This is a guest post by Philip Bradley. Please note that all graphs can be seen full size by clicking on the image.
An analysis of Australian temperature data recorded at fixed times, and the implications for the 'global average land surface temperature' derived from minimum and maximum temperatures
Jonathan Lowe, an Australian statistician, has performed extensive analysis of weather data recorded at fixed times by Australia's Bureau of Meteorology (BoM). This analysis is available at his blog, A Gust of Hot Air. The data comes from 21 weather stations manned by professional meteorologists.
This work needs to be brought to a wider audience because it paints a very different climate picture to the global land datasets based on minimum and maximum temperatures – GISS, HadCRUT and the recent BEST analysis.
I'll present his discoveries in three parts, as there appear to be three significant elements in his work.
Using a minimum and maximum temperature dataset exaggerates the increase in the global average land surface temperature over the last 60 years by approximately 45%
Almost all the warming over the last 60 years occurred between 6am and 12 noon
Warming is strongly correlated with decreasing cloud cover during the daytime and is therefore caused by increased solar insolation
Reduced anthropogenic aerosols (and clouds seeded by anthropogenic aerosols) are the cause of most the observed warming over the last 60 years
I'll then add a part 4 covering some additional analysis of mine
Part 1 - Using a minimum and maximum temperature dataset exaggerates the increase in the global average land surface temperature over the last 60 years by approximately 45%
The BoM records temperature, and other values such at cloud cover, at three-hourly intervals for its main surface weather stations. This dataset goes back 60 years covering the same time period as the recent highly publicized BEST project.
Jonathan's analyses of the fixed time temperature data show that days have become significantly warmer. However, nights have warmed substantially less (only about a third of the day time warming).
Contrast this with the analyses based on minimum and maximum temperatures (Tmin, Tmax) which show warmer days and even warmer nights, based on the assumption Tmin represents nighttime temperatures.
In fact, Tmin usually occurs after dawn at the point when incoming solar insolation exceeds outgoing longwave radiation.
GISS, HadCRUT and BEST wrongly assume the mean of Tmin+Tmax represents average daily (24 hour) temperatures.
At best, the mean of Tmin+Tmax represents average daytime temperatures, and Jonathan's work confirms this.
Note at this point, that any day time warming that does not persist through the following night time is irrelevant to climate warming, as it is not heat that is retained in the climate system for more than 24 hours. So even a correctly derived average temperature is a poor indicator of the amount of warming.
In summary, calculating average temperature from the 3 hourly data instead of the Tmin+Tmax/2 method results in ~45% less warming since 1950.
The graph below illustrates this.
GISS, HadCRUT and Best all refer to their results based on Tmin+Tmax/2 as 'global average temperature'. Clearly, daily temperatures are a range and picking the mid-point of a range (which is what the mean of Tmin and Tmax is) is a poor way to estimate an average. Averaging measurements taken at fixed intervals will provided a more accurate estimate of the daily average.
The ~ 45% warming bias introduced into the 'global average temperature' by using Tmin+Tmax/2 is in itself is a very significant discovery, but there is considerably more to Jonathan's discoveries.
Part 2 – Almost all of the warming over the last 60 years occurred between 6am and 12 noon
Jonathan uses the simple but elegant method of subtracting the temperature at one three-hour interval from the temperature at the next three-hour interval to determine at what times of day the warming over the last 60 years has occurred. This breaks down the total warming over the last 60 years into three-hour slices of the day.
You can see his graphs here - http://gustofhotair.blogspot.com/2009/04/analysis-of-australian-temperature-part_16.html
He finds that warming has occurred in only three of these time slices; 6am to 9am, 9am to 12 noon and 6pm to 9pm. The other five three-hour time slices show either a cooling trend or no significant trend.
In parts 3 and 4, I discuss physical mechanisms that would cause temperature rises to be restricted to these time periods.
Jonathan's analysis shows that there has been no warming occurring at nighttime at all since 1950. Although nighttime temperatures have risen due to residual heat from warmer days.
There are numerous published papers that uncritically state that nighttime temperatures are warming based on the fact daily minimum temperature is increasing.
In 1990, Kukla and Karl published a paper in Environmental Science and Technology that contained the following,
Almost all of the observed increase in mean daily temperature over land over the past 40 years appear to be a result of the increase in early morning minimum (temperature). One is tempted to reason that the efficient blanket of the CO2 rich atmosphere keeps the nighttime temperatures high.
I find it surprising to say the least, that Tom Karl, Director Of The National Climate Data Center hasn't in the last 20 years investigated whether minimum temperatures (which he correctly states occurs in the early morning) do indeed measure nighttime temperatures, as this is perhaps the most important assumption underlying the evidence for AGW.
I could find only one published paper that investigates the relationship between minimum temperatures and measured nighttime temperatures and that was based on hourly temperature measurements taken at one location, the Manua Loa Observatory, and apparently made by the same team that measures CO2 levels. They found that hourly nighttime temperatures did indeed closely correspond with minimum temperatures. Interestingly, they found a slight decrease in daytime temperatures measured at 12 noon, The authors attribute their results to GHG warming. Although they concede other effects could be at work, including clouds.
A study using Anthony Watts' Surface Stations data reaches a couple of interesting, albeit tentative conclusions. Firstly, it found that for the best sited stations, diurnal temperature range (DTR) has not decreased and perhaps even increased over the last century. Secondly, it found that the PDO has a cyclical influence on DTR.
Jonathan's work indicates a marked increase in the range of nighttime to daytime temperatures using the three-hourly data, even though this data doesn't directly measure DTR. Decreasing DTR is considered the signature of GHG warming (more nighttime warming than daytime warming), and this study concludes decreasing DTR at the poorer sited stations may well result from data quality issues.
The Manua Loa study used data from 1977 to 2006, which closely corresponds with the warm phase of the PDO, and this may account for the findings.
Fixed time temperature data is available in the USA. The American Meteorological Society states at its website.
The National Weather Service (NWS) and its predecessor, the U.S. Weather Bureau, have operated a network of weather observation stations and offices in or near many of the large cities in every state, commonwealth, and territory under its jurisdiction. At many of these nearly 300 "first-order stations," systematic measurements of numerous weather elements are made by professional observers. Some of these weather data are recorded hourly,
I assume temperature measurements at fixed times is available in other countries.
Climate science collectively having wrongly assumed minimum temperatures measure nighttime temperatures then goes on to misinterpret the role of clouds, which is the next part of Jonathan's analysis.
Part 3 -Warming is strongly correlated with decreasing cloud cover during the daytime and is therefore caused by increased solar insolation
The BoM includes cloud cover in its three-hourly dataset.
Note that the BoM's website says that cloud cover is still measured by the traditional method of an observer estimating how many eighths of the sky is cloud covered. While I find this surprising, without evidence to the contrary I'll assume this information is correct.
The BoM's three-hourly cloud cover data shows that there has been a marked decrease in cloud cover in the daytime, with a marked increase in cloud cover in the early part of the nighttime (measured at 9pm).
Jonathan then correlates the cloud cover trend with the temperature trend for each three-hourly set of data. Shown in the graphic below.
Unsurprisingly, he finds decreased daytime cloud cover strongly correlated with increased daytime temperatures. There is a weaker correlation between cloud cover and overnight temperatures.
Surprisingly, he finds that the increased cloud cover at 9pm and midnight doesn't correlate with increased warming, which is directly contrary to what is generally accepted as the effect of clouds on nightime temperatures. In part 4, I propose that these clouds are condensation from nighttime radiative cooling, and therefore thin limiting their capacity to reflect longwave radiation. However, clouds and LWR is a complex subject.
The significance of the cloud cover temperature correlation is that it occurs in the daytime and accounts for both increased minimum and maximum temperatures, as decreased clouds result in increased solar insolation and warmer days.
The effect of clouds on minimum and maximum temperature seems to be widely misunderstood in the climate science community. For example,
From 1950 to at least the mid-1990s, minimum temperatures on land have increased about twice as fast as maximum temperatures [Easterling et al., 1997]. This may be attributable in part to increasing cloudiness, which reduces daytime warming by reflection of sunlight while retarding the nighttime loss of heat [Karl et al., 1997].
Karl has got things exactly the wrong way round.
Part 4 - Reduced anthropogenic aerosols (and clouds seeded by anthropogenic aerosols) are the cause of most the observed warming in the land surface over the last 60 years
While Jonathan correctly concludes that increased solar insolation is the primary cause of both increased minimum and increased maximum land surface temperatures, I'd like to extend that conclusion with some analysis of my own.
I mentioned earlier that the daily minimum temperature usually occurs in the early morning at the point when (incoming) solar insolation exceeds net outgoing longwave radiation. In the tropics this is a few minutes after dawn. At higher latitudes in winter it could be an hour or more after dawn.
When the sun rises, solar irradiance initially travels at a low angle through the atmosphere and as a result traverses far more of the atmosphere than it will for most of the day.
Any atmospheric effect that blocks or scatters solar irradiance and hence reduces solar insolation will have its maximum effect just after dawn around the time the minimum temperature occurs. Making the minimum temperature particularly sensitive to atmospheric effects that reduce insolation.
There are two effects at work reducing (especially early morning) solar insolation. One is clouds, particularly low level clouds. The other is aerosol pollution (smoke and haze), particularly low level aerosol pollution
Bear in mind that aerosols act as cloud seeding nuclei. So clouds and aerosols don't vary independently.
A great deal of work has been done on the effect of aerosols on the climate and atmospheric temperatures. However, I am concerned with one specific effect that of aerosols on daily minimum temperature.
Which makes me focus on the effect of black carbon aerosols (smoke to most people).
The Daily University Science News reported,
Black Carbon Aerosol Pollution Cools, Heats, Confuses
New research based on NASA satellite data and a multinational field experiment shows that black carbon aerosol pollution produced by humans can impact global climate as well as seasonal cycles of rainfall.
Because aerosols that contain black carbon both absorb and reflect incoming sunlight, these particles can exert a regional cooling influence on Earth's surface that is about 3 times greater than the warming effect of greenhouse gases.
But even as these aerosols reduce by as much as 10 percent the amount of sunlight reaching the surface, they increase the solar energy absorbed in the atmosphere by 50 percent -- thus making it possible to both cool the surface and warm the atmosphere.
Scientists are concerned that this heating may perturb atmospheric circulation and rainfall patterns.
"When we combined the satellite measurements with surface measurements, we found that the reduction of sunlight reaching the surface was three times larger than the amount of sunlight reflected to space," says V. Ramanathan, director of the Center for Clouds, Chemistry, and Climate at Scripps Institution of Oceanography at the University of California, San Diego. "Averaged over the entire northern Indian Ocean, the man-made pollutants reflected more solar radiation back to space (than pristine skies), but they absorbed up to twice as much radiation in the atmosphere."
The key phrase here is ' both cool the surface and warm the atmosphere' and recall that the daily minimum temperature occurs at the time when solar radiation is traversing the atmosphere at a low angle and any blocking or scattering by aerosols will have its maximum effect.
I discussed this with Jonathan and he confirmed that domestic burning of coal and wood in open fires, agricultural burning and the burning of garden waste was widespread in Australia 50 to 60 years but has since been almost completely eliminated. I grew up in the UK and can confirm the same thing occurred there.
Anyone under the age of 50 will have difficulty appreciating how much smoke an open hearth coal fire produces. I remember in the 1950s and 60s the winter morning ritual of lighting a coal fire. It produced a solid column of smoke up the chimney for 10 to 20 minutes before the coal got properly alight. Multiply that by tens of millions of homes and there was a lot of smoke within a few hundred feet of the ground when the sun rose.
Then there was burning of agricultural and garden waste in the summer and autumn. Burning a pile of wet leaves produces a terrific amount of smoke.
The smoke from a fire is generally pictured as going more or less straight up. It doesn't. Initially it goes upwards but as the smoke cools to the ambient air temperature, it drifts with the wind almost horizontally, forming a layer typically a few hundred feet above the ground.
Starting as early as the 1950s, clean air legislation in the developed world progressively reduced black carbon emissions until almost all routine emissions were eliminated. Which fits with the timing of increasing minimum and maximum temperatures in the global datasets
But what happened in the developing world, particularly rapidly industrializing China and India?
Some studies of temperature trends in China show a larger warming trend than the global average (derived from Tmin+Tmax/2), especially minimum temperature.
On investigation, I found that domestic coal burning which was the ubiquitous form of home heating 50 years is now banned in urban areas. So black carbon emissions will have substantially reduced in and around urban areas in China despite continuing high levels of other aerosol pollutants.
Other studies show substantially less surface warming in China than the global average in recent decades.
The situation in India is complicated by the monsoon, which 'washes' aerosol pollution out the atmosphere. There is no doubt that domestic burning of coal, wood, agricultural waste and dung has substantially increased black carbon aerosol levels outside the monsoon season in recent decades. A study from Pune concludes.
The analysis reveals signiﬁcant decrease in mean annual and mean maximum temperature (since 1900). This decrease in temperature is more pronounced during the winter season, which can be ascribed to a signiﬁcant increase in the amount of suspended particulate matter (SPM) in the ambient air during the last decade. On the contrary, monsoon season shows warming. This warming can be attributed to a signiﬁcant increase in the low cloud amount.
The other area of the world that's of relevance is the old Soviet Block, where pollution controls were minimal. When the collapse occurred, almost all of the highly polluting Soviet era industry was shut down within a few years, and former Eastern Bloc countries rapidly implemented the kinds of clean air initiatives already in place in the West, rapidly reducing aerosol levels toward those in the West.
Overall, the global picture is of substantially reduced aerosols, particularly black carbon, over most of the world since 1950. Although with substantial regional and national variations.
It is generally accepted that black carbon aerosols block sunlight, resulting in less solar insolation but at the same time warming the atmosphere. Decreasing black carbon will increase solar insolation, increasing minimum temperatures, while at the same time cooling the troposphere.
If changes in black carbon aerosol levels are as large as I believe, using land surface minimum temperature to measure global temperatures is giving an entirely false picture of the actual atmospheric warming over land. Increasing minimum temperatures is in fact evidence the atmosphere is cooling (and visa versa).
One final point before I wrap things up.
It is well accepted that aerosols, including black carbon, seed cloud formation. Decrease aerosols and you will decrease clouds. Therefore, I argue that the primary driver of changes in clouds found by Jonathan, and which I infer are primarily low level clouds, is changes in aerosol levels, particularly black carbon. Other studies support this conclusion, including the Pune study referred to above.
The reduced anthropogenic black carbon aerosols hypothesis neatly explains Jonathan's discovery of a sharp increase in cloud cover at 9pm.
If reduced anthropogenic aerosols are reducing cloud seeding and leading to less daytime clouds, more water vapour will remain in the atmosphere. This increased daytime water vapour will be more likely to condensate out as clouds when nightime radiative cooling occurs.
In overall summary, it is clear that increased solar insolation, caused by a combination of decreased clouds and decreased anthropogenic aerosols (and I argue particularly black carbon), is the primary cause of increasing minimum and increasing maximum temperatures, and hence the increase in the land surface 'global average temperature' calculated from these values.
Jonathan's analysis shows deriving 'average temperature' from Tmin+Tmax/2 over-estimates the landsurface average temperature by ~45%. He further shows that most of the remaining warming in the 'average temperature' since 1950 is due to increased solar insolation.
Increased solar insolation from decreased aerosol and aerosol seeded clouds also explains why warming measured by Tmin+Tmax/2 has levelled off in recent years, because in the developed world, parts of the developing world and the ex-Soviet Bloc there is limited remaining black carbon (and other forms of) aerosol pollution left to clean up.
While Jonathan's used Australia-only data, there appears to be no comparable study in any other part of the world and we should assume similar effects exist in the rest of the world.
In conclusion, it is troubling that climate science, including the recent BEST analysis, persists with the minimum and maximum temperature derivation of global land surface average temperature, when the methodology is flawed and the results misleading.
Note that Jonathan also analyses seasonal and tropical versus temperate zone differences. Space considerations made me omit them as they would require considerable discussion, but overall they are supportive of the conclusions above.