Past climate changes are imprinted in natural archives in physical, chemical or biological properties providing local (proxy) temperature information. In NOTICE we will go beyond previous approaches by measuring the noble gas ratios in air bubbles in polar ice cores using a novel sensitive mass spectrometric technique. Based on the straightforward temperature dependence of the solubility of noble gases in the global ocean, this provides temperature information averaged over the entire ocean and thus an integrative measure of its heat content at any point back in time for up to the last 800,000 years.

Lay summary

To understand and predict anthropogenically induced global warming requires detailed knowledge of the variations in the Earth’s climate system and of the coupling processes between climate and radiative forcing. Due to the strong temporal limitations of direct observations, reconstructions of past climate from natural climate archives are key to studying these variations over an extended time scale and to providing crucial validation data for climate models. However, climate reconstructions with global coverage and data-model comparison on a global scale are virtually impossible for long-term climate changes, due to limited data availability and inconsistencies in the chronologies of individual climate records.

Accordingly, a new paleo-thermometer is required that provides global temperature. As most of the heat during climate changes is taken up or released from the ocean, this implies that we are essentially seeking for a global mean ocean thermometer. Interestingly, it is not marine sediments that provide such information but polar ice cores. These cores archive the past atmosphere in little air bubbles, which contain the natural concentration of molecular nitrogen (N2) and of noble gases such as Argon, Krypton and Xenon. Although the large majority of these gases is residing in the atmosphere, a small percentage is dissolved in the ocean. This percentage is controlled by the well-known dependence of their solubility on ocean temperature. Accordingly, we are able to quantify the global mean ocean temperature in the past compared to its present value by very precisely measuring the ratios of molecular nitrogen and noble gases .

We have developed a mass spectrometric technique in recent years, which is able to measure these ratios with the required precision on small air samples extracted from ice cores. In NOTICE we will employ this technique on ice cores from Greenland and Antarctica to reconstruct global mean ocean temperature over up to the last 800,000 years.