The solar energy absorbed by Earth’s surface is more than the energy radiated back to space. That radiation imbalance—predominantly attributable to anthropogenic fossil-fuel emissions—has heated Earth’s surface at an average rate of about 0.5 W/m2 over the past 50 years.
The level of warming is higher than any the planet has experienced since humans first evolved and walked Earth. To better understand the range of possible climate variations, researchers study the geologic past using measurements from ice cores, sediment records, and the like. (For example, see the article by Toby Ault and Scott St. George, Physics Today, August 2018, page 44.)
Marine sediment records include the shells of microsized foraminifera. They live at or near the ocean bottom, and to paleoclimate researchers, they’re good indicators of climate conditions. Foraminiferal, or foram, δ18O is the relative amount of the trace isotope oxygen-18 in the organisms’ shells. That geochemical signal primarily depends on the temperature of the ocean—which affects foraminifer biochemistry—and seawater δ18O, which changes based on the volume of Earth’s ice sheets and glaciers.
Now Princeton University's Sarah Shackleton and colleagues have combed through global foraminiferal δ18O records and found that those data have a new use. Although the ocean temperature and ice-volume signals are distinct, when combined together they reconstruct the energy imbalance at the top of the atmosphere.
When more energy comes into the Earth system than goes out, it has to go somewhere—into the ocean, ice sheets, the atmosphere, and other places. The crucial insight that Shackleton and her colleagues made is that melting ice and rising ocean temperatures have nearly the exact same effect on foram δ18O. That is, when they modeled foram δ18O as a function solely of one variable or the other, the energy changes associated with each were, by pure coincidence, quite similar.
Thanks to that similarity, Shackleton and colleagues inferred the energy imbalance directly from foram δ18O without needing to do the difficult task of deconstructing the isotope measurement into its constituent parts. The researchers confirmed their finding by comparing the energy imbalance calculated from foram δ18O of the past 25 000 years with energy imbalance predicted in previously published results.
The isotope measurements extended the estimate of Earth’s energy imbalance (thick gray line) even farther back in time, for the past 150 000 years (shown below). The periods of positive (red) and negative (blue) energy changes are associated with rising and falling global sea level (orange line) and Heinrich events (vertical gray shading)—periods when icebergs have been documented to break off ice sheets into the ocean.
The new record of energy imbalance also tracks strongly over time with another well-studied climate variable: the amount of sunlight Earth receives during the summer at 65 °N (blue line). Previous research has linked reductions of sunlight at that latitude to the onset of global-scale ice ages, and increased sunlight there triggers deglaciation. (For more on orbitally driven ice ages, see the article by Mark Maslin, Physics Today, May 2020, page 48, and Physics Today, September 2016, page 13.)
With their newly documented approach to measuring Earth’s energy imbalance, Shackleton and colleagues plan to better study millennial-scale perturbations of the climate system and the associated feedback mechanisms. (S. Shackleton et al., Nat. Geosci. 16, 797, 2023.)
