Source: Peter J. Gleckler, Paul J. Durack, Ronald J. Stouffer, Gregory C. Johnson and Chris E. Forest (2016). Industrial-era global ocean heat uptake doubles in recent decades. Nature Climate Change. Advance Online Publication. http://dx.doi.org/10.1038/nclimate2915
The blanket thickens
Carbon dioxide and other greenhouse gases act as a protective blanket surrounding our planet, trapping heat on the Earth and making our climate livable. However, as humans are adding more carbon dioxide to the atmosphere by burning fossil fuels, we are making the blanket thicker and trapping extra heat on Earth. 90% of this heat is absorbed by the ocean while the small amount left over warms up our atmosphere.
The case of the missing heat
Over the past 15 years or so, the temperature of Earth’s atmosphere has not increased significantly despite increases in greenhouse gas levels. The effect is so dramatic it’s been called the “global warming hiatus.” Some argue that because the atmosphere didn’t warm significantly over the past decade human-caused global warming is not happening, but this is inconsistent with the basic science of greenhouse gases.
If the heat trapped by greenhouse gases is not staying in the atmosphere, then where is it? The only other place the extra heat can be stored is in the ocean, so a group of scientists made it their mission to figure out how much heat the ocean is sucking up to see if they can account for that missing heat.
Clues from the deep
The group of scientists from around the US combined different lines of evidence to try find the missing heat stored in the ocean. The best way to look for the heat is to use direct observations of the temperature of the surface and deep ocean. However, up until the recent development of autonomous underwater robots (Argo), there have been very few measurements of ocean temperature, particularly below the surface of the ocean. Most of these measurements are collected on ships which means that remote and extreme regions like Antarctica have very few observations. This makes accurately calculating the global ocean heat content incredibly challenging.
To get around the difficulties of physically measuring ocean temperatures, the authors used another tool, global climate models, to compare with the numbers they found using historical observations. They used a suite of state-of-the-art models of the ocean, atmosphere and land system to recreate Earth’s climate over the past 150 years. Because the models take a lot of computer time to run, they are a few years behind and the historical simulations only go up to 2005; the team also added a model forecast up to 2015 to compare with the latest ocean observations.
Turning up the heat
Different estimates of ocean heat content from ocean observations from the 1960’s onwards show a steady increase. This increase is stronger near the surface of the ocean where heat can enter the ocean directly from the atmosphere, and more slowly in the deep ocean where the sluggish ocean circulation slowly carries the heat to the seafloor.
There are also periods where temperature decreases suddenly. These quick dips in the ocean heat content are caused by volcanic eruptions (marked in Figure 1a) which release particles into the upper atmosphere that have a cooling effect on the atmosphere. The model results show similar changes in heat content to the direct observations and with these two lines of evidence agreeing, the scientists are more confident in the results than with the observations alone. Now that they have confidence in the results, the authors set out to see whether the ocean heat content has increased more rapidly in recent decades, and if this can explain the ‘global warming hiatus’.
Looking at how the heat content has changed over the past 150 years, it turns out that half of the total heat has accumulated in the ocean since 1997, near when the ‘hiatus’ began (Figure 2). They authors also discovered that a large portion (35%) of this heat has been stored in the deep ocean, below 700 m depth.
Case not closed
Because the observations of ocean heat content transitioned from mostly ship-based measurements to Argo around 2005, it is very difficult to determine how heat content changed during this transition. For this reason, the scientists do not have conclusive evidence to say for sure that the ocean has stored all of the missing heat from the atmosphere. The evidence from their investigation does show that the ocean is heating up much faster now than a few decades ago, and it is certainly plausible that ocean heating can explain the hiatus. The case is not closed, and this study highlights the need for more ocean observations, especially in the deep ocean, so that in the future the mystery of where the Earth stores heat can be completely solved.