43, Supplementary Note 13). MIROC-ESM-CHEM missed the “warming hole” over the south central U.S. during the 1950–2011 period because MIROC-ESM-CHEM did not include the AIE on subgrid convective clouds and this effect is dominantly important over the south central U.S.363738. On contrary, MIROC-ESM did not capture the “warming hole” for the 1950–2011 (Supplementary Fig. S39) and other observed features such as Q for the 2000–2011 period (Supplementary Fig. S43) because of different distributions of AOD and SWCF from the MIROC-ESM simulations (Supplementary Figs. S39G and S39F) relative to those from the MIROC-ESM-CHEM (Supplementary Figs. S39C and S39B). Comparisons of distribution patterns from the MIROC-ESM-CHEM and MIROC-ESM simulations (Supplementary Note 13), especially for AOD and SWCF, indicate that the results of AOD and SWCF from the MIROC-ESM simulations are not in right locations as shown in Supplementary Figs. S39G and S39F relative to those from the MIROC-ESM-CHEM (see Supplementary Figs. S39C and S39B) for the 1950–2011 period. The results from the MIROC-ESM-CHEM showed the positive trends in AOD over the central U.S. while MIROC-ESM-CHEM showed the negative trends in AOD over the central U.S. for the 1950–2011 period. This difference causes the different results for the SWCF and Tmax as shown in Supplementary Fig. S39. Since the only difference between MIROC-ESM-CHEM and MIROC-ESM is that the MIROC-ESM simulations used prescribed monthly mean 3-D chemical fields while the MIROC-ESM-CHEM simulations used chemical fields calculated by the online photochemical module (Supplementary Note 13). Since good chemical fields will affect greenhouse gases such as H2O and aerosol (AOD) fields, the much better performance of MIROC-ESM-CHEM relative to MIROC-ESM suggests the attribution of the “warming hole” to aerosol indirect effect (Supplementary Note 13). The U.S. “warming hole” (i.e. the decrease of summer Tmax) over the central and south central U.S. regions in Fig. 4A is caused by the increase of clouds (Fig. 4B) due to increase of aerosols (Fig. 4C) with offset from the greenhouse effect of Q (increase of Q) (Fig. 4D) during 1950 to 2011 (Supplementary Notes 11, 13). The consistent cooling trends in summer Tmax in EUS during 1950–1985 (Fig. 4E) are because of both increase of clouds (Fig. 4F) due to increase of aerosols (Fig. 4G) and decrease of Q (Fig. 4H) (Supplementary Notes 11, 13).

Bottom Line: We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor.These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth.Our results reaffirm that LWCF can warm both winter Tmax and Tmin.

View Article: PubMed Central - PubMed

Affiliation: Research Center for Air Pollution and Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, P.R. China.

ABSTRACT
Aerosols can influence the climate indirectly by acting as cloud condensation nuclei and/or ice nuclei, thereby modifying cloud optical properties. In contrast to the widespread global warming, the central and south central United States display a noteworthy overall cooling trend during the 20(th) century, with an especially striking cooling trend in summertime daily maximum temperature (Tmax) (termed the U.S. "warming hole"). Here we used observations of temperature, shortwave cloud forcing (SWCF), longwave cloud forcing (LWCF), aerosol optical depth and precipitable water vapor as well as global coupled climate models to explore the attribution of the "warming hole". We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor. A global coupled climate model reveals that the observed "warming hole" can be produced only when the aerosol fields are simulated with a reasonable degree of accuracy as this is necessary for accurate simulation of SWCF over the region. These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth. Our results reaffirm that LWCF can warm both winter Tmax and Tmin.

No MeSH data available.


Related in: MedlinePlus