The Effect of Solar Radiation Variations on the Climate of the Earth
M. I. Budyko
Observation and Context
Paleogeographical research shows that for the last two hundred million years, Earth’s climate was remarkably warm and lacked distinct polar cold zones. However, by the end of the Tertiary and into the Quaternary period, an increasing thermal contrast between the equator and the poles triggered the development of large-scale glaciations.
Looking at the modern era, meteorological data from 1881 to 1960 reveals a distinct warming trend of 0.6 °C that abruptly stopped around 1940, followed by a cooling trend of 0.2 °C by the mid-1950s. When compared to solar measurements, this temperature curve closely mirrors historical variations in direct solar radiation arriving at cloudless surface stations. Prior hypotheses attributed these radiation dips to changes in atmospheric transparency from volcanic dust, while the post-1940 cooling has also been linked to human-induced atmospheric pollution.
Hypothesis
M. I. Budyko hypothesized that secular variations in Earth’s global temperature are primarily driven by changes in solar radiation reaching the surface due to atmospheric transparency fluctuations. Furthermore, he posited that because an expanding ice cover increases the planet’s albedo (reflectivity), a highly sensitive, non-linear physical feedback loop exists where comparatively small decreases in solar radiation are sufficient to trigger massive, self-developing glaciations.
Experiment and Methodology
To evaluate these relationships quantitatively, Budyko constructed a simplified, one-dimensional mathematical model of Earth’s surface thermal regime. The experimental framework utilized the following methodology:
- Radiation Parameterization: An empirical formula was derived from historical data to calculate outgoing long-wave radiation based on surface temperature and a mean global cloudiness value of 0.50.
- Horizontal Heat Transfer: Budyko added a dynamic term to link latitudinal temperatures to horizontal heat distribution across the atmosphere and oceans.
- Albedo Feedback Loops: The model explicitly incorporated a variable planetary albedo, which was programmatically linked to ice cover extension. Sea and land ice were assumed to expand southward to any latitude where calculated annual temperatures dropped below the modern freezing threshold observed at 72 °N. Albedo was set to 0.62 over ice-covered zones and 0.32 over ice-free zones.
- Simulation Runs: The model calculated latitudinal temperature distributions under systematic reductions of incoming solar radiation (ΔQp/Qp).
Results and Data
The model successfully replicated modern latitudinal temperature gradients before testing climate stability limits. When incoming solar radiation was reduced, the ice-albedo feedback intensified temperature drops non-linearly:
- A 1.0% decrease in solar radiation dropped the mean global temperature by 5 °C.
- A 1.5% decrease caused a planetary temperature drop of 9 °C and forced the glaciation boundary 10° to 18° of latitude southward, matching the footprint of actual Quaternary ice ages.
- A 1.6% decrease pushed the ice cover past a critical threshold of 50 °N. Past this point, a runaway feedback cycle caused the ice to self-develop and march all the way to the equator, plunging the entire planet into a permanent, sub-zero “White Earth” state.
Conclusion and Climate Impact
Budyko concluded that Earth’s modern climate and glaciations are characterized by extreme instability due to the powerful positive feedback of ice albedo. The experiment demonstrated that historical ice ages can be entirely explained by tiny fluctuations in solar radiation (1.0% to 1.5%) caused by tectonic variations in global volcanic activity.
This study is recognized as a monumental milestone in climate science. By introducing the concept of a runaway “Snowball Earth” scenario, Budyko’s simplified energy balance model proved that the climate system possesses tipping points and multiple stable states, fundamentally altering our understanding of planetary climate vulnerability and anthropogenic global warming.
Citation
Budyko, M. I. (1969). “The effect of solar radiation variations on the climate of the Earth.” Tellus, Vol. 21, No. 5, pp. 611–619.