Arguments
Nuccitelli et al. (2012) Show that Global Warming Continues
Posted on 12 October 2012 by dana1981, robert way, Rob Painting, John Cook
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).
|
Time Period |
0-700 meter OHC (W/m2) |
700-2,000 meter OHC (W/m2) |
LAI Heating (W/m2) |
Net Heat Content Increase (W/m2) |
|
1970-2008 |
0.21 ± 0.063 |
0.082 ± 0.030 |
0.025 ± 0.0012 |
0.31 ± 0.078 |
|
1980-2008 |
0.23 ± 0.062 |
0.12 ± 0.017 |
0.027 ± 0.0019 |
0.37 ± 0.068 |
|
1990-2008 |
0.29 ± 0.082 |
0.14 ± 0.11 |
0.030 ± 0.0031 |
0.46 ± 0.063 |
|
2000-2008 |
0.35 ± 0.13 |
0.15 ± 0.020 |
0.029 ± 0.0068 |
0.53 ± 0.11 |
|
2002-2008 |
0.44 ± 0.17 |
0.26 ± 0.039 |
0.036 ± 0.0044 |
0.73 ± 0.16 |
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. Over 90% of global warming accumulates in the oceans, and there is no indication that it has slowed.
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.
Ocean heat content studies find that global warming has accelerated in recent years – for example, Levitus 2012 finds a rise in OHC of around 10^23 Joules over the last decade, twice that of the previous decade.
At the same time, the growth rate of CO₂ forcing has declined slightly – i.e. we are putting more CO₂ into the atmosphere, but the airborne fraction has declined, so the CO₂ forcing hasn’t been rising quite as steeply since about 1990. This means that the net climate forcing, according to GISS, hasn’t risen since about 2000 -
http://data.giss.nasa.gov/modelforce/
We know that the existing planetary energy imbalance will cause continuing warming for many decades due to ocean thermal inertia, but we wouldn’t expect warming to be *accelerating* if the climate forcing hasn’t increased for 13 years. That’s a puzzle, it seems to me.
One explanation could be that the acceleration in OHC accumulation isn’t real. Another could be that it’s real, and that additional natural positive feedbacks have been kicking in to accelerate the warming despite the known forcings being level for 13 years. A third explanation might be that we have overestimated the negative forcing from atmospheric aerosols – since this is largely assumed or estimated rather than measured, perhaps it’s not been offsetting the greenhouse gas forcing as much as expected in recent years, meaning that GISS are underestimating the net climate forcing.
Does anyone have a view on what the most likely explanation is? Thanks!
Icarus,
Given a climate sensitivity of a mere 2˚C per doubling, the current global temperature increase should currently be 2 * log2(395/285) or 0.94˚C. If climate sensitivity is instead 3, then warming should be 1.4˚C. As such, if we have seen 0.8˚C (using the beginning of the twentieth century as a baseline — if the baseline is 1979, then warming to date due to CO2 forcing is less) then we should expect more.
This implies that there is anywhere from 0.14˚C to 0.8˚C of warming "in the pipeline," (depending on climate sensitivity and start point) yet to be realized even if net forcing freezes at the current level forever. As such, the fact that the oceans are absorbing this warming (for now) is unsurprising. [And, of course, the reason that the net forcing is steady is because of a temporary confluence of negative natural forcings. One should expect (a) net forcing to increase when the negative natural forcings terminate and (b) the ocean to slow its heat uptake and instead allow the atmosphere to come into equilibrium with the ocean.]
Dana, did you, or anyone else on the team, respond to the Douglas and Knox reply published online in Nov 2012? It seems to me they are just saying they prefer their noisy data. Is there any substnce to their claims?
Rugbyguy59 - No. There's basically nothing to respond to. Their 'step-change' claim is nonsensical though, and is easily appreciated by looking at the sea level trend over the period of 2004-2008, a period which Douglass & Knox claim the Earth's radiative balance was negative . The ocean is by far the Earth's largest reservoir of heat (around 93% of global warming goes into it), and a negative radiative balance would imply the thermal expansion component of sea level rise was also negative during that period. Sea level rise should have seen a dramatic plunge, but no such thing happened.
Then there's the problem of the Earth undergoing instantaneous net loss and gain of heat. How would it be possible to instantaneously mix heat into the ocean, and then lose it again? How could that heat escape the ocean without warming the ocean surface, and cooking the atmosphere, on its way out to space?
Drawing lines on a graph, as they did, does not deflect attention away from the physical impossibilty of it all.
Thanks Rob.
I have a question about LAI heating data in figure 1. Is it taken from part 2.3-5 in Church et al. (2011)? If you add those numbers you get 0.4+0.2 *1022J from ice, 0.2 from the atmosphere and 0.67 from land. This adds up to ca 1.5 *1022J, which seems like your result, although your graph implies you have more data from the period and not just an interpolation of these values.
devadatta @69 - basically yes. Church was one of our co-authors and provided his full data set for our paper.