Climate cherry pickers: cooling oceans

Most of global warming, over 80%, go into the oceans. So the amount of heat building up in the oceans is of great interest to climate scientists (and the rest of us climate wonks peering in from the sidelines). Some claim that the oceans are actually cooling, arguing that this proves global warming isn't happening. For example, one paper uses a reconstruction from Argo data as evidence that the oceans are cooling (Loehle 2008):

Figure 1: Ocean heat content smoothed with 1-2-1 filter and overlaid with linear trend (Loehle 2008).

However, this doesn't give you the full picture - there are a number of teams that have reconstructed ocean heat from the various datasets available. In an effort to create the most reliable measure of ocean heat, members from the various teams across the world combined their efforts into a single 'best estimate' of ocean heat (Lyman 2010). What they found was robust warming in the upper ocean over the 16 years from 1993 to 2008.

Upper ocean heat content anomaly
Figure 2: Ocean Heat Content anomaly from various teams. Ocean heat is calculated from 0 to 700 metres (Lyman 2010)

However, even this doesn't give you the full picture. These estimates are of heat content in the upper 700 metres of the ocean. Of course, the ocean goes much deeper than that. A fuller picture of ocean warming analyzes ocean temperature measurements by the Argo network, constructing a map of ocean heat content down to 2000 metres (von Schuckmann 2009). This finds significant ocean warming over the top 2000 metres of the ocean from 2003 to 2008.

Figure 3: Global ocean heat storage (0–2000 metres), measured in 108 Joules per square metre (von Schuckmann 2009).

But wait, there's more! Even ocean heat down to 2000 metres doesn't extend all the way to the ocean depths. A recent paper (Purkey & Johnson 2010) reconstructed ocean heat warming down to abyssal depths, finding significant warming even at the bottom of the ocean (Doug Bostrom wrote a great blog post on this paper).

Figure 4: Rate of ocean warming. Areas of warming are shaded in red and regions of cooling are shaded in blue with intensity scaled by the magnitude of the warming. The basins from south to north are the Southeast Pacific Basin, Chile Basin, Peru Basin, and Pacific Basin (Purkey & Johnson 2010).

To properly understand what's happening with ocean warming, you need to take in the full picture. This means all the data and the whole ocean, not just the upper 700 metres. To claim the oceans are cooling is to ignore the full body of evidence.

Posted by John Cook on Thursday, 11 November, 2010

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