• global warming
• isotherms
• sea surface temperatures
• thermal niche
• biogeochemical cycling
• oceans

The oceans play important roles in climate change, for example, as a heat sink, CO2 store and through active biogeochemical cycling of greenhouse gases. The oceans are warming with an increase in the mean global ocean temperature between the surface and 3000m of 0.037°C between 1955 and 1998. A potentially sensitive index of global ocean conditions is the water temperature at the ocean margins, since marked changes occur over relatively small spatial and temporal scales. I made use of a new sea surface temperature (SST) data-set to analyse seasonal and mean annual positions of the 12, 15 and 18°C isotherms in the North Atlantic, North Pacific, South Atlantic, South Pacific and Indian Ocean over the last 130 years. Consistent long-term trends in the position of isotherms were seen in the northern hemisphere which contrasting to those trends seen in the southern hemisphere. Isotherm position has oscillated in the Northern hemisphere, in lines with the internal climate variability (Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation), reaching maxima in the 1940s and the present day. In the southern hemisphere the pattern is different, with a mainly monotonic southerly shift in isotherm positions over the last 70 years, so that present-day conditions eclipse those experienced at any time in the previous 130 years. These results highlight a strong climate change signal in the oceans of the southern hemisphere, which is likely to be having important implications for biogeochemical cycling and species distributions.