The Science Of Hunga Tonga Global Warming - Part 4

I mentioned Solomon et al, 2010 in Part 3. Using simple arithmetic, we saw that, for a globally averaged increase in stratospheric water vapour of 1.5ppmv (the estimated increase due to the Hunga Tonga eruption), sustained over a period of 5 years, the expected increase in global mean surface temperature according to Solomon et al’s calculations would be 0.3C (assuming 0.2C/decade long term global warming trend). That is a significant warming and would mean the 1.5C ‘dangerous global warming’ threshold would be reached or exceeded within probably the next 5 years - albeit temporarily.

The Guardian ran an article on the Solomon paper at the time.

The writer said:

Scientists have underestimated the role that water vapour plays in determining global temperature changes, according to a new study that could fuel further attacks on the science of climate change.

The research, led by one of the world's top climate scientists, suggests that almost one-third of the global warming recorded during the 1990s was due to an increase in water vapour in the high atmosphere, not human emissions of greenhouse gases. A subsequent decline in water vapour after 2000 could explain a recent slowdown in global temperature rise, the scientists add.

It came at an ‘embarrassing time’ for climate science because of IPCC chief Rajendra Pachauri’s failed 2007 prediction of disappearing Himalayan glaciers, plus Climategate. January 2010, when the Solomon et al paper was published, was just two months after the Climategate scandal broke on November 19th 2009. So ‘difficult’ is probably an understatement! It became even more embarrassing of course when Pachauri was forced to step down after he was found guilty of sexually harassing a female employee, famously claiming effectively that his wandering hands had been hacked somehow by persons unknown (his defence against an allegation of sexual assault was a ridiculous claim that his emails and phone had been hacked)!

The new research comes at a difficult time for climate scientists, who have been forced to defend their predictions in the face of an embarrassing mistake in the 2007 report of the Intergovernmental Panel on Climate Change (IPCC), which included false claims that Himalayan glaciers could melt away by 2035. There has also been heavy criticism over the way climate scientists at the University of East Anglia apparently tried to prevent the release of data requested under Freedom of Information laws.

The new research, led by Susan Solomon, at the US National Oceanic and Atmospheric Administration, who co-chaired the 2007 IPCC report on the science of global warming, is published today in the journal Science, one of the most respected in the world.

Solomon said the new finding does not challenge the conclusion that human activity drives climate change. "Not to my mind it doesn't," she said. "It shows that we shouldn't over-interpret the results from a few years one way or another."

Solomon was forced to defend her own study in light of ‘the man-made global warming consensus’. Rather weakly, she pointed out the bleeding obvious - that natural variations in water vapour can significantly enhance or diminish the observed trend in global warming over decades (what she refers to as “a few years”). The Solomon study showed just how significant even a moderate 0.4ppmv change in stratospheric water vapour could be:

Satellite measurements were used to show that water vapour levels in the stratosphere have dropped about 10% since 2000. When the scientists fed this change into a climate model, they found it could have reduced, by about 25% over the last decade, the amount of warming expected to be caused by carbon dioxide and other greenhouse gases.

They conclude: "The decline in stratospheric water vapour after 2000 should be expected to have significantly contributed to the flattening of the global warming trend in the last decade."

Solomon said: "We call this the 10, 10, 10 problem. A 10% drop in water vapour, 10 miles up has had an effect on global warming over the last 10 years." Until now, scientists have struggled to explain the temperature slowdown in the years since 2000, a problem climate sceptics have exploited.

The scientists also looked at the earlier period, from 1980 to 2000, though cautioned this was based on observations of the atmosphere made by a single weather balloon. They found likely increases in water vapour in the stratosphere, enough to enhance the rate of global warming by about 30% above what would have been expected.

"These findings show that stratospheric water vapour represents an important driver of decadal global surface climate change," the scientists say. They say it should lead to a "closer examination of the representation of stratospheric water vapour changes in climate models".

Solomon’s study still stands today. It has not been withdrawn, or significantly challenged in respect of its scientific robustness, but other scientists have calculated a smaller effect of stratospheric water vapour on global temperature using different methods. Surprise, the science is not settled! The jury is still out, but if global surface temperatures maintain their extraordinary high over the next few years, then real world data might vindicate Solomon et al and thus vindicate her conclusions re. the contribution of stratospheric water vapour to past global warming. For completeness and balance though, I’d like to look at that other study which found that changes in stratospheric water vapour would have only a small effect on surface temperature. The authors mention Solomon et al:

It has been of interest to quantify the contribution of the SWV to the surface temperature change and examine whether the significant radiative perturbation previously reported can substantially influence the surface temperature. Solomon et al. (2010) argued that the observed SWV decrease may have acted to slow the rate of global surface warming over 2000–2009 by about 25%, using an intermediate-complexity model prescribed with observed SWV changes as a forcing. Wang et al. (2017) used the Community Earth System Model (CESM) in a nudging experiment to measure the surface temperature variability in response to the observed SWV change and argued the SWV-induced surface warming accounts for 19% of the observed surface temperature increase. These works suggested that there may be significant surface temperature response to SWV perturbations in observed (transient) climate change, although it remains unclear how, that is, via what processes, SWV change leads to surface temperature change.

Note they also mentioned a later study which found that SWV accounted for 19% of an observed decadal increase in temperature (Wang et al, 2017). The authors of the new study found only a 5.4% contribution to global warming from SWV:

We take the SWV increase as an external forcing and prescribe the SWV concentrations in the coupled slab-ocean-atmosphere model. The SWV increase causes 0.42 K global mean surface temperature increase, accounting for 5.4% of the total warming (7.7 K) caused by the quadrupling of the CO₂. The SWV average mixing ratio change is 5.2 ppmv, yielding a normalized surface temperature change, which is defined as the surface temperature change per unit SWV mixing ratio change, of 0.08 K ppmv⁻¹.

That’s not much, in fact it seems puny. Solomon et al results in a maximum warming of 0.2C per 1ppmv increase in SWV (0.3C per 1.5ppmv - see Part 3). If we assume a more realistic lower rate of global warming (in line with UAH lower tropospheric trend) due to GHGs of 0.13C/decade, we get 0.13C per 1ppmv increase. That results in an increase in GMST over 5 years of nearly 0.2C due to Hunga Tonga which, as explained in Part 3, is still 3 times the expected increase in surface temperature due to GHGs over that short period. According to the new authors’ calculations though, we can only expect an increase in temperature of 1.5x0.08=0.12C over the 5 year period; however, even this lower forcing estimate still means the temperature increase due to Hunga Tonga is more than the expected increase in surface temperature due to GHGs alone (1.84 times to be exact). So even though it seems puny, in comparison to GHG forcing over 5 years, it’s not. But there is good reason to suspect that the new authors’ calculations are not applicable in the case of a sudden increase in stratospheric water vapour due to volcanic activity, as the authors themselves explain:

Our conclusion, regarding the equilibrium climate, differs from Wang et al. (2017) in which the authors showed the contribution from SWV could explain 19% of the observed surface warming prior to the warming hiatus, almost 4 times of the contribution in our estimation, although we use the same model.

To understand this difference, we compare the normalized SWV effect, defined as the surface temperature change per unit SWV mixing ratio change. The result in section 3.1 renders a normalized SWV effect of 0.08 K ppmv⁻¹ in our simulation. In Wang et al. (2017), a uniform 1 ppmv increase of SWV produced a 0.12 K increase in surface temperature, that is, a normalized effect of 0.12 K ppmv⁻¹. This quantitative difference may result from the different techniques used to prescribe the SWV perturbation as well as the different patterns of the SWV. Wang et al. (2017) used a nudging method to add a water vapor tendency term in the water vapor budget equation to simulate the climate response to SWV. Essentially, this represents a virtual source constantly supplying water vapor in the stratosphere and keeping the system out of equilibrium. In this paper, we prescribe the SWV concentration representing the GCM-simulated stratospheric moistening in the radiation scheme to simulate the equilibrium climate response to the SWV radiative perturbation.

Nevertheless, the difference in normalized warming (0.08 vs. 0.12 K ppmv⁻¹) cannot fully explain the larger difference in the surface warming attribution (5% vs. 19%) between the two studies. The primary reason is likely the different warming contexts used in the two studies. Our study is in the context of the quadrupled CO₂. The climate change only arises from the CO₂ increase and the 5% attribution is obtained by comparing the partial surface warming induced by the SWV change associated with the CO₂ perturbation to the total surface warming driven by the CO₂ perturbation, both simulated by the same model. The context of Wang et al. (2017) is the observed surface temperature change, which is subject to the effects of different forcing agents that may lead to different associations of the SWV and surface temperature changes.

So the authors make a fairly good job of explaining why their surface temperature response to an increase in SWV is so very different from Wang et al (and Solomon et al): it comes down to the fact that they are simulating the equilibrium response after a quadrupling of CO2 concentration (resulting in a long term increase in SWV concentration) whereas Wang et al were considering the non-equilibrium response to different radiative forcings (e.g. short term natural fluctuations in SWV - including those due to volcanoes).

Guardian Climate Change Journalists Don’t (or Can’t) Read Science Papers

Lastly, I just want to point out what idiots they are at the Guardian. I’m pretty sure the following is not a case of deliberate misinformation on the part of the journalist, but simply incompetence. In January 2010, David Adam, the Environment Correspondent wrote:

The scientists also looked at the earlier period, from 1980 to 2000, though cautioned this was based on observations of the atmosphere made by a single weather balloon. They found likely increases in water vapour in the stratosphere, enough to enhance the rate of global warming by about 30% above what would have been expected.

But if you read Solomon et al, this is what the authors actually say about the period 1980 to 1996-2000:

In a second case, it was assumed that water vapor had increased uniformly by 1 ppmv at all latitudes and altitudes above the tropopause between 1980 and the 1996–2000 period. A total globally averaged radiative forcing including stratospheric adjustment of +0.24 W m–2 was obtained for this assumed 1 ppmv increase, close to the value of +0.29 W m–2 reported for example in (3). This can be compared to the radiative forcing increase due to the growth of carbon dioxide of about +0.36 W m–2 from 1980–1996.

0.24 is two thirds 0.36, i.e. SWV vapour forcing over the period 1980 to 2000 was a huge 66% of the forcing due to GHGs alone. That’s not a 30% enhancement! The 30% increase occurred over the 1990s:

Figure 3 also shows that an increase in global stratospheric water vapor at the upper end of the range suggested by the balloon measurements should be expected to have steepened the rate of global warming from 1990–2000 by about 30% compared to a case neglecting stratospheric water changes.

To be fair to our hopelessly confused Guardian Environment Correspondent, the headline, just for a change, is more accurate than the text:

Water vapour caused one-third of global warming in 1990s, study reveals

Still, 13 years later and they haven’t got around to correcting their article - and probably won’t now as it would mean admitting that stratospheric water vapour increases can enhance by as much as two thirds the expected amount of global warming over a period of 20 years due to GHGs alone. Not good, especially when you’ve got a Hunga Tonga elephant in the room.