I few months ago, I had a paper accepted in the Journal of Geophysical Research. Since its repercussions are particularly interesting for the general public, I decided to write about it. I would have written earlier, but as I wrote before, I have been quite busy. I now have time, sitting in my hotel in Lijiang (Yunnan, China).

A calorimeter is a device which measures the amount of heat given off in a chemical or physical reaction. It turns out that one can use the Earth's oceans as one giant calorimeter to measure the amount of heat Earth absorbs and reemits every solar cycle. Two questions probably pop in your mind,
a) Why is this interesting?
and,
b) How do you do so?
Let me answer.

One of the raging debates in the climate community relates to the question of whether there is any mechanism amplifying solar activity. That is, are the solar synchronized climatic variations that we see (e.g., take a look at fig. 1 here) due to changes of just the solar irradiance, or, are they due to some effect which amplifies the solar-climate link. In particular, is there an amplification of some non-thermal component of the sun? (e.g., UV, solar magnetic field, solar wind or others which have much larger variations than the 0.1% variations of the solar irradiance). This question has interesting repercussions to the question of global warming, which is why the debate is so fierce.

If only solar irradiance is the cause of the solar-related climate variations, it would imply that the small solar variations cause large temperature variations on Earth, and therefore that Earth has a very sensitive climate. If on the other hand there is some amplification mechanism, it would imply that solar variations induce much larger variations in the radiative budget, and that the observed temperature variations can therefore be explained with a smaller climate sensitivity.

Since global warming alarmists want a large sensitivity, they adamantly fight any evidence which shows that there might be an amplification mechanism. Clearly, a larger climate sensitivity would imply that the same CO₂ increase over the 21^st century would cause a larger temperature increase, that is, allow for a more frightening scenario, more need for climate research and climate action, and more need for research money for them. (I am being overly cynical here, but it some cases it is not far from the truth). Others don't even need research money, don't really care about the science (and certainly don't understand it), but make money from riding the wave anyway (e.g., a former vice president, without naming names).

On the other end of the spectrum, politically driven skeptics want to burn fossil fuels relentlessly. A real global warming problem would force them to change their plans. Therefore, any argument which would imply a small climate sensitivity and a lower predicted 21^st century temperature increase is favored by them. Just like their opponents, they do so without actually understanding the science.

I of course, don't get money from oil companies. In fact, I am not a republican (hey, I am even the head of a workers union). I care about the environment (I grew up in a solar house) and think there are a dozen good reasons why we should burn less fossil fuels, but as you will see below, global warming is not one of them. In fact, I am driven by something strange... the quest for the knowledge!

With this intro, you can realize why answering the solar amplification question is very important (besides being a genuinely interesting scientific question), and why answering it (either way) would make some people really annoyed.

So, what do the oceans tell us?

Over the 11 or so year solar cycle, solar irradiance changes by typically 0.1%. i.e., about 1 W/m² relative to the solar constant of 1360 W/m². Once one averages for the whole surface of earth (i.e., divide by 4) and takes away the reflected component (i.e., times 1 minus the albedo), it comes out to be about 0.17 W/m² variations relative to the 240 W/m². Thus, if only solar irradiance variations are present, Earth's sensitivity has to be pretty high to explain the solar-climate correlations (see the collapsed box below).

However, if solar activity is amplified by some mechanism (such as hypersensitivity to UV, or indirectly through sensitivity to cosmic ray flux variations), then in principle, a lower climate sensitivity can explain the solar-climate links, but it would mean that a much larger heat flux is entering and leaving the system every solar cycle.
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The IPCC's small solar forcing and the emperor's new clothes.

With the years, the IPCC has tried to downgrade the role of the sun. The reason is stated above - a large solar forcing would necessarily imply a lower anthropogenic effect and lower climate sensitivity. This includes perpetually doubting any non-irradiance amplification mechanism, and even emphasizing publications which downgrade long term variations in the irradiance. In fact, this has been done to such an extent, that clear solar/climate links such as the Mounder minimum are basically impossible to explain with any reasonable climate sensitivity. Here are the numbers.

According to the IPCC (AR4), the solar irradiance is responsible for a net radiative forcing increase between the Maunder Minimum and today of 0.12 W/m² (0.06 to 0.60 at 90% confidence). We know however that the Maunder minimum was about 1°C colder (e.g., from direct temperature measurements of boreholes - e.g., this summary). This requires a global sensitivity of 1.0/0.12°C/(W/m²). Since doubling the CO₂ is thought to induce a 3.8 W/m² change in the radiative forcing, irradiance/climate correlations require a CO₂ doubling temperature of ΔT_x2 ~ 31°C !! Besides being at odds with other observations, any sensitivity larger than ΔT_x2 ~ 10°C would cause the climate to be unconditionally unstable (see box here).

Clearly, the IPCC scientists don't comprehend that their numbers add up to a totally inconsistent picture. Of course, the real story is that solar forcing, even just the irradiance change, is larger than the IPCC values. [/collapse] Now, is there a direct record which measures the heat flux going into the climate system? The answer is that over the 11-year solar cycle, a large fraction of the flux entering the climate system goes into the oceans. However, because of the high heat capacity of the oceans, this heat content doesn't change the ocean temperature by much. And as a consequence, the oceans can be used as a "calorimeter" to measure the solar radiative forcing. Of course, the full calculation has to include the "calorimetric efficiency" and the fact that the oceans do change their temperature a little (such that some of the heat is radiated away, thereby reducing the calorimetric efficiency).

It turns out that there are three different types of data sets from which the ocean heat content can derived. The first data is is that of direct measurements using buoys. The second is the ocean surface temperature, while the third is that of the tide gauge record which reveals the thermal expansion of the oceans. Each one of the data sets has different advantages and disadvantages.

The ocean heat content, is a direct measurement of the energy stored in the oceans. However, it requires extended 3D data, the holes in which contributed systematic errors. The sea surface temperature is only time dependent 2D data, but it requires solving for the heat diffusion into the oceans, which of course has its uncertainties (primarily the vertical turbulent diffusion coefficient). Last, because ocean basins equilibrate over relatively short periods, the tide gauge record is inherently integrative. However, it has several systematic uncertainties, for example, a non-neligible contribution from glacial meting (which on the decadal time scale is still secondary).

Nevertheless, the beautiful thing is that within the errors in the data sets (and estimate for the systematics), all three sets give consistently the same answer, that a large heat flux periodically enters and leaves the oceans with the solar cycle, and this heat flux is about 6 to 8 times larger than can be expected from changes in the solar irradiance only. This implies that an amplification mechanism necessarily exists. Interestingly, the size is consistent with what would be expected from the observed low altitude cloud cover variations.

Here are some figures from the paper:

fig. 1: Sea Surface Temperature anomaly, Sea Level Rate, Net Oceanic Heat Flux, the TSI anomaly and Cosmic Ray flux variations. In the top panel are the inverted Haleakala/Huancayo neutron monitor data (heavy line, dominated by cosmic rays with a primary rigidity cutoff of 12.9 GeV), and the TSI anomaly (TSI - 1366 W/m² , thin line, and based on Lean [2000]). The next panel depicts the net oceanic heat flux, averaged over all the oceans (thin line) and the more complete average heat flux in the Atlantic region (Lon 80°W to 30°E, thick line), based on Ishii et al. [2006]. The next two panels plot the SLR and SST anomaly. The thin lines are the two variables with their linear trends removed. In the thick lines, the ENSO component is removed as well (such that the cross-correlation with the ENSO signal will vanish).

fig 2: Sea Level vs. Solar Activity. Sea level change rate over the 20th century is based on 24 tide gauges previously chosen by Douglas [1997] for the stringent criteria they satisfy (solid line, with 1-σ statistical error range denoted with the shaded region). The rates are compared with the total solar irradiance variations Lean [2000] (dashed line, with the secular trends removed). Note that unlike other calculations of the sea level change rate, this analysis was done by first differentiating individual station data and then adding the different stations. This can give rise to spurious long term trends (which are not important here), but ensure that there are no spurious jumps from gaps in station data. The data is then 1-2-1 averaged to remove annual noise. Note also that before 1920 or after 1995, there are about 10 stations or less such that the uncertainties increase.

fig 3: Summary of the “calorimetric” measurements and expectations for the average global radiative forcing F_global. Each of the 3 measurements suffers from different limitations. The ocean heat content (OHC) is the most direct measurement but it suffers from completeness and noise in the data. The heat flux obtained from the sea surface temperature (SST) variations depends on the modeling of the heat diffusion into the ocean, here the diffusion coefficient is the main source of error. As for the sea level based flux, the largest uncertainty is due to the ratio between the thermal contribution and the total sea level variations. The solid error bars are the global radiative forcing obtained while assuming that similar forcing variations occur over oceans and land. The dotted error bars assume that the radiative forcing variations are only over the oceans. These measurements should be compared with two different expectations. The TSI is the expected flux if solar variability manifests itself only as a variable solar constant. The “Low Clouds+TSI” point is the expected oceanic flux based on the observed low altitude cloud cover variations, which appear to vary in sync with the solar cycle (while assuming several approximations). Evidently, the TSI cannot explain the observed flux going into the ocean. An amplification mechanism, such as that of CRF modulation of the low altitude cloud cover is required.

So what does it mean?

First, it means that the IPCC cannot ignore anymore the fact that the sun has a large climatic effect on climate. Of course, there was plenty of evidence before, so I don't expect this result to make any difference!

Second, given the consistency between the energy going into the oceans and the estimated forcing by the solar cycle synchronized cloud cover variations, it is unlikely that the solar forcing is not associated with the cloud cover variation.

Note that the most reasonable explanation to the cloud variations is that of the cosmic ray cloud link. By now there are many independent lines of evidence showing its existence (e.g., for a not so recent summary take a look here). That is, the cloud cover variations are controlled by an external lever, which itself is affected by solar activity.

Incidentally, talking about the oceans, Arthur C. Clarke made once a very cute observation:


References:
1) Nir J. Shaviv (2008); Using the oceans as a calorimeter to quantify the solar radiative forcing, J. Geophys. Res., 113, A11101, doi:10.1029/2007JA012989. Local Copy.