Instrumental measurements of ocean surface temperature show that warming over the course of the 20thcentury has generally paralleled that of the atmosphere. Some series go back as far as the 19thcentury for certain port sites. Temperature measurements from early scientific expeditions, such as the HMS Challenger from 1872 to 1876, can be compared with recent data from the Argo program. To cover the last few centuries, it is also necessary to use paleothermometric indicators measured in marine archives (sediments, corals, molluscs, etc.) with variable temporal resolutions (monthly to decadal).
Dozens of series from the three main oceans have been compared and compiled as part of the PAGES2k program. The averages show that the surface ocean cooled over the historical period in parallel with the atmosphere, reaching a minimum around the beginning of the 19thcentury before the trend reversal marked by global warming.
In addition to direct paleoceanographic data, it is also possible to use continental series dominated by dendroclimatic data. By studying ocean-continent spatial covariances over the last century, we can extrapolate the spatial coverage of oceanic variations for earlier periods. This leads to the mapping of temperature anomalies for the PAG and OCM periods. The difference in these anomalies confirms that the CMO period was warmer overall than the PAG period, particularly over the continents (Europe and North America) and the oceans, especially the Atlantic and North Pacific. The map of differences between the OCM and the PAG suggests the presence of a negative anomaly(i.e. OCM colder than PAG) in the equatorial Pacific, which would be linked to a modulation of the ENSO phenomenon during the OCM-PAG transition. However, paleoceanographic series based on seasonally resolved archives such as massive corals do not confirm any marked variation in ENSO during the PAG.
