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Coby,

I accept that I should not have said [in my previous blog post, which can be accessed by clicking here] that well mixed GHGs “presumably have a similar warming impact in both hemispheres” (Figure 11 of Hansen et al, third map down in right column refers).

But I don’t agree that ‘there is no significant discrepancy between modeled and observed behaviours.’ For example, on my reading of Figure 18 of Hansen et al (2005), to which Gavin directed me, the observed warming exceeded modelled warming by a significant margin over much the greater part of the northern hemisphere in the 1979-2003 period. In the Antarctic the discrepancy was in the opposite direction.

For the world as a whole, both the observed and the modelled warming in this period was about 0.4°C – i.e., equivalent to less than one-seventh of the vertical axis of the Figure. Visual inspection suggests that, over about one-half of the globe, the DIFFERENCE between the observed and modelled warming for this 24-year period was similar to, or greater than, the observed average warming for the world as a whole for this period. I’m surprised that you think that these aren’t significant discrepancies.

I did not misinterpret the mapping of aerosols and their effects on the ‘Sulphur Cycle Experiment’ page of the climateprediction.net website. The description is quite clear: the map shows ‘the model’s surface temperature response to increasing sulphur emissions from pre-industrial levels.. to present day levels..’. The description also says that ‘The cooling effect of sulphate aerosol can be seen throughout the whole northern hemisphere’ (which isn’t strictly true: there is a warming effect in Northern Scandinavia).

The accompanying text says that ‘a PREDICTION of the climate of the 21st century needs to contain the effects of sulphate aerosol otherwise the warming trend may be OVERestimated’ (EMPHASES added). There are two errors here. First, the climateprediction.net simulations aren’t predictions; and secondly, the statement assumes, contrary to most expectations, that sulphate aerosol emissions will increase in the 21st century.

In fact, nearly all of scenarios project that emissions of sulphur oxides will DECREASE in this century. For the four SRES markers, the projected decreases between 2000 and 2100 are: A1, 60%; A2, 13%; B1, 84%; and B2, 31%. For the two illustrative A1 scenarios, the projected decreases are: A1FI, 42%; and A1T, 71%. These decreases lead to significant positive (negative of a negative) forcing in the 21st century, and concomitant WARMING in the IPCC scenarios.

There are huge uncertainties in relation to aerosols, both in respect of the trends in emissions in recent times and in the effects on climate. Having already tripped myself up once in a confusion of forcings and temperatures, I’ll avoid drawing my own conclusions and will simply draw a contrast between what Hansen et al (2005) say and what is posted on the climateprediction.net website:

(a) Hansen et al (2005) assess the total 1880-2003 negative aerosol forcing, including the indirect effect, as equivalent to more than one-half of the effective forcing for the total of the well-mixed GHGs (CO2, CH4, N2O and the CFCs) over the same period (pps. 5, 7). But they say that ‘empirical data for checking model-based temporal changes of tropospheric aerosol amount.. are meager’, and ‘Our largely subjective estimate of the uncertainty in the net aerosol forcing is at least 50%’ (p. 7).

(b) Hansen et al (2005) also say that: ‘Observed global warming, as well as the global warming in the model driven by all forcings, has been nearly constant at almost 0.15°C/decade over the past 3-4 decades, except for temporary interruptions by large volcanoes. This high warming rate has been maintained in the recent decade despite a slowdown in the growth rate of climate forcing by well-mixed GHGs.. The warming rate in the model is maintained because, BY ASSUMPTION, TROPOSPHERIC AEROSOLS STOP INCREASING IN 1990.. The ASSUMPTION that global aerosol amount approximately levelled off after 1990 IS UNCERTAIN, because adequate aerosol observations are not available.. An implicit well-known conclusion is that future global warming may depend substantially on how the global aerosol amount continues to evolve, as well as on the GHG growth rate’ (EMPHASES added).

(c) By (apparent) contrast, the simulations on the climateprediction.net website for the average of 66 models that had ‘made it to at least 2005’, to which I referred in an earlier post on this thread, show ‘overheating’ as a result of the non-inclusion of sulphate aerosols on what seems to be a much larger scale than implied in Hansen et al. Moreover, the widening of the gap between the ‘without aerosols’ temperature simulations and observations appears to be at least as great between 1990 and 2005 as in the decades preceding 1990.

To me, this suggests that the climateprediction.net estimates of aerosol emissions do NOT level off after 1990, and that a continuing growth in such emissions is reflected in the ‘Sulphur Cycle Experiment’ map.

In your initial post, Coby, you told me that for my follow up questions I might find Chapter 12 of the TAR informative, and provided a link. I am in fact quite familiar with what is said in that Chapter on regional climate projections, for which Australia’s John Zillman was Review Editor. If this post was not already overlong, I’d draw on the conclusions of that chapter, and on some of John’s subsequent statements on this subject, to reinforce some of the points made above.

I’m sorry if you think that I am again raising ‘the most elementary complication’ and that, because I ‘have never done anything but the most cursory research, [I] assume no one ever before has ever thought of it’ – and am ‘triumphantly pronounc[ing] climate science as an ignorant religion.’

You alleged, referring to me, ‘that it was completely understandable that most climate scientists are not interested in responding to people who come with pre-formed conclusions that imply they are stupid or frauds’, and said that it is ‘a credit to people like Gavin that they understand the importance of this issue and therefore the importance of overlooking, for the most part, such egregious behaviour.’

And you concluded that ‘the primary consequence is that the real experts in general decide not to waste their time with prejudice matched by ignorance and it falls on non-experts like [you] to spend the time.’

Let me assure you that I don’t come to climate change science issues with pre-formed conclusions that imply that climate change scientists are stupid or frauds. I tried for a long time to stick to my knitting, but found that I was being criticised (notably on this blog) for pleading ignorance of climate change science.

I’ve therefore decided that we should all be prepared to ask some dumb questions in areas outside our area of specialisation and that’s what I’m doing. I’ll have some separate questions to address to Gavin, but in the meantime I hope that you are able to take some time to respond to the issues raised above.

Ian Castles

According to the New Zealand Herald:

“A group of leading climate scientists has announced the formation of the New Zealand Climate Science Coalition, aimed at refuting what it believes are unfounded claims about man-made global warming.

“We believe this is a significant development in opening up the debate about the real effects of climate change and the justification for the costs and other measures prescribed in the Kyoto protocols,” said the coalition’s secretary, Terry Dunleavy.

He said members of the coalition had had enough of “over-exaggerated” claims about the effects of man-made global warming and aimed to provide a balance to “what is being fed to the people of New Zealand”.

He said that the coalition’s three main roles would be:

* To publish and distribute papers and commentaries produced by members of the coalition;

* To audit statements by other organisations, both in New Zealand and overseas, which are published in New Zealand, or are expected to influence New Zealand public policy and public opinion;

* To audit the forthcoming United Nations Intergovernmental Panel on Climate Change (IPCC) report.

The coalition has registered a website domain name, www.climatescience.org.nz, which it expects to have running within a day or two.”

And I received the following comment from a reader of this weblog with the link to the newspaper:

“A newspaper snippet on New Zealand contrarians banding together to defeat the IPCC forces of darkness !

I can only hope you give these contrarian guys as much stick as Hansen and the IPCC. Any spurious arguments or hanging one on, and you should be up them for the rent.

And have a look how many contrarian blogs still have the MSU satellite* story the wrong way around.”

I am of course keen to publish criticisms and comments on information at the new New Zealand Climate Science Coalition website, email short essays to jennifermarohasy@jennifermarohasy.com .

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* For those wondering what the MSU data is, here’s a snippet from ABC Online last August, click here. The article explains how satellite measurements suggesting cooling rather than warming in the troposphere were an artifact of a wrongly calibrated satellite.

It is interesting to read what the explanation HAD BEEN at Global Hydrology and Climate Centre on 14 June 2000 before the calibration problem was discovered:

Global temperatures have been monitored by satellite since 1979 with the Microwave Sounding Units (MSU) flying on the National Oceanic and Atmospheric Administration’s (NOAA) TIROS-N series of polar-orbiting weather satellites. Data from nine separate satellites have been combined to provide a global record of temperature fluctuations in the lower troposphere (the lowest 5 miles of the atmosphere) and the lower stratosphere (covering an altitude range of about 9-12 miles). The global image above shows monthly-averaged temperature anomalies (departure from seasonal normals), while the graph shows point or area-averaged anomalies for the entire period of record (since January 1979).

The lower tropospheric data are often cited as evidence against global warming, because they have as yet failed to show any warming trend when averaged over the entire Earth. The lower stratospheric data show a significant cooling trend, which is consistent with ozone depletion. In addition to the recent cooling, large temporary warming perturbations may be seen in the data due to two major volcanic eruptions: El Chichon in March 1982, and Mt. Pinatubo in June 1991.

Was this explanation just sweep under the carpet when the scientists found that the satellite data was showing a warming trend? In hindsight how credible was this explanation?

Climatologists at Duke University in the US have been running some simulations for the Northern Hemisphere and in their latest press release are suggesting that the higher range temperature scenarios are unlikely but nevertheless give a probability for a 4.5C increase and suggest that there were plenty of “ups and downs” in temperature before this “modern era”. Here’s the press release:

Durham, N.C. — Instrumental readings made during the past century offer ample evidence that carbon dioxide and other “greenhouse gases” in the atmosphere are warming Earth’s climate, a team led by Duke University scientists has reported. But by analyzing indirect evidence of temperature fluctuations over six previous centuries, the team also found that the magnitude of future global warming will likely fall well short of current highest predictions.

In making their deductions, the researchers ran some 1,000 computer simulations, covering 1,000 years, that took into account a range of modern and ancient climate records. Modern records are based on thermometer readings, while measurements derived from such sources as tree rings and ice cores served as markers of warm and cold spells over prior centuries.

The investigators evaluated the data using an “energy balance model” that they describe as a slimmed-down version of the heavy-duty computer models typically used to analyze climate trends. It is the model’s streamlined nature that enabled the researchers to perform such large numbers of simulations over such a long period in such detail, they said.

The group used thousands of different versions of this model, each version varying in some of its properties, in order to determine which variants best matched actual observations. One key property that varied was what the researchers termed “sensitivity” — that is, how much the simulations’ temperatures would change in response to increasing greenhouse gas levels.

“What I can say very confidently is that the present-day sensitivity is not zero, meaning that there is a positive, warming response to greenhouse gases,” said climate analyst Gabriele Hegerl, an associate research professor at Duke’s Nicholas School of the Environment and Earth Sciences. “Our work also substantially reduces the probability of very high climate sensitivities.”

Hegerl is lead author of the study, published April 20, 2006, in the journal Nature. Her co-authors are Thomas Crowley, Duke’s Nicholas Professor of Earth Systems Science; William Hyde, a former Nicholas School research scientist now at the University of Toronto; and David Frame, a researcher at the University of Oxford.

Their work was supported by the National Oceanic and Atmospheric Administration, the U.S. Department of Energy and the National Science Foundation.

Many scientists expect that the level of carbon dioxide in the atmosphere will sometime this century reach double the levels that were present during preindustrial times. Because carbon dioxide traps outgoing heat energy similarly to the glass in a greenhouse, the additional human-created outputs of the gas — mostly from fossil-fuel burning — are expected to warm Earth’s climate. The key question is: by how much?

The commonly accepted range for how much average global temperatures will rise in response to a doubling of atmospheric carbon dioxide is between 1.5 degrees and 4.5 degrees Celsius, according to the researchers. But some observational studies, they noted, suggest the possibility that average temperatures might rise more than 9 degrees.

However, the new study — using “reconstructions” of Northern Hemisphere temperatures since the year 1270 — indicates a 90 percent probability that a doubling of carbon dioxide levels will result in temperature increases of between 1.5 degrees and 6.2 degrees, the team reported.

In turn, the study showed a reduced likelihood that the actual maximum increase will exceed 4.5 degrees — “from 36 percent to 15 percent or less,” the researchers said. A 4.5 degree increase is the highest maximum currently predicted by the international Intergovernmental Panel on Climate Change.

Hegerl said her group confined its study largely to the Northern Hemisphere because only there have scientists collected enough data to reconstruct temperature variations over the entire past millennium.

According to Hegerl, some studies claim that preindustrial temperatures fluctuated very little until the past century, and have risen sharply since.

“But our reconstruction supports a lot of variability in the past, as well as an upward trend in the 20th century,” she said. And a record with plenty of ups and downs before the modern era “shows a climate reacting then and now to a variety of ‘external forcing,'” she said.

The term “external forcing” refers to all those outside influences that can perturb the climate. Understanding how temperatures responded to such forcings in the premodern era — when the impact of carbon dioxide and other heat-trapping gases varied relatively little — helps scientists predict future forcings by greenhouse gases, Hegerl said.

“Looking back longer in time makes it possible to more confidently rule out responses that are very high or very low,” she said.

The researchers consulted instrumental records of the various forcings that have occurred in modern times, with the aim of comparing those to actual recorded temperatures.

In order to reconstruct temperatures from the centuries before 1850, the team used various lines of indirect evidence. They looked, for example, at particulates trapped in ice cores as measures of past volcanic eruptions. Such eruptions eject clouds of particles high into the atmosphere. By reducing the amount of sunlight that can pass through the atmosphere, the particles tend to cool the climate for a time, Hegerl said.

They also consulted a number of tree ring studies that reveal hot and cold spells in ancient growth variations, as well as studies that can estimate temperatures as far back as the 1600s based on readings obtained from holes bored deep into the ground.

Although the researchers collected data spanning a full millennium, because of some technical limitations they actually simulated temperature variations over a roughly 700-year period beginning in 1270.

All in all, the researchers considered four different detailed reconstructions of past climates, including a new reconstruction done by Crowley and Hegerl, to deduce probable temperatures before reliable instruments were available.

According to Hegerl, past volcanic eruptions provided the strongest tie between past climate forcings and temperatures. “You can see downturns in temperature exactly where you see volcanic eruptions,” she said.