Contribution of solar radiation to decadal temperature variability over land
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Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved August 2, 2013 (received for review June 18, 2013)

Significance
Global air temperature has become the primary metric for judging global climate change. The variability of global temperature on a decadal timescale is still poorly understood. This paper shows that surface incident solar radiation (Rs) over land globally peaked in the 1930s, substantially decreased from the 1940s to the 1970s, and changed little after that. The cooling effect of this reduction of Rs accounts in part for the near-constant temperature from the 1930s into the 1970s. Since then, neither the rapid increase in temperature from the 1970s through the 1990s nor the slowdown of warming in the early twenty-first century appear to be significantly related to changes of Rs.
Abstract
Global air temperature has become the primary metric for judging global climate change. The variability of global temperature on a decadal timescale is still poorly understood. This paper examines further one suggested hypothesis, that variations in solar radiation reaching the surface (Rs) have caused much of the observed decadal temperature variability. Because Rs only heats air during the day, its variability is plausibly related to the variability of diurnal temperature range (daily maximum temperature minus its minimum). We show that the variability of diurnal temperature range is consistent with the variability of Rs at timescales from monthly to decadal. This paper uses long comprehensive datasets for diurnal temperature range to establish what has been the contribution of Rs to decadal temperature variability. It shows that Rs over land globally peaked in the 1930s, substantially decreased from the 1940s to the 1970s, and changed little after that. Reduction of Rs caused a reduction of more than 0.2 °C in mean temperature during May to October from the 1940s through the 1970s, and a reduction of nearly 0.2 °C in mean air temperature during November to April from the 1960s through the 1970s. This cooling accounts in part for the near-constant temperature from the 1930s into the 1970s. Since then, neither the rapid increase in temperature from the 1970s through the 1990s nor the slowdown of warming in the early twenty-first century appear to be significantly related to changes of Rs.
Global temperature has become the primary metric for judging global climate change, although many other factors are recognized to be of comparable importance. The overall increase of global temperature over the last century has been largely attributed to the increase of greenhouse gases (1). Less well understood are the reasons for the variability of this increase on a decadal timescale. In particular, warming from 1900 to 1940 was followed by three decades of flat or slightly decreasing temperature, then three decades of very rapid temperature increase, then so far in this century, very little additional increase. The two most plausible explanations for the decadal variability are natural climate variability and variable degrees of cooling from anthropogenic releases of sulfur gas producing sulfate aerosols (2). This effect has long been proposed as a mechanism to counter greenhouse warming (3), has become the basis for many geoengineering proposals (4), and has been used to attribute the lack of warming so far this century to the rapid growth of aerosols in Asia (5).
Besides the difference in sign of their temperature effects, sulfate aerosols are distinguished from greenhouse gases in that they only affect daytime radiation, i.e., surface incident solar radiation (Rs). Some kinds of natural variability can also act through affecting Rs, i.e., those involving cloud properties.
Changes of aerosol loading and cloud properties likely caused the rapid decrease of Rs, measured at the surface from the 1950s to the 1980s, referred to as “global dimming,” and its partial recovery after that (6). The plausible suggestion was made by Wild et al. (7) that the rapid warming in the late twentieth century was a consequence of the cessation of global dimming, possibly in part from the imposition of controls on sulfur emission in the industrialized nations (8, 9).
This paper examines further the hypothesis that variations in Rs have caused much of the observed decadal variability in the rate of warming. Direct measurements of Rs cannot be quantitatively related to such variability because they have been limited in their geographical coverage. The approach used here is to examine a global land dataset of diurnal temperature range (DTR). This concept is not new, indeed, Wild et al. (7) noted (compare with their figure 2) that the global pattern of DTR was similar to that of their global dimming and brightening. The present paper develops the longest and most comprehensive dataset for DTR possible, and, with some plausible assumptions, establishes what the contribution of Rs has been to decadal temperature variability. It indicates that a decrease of Rs from the 1940s through the 1970s reduced the global temperature trend over that period. However, global temperature does not appear to have been significantly affected by changing Rs after that. The method is limited in that it is only applicable over land. As the effects of aerosols are likely to be less over ocean, especially in the Southern Hemisphere, this approach may exaggerate the actual effect of aerosols on global temperature trends.
Results
Relationship Between Rs and DTR.
This section establishes that locally DTR is highly correlated with Rs, but that spatial and seasonal variability precludes direct use of this correlation to infer Rs where it is not already measured. In the absence of weather variability, near-surface air temperature Ta over land decreases with time at night from longwave radiative cooling and reaches Tmin before sunrise. After sunrise, the surface is heated by Rs and this heat is transferred as sensible heat H to the overlying air, raising Ta to Tmax in early afternoon. Therefore, changes of DTR = Tmax − Tmin, have been interpreted as directly related to changes of Rs (6, 7, 10⇓⇓–13). Here we explain how Rs and DTR connect physically and how their relationship varies with environment.
Fig. 1 shows the correlation of monthly anomalies of Rs collected by the Global Energy Balance Archive (GEBA) (14) with DTR from 1950 to 2005 at 524 globally distributed stations (see SI Text and Fig. S1 for DTR data sources and their quality control). The correlation coefficients between Rs and DTR are the highest in humid areas and lower in arid or semiarid areas because the fraction of absorbed Rs generating H also depends on variable soil moisture resulting from the frequency and intensity of precipitation (15, 16). Besides its dependence on surface wetness (17), the partitioning of surface-absorbed Rs between H and latent heat flux (λE) depends on land-cover conditions (18, 19) and atmospheric evaporative demand (20). In humid areas, both H and λE generally increase with Rs (21, 22), but under warm conditions the latter increases more (23). In arid or semiarid regions, λE is limited by soil water supply and H can account for a higher portion of surface absorbed Rs. Fig. 2 shows, as expected from the above discussion, that the sensitivity of DTR to Rs is higher in arid or semiarid areas than in humid areas.
Correlation coefficients between monthly anomalies of DTR and Rs. The Rs observations are from the GEBA, and the homogenized maximum and minimum air temperature at 2 m are from the Global Historical Climatology Network (GHCN). Both datasets cover the period from 1950 to 2005. Each point in the figure represents a weather station where both Rs and DTR are available for more than 120 mo. There are 524 stations in total.
The sensitivity of DTR to Rs (in °C per Wm−2) calculated from the monthly anomalies of Rs and DTR. The data used here are the same as in Fig. 1.
Surface aridity changes seasonally for most monsoon areas, i.e., where it is wet only in a rainy season, but its interannual variability is expected to be much less than such seasonal changes. To reduce the impact of seasonality, we used monthly and annual anomalies rather than absolute values of DTR and Rs. In the following discussion, we also divide a year into boreal warm seasons (May to October) and boreal cold seasons (November to April).
The correlations and sensitivity shown in Figs. 1 and 2 are the lowest in coastal areas. Evidently the impact of Rs on DTR in these areas is masked by the impact of energy advection with regular alteration between land breezes and ocean breezes. This masking can be substantially reduced by regional averaging of DTR and Rs (6, 24).
Figs. 3 and 4 compare Europe’s and China’s regional average annual anomalies of DTR with those of Rs. These quantities agree quite well, partly because of their better data density and data continuity (Fig. S3). The agreement between regional DTR and Rs over Europe has also been confirmed by both data analysis (10) and model simulation (24). In China, the decrease of Rs is in good agreement with the reduction of DTR before 1990 (11). However, Rs in China increased suddenly during the early 1990s but not DTR and sunshine duration (25). The introduction of new pyranometers from 1990 to 1993 introduced this inhomogeneity into the Rs observations (25, 26).
Scatterplots of annual anomalies of regional DTR as a function of annual anomaly of Rs during warm seasons (May to October) and cold seasons (November to April) from 1950 to 2005. The correlation coefficients are 0.61 and 0.83 over China and 0.86 and 0.73 over Europe during the warm and cold seasons, respectively.
Fig. 4 also shows that DTR has had a larger temporal variability than Rs, a consequence of the annual variability of precipitation leading to variations in the partitioning of surface absorbed Rs between λE and H. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) concluded that precipitation has had large annual variability during the last century, but that its long-term trend and thus its impact on the long-term trend of DTR has been negligible (1), as confirmed by Figs. 3 and 4 and the following sections. The impact of annual variability of precipitation is largely removed by using 5-y smoothing of the anomalies of DTR as in the following.
Variability of DTR, a Proxy of Rs, from 1900 to 2010.
This section establishes what is available as a global record for DTR variability. For estimation of DTR over land with optimum spatial and temporal coverage and the highest quality, we combined three data sources (27⇓–29) (see SI Text for detailed information) for the past 110 y. Monthly anomalies of DTR were derived by removing its seasonal cycle. Observations of DTR had the highest density in North America. To mitigate the impact of the different data densities, monthly anomalies of DTR were binned into 5° × 5° grids. Given the low correlation between DTR and Rs in coastal areas, we only selected the grids with more than 50% of their area over land, as shown in Fig. S4. The monthly anomalies at each grid were averaged into regional monthly anomalies, and then into annual values and 5-y average annual anomalies at each region, as plotted in Fig. 5.
Five-year average of annual anomaly (black) of regional DTR from 1900 to 2010 averaged from the monthly anomalies at 5° × 5° grids (Fig. S4), which is calculated from weather stations. For comparison, anomalies during boreal warm seasons (May to October, red) and boreal cold seasons (November to April) are shown.
Europe is the only region where measurements of both DTR and Rs extend back to the 1920s (6). DTR generally increased in Europe from the 1920s to the 1950s. After the late 1950s, it began to decrease until the 1980s, and since the 1990s increased. These variations of DTR are consistent with those of observed Rs (6, 24, 30). The better agreement of warm-season variability of DTR with that of Rs is consistent with the larger Rs during warm seasons.
Attempts have been made to correlate annual Rs and DTR both at regional and global scales (6, 7). However, existing studies have not recognized that DTR and Rs have different seasonal cycles; Rs is largest in summertime as a result of higher solar elevation. However, DTR is relatively low in moist summers because of the small fraction of Rs that is partitioned into H. Therefore, annual variability of DTR is primarily determined by its variability during seasons other than summer. DTR and Rs agree well both for warm and cold seasons, and variability of Rs over warm seasons is more representative of its annual variability. The reported annual variability of Rs, therefore, agrees better with DTR over warm seasons over Europe (and other regions) than that over an entire year or cold seasons. Variability of DTR over warm and cold seasons is substantially different at both the regional scale (Fig. 5) and the global scale (Fig. 6). For this reason, it is essential to consider these differences in reconstructing variability of Rs from DTR.
(A) The 5-y smoothed impact of Rs on mean air temperature (Ta) over global land during boreal warm seasons (May to October, red), boreal cold seasons (November to April, blue), and an entire year (black). The mean air temperatures over global land are also shown in B.
In Asia, DTR substantially decreased from the 1950s to the 1980s, was stable until 2000, and then decreased again, consistent with Rs derived from sunshine duration (25) and the dimming of directly measured Rs between 1960 and 1990 in China (11, 31). As already mentioned, after the 1990s, direct observations of Rs became inconsistent with those of DTR and sunshine (31), a result of the urban bias of Rs observations. When averaged over all stations (∼400 stations) rather than over the ∼50 urban stations in China with direct observations of Rs, Rs derived from sunshine duration was stable during the 1990s and decreased after 2000 (25).
DTR substantially decreased in North America from 1900 to 2010, consistent with the increase of cloudiness, in particular, of low clouds (32), and decrease of sunshine duration (33). Cloud cover alone accounted for up to 63% of the regional annual DTR variability in the United States from 1902 to 2002, with cloud-cover trends especially driving DTR in northern United States (34). Aerosol loading over North America was relatively light (35) and rather stable during the past few decades (8). Observations at six stations in the United States showed that Rs significantly increased from 1995 to 2007 (36, 37), primarily in the 1990s (38).
As there is a good agreement between Rs and DTR, changes of Rs are expected to be similar to those of DTR, especially during the warm seasons. Fig. 5 shows the variability of DTR over land during the past century, and hence provides qualitative estimates of Rs variability over this period. However, it is difficult to reconstruct Rs quantitatively using the variability of DTR because of the changes of their relationship with time (e.g., from wet to dry seasons; Fig. 3) and region (e.g., from humid to arid regions) (Figs. 1 and 2). Below, we describe another approach for using DTR to estimate the impact of Rs on Ta.
Estimation of the Impact of Rs on Ta from 1900 to 2010.
Elevated greenhouse gases (GHG) have increased atmospheric downward longwave radiation (Ld) (39, 40) and Ta (41) during the twentieth century. However, variability in radiative forcing from aerosols and clouds complicates the attribution of the observed climate change to the elevated GHG. The previous sections have established a more comprehensive climatology for DTR than that available previously and its long-term variability is highly consistent with that of Rs. This climatology allows us to address the question of how much of the observed temperature change has been a result of changes of Rs. For the following analysis, we assume: (i) Tmin is not changed by Rs; and (ii) DTR is only changed by changes of Rs (elaborated on in Discussion and Conclusions).
The globally averaged anomaly of DTR is calculated directly from its grid values. Daily mean air temperature Ta is commonly estimated by Ta = 0.5 × (Tmax + Tmin). As DTR = Tmax − Tmin, we obtain Ta = Tmin + 0.5 × DTR. For the given assumptions, the impact of Rs on daily mean Ta is 0.5 × DTR (Fig. 6). These assumptions can be inaccurate for various reasons, e.g., changes of daytime radiation can be stored and released to change nighttime temperature. Observations from global flux networks show that storage fraction is less than 10% of Rs at most surfaces (42, 43). Allowing for this effect would likely amplify our estimate of the impact of Rs on Ta over global land by a factor of ∼1.1.
We calculate the impact of Rs on mean Ta during the three time periods: (i) 1900–2010 (the whole time period when data are available); (ii) 1940–1984 (the global dimming period) and (iii) 1985–2010 (the global brightening period). The results are summarized in Table 1.
The impact of Rs on daily mean air temperature (Ta) during three periods, 1900–2010, 1985–2010, and 1940–1984 (in °C per 100 y)
Table 1 and Fig. 6 indicate that a reduction in Rs has reduced Ta and that it decreased most rapidly during the dimming period of 1940–1984. The rate of temperature increase during the cold seasons has been reported to be much higher than that during the boreal warm seasons (May to October) (44). Fig. 6 shows that warm-season Rs substantially decreased from the 1940s to early 1950s and during the 1970s, resulting in a reduction of more than 0.2 °C in Ta. Similarly, cold-season Rs substantially decreased from the 1960s through the 1970s, resulting in a decrease of nearly 0.2 °C in Ta. A subsequent increase of Rs was only significant over Europe. In conclusion, the variations of Rs partly accounted for the near absence of warming from midcentury through the 1970s. The maximum cooling seen in the early 1980s and early 1990s were consistent with the effects expected from the El Chichón and Pinatubo volcanoes, respectively. Fig. 6 also shows that the results are substantially different for warm seasons, cold seasons, and the entire year when using DTR to quantify the impact of Rs on air temperature (7).
Discussion and Conclusions
This paper shows, using direct Rs observations (6, 45) and sunshine duration observations (25), that the interannual variability of DTR can be used as a proxy for the long-term variability of Rs. In principle, this relationship should also be applicable to model simulations. AR4 climate models (46) show a weak monotonic increase of DTR from 1950 (44), compare with their figure 5, suggesting that many of the models examined applied a slow constant ramp-up of aerosol forcing rather than concentrated increases before 1980 as indicated here. Changes of DTR are expected to be directly related to H from surface to overlying air but the magnitudes of these turbulent fluxes are not readily estimated (22). Many parameters affect the relationship between Rs and H, and consequently, the relationship between Rs and DTR.
Impacts of land-cover and land-use change (i.e., urbanization and irrigation) have been ignored here. In developing countries, such as China and India (47), there has been substantial urbanization and increased irrigation activity (48) since 1900 with opposing and possibly largely cancelling effects on DTR (16, 49, 50), so with impacts likely to be important locally, but likely to be small at a regional scale (1). Precipitation had a large annual variability but its long-term trend was negligible during the last century (1), and so likely also its impact on the long-term trend of DTR. At annual timescale or station-scale changes of precipitation and land-cover/land introduce substantial uncertainties. Therefore, use of DTR for estimates of variability of Rs and its impact on Ta should be confined to decadal timescale and regional space scale.
Because of the sparse distribution of measurement stations (1) and changes in measurement methods (38) and instruments (25, 51), direct observations cannot provide a reliable estimate of Rs over land during the past century, nor do current climate models generate long-term variability of Rs (52). This study qualitatively reconstructs Rs over land from 1990 to 2010 using the latest homogenized DTR observations at globally distributed weather stations. It infers that Rs over land globally peaked in the late 1930s, substantially decreased from the 1940s to the 1970s, and changed little after that. These estimates are consistent with observations of Rs and sunshine duration where these observations are available.
More importantly, the DTR observations allow us to estimate the impact of Rs on the observed changes of Ta. Only changes before 1984 appear related to the observed temperature trends and DTR variability after 1995 indicates a negligible global impact of Rs variability. The small impact of Rs on Ta may be partly a result of the low sensitivity of Ta to Rs, much lower than the sensitivity of Ta to longwave radiation caused by greenhouse gases (53).
The surface energy budget directly determines the Earth’s surface climate and its changes, but on more local scales strongly interacts with transport processes. In consequence, most existing studies have focused on the energy balance at the top of the atmosphere (5), which is indirectly related to surface Ta, depending on how clouds (54, 55), aerosols, and other feedbacks work. This paper provides a direct and simple method to estimate the variability of Rs over land, which is applied from 1900 to 2010 and estimates the impact of this variability on surface temperature change.
Changes of Rs are primarily determined by changes of clouds and aerosols. Aerosols are known to have accounted for variability of Rs in Europe and China (25, 56), while clouds have been used to explain changes of Rs in the United States (36, 37) during the last two decades. Natural variability from clouds is expected to be more regional and of shorter timescale than the trends from aerosols, but otherwise we are not able to separate their effects. This paper also does not address the mechanisms through which clouds and aerosols respond to climate change (57), i.e., through changes of cloud-cover fraction or cloud height (58).
Our analysis of impact of Rs on Ta does not account for warming effect of solar radiation absorbed by aerosols, i.e., from black carbon (59⇓–61). To zeroth order, aerosol absorption within the daytime boundary layer will return the solar energy removed from the surface, so will not change DTR but will contribute to warming Ta. Our analysis, in principle, cannot include the warming of absorbing aerosols in the aerosol layer although their scattering and absorption effects on surface Rs are included.
Acknowledgments
Chinese homogenized daily maximum and minimum temperature at 549 stations were provided by Prof. Zhongwei Yan. GEBA surface incident solar radiation data were kindly provided by Prof. Martin Wild. We thank Dr. Qian Ma for processing some data for this study. This study was supported by the National Basic Research Program of China (2012CB955302), the National Natural Science Foundation of China (41175126), and the US Department of Energy (BER) Grant DE-FG02-09ER64746.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: kcwang{at}bnu.edu.cn.
Author contributions: K.W. designed research; K.W. performed research; K.W. and R.E.D. analyzed data; and K.W. and R.E.D. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1311433110/-/DCSupplemental.
Freely available online through the PNAS open access option.
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