Although scientists understand that the record of Earth’s surface and atmospheric temperatures over the past century show a signature of warming correlated with rising carbon dioxide levels, there remain many doubters and deniers. One problem is that the land and atmosphere temperature data contain many sources of “noise” such as solar variability, El Niño cycles, and weather. This noise must be identified and corrected to clearly reveal the warming signal. A new, alternative approach to analyzing the data has recently been described by a team of international climate scientists led by Lijing Cheng. Their paper appears in the September 2017 issue of EOS, published by the American Geophysical Union. This alternative is based on measurements of the ocean.
Most of the additional heat associated with global warming is deposited in the ocean. The very large mass and heat capacity of the ocean also minimizes external “noise.” These scientists suggest two ways to measure the heating of the ocean. The first uses data available since 2006 on ocean temperatures, using the Argo autonomous floats that measure temperature of the top 2 km of water. The second measures long-term changes in sea level from satellite altimetry, which has been possible since the early 1990s. Both of these data sets show clear signatures of heat deposition in the ocean, from the temperature changes in the top 2 km of water and from the expansion of the ocean water due to heating. These two measures are less noisy than land and atmospheric temperatures.
What time interval is necessary to show a statistically sound signature of global warming? They find that the warming signal is clear from just three to four years of data on either ocean temperatures or sea level, while the same significance requires more than two decades of surface and atmosphere temperatures. In addition, there is the advantage that it is ocean changes that currently dominate discussion of the hazards of global warming. Sea level rise is directly connected to coastal flooding from storm surges, and water temperatures are also implicated in the strength of tropical storms. Warmer water evaporates more quickly and deposits more energy into tropical storms forming over water. At a time when storm damage and flooding are in the headlines, using direct measurements of the ocean itself may inform and influence the broader political discussions of the hazards of climate change.