5. Why does CO2 lag temperature?
Through Earth’s history, the climate has changed considerably over a wide range, from ice ages to warm periods without any ice at the poles. These climate variations had several triggering mechanisms, like variation in solar activity, Milanković cycles, volcanic activity and changes in atmospheric composition. With data from ice cores, scientists were able to recover atmospheric data from the past 400,000 years. The data from these ice cores included the annual atmospheric composition over the period of these 400,000 years, with the different concentrations of atmospheric gases. After analyzing the data, a correlation between CO2 and temperature can be reconstructed (see figure below). However, a rise in CO2 did not precede a rise in temperature but is lagging behind the temperature curve by 200 to 1000 years. At the first glance, the temperature seems to be the driving factor causing the increase in CO2 concentration, which is contrary to the common point of view that CO2 drives global warming. A study by Shakun et al. (2012) investigates this correlation in detail.
The study shows that the initial stages of increasing temperatures after the last ice age were triggered by the Milanković cycles. This initiated a reaction chain, leading to the heating of the oceans, which then released CO2. With the increasing greenhouse effect, temperatures started to increase and the emission of CO2 from the oceans into the atmosphere accumulated. The time lag between CO2 and temperature is caused by a temporal offset between the oceans heating up and the constant release of oceanic CO2. Through this accumulating effect, CO2 became the primary driver of temperature during the glacial-interglacial warming. The increasing CO2 levels then become both, the cause and effect of further warming. This positive feedback is necessary to trigger the shifts between glacials and interglacials as the effect of orbital changes is too weak to cause such variation.
The main statement of Shakun et al. (2012) is that CO2 has led global warming, rather than following it. The actual interactions between temperature and CO2 described in this study are more complex. The data acquired for this study is based on an Antarctic ice core record for atmospheric CO2 data as well as sediment cores from around the globe, which extend back to the last glacial-interglacial transition 18,000 years ago. From these proxy records, sea surface temperatures for marine records and surface air temperatures were collected.
Comparing the CO2 increase to the different temperature records it is possible to estimate whether CO2 led or lagged temperature in different geographic distributions. The result was that CO2 lags and leads temperature changes at the same time. The southern hemisphere showed temperature rising before CO2 concentration, while the northern hemisphere showed the opposite (see figure below). This result can be explained by several factors.
The initial warming, which was triggered by the Milanković cycles, is reflected in the highest latitudes and started approximately 19,000 years ago. The arctic warming then melted large quantities of ice, causing a strong freshwater input into the oceans. This influx of fresh water disrupted the Atlantic meridional overturning circulation (AMOC), which in return caused a seesawing of heat between the hemispheres. The AMOC is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic and a southward flow of colder, deep waters that are part of the thermohaline circulation. The bipolar seesaw concept describes the anti-phasing of Greenland and Antarctic temperature changes along with Dansgaard-Oeschger climatic oscillations and Heinrich Events during the last glacial period. Sudden changes in the thermohaline circulation affect the polar climate in both hemispheres through changes in the south-northward heat transport. With a freshwater influx in the North Atlantic, the thermohaline circulation (AMOC) is turned off and the northern hemisphere cools down, while the southern hemisphere and tropics heat up at around 18,000 years ago. Once the deep-water formation turns on again, the meridional heat transfer resumes and the northern hemisphere warms while the southern hemisphere loses heat.
The warming of the southern ocean 18,000 years ago caused a decrease of CO2 solubility in the water. The lower solubility resulted in the emission of CO2 from the ocean into the atmosphere at around 17,500 years ago, which in return caused a global warming due to the greenhouse effect. This time lag of around 500 years is what causes the CO2 lagging behind temperature in the ice core record. With the increasing CO2 concentrations, deriving from the heating oceans, the greenhouse effect starts accumulating, making CO2 the driver of further temperature changes.
Knutti, R., Flückiger, J., Stocker, T.F. and Timmermann, A., 2004. Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation. Nature, 430, 851-856.
Shakun, Jeremy D.; Clark, Peter U.; He, Feng; Marcott, Shaun A.; Mix, Alan C.; Liu, Zhengyu et al. (2012): Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. In Nature 484 (7392), pp. 49–54. DOI: 10.1038/nature10915.
Muryshev, K. E.; Eliseev, A. V.; Denisov, S. N.; Mokhov, I. I.; Arzhanov, M. M.; Timazhev, A. V. (2019): Time lag between changes in global temperature and atmospheric CO2 content under anthropogenic emissions of CO2 and CH4 into the atmosphere. In IOP Conf. Ser.: Earth Environ. Sci. 231, p. 12039. DOI: 10.1088/1755-1315/231/1/012039.
Pedro, Joel B.; Jochum, Markus; Buizert, Christo; He, Feng; Barker, Stephen; Rasmussen, Sune O. (2018): Beyond the bipolar seesaw: Toward a process understanding of interhemispheric coupling. In Quaternary Science Reviews 192, pp. 27–46. DOI: 10.1016/j.quascirev.2018.05.005.