A top economic advisor to the Australian Government recently suggested in an op-ed published by the national daily The Australian that one scenario we should be prepared for is global cooling due to a possible change in solar activity. Certain reports in the U.S. media have also suggested cooling may already be upon us because of the sun. What is one to make of this?
Should we be prepared for global cooling driven by changes in solar output?
There is much to be learned about the sun’s role in climate. However, best estimates suggest the sun’s variability over recent decades has played a small role in observed climate change] (footnote 1). Though it is still difficult to predict changes in the sun, modeled projections suggest even a return to the “Maunder Minimum” (footnote 2) would provide only minor compensation for anticipated greenhouse-gas warming.
The attention on the sun serves as a reminder that modern climate science arose, in part, from efforts to address societal concerns over global cooling, setting the stage for advances in climate science over the past forty years. There was concern among some policy makers that the decades of cooling from the mid-1940s through the mid 1970’s represented the onset of a new ice age.
In the 1970s the Little Ice Age, and the “Year Without a Summer” following the 1815 Tambora eruption, were known. So the dangers of a cold climate were perhaps more current in peoples’ minds than the dangers of warming, and historically people have been more worried about being too cold than too warm. The idea of an impending ice age did feature in some media pieces.
The role of the earth’s orbit in climate (footnote 3) was being confirmed. These long-term changes in the distribution of solar radiation over the seasons and latitudes were discovered to be imprinted upon the record of climate, leading to the suggestion that, all else being equal, the earth might be headed towards an ice age. Some scientists did suggest another ice age might be on the way (footnote 4), but it was not clear when. It was known by then the earth was in an “interglacial” stage, and these warm stages were relatively short excursions in the generally cool Pleistocene epoch (footnote 5).
Another “cold climate” worry came from the fear of “nuclear winter,” and also drove the development of climate models.
All these concerns drove an examination of scientific understanding about climate in the 1970s. The scientific community said they were not in a position to confidently predict which way climate would go from that point. Would the long slow orbital changes plus anthropogenic aerosol emissions cool the planet? Or would greenhouse gases become the dominant forcing? [See Spencer Weart’s “The Discovery of Global Warming” for a thorough and comprehensive treatment of the history of climate science.]
A 1975 U.S. National Academy of Sciences report on climate change is blunt about the state of understanding of the climate system at the time:
“We do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate. … The climates of the earth have always been changing, and they will doubtless continue to do so in the future. How large these future changes will be, and where and how rapidly they will occur, we do not know.”
The report also notes that:
“There seems little doubt that the present period of unusual warmth will eventually give way to a time of colder climate, but there is no consensus with regard to either the magnitude or rapidity of the transition. The onset of this climatic decline [i.e. cooling] could be several thousand years in the future, although there is a finite probability that a serious worldwide cooling could befall the earth within the next hundred years.”
Some commentators have seized upon statements like the latter as evidence there was a “consensus” among scientists about a coming ice age. But actually there was not a clear sense which way the climate would go in the coming decades. Climate scientists were already aware of the potential of global warming from carbon dioxide emissions, but when and by how much would their influence be felt?
A report to the US Johnson Administration in 1965 (footnote 6) is prescient, and predicts correctly, a rise in the CO2 concentration of the atmosphere. It also anticipates some of the effects of that rise, including warming of the atmosphere and ocean, sea-level rise, and melting of ice caps. The authors also envision the possibility of deliberate manipulation of the climate system, what we would now call “geoengineering“:
“The climatic changes that may be produced by the increased CO2 content could be deleterious from the point of view of human beings. The possibilities of deliberately bringing about countervailing climatic change therefore need to be thoroughly explored.”
In 1975 paleoclimatologist Wally Broecker (also an author of the NAS report) made a bold prediction of warming (footnote 7):
“… a strong case can be made that the present cooling trend will, within a decade or so, give way to a pronounced warming induced by carbon dioxide. By analogy with similar events in the past, the natural climatic cooling which, since 1940, has more than compensated for the carbon dioxide effect, will soon bottom out. Once this happens, the exponential rise in the atmospheric carbon dioxide content will tend to become a significant factor and by early in the next century will have driven the mean planetary temperature beyond the limits experienced during the last 1000 years.”
Pioneers like Svante Arrhenius in the late 1800’s and Mikhail Budyko (footnote 8) in the 1960s had already anticipated the warming effect of greenhouse gases, though they saw global warming as beneficial.
Climate science has evolved. We now see the warming effects of accumulating greenhouse gases as a greater risk than global cooling. We also recognize a broader spectrum of impacts than warming alone, such as the acidification of the ocean due to its absorption of CO2.
New studies of climate variability have provided timely reminders that we need to be able to adapt to climate changes, no matter what drives them.
1. Lean, J. L. (2010), Cycles and trends in solar irradiance and climate, Wiley Interdisciplinary Reviews: Climate Change, 1(1), 111-122, doi:10.1002/wcc.18.
2. A 17th Century low in solar activity associated with the “Little Ice Age;” Feulner, G., and S. Rahmstorf (2010), On the effect of a new grand minimum of solar activity on the future climate on Earth, Geophys. Res. Lett., 37(5), L05707, doi:10.1029/2010gl042710.
3. Hays, J. D., J. Imbrie, and N. J. Shackleton (1976), Variations in the Earth’s orbit: Pacemaker of the ice ages, Science, 194(4270), 1121-1132, doi:10.1126/science.194.4270.1121.
4. Kukla, G. J., and R. K. Matthews (1972), When Will the Present Interglacial End?, Science, 178(4057), 190-202, doi:10.1126/science.178.4057.190.
5. Tzedakis, P. C., E. W. Wolff, L. C. Skinner, V. Brovkin, D. A. Hodell, J. F. McManus, and D. Raynaud (2012), Can we predict the duration of an interglacial?, Clim. Past, 8(5), 1473-1485, doi:10.5194/cp-8-1473-2012.
6. Environmental Pollution Panel (1965), Restoring the Quality of Our Environment, 133 pp, President’s Science Advisory Committee, Washington D.C..
7. Broecker, W. S. (1975), Are We on the Brink of a Pronounced Global Warming?, Science, 189(4201), 460-463, doi:10.1126/science.189.4201.460.
8. Budyko, M. I. (1969), The effect of solar radiation variations on the climate of the Earth, Tellus, 21(5), 611-619, doi:10.1111/j.2153-3490.1969.tb00466.x.