Natural climate forcings and the relevance for modern climate change

Jack Wilkin (UK)

Climate sceptics often argue that, because the climate has changed in the past, contemporary global warming must therefore also be natural. However, in this context, paleoclimatologists study the causes and effects of climate change throughout geological time to better understand human-induced (anthropogenic) warming. And it is this that this article will discuss.

Carbon dioxide

Carbon dioxide lets through the short-wave light emitted by the sun but traps the long-wave light that would ordinarily be reflected back into space. The more CO₂ you put in the atmosphere, the more of this long-wavelength light is trapped, and the warmer the atmosphere becomes.

The ability for CO₂ to trap long-wavelength light and cause warming can be experimented on practically in the laboratory. This was first done by Arrhenius (1896), who concluded that higher levels of CO₂ in the atmosphere would prevent outgoing radiation, thus heating the atmosphere. And, in the 1950s scientists began to become concerned about rising CO₂ concentrations in the atmosphere and global warming (for example, Plass 1956; Revelle and Suess, 1957), so concerns over anthropogenic warming are nothing new.

In the geological past, CO₂ levels were higher than today and have been slowly decreasing over time. CO₂ was taken out of the atmosphere by chemical weathering and deposited as limestones. In fact, enhanced weathering has been suggested as a method to help combat anthropogenic warming (Beerling et al., 2018) showing how knowledge of past climates can help us today and in the future.

This long-term decrease is punctuated by sporadic increases in CO₂ values by large-scale flood basalts, the largest volcanic events in Earth’s history (for example, Berner and Kothavala, 2001). Some of the most dramatic flood basalt events in the geological record include the Siberian Traps at the end of the Permian (about 250 million years ago) and the Deccan Traps at the end of the Cretaceous (about 66 million years ago), and both are associated with mass extinctions (for example, Clapham and Renne, 2019; see also MacLeod, 2013, which is an excellent book on mass extinctions).

By burning fossil fuels, humans have been releasing this buried carbon accumulated over hundreds of millions of years back into the atmosphere in just a few decades (Fig. 1). It is no wonder then that CO₂ concentrations have increased so dramatically and with it global temperatures.

Solar irradiance and the Milankovitch Cycles

Solar irradiance – the amount of solar radiation that reaches the Earth per unit distance – can be influenced by changes in the Earth’s orbit (Milankovitch Cycles) and the amount of energy the Sun emits (solar luminosity). Milankovitch Cycles describe predictable and cyclical changes in the shape of Earth’s orbit. Such cyclicity caused the growth and retreat of ice sheets during the ice age to the point that they are nicknamed the “pacemaker of the Ice Ages” (Hays e tal., 1976).

There are three Milankovitch Cycles (Fig. 2): eccentricity, obliquity and precession. Eccentricity describes the change in the shape of Earth’s orbit around the sun and is the longest cycle taking roughly 100,000 years. Obliquity describes the changes in the tilt of the Earth’s axis over a 41,000 year period. Finally, we have the precession of the equinoxes, which describes changes in the axis wobble and turning ellipse occurring over a 19,000 to 23,000-year cycle, changing 1° every 72 years and affecting both poles equally.

Solar luminosity has also been slowly increasing over geological time. The solar luminosity changes in the short term and can causes hort-term warming and cooling (Feulner, 2012). Over the long term, the sun’s energy was lower in the past and steadily increased over time. This means that global temperature in the past could be lower than what it is today despite the higher CO₂.

One example of this high CO₂ but cool climate is the Late Ordovician ice age 450 million years ago. During this time, solar output was about 4.5% lower than today (Crowley and Baum, 1995). However, the climate on either side of the ice age was warmer than today despite solar irradiance being much lower. This was because CO₂ levels were much higher than today, so this could counter the effects of lower luminosity. The Late Ordovician ice age was caused by CO₂ being removed from the atmosphere due to a series of mountain building events that sped up chemical weathering (Kemp et al.,1999).

Critics of anthropogenic warming sometimes argue that the sun is the main driving force for recent warming in the late twentieth and early twenty-first centuries. However, solar irradiance has decreased since the 1960s, yet global temperatures continue to rise (for example, Liepert et al.,1994; Russak, 2009). The rise in global temperatures during the last 50 years coincides with increases in CO₂ levels. Because the increase in solar luminosity has been slow and steady throughout time, scientists have found that sporadic increases and decreases in the CO₂ level in the atmosphere is the primary driver in global temperature (Royer et al., 2004).

One way to visualise the relative effects of the Sun and CO₂ on climate is to imagine them as two radiators in a room. The radiator marked as the Sun has been slowly warming but the CO₂ radiator has been slowly dropping. If one or the other suddenly has its temperature dial turned up, the room heats up.

Ice caps

The amount of solar energy reaching or remaining in the atmosphere can also be influenced by the presence of large ice caps. Because ice and snow are white, they have a higher albedo effect than other materials. This means they reflect more solar radiation back into space (Fig. 3). This can create a positive feedback loop, where lower temperatures cause ice caps to go which increases albedo causing temperatures to decrease further, which in turn expands the ice caps.

Likewise, warmer temperatures melt ice decreasing albedo creating a positive feedback loop of increasing temperatures. Increasing albedo limits the amount of shortwave radiation from the Sun that reaches the ground. Shortwave radiation passes through the atmosphere and it becomes absorbed by the Earth’s surface and some compounds in the atmosphere, and is re-radiated as long wave radiation. It is this longwave radiation that we feel as heat and becomes trapped by greenhouse gasses (UGS Berkeley, 2024).

Aerosols

Another way that solar radiation can be reflected into space is by aerosols. Aerosols scatter incoming solar radiation; and this has a cooling effect by enhancing the total reflected solar radiation back into space (Myhre et al., 2013). For instance, volcanoes release both greenhouse gases and aerosols producing both a warming and cooling effect, resulting in a volcanic winter (Gerlach, 2011). The net temperature changes are dependent on the type of volcano and the relative amounts of greenhouse gases/aerosols they emit.

Large meteor impacts also release a lot of dust into the atmosphere. It is estimated that the K-Pg impact that killed the dinosaurs released a lot of sulphur (due to the underlying geology of the Yucatan Peninsula), which quickly formed aerosols (as well as acid rain) that back scattered and absorbed solar radiation causing rapid cooling on the Earth’s surface in the short term. However,the CO₂ locked up invaporised limestones was released into the atmosphere contributing to long-term warming in the early Palaeocene (Kring, 2007).

But why is this relevant?

The Earth’s climate is in a perpetual state of change. Research has revealed that there have been many cool periods and periods of warming in the past. The global climate system is very dynamic, and past events might help us better understand the impact of anthropogenic climate change.

A decent analogue for the twenty-first century may be the Palaeocene-Eocene Thermal Maximum (PETM) 56 million years ago. But we need to remember that, even though the PETM was a time of rapid CO₂ rise (one of the fastest ever), current rates of change caused by human activity is nearly ten times as fast as during the PETM (Cui et al., 2011).

Palaeoclimate data shows CO₂ is the primary cause of temperature changes throughout the Phanerozoic (the last 540 million years). Therefore, by looking at how climate systems responded to changes in CO₂ levels in the past, we can develop models to predict what anthropogenic CO₂ rise could mean for the future. These models have successfully predicted how human-induced climate change will affect future generations, and more data will inevitably lead to even more accurate models.

References

Arrhenius. S. (1896) On the influence of carbonic acid in the air upon the temperature of the ground. Philosophical Magazine and Journal of Science 5(41): 237–276: https://doi.org/10.1080/14786449608620846.

Beerling, D. Leaker, J.R., Long, S.P., Scholes, J.D., Ton, J., Nelson, P.N., Bird, M., Kantzas, E., Taylor, L.L., Sarkar, B., Kelland, M., DeLucia, E., Kantola, I., Müller, C., Rau, G. and Hansen, J. (2018) Farming with crops and rocks to address global: climate, food and soil security. Nature Plants 4: 138–147. https://doi.org/10.1038/s41477-018-0108-y.

Berner, R.A. and Kothavala, Z. (2001) Geocarb III: A revised model of atmospheric CO2 over Phanerozoic time. American Journal of Science 301: 182–204: https://ajsonline.org/article/60629.

Clapham, M.E. and Renne, P.R. (2019) Flood basalts and mass extinctions. Annual Review of Earth and Planetary Sciences 47: 275–303: https://www.annualreviews.org/content/journals/10.1146/annurev-earth-053018-060136.

Crowley, T.J. and Baum, S.K. (1995) Reconciling Late Ordovician (440 Ma) glaciation with very high (14X) CO2 levels. Journal of Geophysical Research: Atmospheres 100(D1): 1093–1101: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/94JD02521.

Cui, Y., Kemp, L.R., Ridgewell, A.J., Charles, A.J., Junium, C.K., Diefendorf, A.F., Freeman, K.H., Urban, N.M. and Harding, I.C. (2011) Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum. Nature Geoscience 4: 481–485: https://doi.org/10.1038/ngeo1179.

Feulner, G. (2012) The faint young sun problem. Reviews of Geophysics 50(2): http://doi.org/10.1029/2011RG000375.

Gerlach, T. (2011) Volcanic versus anthropogenic carbon dioxide. EOS 92(24): 201–208: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011EO240001.

Hays, J.D., Imbrie, J. and Shackleton, N.J. (1976) Variations in the Earth’s Orbit: Pacemaker of the Ice Ages: For 500,000 years, major climatic changes have followed variations in obliquity and precession. Science 194(4270): 1121–1132: https://www.science.org/doi/10.1126/science.194.4270.1121.

Keith, D.W., Weisenstein, D.K., Dykema, J.A. and Keutsch, F.N. (2016) Stratospheric solar geoengineering without ozone loss. PNAS 113(53): 14910–14914: https://www.pnas.org/doi/full/10.1073/pnas.1615572113.

Kemp, L.R., Arthur, M.A., Patzkowsky, M.E., Gibbs, M.T., Pinkus, D.S. and Sheehan, P.M. (1999). A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 152(1/2): 173–187: https://www.sciencedirect.com/science/article/abs/pii/S0031018299000462?via%3Dihub.

Kring, D.A. (2007) The Chicxulub impact event and its environmental consequences at the Cretaceous-Tertiary boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 255(1/2): 4–21: https://www.sciencedirect.com/science/article/abs/pii/S0031018207003173?via%3Dihub.

Liepert, B., Fabian, P. and Grassl, H. (1994) Solar radiation in Germany – Observed trends and an assessment of their causes. Part I: Regional approach. Contributions to Atmospheric Physics 67(1): 15–29.

MacLeod, N. (2013) The great extinction: What causes them and how they shape life. Natural History Museum: London.

Myhre, G., Myhre, C. E.L., Samset, B.H. and Storelvmo, T. (2013) Aerosols and their Relation to Global Climate and Climate Sensitivity. Nature Education Knowledge 4(5):7. Available at: https://www.nature..com/scitable/knowledge/library/aerosols-and-their-relation-to-global-climate-102215345/.

Plass, G.N. (1956) The carbon dioxide theory of climate change. Tellus 8(2): 140–154.

Revelle, R. and Suess, H.E. (1957) Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO₂ during the past decades. Tellus 9(1): 18–27: https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.2153-3490.1957.tb01849.x.

Royer, D.L., Berner, R.A., Montañez, I.P., Tabor, N.J. and Beerling, D.J. (2004) CO2 as a primary driver of Phanerozoic climate. GSA Today 14(3): 4–10.doi: 10.1130/1052-5173(2004)0142.0.CO;2.

Russak, V. (2009) Changes in solar radiation and their influence on temperature trend in Estonia (1955–2007). Journal of Geophysical Research: Atmospheres 114(D10): https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2008JD010613.

USG Berkeley. (2024) Re-radiation of heat. Available at: https://ugc.berkeley.edu/background-content/re-radiation-of-heat/ [last accessed 27/04/24].