News flash: global warming continues (Nuccitelli et al. 2012)

from SkS:

We are very pleased to report on a new paper in press at Physics Letters A (PLA) by the Skeptical Science team and oceanography expert John Church.  In typical SkS international coordination style, the paper’s authors included an American (Dana Nuccitelli), a Canadian (Robert Way), a New Zealander (Rob Painting), and two Australians (John Cook and John Church).

The paper is a Comment on another paper, Douglass & Knox 2012 (DK12).  We originally began examining this paper in a blog post which can be viewed here.  DK12 used ocean heat content (OHC) data for the upper 700 meters of oceans to draw three main conclusions: 1) that the rate of OHC increase has slowed in recent years (the very short timeframe of 2002 to 2008), 2) that this is evidence for periods of ‘climate shifts‘, and 3) that the recent OHC data indicate that the net climate feedback is negative, which would mean that  climate sensitivity (the total amount of global warming in response to a doubling of atmospheric CO2 levels, including feedbacks) is low.

Our original draft blog post noted that DK12 had effectively been “pre-bunked,” as several recent studies have reconciled global heat content data with top of the atmosphere (TOA) energy imbalance measurements with no evidence of a long-term slowdown in global warming.  Several recent studies have also concluded that it is necessary to include data from the deep ocean in order to reconcile global heat content and the TOA energy imbalance, which DK12 failed to do.  Ultimately we decided that it was worth writing up our findings and submitting them to PLA as a comment on DK12.

We used pentadal (5-year average) OHC data to a depth of 2,000 metres from Levitus et al. (2012), and land, atmosphere, and ice (LAI) heating data from Church et al. (2011).  Our results are shown in Figure 1.

Figure 1: Land, atmosphere, and ice heating (red), 0-700 meter OHC increase (light blue), 700-2,000 meter OHC increase (dark blue).  From Nuccitelli et al. (2012), and added to the SkS Climate Graphics Page.

As this figure shows, there has been no significant slowing in global heat content.  We quantify this result in Table 1.

Table 1: Global Flux Imbalance During Selected Periods.  From Nuccitelli et al. (2012).

In fact the rate of net global heat content increase has risen.  The data also show that failing to account for increases in deep OHC is a problematic omission.

“We find that the OHC increase for the 700-2000 meter layer neglected by DK12 accounts for approximately 30% of the 0-2000 meter increase in recent decades.”

Mistaken Analysis Begets Mistaken Conclusions

Thus the DK12 conclusion that ocean heating slowed from 2002 to 2008 was a result of cherrypicking both a short timeframe and only part of the global heat content data.  As a result of cherrypicking noisy short-term data, DK12 argued that the apparent slowing in the rate of OHC increase was a result of a ‘climate shift‘ in 2002.  However, our Figure 1 and Table 1 illustrate that the long-term global heat content trend has risen at a steady, increasing rate over the past 4 decades.

DK12 compounded their erroneous analysis by attempting to calculate the net climate feedback based solely on their estimated 2002-2008 OHC increase for the uppermost 700 meters, and only considering the CO2 and solar radiative forcings, ignoring the significant aerosol forcing, for example.  As Nuccitelli et al. (2012) discusses, this attempted analysis is problematic for several reasons.

“A key conclusion in DK12, that the net CO₂ feedback is negative, is also based exclusively on an analysis of data during one of their proposed ‘climate shift’ periods (2002-2008) with a negative flux imbalance.  However, this conclusion does not hold during the ‘climate shift’ periods with a larger positive flux imbalance, and thus the conclusion is not robust.  Additionally, accounting for the heating of the oceans from 700 to 2,000 meters and LAI nullifies the DK12 conclusion even during the 2002-2008 timeframe.  The CO₂ feedback is effectively a constant value, and thus should not be calculated using such a short timeframe when data over a longer period are available.  The DK12 feedback calculation is invalidated by focusing on noisy short-term data and failing to account for all radiative forcings at work, as well as all heat reservoirs, in particular the oceans below 700 meters.”

Nuccitelli et al. Show that Global Warming Continues

Ultimately our paper shows that all three of the main conclusions in DK12 are faulty: the rate of OHC increase has not slowed in recent years, there is no evidence for ‘climate shifts‘ in global heat content data, and the recent OHC data do not support the conclusion that the net climate feedback is negative or that climate sensitivity is low.

I would like to conclude by once again thanking my co-authors for their work in successfully completing this paper.  We at SkS appreciate that John Church was willing to join our team, that his colleague Neal White was willing to provide us with their global heat content data set, and that their colleague and fellow oceanography expert Catia Domingues was willing to review our paper and provide valuable feedback to improve the paper.