Determination of surface albedo and snow/ice content

Determination of surface albedo and snow/ice contentvariation using the MODIS data in the past two decades(2001–2020)

D SINGH

Centre of Studies in Resources Engineering, Indian Institute of Technology Bombay, Mumbai 400 076, India.e-mail: [email protected]

MS received 6 November 2020; revised 21 January 2021; accepted 27 January 2021

Snow/ice cover is an integral part of the Earth’s climate system. Earth’s snow/ice cover helps regulate theenergy exchange between the Earth’s surface and the atmosphere, directly regulating the surface (andnear-surface) temperatures. This study utilizes measurements from the Moderate Resolution ImagingSpectroradiometer (MODIS) MCD43C3 dataset to derive the albedo and snow cover variation over thepast two decades (2001–2020). The snow cover and the albedo vary with local seasonal variation. Theglobal and the southern hemisphere albedo drops by about 17.4% and 26.9%, respectively, during2017–2020. However, the northern hemisphere albedo drops by about 10.3% from 2017 to 2019 and thenincreases by about 14.2% in 2020. For non-glaciated regions, the albedo drops from 2017 to 2019 ismaximum during JJA (23.5%), followed by SON (19.2%), DJF (12.7%), and MAM (7.1%) monthsaverages. The non-glaciated albedo in the southern hemisphere falls (2016 onwards) by about 40.6%,47.5%, 55.8%, 30.8% for DJF, MAM, JJA, and SON month averages, respectively. Since the snow covervariation is minimal in the past two decades, an increase in the Northern Hemisphere’s albedo in 2020indicates a relatively fresher snow presence on the surface compared to previous years.

Keywords. Earth; albedo; snow; ice; MODIS; climate change.

1. Introduction

Earth receives and releases energy primarily by theradiative transfer processes. Therefore, the totalEarth’s top of atmosphere (TOA) solar energybudget depends on the absorbed/reCected solarradiation and emitted Earth’s radiation. The EarthreCects (and scatters) back about 29% of theincoming solar radiation (Stephens et al. 2015),also known as the Earth’s net albedo. Almost 70%of Earth’s surface is covered with open waterbodies such as ocean and lakes. Open-ocean has analbedo of about 0.07, which is almost an order

magnitude lower than snow/ice-covered areasalbedo (*0.72) (Singh et al. 2015). Only 12% of theEarth’s surface is permanently covered in snow/iceand can go up to 33% coverage depending on theseason. Therefore, the snow/ice albedo contributessignificantly to the global albedo, although the areacovered is much smaller. Snow and ice are presentin high abundance on Earth’s surface and signifi-cantly impact the Earth’s climate due to their highalbedo (reCectivity) and substantial seasonal vari-ations. Therefore, snow/ice albedo significantlyaAects the planet’s radiative energy budget andglobal temperatures (North et al. 1981; Flanner

J. Earth Syst. Sci. (2021) 130:80 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-021-01592-4 (0123456789().,-volV)(0123456789().,-volV)

et al. 2007, 2009, 2011; Hudson 2011; Perket et al.2014; Singh et al. 2015; Singh and Flanner 2016;Schneider et al. 2018).The snow with higher impurity content is darker

than fresh snow and absorbs higher solar insolation,leading to higher snowmelting.When the snowmelts,it reveals relatively darker underneath surfaces. Thesedarker surfaces absorb more solar radiation andenhance the melting eAect, leading to a temperaturerise in the atmosphere. This phenomenon is known aspositive albedo feedback.The albedo feedback is one ofthe most potent positive feedback mechanisms (onlywater-vapour and cloud feedback are stronger thanalbedo feedback) operating within the Earth’s climatesystem (Bony et al. 2006; Winton 2006; Randall et al.2007; Shell et al. 2008; Soden et al. 2008; Flato et al.2013; Singh et al. 2015). Aerosols are among the mostcritical inCuencers of the snow albedo (and, therefore,TOA energy budget) in Earth’s climate system(WarrenandWiscombe1980;Aoki et al. 2006;He et al.2017; Lee et al. 2017; Ward et al. 2018). Lightabsorbing aerosols, such as black carbon, eDcientlyabsorb solar radiation and significantly impact thelocal energy budget (Koch et al. 2009;Bond et al. 2013;Ward et al. 2018). Therefore, anthropogenic and otherexternal forcings can determine the sensitivity of theclimate to the albedo changes.Thiswork focussesonanalysingandunderstanding

thevariationof snow/ice cover and the surface albedo(snow-covered) on the Earth’s surface in the past twodecades (2001–2020). Here, I analyse the global,seasonal, annual, and hemispheric (northern andsouthern) changes of the surface albedo and thesnow/ice cover. In this work, I also include the vari-ations and impact on the surface albedo due to gla-ciated and non-glaciated regions. Glaciated regionsinclude the areas on Earth’s surface that are perma-nently covered (e.g., Greenland andAntarctica) withice throughout the year (or for multiple years). Non-glaciated regions include areas that receive seasonalsnow, which disappears during the local summerseason. I also perform the trend analysis to observethe significance of increasing or decreasing snowcover on the surface albedo. The surface albedomentioned hereafter refers to the albedo of snow/ice-covered regions only, unless stated otherwise.

2. Data and methodology

This work utilizes the Moderate Resolution ImagingSpectroradiometer (MODIS) MCD43C3 Version 6(v006) (Schaaf and Wang 2015) Bidirectional

ReCectance Distribution Function and Albedo(BRDF/Albedo) dataset for the analysis. TheMCD43C3 data are produced daily using 16 days ofTerra and AquaMODIS data in a 0.05�90.05� spatialresolution Climate Modelling Grid (CMG) (Schaafet al. 2002; Gao et al. 2005; Schaaf and Wang 2015).Observations are temporally weighted to the ninthday of the 16-day retrieval period and cover the entireglobe. TheMCD43C3 provides both black-sky albedo(directional hemispherical reCectance) and white-skyalbedo (bi-hemispherical reCectance) at seven differ-ent spectral bands as well as the visible, near-infrared(NIR), and shortwave bands. Among other ancillarylayers, the MODIS data also provides snow cover (%)and quality Cags.Black-sky albedo is deBned as albedo in the

absence of a diffuse component and is a function ofsolar zenith angle. White-sky albedo is deBned asalbedo in the absence of a direct component whenthe diffuse component is isotropic. Black-skyalbedo and white-sky albedo mark the extremecases of completely direct and completely diffuseillumination. Actual albedo is a value interpolatedbetween these two as a function of the fraction ofdiffuse skylight, which is a function of the aerosoloptical depth (Lewis and Barnsley 1994; Luchtet al. 2000). This study utilizes only the white-skyalbedo of the visible, NIR, and shortwave bandswith quality Cag 2 or better. The observation withquality Cag 2 or better contains relatively gooddata with 75% or more data with full inversionsrather than Bll values. This quality Cag ensures theusage of only high-quality data for the study.However, using such a high-quality Cag signifi-cantly reduces the albedo measurements’ spatialand temporal coverage. The International Geo-sphere–Biosphere Programme (IGBP) deBnes var-ious land classes used in the MODIS MCD12C1(v006) product (Friedl and Sulla-Menashe 2015).For this study, the region classiBed as snow or icein the MODIS MCD12C1 land type dataset isconsidered as permanently glaciated (Singh et al.2015). A separate land classiBcation Ble(2001–2020) for each year is used to identify andBlter each year’s surface albedo.First, I Blter out the albedo for the visible, NIR,

and shortwave band and the snow cover data withquality Cags mentioned earlier. After that, I anal-yse the snow cover and the surface albedo variationduring the 2001–2020 time period. Lastly, I per-form the trend analysis using the Mann–Kendallregression technique with a 95% significance levelto understand the significance of variations for the

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surface albedo and the snow cover (Hamed andRamachandra Rao 1998; Yue et al. 2002; Hamed2008).

3. Results and discussion

3.1 Global and seasonal variability

The global and seasonal variability of the albedoprimarily depends on the change in the snow cover.The incident solar energy and the local atmo-sphere’s physical state, such as temperature,humidity, and pressure, direct the amount andduration of snowfall in a particular region. Figure 1shows the tri-monthly seasonal averages of snowcover for the entire data (2001–2020). During thenorthern hemisphere winter (DJF – December,January, February), the snow cover is maximum.The seasonal snow cover gradually decreases as wemove towards the northern hemisphere summer(JJA – June, July, August). A similar seasonaleAect can also be seen for the southern hemisphere.However, the eAect is not prominent due to theabsence of a significant amount of land atmid-latitudes in the southern hemisphere. Thetri-monthly seasonal averages of the surface albedo(Bgure 2) also follow the same seasonal trend as thesnow cover. The mean global snow surface albedois about 0.62, which is about double the net

(combined all land masses and water bodies) globalalbedo. Some data is missing in the polar regionsdue to sunlight’s absence during the local winter.Figure 3 shows the monthly-averaged snow

albedo for the visible, NIR, and shortwave bands.During the northern hemisphere winter, the visiblealbedo is significantly higher (*0.2) than the NIRalbedo because of fresh snow deposition over landareas. The NIR albedo is also significantly lowerthan the visible albedo for fresh H2O snow (Singhand Flanner 2016; Warren and Brandt 2008;Wiscombe and Warren 1980). Therefore, the netshortwave albedo variation is dominated by visiblealbedo variations. The seasonal impact is also evi-dent in the monthly albedo changes and follows theseasonal changes in the northern hemisphere. Thesurface albedo is highest during the northernhemisphere winter and gradually decreases as theseason approaches local summer.The southern hemisphere’s contribution towards

the net surface shortwave albedo during May,June, and July (MJJ) months is almost negligiblecompared to the northern hemisphere contribution(Bgure 4). However, the same contribution is sig-nificantly higher during other months (Bgure 4).This seems contradictory as we expect moresnowfall during local hemispheric winter. Also, thepermanently glaciated polar regions are highlyreCective. However, during the local winter, polar

Figure 1. Tri-monthly seasonally averaged snow cover (DJF: December, January, February; MAM: March, April, May;JJA: June, July, August; SON: September, October, November). White areas indicate either the absence of snow or no-data wasavailable.

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regions are deprived of solar illumination andunable to contribute towards the eAective surfacealbedo. For the northern hemisphere, the contri-bution variation is not as extreme as the southernhemisphere because the northern hemisphere has asignificant amount of land-mass in mid- and high-latitude regions compared to the southern hemi-sphere. The seasonal snow cover over the northern

hemisphere mid- and high-latitude land-massregion contributes significantly towards the netsurface albedo.The snow deposition cycle does not vary much

(Bgure 5) because Earth’s motion around the Sunand the average solar illumination remain almostthe same from one year to another. However, weobserve small Cuctuations in the surface albedo

Figure 2. Tri-monthly seasonally averaged shortwave snow albedo (DJF: December, January, February; MAM: March, April,May; JJA: June, July, August; SON: September, October, November). White areas indicate either the absence of snow orno-data was available.

Figure 3. Monthly (globally and annually averaged) variation of the snow albedo for the visible (VIS), NIR, and shortwave band.


over the years. From 2017 to 2019, there is a slightdrop (*2–6%) in the surface albedo for each tri-mester average with a maximum (*22.5%) beingin JJA (June, July, and August). In 2020, thistrend changes, and we observe an increase in thesurface albedo with a maximum (*5.4%) againbeing in JJA (June, July, and August). Since thereis no significant change in the snow cover over theyears, the albedo increase indicates that the snow

has less impurity than recent previous years. Asthe data for September, October, and November(SON) months (for 2020) is not available at thetime of this study, one can expect that the SONmonths’ albedo and snow cover will follow a similartrend to that of MAM months.In Bgure 6, we do not observe any significant

variation in the northern hemisphere, southernhemisphere, and global albedos until 2017. From

Figure 4. Contribution of the northern and the southern hemisphere snow cover on the monthly (globally and annuallyaveraged) shortwave surface snow albedo.

Figure 5. Annual variation of the net shortwave surface albedo (top) and the snow cover (bottom). Each plot includes thetri-monthly global average for the respective variables.


2017 to 2020, the global and the southern hemi-sphere albedo drops by about 17.4% and 26.9%,respectively. However, the northern hemispherealbedo drops by about 10.3% from 2017 to 2019and then increases by about 14.2% in 2020.Figure 6 indicates that the southern hemispherestrongly inCuences the global albedo and con-tributes to about 74% to the global albedo.Therefore, an increase in the northern hemispherealbedo in 2020 has only a minor eAect on the globalalbedo. The drop in albedo in the northern andsouthern hemispheres is directly related to themelting of the Arctic ice (Pistone et al. 2014; Caoet al. 2015) and the Antarctic ice (Schneider et al.2018).

3.2 Glaciated and non-glaciated regions

On a global scale, both glaciated and non-glaciatedregions contribute almost equally towards the netshortwave surface albedo (Bgure 7). During thenorthern hemisphere winter, glaciated regionsdominate the surface albedo more because of theAntarctic region’s substantial albedo contribution.In the northern hemisphere, the glaciated regionhas less than a 5% impact, and the net albedoalmost follows the non-glaciated region’s surfacealbedo. On the contrary, glaciated regions com-pletely dominate the net surface albedo for the

southern hemisphere except during MJJ months.During MJJ months, the Antarctic ice sheet’simpact is almost negligible due to the non-avail-ability of solar illumination during the local winter.Figure 8 shows the surface albedo’s annual

variability and the snow cover for different landclasses and global averages. The non-glaciatedregion snow cover remains almost constantthroughout the two decades in this study. How-ever, the albedo drops significantly (*10%) from2017 to 2019 and rises again a bit (*4.3%) in 2020(similar to JJA months in Bgure 5). The snow coverremains relatively constant for glaciated regionsuntil 2019 and then drops slightly (*3.7%) for2020. As a result, the global snow cover dropssignificantly by about 15%. The snow cover dropcauses a simultaneous drop of about 11% in theglobal albedo in 2020. This indicates that glaciatedregions dominate the net surface snow albedo. Thedata for September onwards (for 2020) was notavailable at the time of this study, which couldprobably cause a drop in the total snow cover andthe albedo estimation in 2020.Tri-monthly averages of global, glaciated, and

non-glaciated areas provide detailed insight intothe seasonal albedo variation (Bgure 9). For non-glaciated regions, the albedo also drops between2017 and 2019 for all seasons. The JJA albedo dropagain remains maximum (23.5%), followed by SON

Figure 6. Annual variation of the surface albedo for northern hemisphere and southern hemisphere regions, and global scale.


Figure 7. Monthly (globally and annually averaged) variation of the snow albedo for global (top), northern hemisphere (NH,middle), and southern hemisphere (SH, bottom) regions. Each plot also includes the comparison of the surface albedo forglaciated and non-glaciated regions.

Figure 8. Annual variability of the surface albedo and snow cover for glaciated, non-glaciated, and global regions.


(19.2%), DJF (12.7%), and MAM (7.1%) monthsaverages. During MAM and JJA, the global albedoalmost replicates the non-glaciated albedo. Theglaciated albedo also drops during MAM (althoughvery small, has a significant impact on globalalbedo) and JJA months, causing a significant dropin the global albedo. For DJF and SON months,the glaciated albedo remains relatively constantfrom one year to another. Therefore, we observe amixed impact of glaciated and non-glaciated albe-dos on the global albedo. The surface snow albedois primarily dependent on two factors: (1) Varia-tion in the amount of surface snow/ice depositionand (2) variation in the impurity content withinthe snow. We again observe a rise in surface albedofor 2020, but no significant change is observed inthe surface snow cover. This indicates the presenceof fresher snow (both glaciated and non-glaciated)on the surface. We also expect a similar trend forSON months.Figure 10 shows a similar picture of albedo as

Bgure 9, but for northern hemisphere only. Theannual albedo variation follows a similar trend(and similar values) during MAM and JJA months.However, for DJF and SON months, the glaciated

regions’ albedo falls significantly, causing a signif-icant drop in the net northern hemisphere albedovalues. During the northern hemisphere winter,glaciated regions do not significantly contribute tothe net albedo due to the absence of sunlight inhigh latitude areas.For the southern hemisphere, the albedo varies

quite interestingly, especially from 2016 onwards(Bgure 11). The non-glaciated albedo is very sen-sitive (due to small land availability) and falls byabout 40.6%, 47.5%, 55.8%, 30.8% for DJF, MAM,JJA, and SON month averages, respectively. Thenet albedo for MAM and JJA months also fallssignificantly due to a more non-glaciated region’scontribution. However, the glaciated regions com-pletely dominate the net albedo during DJF andSON months (due to the Antarctic ice sheet)and have a negligible contribution from thenon-glaciated regions.

3.3 Trend analysis

The trend analysis of data allows us to predictfuture variation in a variable using past informa-tion. In this study, I utilize the Mann–Kendall

Figure 9. Tri-monthly averages for glaciated, non-glaciated, and global regions.


Figure 10. Same as Bgure 9, but for northern hemisphere only.

Figure 11. Same as Bgure 10, but for southern hemisphere only.


regression technique (Hamed and RamachandraRao 1998; Yue et al. 2002; Hamed 2008; Singh et al.2015) with a 95% significance level to perform thetrend analysis on the albedo and the snow coverdata. The albedo data do not show any significantinter-annual trends during the study duration.Only some speciBc scenarios of snow cover showsome significant trends for the study period in thisstudy. Table 1 lists the slope of significant trends inthe respective scenarios for the snow cover data.Negative slope values indicate a decrease in snowcover, and positive slope values indicate increasedsnow cover.During the JJA months, the trends are most

noteworthy and are consistent with the results dis-cussed in the previous section. Global and northernhemisphere trends are all negative, which indicatesthat the snow cover has slightly depreciated globallyand in the northern hemisphere over the years. Thesnow cover drop (especially in the northern hemi-sphere) indicateswarmer global temperatures causeddue topositive snow feedback.However, the southernhemisphere scenarios are opposite and observe apositive snow cover trend, indicating a slight increasein the snow cover. The Antarctic sheet’s strongfeedback primarily causes a positive trend in thesouthern hemisphere. All the significant trends in thesouthern hemisphere are observed either during thelocal spring (SON) or during the local autumn(MAM). This suggests that the snow starts accumu-lating slightly earlier than the localwinter and stays abit longer after the local winter passage.

4. Conclusions

The seasonal snow cover follows the solar insola-tion cycle primarily guided by the Earth’s motionaround the Sun. Therefore, the surface snow albedofollows a similar trend to the snow cover. This

study surveys the snow cover and the snow albedovariation on Earth’s surface over the past twodecades (2001–2020). On average, the visiblealbedo is higher by about 28% (ranges between *3and 40% depending upon the season) than the NIRalbedo. The visible albedo of fresh snow is muchhigher than the settled snow, whereas snow con-solidation has minimal impact on the NIR albedo.Due to a lack of sunlight during local winter, thesouthern hemisphere contribution to the globalalbedo is almost negligible compared to thenorthern hemisphere. The southern hemispherealso lacks a significant amount of land masses inmid- and high-latitude regions. Therefore, theimpact of temporal snow on the net albedo is quitesignificant in the northern hemisphere. TheAntarctic region completely dominates the south-ern hemisphere albedo.From 2017 to 2019, a significant drop has been

observed in the global, the northern hemisphere,and the southern hemisphere surface albedos. In2020, this drop continues for the global and thesouthern hemisphere albedos, but the albedoincreases for the northern hemisphere. The drop inalbedo is most significant during JJA months,while the least significant during DJF months. Thedrop in albedo is directly related to the melting ofthe Arctic ice and the Antarctic ice. However, anincrease in the albedo indicates a relatively freshersnow presence on the surface compared to previousyears.Glaciated regions completely dominate the net

surface albedo for the southern hemisphereexcept during MJJ months, while non-glaciatedregions fully control the net albedo in thenorthern hemisphere throughout the year. Bothglaciated and non-glaciated regions show a dropin the albedo values from 2017 to 2019. However,the non-glaciated regions show a slight increaseof about 4.3% in the albedo, although there is no

Table 1. Slope values for significant scenarios of snow cover data.

Region Land class Months group Slope

Global All MAM �0.098

Global All JJA �0.140

Global Non-glaciated JJA �0.104

Northern hemisphere Non-glaciated DJF �0.121

Northern hemisphere Non-glaciated MAM �0.145

Northern hemisphere Non-glaciated JJA �0.132

Southern hemisphere Non-glaciated MAM 0.157

Southern hemisphere Non-glaciated SON 0.164

Southern hemisphere Glaciated MAM 0.102


significant snow cover change indicating thepresence of fresher snow. The albedo drop isagain maximum during JJA months, followed bySON, DJF, and MAM months. For the southernhemisphere, the non-glaciated albedo is verysensitive (due to small land availability) and fallsconsiderably for all seasons.The data shows some significant trends for some

speciBc scenarios of snow cover only. All trends arenegative during JJA months, which indicates adrop in the snow cover. Some southern hemispherescenarios show a positive trend, indicating anincrease in the snow cover. A better understandingof the southern hemisphere would be possible whenthe remaining data (for 2020) will be available inthe future. A significant change in the albedo val-ues is observed towards the end of the last decade(2016 onwards). In future, a better estimate of thealbedo trend would be possible with more dataavailability.

Acknowledgement

This work was partially supported by the DST-INSPIRE Faculty Award.

Author statement

DS performed conceptualization, methodology,analysis, writing-reviewing and editing of themanuscript.

Data availability

The complete MODIS MCD43C3 data can bedirectly accessed by visiting the following link:https://doi.org/10.5067/MODIS/MCD43C3.006.The MCD12C1 dataset can be accessed viahttps://doi.org/10.5067/MODIS/MCD12C1.006.The plots and maps generated in this study can beobtained by directly contacting the correspondingauthor.

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Corresponding editor: C GNANASEELAN


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Determination of surface albedo and snow/ice content