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Wednesday, October 14, 2009

Joseph Romm: Skeptical Science explains how we know global warming is happening: It’s the oceans, stupid!

Skeptical Science explains how we know global warming is happening: It’s the oceans, stupid!

by Joseph Romm, Climate Progress, October 10, 2009
The empirical data has spoken. Cancel the global cooling party. Global warming is still happening.
The planet is heating up, thanks to human-generated emissions of greenhouse gases.  But as a new NOAA-led study, “An observationally based energy balance for the Earth since 1950” (subs. req’d, release here) concluded:
[S]ince 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.
Note that this Journal of Geophysical Research-Atmospheres study was done “without using global climate models.”

Figure 1: “Total Earth Heat Content [anomaly] from 1950 (Murphy et al. 2009). Ocean data taken from Domingues et al 2008.”

That figure comes from the first of two posts by the terrific website Skeptical Science, which I repost below.  Skeptical Science is an excellent, well-organized site to send convincible people for a shredding of the standard, long-debunked denier talking points.

Now I’m sure the deniers and delayers out there are shrieking, “There are peer reviewed analyses that document that upper ocean warming has halted since 2003!” — a claim I dealt with in my July post, “Like father, like son: Roger Pielke Sr. also doesn’t understand the science of global warming — or just chooses to willfully misrepresent it.”

Subsequently, however, another JGR article, “Global hydrographic variability patterns during 2003–2008” (subs. req’d, draft here) details an analysis of “monthly gridded global temperature and salinity fields from the near-surface layer down to 2000 m depth based on Argo measurements.”  Background on Argo here.  

Their findings are summed up in this figure:

Figure [2]: Time series of global mean heat storage (0–2000 m), measured in 108 Jm-2.

Still warming, after all these years!  And just where you’d expect it.  The study makes clear that upper ocean heat content, perhaps not surprisingly, is simply far more variable than deeper ocean heat content, and thus an imperfect indicator of the long-term warming trend.

UPDATE:  Yes, I am aware of the recent upper-ocean heat content data on the web.  Please note that plots of very recent, highly variable upper-ocean content heat data down to 700 meters from unpeer-reviewed sources do not trump peer-reviewed analysis of much longer-term data down to 2000 m.  Is it too much to ask people to actually read this entire post before posting comments?

What follows is a repost of two articles from Skeptical Science discussing these figures and the recent studies in more detail:

[I have renamed the figure in Part 2, "Figure 2" for the sake of clarity.]

How we know global warming is still happening, Part 1

Skeptics proclaim that global warming stopped in 1998. That we’re now experiencing global cooling. However, these arguments overlook one simple physical reality — the land and atmosphere are only one small fraction of the Earth’s climate (albeit the part we inhabit). Global warming is by definition global. The entire planet is accumulating heat due to an energy imbalance. The atmosphere is warming. Oceans are accumulating energy. Land absorbs energy and ice absorbs heat to melt. To get the full picture on global warming, you need to view the Earth’s entire heat content.
This analysis is performed in An observationally based energy balance for the Earth since 1950 (Murphy 2009) which adds up heat content from the ocean, atmosphere, land and ice. To calculate the Earth’s total heat content, the authors used data of ocean heat content from the upper 700 metres. They included heat content from deeper waters down to 3000 metres depth. They computed atmospheric heat content using the surface temperature record and the heat capacity of the troposphere. Land and ice heat content (eg – the energy required to melt ice) were also included.
[See Figure 1 above.]
A look at the Earth’s total heat content clearly shows global warming has continued past 1998. So why do surface temperature records show 1998 as the hottest year on record? Figure 1 shows the heat capacity of the land and atmosphere are small compared to the ocean (the tiny brown sliver of “land + atmosphere” also includes the heat absorbed to melt ice). Hence, relatively small exchanges of heat between the atmosphere and ocean can cause significant changes in surface temperature.
In 1998, an abnormally strong El Nino caused heat transfer from the Pacific Ocean to the atmosphere. Consequently, we experienced above average surface temperatures. Conversely, the last few years have seen moderate La Nina conditions which had a cooling effect on global temperatures. And the last few months have swung back to warmer El Nino conditions. This has coincided with the warmest June-August sea surface temperatures on record. This internal variation where heat is shuffled around our climate is the reason why surface temperature is such a noisy signal.
Figure 1 also underscores just how much global warming the planet is experiencing. Since 1970, the Earth’s heat content has been rising at a rate of 6 x 1021 Joules per year. In more meaningful terms, the planet has been accumulating energy at a rate of 190,260 GigaWatts. Considering a typical nuclear power plant has an output of 1 GigaWatt, imagine 190,000 nuclear power plants pouring their energy output directly into our oceans.

Figure 1 only goes as far as 2003 as the ocean heat data used (Domingues 2008) only goes that far. What has global warming been doing since then? Since 2003, ocean heat data has been measured by the newly deployed Argo network. However, there have been teething problems with the Argo buoys experiencing pressure sensor issues that impose a cooling bias on the data.
Consequently there have been several data analyses on ocean heat since 2003. One reconstruction of ocean heat show cooling since 2003 (Willis 2008). Other analyses of the Argo data show ocean warming (Levitus 2009, Leuliette 2009, Cazenave 2009).
How do we determine which analyses are more accurate? Ocean heat data can also be independently determined through other empirical means. Cazenave 2009 uses satellite gravity measurements to create two independent estimates of ocean heat – both find warming. Sea level has been inexorably rising since 2003. As a large portion of sea level rise is due to thermal expansion from ocean warming, this is an indirect confirmation of warming.

Lastly, the planet’s energy imbalance is confirmed by satellite measurements of incoming and outgoing radiation. Earth’s Global Energy Budget (Trenberth 2009) examines satellite measurements for the Mar 2000 to May 2004 period and finds the planet is accumulating energy at a rate of 0.9 ± 0.15 W m−2. This is consistent with the amount of heat accumulating in the ocean. Preliminary analysis on the latest CERES satellite data shows an increasing energy imbalance from 2004 to the end of 2008 (although this data is yet to be published, more on this later).
So the point to remember when considering short term cooling trends in surface temperature records is that the atmosphere is only one small part of a planet which is in energy imbalance. Empirical measurements show the planet continues to accumulate heat. More energy is coming in than is radiating back out to space. Global warming continued past 1998 and is still happening.

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:
[See Figure 2 above.]
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.

Link to Climate Progress blog:

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