Abstract
Currently, mass-producible passive daytime radiative cooling materials (including metamaterial films and coatings) with solar reflectivity exceeding 95% have been applied in various fields such as building thermal management and mitigation of the urban heat island effect. Previous studies have indicated that this type of technology can indirectly mitigate global warming by reducing anthropogenic greenhouse gas emissions associated with cooling energy consumption. However, quantitative research is lacking regarding the potential of large-scale radiative cooling technology to directly mitigate global warming by reducing Earth's energy imbalance. To fill this research gap, this study aims to quantify the immediate impact of land-based large-scale radiative cooling technology on planetary albedo. Analysis shows that in most regions, the contribution of increased surface albedo to planetary albedo is significantly weakened due to the atmosphere's shielding effect (reflection and absorption) of solar radiation reflected from the surface. Further quantitative analysis reveals that even large-scale deployment (e.g., covering 5% of the global land area) of daytime radiative cooling technology cannot directly and completely offset the currently observed trend of Earth's energy imbalance.
0 Introduction
Anthropogenic carbon emissions are likely the main cause of global warming and energy imbalance on Earth [1] . Researchers have proposed a variety of climate engineering methods, including carbon dioxide removal [2], the controversial solar geoengineering [3-4], and passive daytime radiative cooling [5], all of which aim to address global warming through direct or indirect means. Carbon dioxide removal addresses the problem at its root, and its initial purposeis to reduce the positive radiative forcing generated by reducing the concentration of greenhouse gases in the atmosphere [6]. Solar geoengineering takes a more direct intervention strategy, and its core mechanism is to enhance the reflectivity of the Earth system (especially the atmosphere) to solar radiation, thereby attempting to directly offset the positive radiative forcing generated by factors such as the accumulation of greenhouse gases [7]. Unlike the first two technologies that aim to directly intervene in the global climate system, the design intention of passive daytime radiative cooling technology is not a macro-level climate engineering technology, but an energy-saving technology for local applications [8-11]. Its core mechanism is based on the high solar reflectivity of the material itself and its high emissivity in the mid-infrared atmospheric window, which directly emits heat into the vast outer space in the form of infrared radiation. Therefore, the fundamental goal of this technology is to utilize this passive physical process to replace or assist traditional refrigeration methods that are high in energy consumption, high in carbon emissions, or high in water consumption in specific scenarios such as building cooling and industrial energy conservation, thereby achieving the purpose of energy conservation and carbon reduction [12-20]. Passive daytime radiative cooling technology has been applied to fields such as enhanced building envelope [21-23], low-energy refrigeration systems [24-27], freshwater generation [28-31], personal thermal management [32-36], and mitigation of the urban heat island effect [37-41].
The above methods differ significantly in their mechanisms for mitigating Earth's energy imbalance: (1) Carbon dioxide removal aims to address global warming at its root by reducing atmospheric greenhouse gas concentrations. (2) Solar geoengineering attempts to directly counteract the positive trend of Earth's energy imbalance by increasing the amount of solar radiation reflected back into space. (3) Passive daytime radiative cooling is a hybrid approach that can indirectly reduce carbon emissions by reducing energy consumption and may also directly increase the solar reflectivity of the Earth system.
Different climate mitigation methods require different infrastructures: (1) Carbon dioxide removal relies on specialized infrastructure capable of extracting carbon dioxide from the atmosphere through chemical or physical processes [42-44]. (2) The controversial technology of solar geoengineering requires injecting reflective aerosols into the stratosphere or deploying reflectors at the top of the atmosphere [45-46]. (3) The deployment of passive daytime radiative cooling technology is relatively simple, and can be achieved by covering roads or roofs with highly reflective cooling coatings or installing radiative cooling panel [47-48].
Clearly, large-scale implementation of passive daytime radiative cooling (RC) technology can be considered a potential land-based solar geoengineering technology with the potential to directly and indirectly mitigate global warming [8, 49]. Numerous studies have demonstrated and quantified its indirect global warming mitigation potential using various methods: such technologies reduce greenhouse gas emissions by decreasing cooling energy consumption [22-23, 38, 50]. In contrast to the aforementioned indirect energy-saving and energy-reducing pathways, land-based radiative cooling technology may also reduce the current energy imbalance in the Earth system by enhancing planetary albedo, thus exhibiting a more direct potential to mitigate global warming [8, 22, 37]. To assess this direct potential, this paper focuses on a specific and crucial entry point: quantifying the immediate impact of large-scale radiative cooling technology applications on the planetary albedo of Earth.
1 Relationship between surface albedo and planetary albedo
There is a fundamental difference between planetary albedo and surface albedo in the Earth system: planetary albedo directly determines the total amount of solar energy entering the top of the Earth system (TOA) , and is a fundamental variable for measuring global energy balance; while the application and effect evaluation of most radiation cooling technologies are mainly focused on the regulation of solar irradiance on the surface of Earth (SOE) . In order to assess and compare the contributions of the surface and atmosphere to planetary albedo, we first conduct an analysis based on the principle framework shown in Figure1. The data used in this study comes from the geospatial historical dataset provided by the Copernicus Climate Change Service [51], which includes the monthly average values of incident and outgoing solar irradiance at the top of the atmosphere and the Earth's surface, with a time resolution at the hourly level.
Fig.1: Schematic diagram of solar irradiance and albedo in the Earth system.Wherein, Psolar in, TOA and Psolar out, TOA represent incident and emitted solar irradiance at the top of the atmosphere; Psolar in, SOE and Psolar out, SOE represent incident and reflected solar irradiance at the Earth's surface; RTOA represents planetary solar albedo; RSOE denotes surface solar albedo; rATM and aATM correspond to atmospheric single solar reflectance and absorptivity.
Based on the method in reference [52], the planetary albedo (RTOA) and the surface albedo (RSOE) can be correlated through the atmospheric single solar reflectance (rATM) and absorptivity (aATM) :
(1)
Meanwhile, the ratio of solar irradiance incident at the Earth's surface to that incident at the top of the atmosphere is defined as:
(2)
It should be clarified that RTOA is determined by the ratio of incident to emitted irradiance at the top of the atmosphere in the database (Psolar in, TOA to Psolar out, TOA) , while RSOA is derived from the ratio of incident to reflected irradiance at the Earth's surface (Psolar in, SOE to Psolar out, SOE) . Based on Equations (1) and (2) , the atmospheric single reflectivity (rATM) and absorptivity (aATM) can be calculated.
At the same time, planetary albedo can be decomposed into the sum of the contributions from the atmosphere and the surface:
(3)
Clearly, atmospheric single reflectance contributes entirely to planetary albedo; in contrast, surface albedo contributes only partially because solar irradiance is attenuated by the atmosphere both on its path to the surface and back into space.
In addition to the aforementioned geospatial solar attributes, we also focus on their global average. Here, we ignore the influence of the Earth's oblateness and its temporal variation, and use the latitudinal cosine as a weighting factor when integrating global geospatial solar irradiance.
2 Baseline Surface and Planetary Albedo
Figure2 shows the geospatial distribution maps of the calculated annual mean planetary albedo, surface albedo, atmospheric single solar reflectance and absorptivity, as well as the contributions of the surface and atmosphere to planetary albedo. These distribution maps will serve as the benchmark for subsequent albedo enhancement analysis. As shown in Figure2 (a) , planetary albedo is significantly higher (40%–70%) in the polar regions and their surrounding areas, some high-altitude dry and cold regions, some desert climate zones, and isolated humid regions. This spatial distribution pattern is dominated by different factors in different regions: in the polar regions, high planetary albedo is due to the combined effect of highly reflective snow-covered surfaces (see Figure2 (b) ) and relatively high atmospheric reflectance (see Figure2 (c) ) ; in dry deserts or high-altitude regions, it is mainly attributed to high surface albedo (see Figure2 (b) ) ; while in isolated humid regions, it is mainly caused by high atmospheric reflectance (see Figure2 (c) ) . Furthermore, the results in Figure2 (a) and Figure2 (b) are completely consistent with the report in reference [52] .
In contrast, planetary albedo is low in most other regions (10%–30%) , as shown in Figure2 (a) .This is mainly attributed to the low surface and atmospheric reflectivity in most parts of the world, especially in almost all oceans and densely populated areas (see Figures 2 (b) and 2 (c) ) . The global distribution of atmospheric solar absorptivity obtained by the solution is shown in Figure2 (d) , which is generally low, in stark contrast to the significant spatial variability of atmospheric albedo in Figure2 (c) . The spatial distribution patterns of atmospheric reflectivity and absorptivity are consistent with global cloud cover observed by satellites [53].
Figures 2 (e) and 2 (f) illustrate the global distribution of the contributions of surface albedo and atmospheric reflectivity to planetary albedo. It can be seen that the surface contribution is significantly low, reaching only 10% in most regions (see Figure2 (e) ) ; while the atmospheric contribution is relatively high, ranging from 15% to 40% (see Figure2 (f) ) . Although surface albedo is high in the polar regions, its contribution remains small because it is largely attenuated by the atmosphere. Furthermore, since the solar irradiance received in the polar regions is much lower than in low-latitude regions, their contribution to the global average planetary albedo is negligible. The comparison of surface and atmospheric contributions suggests that, theoretically, increasing atmospheric reflectivity (e.g., through aerosol injection) may be a more effective approach than increasing surface albedo (e.g., through large-scale deployment of passive daytime radiative cooling materials) .
Fig.2: Global Surface and Planetary Albedo: (a) planetary solar albedo (RTOA) ; (b) Surface solar albedo (RSOE) ; (c) – (d) Derived atmospheric single-pass solar reflectance (rATM) and absorption (aATM) ; (e) – (f) Actual contributions of surface and atmosphere to planetary albedo (RTOA←ATM and RTOA←SOE) .
3 Immediate effects of increased surface albedo
Next, we introduce a small perturbation (e.g., a1% or 0.01 increase) to the surface albedo and atmospheric single-scattering albedo to quantify their respective instantaneous effects on the planetary albedo. This perturbation is applied uniformly across all geographic coordinates. Here we assume that atmospheric single-pass solar absorptivity remains constant in both cases because its global variability is minimal even if atmospheric reflectance changes significantly (see Figures 2 (c) and 2 (d) ) . Furthermore, the specific implementation techniques are beyond the scope of this paper.
As shown in Figure3 (a) , when the surface albedo increases by 1 percentage point, the resulting planetary albedo enhancement is generally low across most regions (less than 0.002, or 0.2percentage points) . In arid and desert climate zones, the contribution is slightly higher, reaching approximately 0.005 (0.5 percentage points) . However, as shown in Figure3 (b) , an equivalent increase (1 percentage point) in atmospheric reflectance proves more effective, with its contribution over oceanic regions exceeding 0.009 (0.9 percentage points) .
Figure3 (c) compares the enhancing effects of increases in surface albedo and atmospheric reflectivity on the global average planetary albedo. It can be seen that for every 1 percentage point increase in surface albedo, the average planetary albedo increases by approximately 0.003 (0.3 percentage points) ; while an increase of the same magnitude in atmospheric reflectivity produces an average increase of over 0.008 (0.8 percentage points) , approximately 2.7 times that of the former. This result clearly shows that the effect of increasing surface albedo is far less than that of increasing atmospheric reflectivity.
Fig.3: The impact of increased surface solar albedo or atmospheric single solar reflectance on planetary albedo.
4 Immediate impact of large-scale deployment of land-based radiative cooling technology
Furthermore, we assess the immediate impact on planetary albedo by assuming that a certain percentage (e.g., 5%) of the land surface is covered with radiative cooling (RC) technology, which has a high solar reflectivity of 95%. Let fRC denot RC area coverage, then the albedoincrement applied to each geographic grid cell can be expressed as:
(4)
Wherein
(5)
Here, H1 (θ, ϕ) is a binary masking function, taking a value of 1 in land areas and 0 in non-land areas. Figure4 (a) shows the geographical distribution of the instantaneous increase in planetary albedo after 5% of the global land area is covered with high solar reflectivity radiative cooling technology. The effect is uneven: in areas with low baseline surface albedo, the increase in surface albedo is greater, and correspondingly, the increase in planetary albedo can reach up to 0.025 (i.e., 2.5 percentage points) ; this effect is also more significant in areas with less cloud cover. Therefore, strategic geographical deployment is key to maximizing the mitigation potential of radiative cooling technology (i.e., mitigating Earth's energy imbalance by increasing planetary albedo) .
Figure4 (b) further reveals the relationship between the global mean albedo enhancement (ΔRTOA, mean) and the total land coverage of radiative cooling technologies (fRC) . The results indicate that for every 1% increase in RC coverage, the global average planetary albedo increases only marginally by 0.0062 (0.062 percentage points) in a nearly linear manner. This corresponds to an instantaneous radiative forcing (RF=ΔRTOA, mean∙ PTOA in, mean) of –0.21 W·m-2 for every 1% of total land area covered . A 5% RC coverage corresponds to an instantaneous radiative forcing of approximately –1.05 W·m-2. This value is slightly higher than the global average energy imbalance observed in 2025 (approximately 0.75 W·m-2) , but significantly lower than the energy imbalance observed in 2023 (approximately 1.8 W·m-2) .
Fig.4: Effect of Radiative Cooling Technology Land Cover on Planetary Albedo Enhancement.
5 Conclusion
In summary, due to the shielding effect of the atmosphere on the reflection of solar radiation from the Earth's surface (reflection and absorption) , the actual contribution of surface albedo to planetary albedo is significantly weakened. Specifically, the contribution of surface albedo to planetary albedo is generally low (below 10% in most areas) , far lower than the contribution of atmospheric reflectivity (15%–40%) .
An increase of 1 percentage point in surface albedo only increases the global average planetary albedo by about 0.003 (0.3 percentage points) ; this effect is significantly less than the gain of more than 0.008 (0.8 percentage points) brought about by an equivalent increase in atmospheric reflectivity.
An evaluation of a large-scale theoretical deployment scheme for radiative cooling technology revealed that covering5% of the global land area with a high reflectivity of 95% would generate a global average negative radiative forcing of approximately-1.05 W·m-2. While this figure is considerable, its direct offsetting capacity is limited compared to the severe energy imbalances currently observed on Earth (such as approximately 1.8 W·m-2 in 2023) [54].