The Little Ice Age (LIA) is a period of cold global average surface temperatures from around 1600 to 1850, following the Medieval Warm Period (950 ~ 1250). A new study published in Science suggested that since the ocean adjusts to the surface thermal anomalies with the time scales of 100 ~ 1,000 years, some parts of the global deep ocean may still experiencing the cooling from the LIA. More specifically, the study used an ocean model with modern and paleo data to find that the deep Indo-Pacific Oceans are still adjusting to the cooling from the LIA. This is because the surface thermal anomalies in the Indo-Pacific cannot penetrate directly into the deep oceans (e.g., Lumpkin et al., 2007; Lee et al., 2019). This is in contrast with the Atlantic and Southern Oceans where the surface thermal anomalies are effectively carried into the deep oceans via the strong meridonal overturning circulations. Therefore, in the Atlantic and Southern Oceans, the anthropogenic surface warming of the 20th century has already permeated the deep oceans. The study further concluded that heat loss in the deep Indo-Pacific Oceans since 1750 offset around one quarter of the global heat gain in the upper ocean.
Figure 1 from Gebbie and Huybers (2019):A) Global average (black line) and regionally averaged (colored lines) surface temperature time series, for a simulation initialized from equilibrium in 15 CE (EQ-0015). Regional variations are plotted for the Antarctic (ANT), North Atlantic (NATL), sub-Antarctic (SUBANT), and North Pacific (NPAC). Prior to globally available instrumental surface temperatures beginning in 1870 CE, global changes are prescribed according to estimates from paleoclimate data. (B) Time evolution of the Pacific-average potential temperature profile from EQ-0015. Pacific average is taken north of 45°S, and color shading has a 2.5-cK interval from –35 to 35 cK. Note the expanded time axis after 1750 CE.
Gebbie, G., & Huybers, P. (2019). The Little Ice Age and 20th-century deep Pacific cooling. Science, 363(6422), 70-74. https://science.sciencemag.org/content/363/6422/70