From your comments, you've misread the article you linked to - either that or you aren't aware that carbon dioxide has been increasing in the atmosphere since the industrial revolution. (Or maybe you aren't aware of when the industrial revolution began.) Prior to the late 1950s CO2 has been measured from ice cores - going back nearly 800,000 years. Since the late 1950s it is measured from very precise instrumentation, which over the past few decades has been set up around the globe.
The study you quote provides data about changes in ocean heat between the Challenger expedition (1872–76) and today. It shows that the ocean has been gaining heat as greenhouse gases increase. For a long time it has been known that a lot of heat has been accumulating in the ocean as a result of all the extra greenhouse gases we are putting into the air. This study provides more evidence of how much energy has been stored in the oceans since 135 years ago.
This, the first global-scale comparison of Challenger and modern data, shows spatial mean warming at the surface of 0.59?°C±0.12, consistent with previous estimates5 of globally averaged sea surface temperature increase. Below the surface the mean warming decreases to 0.39?°C±0.18 at 366?m (200?fathoms) and 0.12?°C±0.07 at 914?m (500?fathoms). The 0.33?°C±0.14 average temperature difference from 0 to 700?m is twice the value observed globally in that depth range over the past 50 years6, implying a centennial timescale for the present rate of global warming. Warming in the Atlantic Ocean is stronger than in the Pacific. Systematic errors in the Challenger data mean that these temperature changes are a lower bound on the actual values. This study underlines the scientific significance of the Challenger expedition and the modern Argo Programme and indicates that globally the oceans have been warming at least since the late-nineteenth or early-twentieth century.
