How we know global warming is happening, Part 2

In our last post, we determined whether global warming was still happening by adding up all the heat content of the climate system.  Murphy 2009 performed this analysis and found that that planet has been accumulating heat up to 2003. Unfortunately their data ends there as the ocean data they used from Domingues 2008 stops at the end of 2003. So how do we find out what's happened from 2003 until now? Unfortunately, there is no time series (that I know of) of the planet's total heat content up to present time. However, we do have the next best thing.

Global hydrographic variability patterns during 2003–2008 (Schuckmann 2009) analyses ocean temperature measurements by the Argo network, constructing a map of ocean heat content down to 2000 metres (H/T to Chris for bringing it to my attention). This is significantly deeper than other recent papers that focus on upper ocean heat, only going down to 700 metres. They constructed the following time series of global ocean heat:

Figure 1: Global ocean heat anomaly (0–2000 m), measured in 108 Jm-2. Anomaly is defined as the deviation from the average heat content over 2003 to 2008.

Globally, the oceans have still been steadily accumulating heat right to the end of 2008. Combined with the results of Murphy 2009 who finds the planet accumulating heat right to 2003, we now see a picture of unbroken global warming. Over the last 5 years, the oceans have been absorbing heat at a rate of 0.77 ± 0.11 Wm?2.

So what is our planet's total energy imbalance? Indulge me as I perform some rough back-of-a-napkin calculations. Murphy 2009 found that about 5.6% of the planet's energy imbalance went into the land and atmosphere. In other words, 94.4% of global warming goes into the oceans. So if the ocean is absorbing 0.77 ± 0.11 Wm?2, this puts the total energy imbalance at around 0.82 ± 0.12 Wm?2. This is a slight underestimate as Murphy 2009 included ocean heat down to 3000m (remember this is back-of-a-napkin stuff).

How does this value compare to other estimates of energy imbalance? Hansen 2005, using ocean heat data, calculated the planet's energy imbalance around 2003 to be 0.85 Wm?2. Trenberth 2009 examined satellite measurements of incoming and outgoing radiation for the March 2000 to May 2004 period and found the planet accumulating energy at a rate of 0.9 ± 0.15 Wm?2.

All these estimates are consistent with each other. Most importantly, all find a statistically significant positive energy imbalance. The empirical data has spoken. Cancel the global cooling party. Global warming is still happening.

Posted by John Cook on Friday, 2 October, 2009

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