Lord Monckton...hero of Denialism
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Medieval Warm Period was warmer
The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made......Not Really a Lord Monckton.
While the Medieval Warm Period saw unusually warm temperatures in some regions, globally the planet was cooler than current conditions. ....science and facts.
The Medieval Warm Period spanned 950 to 1250 AD and corresponded with warmer temperatures in certain regions. During this time, ice-free seas allowed the Vikings to colonize Greenland. North America experienced prolonged droughts. Just how hot was the Medieval Warm Period? Was the globe warmer than now? To answer this question, one needs to look beyond warming in a few regions and view temperatures on a global scale.
Prior temperature reconstructions tend to focus on the global average (or sometimes hemispheric average). To answer the question of the Medieval Warm Period, more than 1000 tree-ring, ice core, coral, sediment and other assorted proxy records spanning both hemispheres were used to construct a global map of temperature change over the past 1500 years (Mann 2009). The Medieval Warm Period saw warm conditions over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America. In these regions, temperature appears to be warmer than the 1961–1990 baseline. In some areas, temperatures were even as warm as today. However, certain regions such as central Eurasia, northwestern North America, and the tropical Pacific are substantially cooler compared to the 1961 to 1990 average.
Figure 1: Reconstructed surface temperature anomaly for Medieval Warm Period (950 to 1250 A.D.), relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable.
How does the Medieval Warm Period compare to current conditions? Here is the temperature pattern for the last decade (1999 to 2008). What we see is widespread warming (with a few exceptions such as regional East Antarctic cooling)
Figure 3: Surface temperature anomaly for period 1999 to 2008, relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable (NOAA).
The Medieval Warm Period was not a global phenomenon. Warmer conditions were concentrated in certain regions. Some regions were even colder than during the Little Ice Age. To claim the Medieval Warm Period was warmer than today is to narrowly focus on a few regions that showed unusual warmth. However, when we look at the broader picture, we see that the Medieval Warm Period was a regional phenomenon with other regions showing strong cooling. Globally, temperatures during the Medieval Period were less than today.
The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made......Not Really a Lord Monckton.
While the Medieval Warm Period saw unusually warm temperatures in some regions, globally the planet was cooler than current conditions. ....science and facts.
The Medieval Warm Period spanned 950 to 1250 AD and corresponded with warmer temperatures in certain regions. During this time, ice-free seas allowed the Vikings to colonize Greenland. North America experienced prolonged droughts. Just how hot was the Medieval Warm Period? Was the globe warmer than now? To answer this question, one needs to look beyond warming in a few regions and view temperatures on a global scale.
Prior temperature reconstructions tend to focus on the global average (or sometimes hemispheric average). To answer the question of the Medieval Warm Period, more than 1000 tree-ring, ice core, coral, sediment and other assorted proxy records spanning both hemispheres were used to construct a global map of temperature change over the past 1500 years (Mann 2009). The Medieval Warm Period saw warm conditions over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America. In these regions, temperature appears to be warmer than the 1961–1990 baseline. In some areas, temperatures were even as warm as today. However, certain regions such as central Eurasia, northwestern North America, and the tropical Pacific are substantially cooler compared to the 1961 to 1990 average.
Figure 1: Reconstructed surface temperature anomaly for Medieval Warm Period (950 to 1250 A.D.), relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable.
How does the Medieval Warm Period compare to current conditions? Here is the temperature pattern for the last decade (1999 to 2008). What we see is widespread warming (with a few exceptions such as regional East Antarctic cooling)
Figure 3: Surface temperature anomaly for period 1999 to 2008, relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable (NOAA).
The Medieval Warm Period was not a global phenomenon. Warmer conditions were concentrated in certain regions. Some regions were even colder than during the Little Ice Age. To claim the Medieval Warm Period was warmer than today is to narrowly focus on a few regions that showed unusual warmth. However, when we look at the broader picture, we see that the Medieval Warm Period was a regional phenomenon with other regions showing strong cooling. Globally, temperatures during the Medieval Period were less than today.
I see,so you believe in CO2 driven global warming when there is no scientific proof and I'm just plain looney. I can't argue that with you but I can suggest deworming and distemper shots may help with your symptoms. I hope it isn't rabies.
Monckton....
It's cooling
"Global warming has stopped and a cooling is beginning. No climate model has predicted a cooling of the Earth – quite the contrary. And this means that the projections of future climate are unreliable." (source: Henrik Svensmark)
What the science says....
To say we're currently experiencing global cooling overlooks 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 (the energy required to melt ice) were also included.
Figure 1: Total Earth Heat Content from 1950 (Murphy 2009). Ocean data taken from Domingues et al. 2008.
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.
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. Schuckmann 2009 analyzes ocean temperature measurements by the Argo network, constructing a map of ocean heat content down to 2000 metres. 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:
Figure 2: Time series of global mean heat storage (0–2000 m), measured in 108 Joules per square metre.
Globally, the oceans continued to accumulate heat right to the end of 2008. Over the last 5 years, the oceans have been absorbing heat at a rate of 0.77 Watts per square metre. Combined with the results of Murphy 2009, we now see a picture of continued global warming.
How does this value compare to other estimates of energy imbalance? Willis 2004 combines satellite altimetry with ocean heat measurements to find an ocean warming rate of 0.85 Watts per square metrefrom 1993 to 2003. Hansen 2005, using ocean heat data, calculated the planet's energy imbalance in 2003 to be 0.85 Watts per square metre. 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 Watts per square metre.
These results all find broad agreement and all find a statistically significant positive energy imbalance. Our climate is still accumulating heat. Global warming is still happening.
It's cooling
"Global warming has stopped and a cooling is beginning. No climate model has predicted a cooling of the Earth – quite the contrary. And this means that the projections of future climate are unreliable." (source: Henrik Svensmark)
What the science says....
To say we're currently experiencing global cooling overlooks 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 (the energy required to melt ice) were also included.
Figure 1: Total Earth Heat Content from 1950 (Murphy 2009). Ocean data taken from Domingues et al. 2008.
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.
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. Schuckmann 2009 analyzes ocean temperature measurements by the Argo network, constructing a map of ocean heat content down to 2000 metres. 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:
Figure 2: Time series of global mean heat storage (0–2000 m), measured in 108 Joules per square metre.
Globally, the oceans continued to accumulate heat right to the end of 2008. Over the last 5 years, the oceans have been absorbing heat at a rate of 0.77 Watts per square metre. Combined with the results of Murphy 2009, we now see a picture of continued global warming.
How does this value compare to other estimates of energy imbalance? Willis 2004 combines satellite altimetry with ocean heat measurements to find an ocean warming rate of 0.85 Watts per square metrefrom 1993 to 2003. Hansen 2005, using ocean heat data, calculated the planet's energy imbalance in 2003 to be 0.85 Watts per square metre. 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 Watts per square metre.
These results all find broad agreement and all find a statistically significant positive energy imbalance. Our climate is still accumulating heat. Global warming is still happening.
The skeptic argument...
There's no tropospheric hot spot
The IPCC confirms that computer modeling predicts the existence of a tropical, mid-troposphere “hot spot” about 10km above the Earth’s surface. Yet in the observed record of the Hadley Centre’s radiosondes, the predicted “hot-spot” signature of anthropogenic greenhouse warming is entirely absent (source: Christopher Monckton)
What the science says...
Satellite measurements match model results apart from in the tropics. There is uncertainty with the tropic data due to how various teams correct for satellite drift. The U.S. Climate Change Science Program conclude the discrepancy is most likely due to data errors.
Part 1: The “Hotspot” as an Alleged Fingerprint of Anthropogenic Warming
A great deal of the confusion surrounding the issue of temperature trends in the upper troposphere comes from the mistaken belief that the presence or lack of amplification of surface warming in the upper troposphere has some bearing on the attribution of global warming to man-made causes.
It does not.
Attribution of anthropogenic origins of the current climatic changes can be tested from many different directions. On of the most clear examples for those with some familiarity with the Earth’s atmosphere is the issue of stratospheric cooling. If the sun were to suddenly increase its output by 2%, we would rightfully expect the atmosphere as well as the surface to warm up in response. This can be examined, for instance, by looking at the response in a GCM like GISS ModelE:
2% increase in solar forcing (via RealClimate)
Likewise, if we were to double preindustrial levels of CO2, we would expect the surface and the lower atmosphere to warm. However, unlike the case of increasing solar influence, we would not expect the lower atmosphere to warm through at all levels. Increasing the greenhouse effect should warm the surface and troposphere, but cool the lower stratosphere.
Doubling of CO2 (via RealClimate)
In the doubled CO2 scenario, there is a pronounced cooling of higher altitudes, i.e. the stratosphere, and this feature is entirely absent in the +2% solar scenario.
This stratospheric cooling is a fingerprint of increased greenhouse (as opposed to solar) warming. For a more in depth discussion of why the stratosphere cools under enhanced greenhouse warming, see discussions at Skeptical Science and The Science of Doom. In other words, the difference in the two simulations is not the presence of a "hot spot" in one and its absence in the other, it's the stratospheric cooling apparent in the increased CO2 simulation.
In the IPCC Fourth Assessment Report (AR4), historical forcings were simulated in the Parallel Climate Model, and and the zonal mean temperature responses to each were broken out in separate panels. There was some increase in solar irradiance during the period, which shows up as a modest amount of warming throughout the atmosphere, with some amplification in the upper troposphere (the sort of greenish-yellow and yellow patterns respectively in panel a). As we all know, there was a significant change in GHG forcing during that time, which manifests as surface warming, amplified upper troposphere warming, and stratospheric cooling (panel c), and the net effect of all forcings was shown (panel f).
Fig 9.1: Zonal mean atmospheric temperature change from 1890 to 1999 (°C per century) as simulated by the PCM model from (a) solar forcing, (b) volcanoes, (c) well-mixed greenhouse gases, (d) tropospheric and stratospheric ozone changes, (e) direct sulphate aerosol forcing and (f) the sum of all forcings. Plot is from 1,000 hPa to 10 hPa (shown on left scale) and from 0 km to 30 km. (IPCC AR4 WG1)
So far so good. Right? Well, this is actually where things went off the rails.
Climate “skeptics” apparently became convinced that the “hot spot” in Figure 9.1c was the fingerprint of anthropogenic warming the IPCC was referring to, rather than stratospheric cooling coupled with tropospheric warming.
As he so often does, Monckton serves as a useful example of getting things wrong, claiming:
The mistaken belief in “skeptic” circles is that the existence of anthropogenic warming somehow hinges on the existence of the tropospheric “hot spot”- it does not. Period. Tropospheric amplification of warming with altitude is the predicted response to increasing radiative forcing from natural sources, such as an increase in solar irradiance, as well. Stratospheric cooling is the real "fingerprint" of enhanced greenhouse vs. natural (e.g. increased solar) warming.
Part 2: The Existence of Amplified Warming in the Upper Tropical Troposphere
So, does the “hot spot” actually exist? That is to say, is the tropsosphere actually warming as expected? Unfortunately, the answer to this is much less cut and dry.
There is a good theoretical basis for this expectation of amplification in the upper troposphere relative to the surface. We expect that an increase in radiative forcing would result in a moist adiabatic amplification of warming with altitude, i.e. that the troposphere would warm faster with height. This also appears as an emergent property in climate models, which show a similar vertical profile of warming to that expected under the moist adiabatic lapse rate.
Unfortunately, actually determining what is happening in the real tropical troposphere has proven to be quite difficult. Perhaps the largest reason for this is the quality of data from the main source of our information from this region for long time periods- radiosonde networks.
Although on seasonal and annual scales, some radiosonde records are in relatively good agreement with theoretical and modeling expectations, on decadal timescales, they show less warming or even cooling of the upper troposphere. However, the tropics, especially at higher altitudes, are a notorious problem area for most if not all of the older radiosonde networks. And attempts to stitch together longer records from multiple networks (and integrate them with newer satellite records) have introduced problems as well. There have been many attempts to quantify and remove these biases (e.g. Randel 2006, Sherwood 2008). Although these attempts have managed to reconcile the observational data with theoretical and model expectations within overlapping uncertainty intervals, the real world behavior of the troposphere is still unclear (Bengtsson 2009, Thorne 2010).
Allen and Sherwood sought to side step the problems associated with the radiosonde data entirely, and examined the “dynamical relationship known as the thermal-wind equation, which relates horizontal temperature gradients to wind shear”. Thermal wind speed data, in contrast to the temperature data, lacked many of the systematic adjustment issues and other errors, and were used as a proxy for temperature. Allen and Sherwood found that the troposphere appeared to be warming in reasonable agreement to theoretical and modeling expectations.
Vertical profile of tropical mean temperature trends. Trends reflect the mean change in temperature (in K per decade) between 20° N and 20° S for the period 1979–2005, obtained from radiosonde temperature measurements5 (blue and green colours), climate models8 (dashed orange, with grey shading indicating 2-sigma range) and the new reconstructions from radiosonde winds4 (pink, with error bars indicating 2-sigma range). The surface temperature change11 from 1979–2005 (grey asterisk) and the vertical profile inferred from the moist adiabatic lapse rate (dashed yellow) are also shown. The model range was derived by scaling the model vertical trend behaviour (which has been shown to be tightly constrained8) and its uncertainties8 by the surface trend. Prior to 2007, only the HadAT and RATPAC estimates existed, and a case could be made for a fundamental discrepancy between modelled and radiosonde observed behaviour. (Thorne 2008)
Recently, Johnson and Xie have approached the question from a different but similarly indirect angle. They examined trends in tropical sea surface temperatures (SSTs) and precipitation, which have direct implications for the behavior of the vertical tropical tropospheric temperature profile:
As the SST threshold for convection is tied to convective instability, this threshold must be strongly related to the tropical upper-tropospheric temperature. Observations show that tropospheric temperatures in the tropics approximately follow a moist-adiabatic temperature profile, which suggests an adjustment of upper-tropospheric temperatures in response to surface temperatures in the tropics. This hypothesis of moist-adiabatic lapse rate (MALR) adjustment predicts a close covariability between the SST threshold and tropical mean SST. If true, the variability and long-term trend of the SST threshold may reveal important information about the variability and trends in the tropical troposphere.
Climate warming over the tropical oceans [exaggerated]: a) In a climate before warming, convection and heavy tropical rain is restricted to a region where SSTs exceed a threshold value (dotted line), and temperatures decrease with altitude. b) Johnson and Xie show that this SST threshold has risen in tandem with mean SSTs over past decades, and that the area of surface ocean where convection occurs has remained constant. As a result of warming at the sea surface, air temperatures rise most at high altitudes. (Sobel 2010)
Tropical convection and thus precipitation is heavily dependent on sea surface temperatures (SSTs). Thus the absence of increased precipitation is indicative of stability upwards through the troposphere, which suggests that the upper tropical troposphere is indeed warming faster than surface temperatures.
[T]he similarity between the trends of SST and the SST threshold for convection in [the following figure] is consistent with approximate MALR adjustment in observations and inconsistent with reduced upper tropospheric warming relative to the surface, as indicated in some observational data sets. Although the statistical uncertainty of 30-year trends is rather high, the clean relationship between the SST threshold and tropical mean SST at all timescales in both observations and models increases confidence that the tropical atmosphere is warming in a manner that is broadly consistent with theoretical MALR expectations.
Time series of tropical mean SST and the SST threshold for convection. Thirty-year time series of annual tropical mean (20° S to 20° N) SST (black diamonds) and two estimates of the SST threshold for convection (blue squares and red stars). Linear trend lines are also shown. The linear trends with 95% confidence intervals for the tropical mean SST, the PD2mmd^-1 SST threshold estimate and the linear P fit SST threshold estimate are 0:088±0:057;0:083±0:076 and 0:080±0:113 °C per decade, respectively. The effective degrees of freedom in the 95% confidence interval calculations account for the lag-1 autocorrelation in the residual time series. (Johnson 2010)
Is this the “final word” on amplified tropospheric warming? Of course not. Ideally, instrumental biases and gaps in the satellite and radiosonde records can be sorted out, longer records from newer networks will provide more confident results, and we can get an even clearer picture of what’s going on in the tropical troposphere. In the meantime, however, this is further evidence that things are behaving more or less as we’d expect.
But moreover, these papers illustrate some key aspects of science (and particularly climate science), that could use some emphasizing. Science is iterative, not dictative or supernaturally revelatory. There’s no single, infallible decree. Science is the process by which we strive to best approximate reality. The first results are not necessarily the “best” results, and they certainly are not written in stone. Our monitoring systems, particularly (ironically?) the ones with multidecadal records, were not designed for the kind of questions we may be trying to investigate with them. Or, to paraphrase a certain former Secretary of Defense, you study the world with the instruments you have, not the instruments you might want or wish to have at a later time. Would life be a lot easier if we had designed and implemented global climatic monitoring systems in the 60s and 70s with an eye for explicitly addressing the questions we have now? Of course! But we have to make do with what we’ve got, and that means working with problematic data and finding creative ways to work around them. To that end, it’s worth pointing out, proxies aren’t just used to study the past.
Through comments here and at other blogs, I get the impression that people think using proxies is restricted to paleo questions. The presumption seems to be that in our digital, high-speed, satellite-monitored age, direct observations are the only game in town. As this case shows, however, this is decidedly not true. Indirect methods of assessing an issue are sometimes the only (or only alternate in the case of suspect data) methods available. And that’s not necessarily a bad thing! Sometimes looking at a question from a different angle can avoid some potential complications altogether. And finally, there is a pernicious lie that can be heard in climate denialist circles, typified by remarks like these from Dick Lindzen:
This does illustrate a germ of truth buried in Lindzen’s conspiratorial falsehood, however. Climate models and theoretical climate dynamics/meteorology are constrained by physics, and for the most part, models tend to agree with physics-based, theoretical underpinnings of meteorology/climate dynamics. When there is an apparent discrepancy between “models” and observations, that often (but not always) means there is a discrepancy between general, theoretical meteorological expectations and the observational data. It’s not a case of trying to reconcile the observations with climate models, but rather trying to reconcile observational data (which often have well known biases) with our physics-based understanding of the climate system.
When people are quick to point out some alleged contradiction between climate models and a data set, they don’t realize that often as not they are pointing out a contradiction between the observations and our fundamental explanations of the climate system irrespective of the question of anthropogenic influence. And far from justifying a position of “nothing to worry about”, significant flaws in our understanding of the climate system would greatly strengthen the case for mitigation from a risk management perspective, as uncertainty and ignorance of consequences increase the relative value of insurance. But that’s a topic for a different day…
References:
There's no tropospheric hot spot
The IPCC confirms that computer modeling predicts the existence of a tropical, mid-troposphere “hot spot” about 10km above the Earth’s surface. Yet in the observed record of the Hadley Centre’s radiosondes, the predicted “hot-spot” signature of anthropogenic greenhouse warming is entirely absent (source: Christopher Monckton)
What the science says...
Satellite measurements match model results apart from in the tropics. There is uncertainty with the tropic data due to how various teams correct for satellite drift. The U.S. Climate Change Science Program conclude the discrepancy is most likely due to data errors.
Part 1: The “Hotspot” as an Alleged Fingerprint of Anthropogenic Warming
A great deal of the confusion surrounding the issue of temperature trends in the upper troposphere comes from the mistaken belief that the presence or lack of amplification of surface warming in the upper troposphere has some bearing on the attribution of global warming to man-made causes.
It does not.
Attribution of anthropogenic origins of the current climatic changes can be tested from many different directions. On of the most clear examples for those with some familiarity with the Earth’s atmosphere is the issue of stratospheric cooling. If the sun were to suddenly increase its output by 2%, we would rightfully expect the atmosphere as well as the surface to warm up in response. This can be examined, for instance, by looking at the response in a GCM like GISS ModelE:
Likewise, if we were to double preindustrial levels of CO2, we would expect the surface and the lower atmosphere to warm. However, unlike the case of increasing solar influence, we would not expect the lower atmosphere to warm through at all levels. Increasing the greenhouse effect should warm the surface and troposphere, but cool the lower stratosphere.
In the doubled CO2 scenario, there is a pronounced cooling of higher altitudes, i.e. the stratosphere, and this feature is entirely absent in the +2% solar scenario.
This stratospheric cooling is a fingerprint of increased greenhouse (as opposed to solar) warming. For a more in depth discussion of why the stratosphere cools under enhanced greenhouse warming, see discussions at Skeptical Science and The Science of Doom. In other words, the difference in the two simulations is not the presence of a "hot spot" in one and its absence in the other, it's the stratospheric cooling apparent in the increased CO2 simulation.
In the IPCC Fourth Assessment Report (AR4), historical forcings were simulated in the Parallel Climate Model, and and the zonal mean temperature responses to each were broken out in separate panels. There was some increase in solar irradiance during the period, which shows up as a modest amount of warming throughout the atmosphere, with some amplification in the upper troposphere (the sort of greenish-yellow and yellow patterns respectively in panel a). As we all know, there was a significant change in GHG forcing during that time, which manifests as surface warming, amplified upper troposphere warming, and stratospheric cooling (panel c), and the net effect of all forcings was shown (panel f).
Fig 9.1: Zonal mean atmospheric temperature change from 1890 to 1999 (°C per century) as simulated by the PCM model from (a) solar forcing, (b) volcanoes, (c) well-mixed greenhouse gases, (d) tropospheric and stratospheric ozone changes, (e) direct sulphate aerosol forcing and (f) the sum of all forcings. Plot is from 1,000 hPa to 10 hPa (shown on left scale) and from 0 km to 30 km. (IPCC AR4 WG1)So far so good. Right? Well, this is actually where things went off the rails.
Climate “skeptics” apparently became convinced that the “hot spot” in Figure 9.1c was the fingerprint of anthropogenic warming the IPCC was referring to, rather than stratospheric cooling coupled with tropospheric warming.
As he so often does, Monckton serves as a useful example of getting things wrong, claiming:
the models predict that if and only if Man is the cause of warming, the tropical upper air, six miles above the ground, should warm up to thrice as fast as the surface, but this tropical upper-troposphere “hot-spot” has not been observed...
This unequivocally incorrect claim was also made in the NIPCC "skeptic" report (Section 3.4), which was signed off on by such supposedly "serious" contrarians as Craig Idso and S. Fred Singer. The mistaken belief in “skeptic” circles is that the existence of anthropogenic warming somehow hinges on the existence of the tropospheric “hot spot”- it does not. Period. Tropospheric amplification of warming with altitude is the predicted response to increasing radiative forcing from natural sources, such as an increase in solar irradiance, as well. Stratospheric cooling is the real "fingerprint" of enhanced greenhouse vs. natural (e.g. increased solar) warming.
Part 2: The Existence of Amplified Warming in the Upper Tropical Troposphere
So, does the “hot spot” actually exist? That is to say, is the tropsosphere actually warming as expected? Unfortunately, the answer to this is much less cut and dry.
There is a good theoretical basis for this expectation of amplification in the upper troposphere relative to the surface. We expect that an increase in radiative forcing would result in a moist adiabatic amplification of warming with altitude, i.e. that the troposphere would warm faster with height. This also appears as an emergent property in climate models, which show a similar vertical profile of warming to that expected under the moist adiabatic lapse rate.
Unfortunately, actually determining what is happening in the real tropical troposphere has proven to be quite difficult. Perhaps the largest reason for this is the quality of data from the main source of our information from this region for long time periods- radiosonde networks.
Although on seasonal and annual scales, some radiosonde records are in relatively good agreement with theoretical and modeling expectations, on decadal timescales, they show less warming or even cooling of the upper troposphere. However, the tropics, especially at higher altitudes, are a notorious problem area for most if not all of the older radiosonde networks. And attempts to stitch together longer records from multiple networks (and integrate them with newer satellite records) have introduced problems as well. There have been many attempts to quantify and remove these biases (e.g. Randel 2006, Sherwood 2008). Although these attempts have managed to reconcile the observational data with theoretical and model expectations within overlapping uncertainty intervals, the real world behavior of the troposphere is still unclear (Bengtsson 2009, Thorne 2010).
Allen and Sherwood sought to side step the problems associated with the radiosonde data entirely, and examined the “dynamical relationship known as the thermal-wind equation, which relates horizontal temperature gradients to wind shear”. Thermal wind speed data, in contrast to the temperature data, lacked many of the systematic adjustment issues and other errors, and were used as a proxy for temperature. Allen and Sherwood found that the troposphere appeared to be warming in reasonable agreement to theoretical and modeling expectations.
Recently, Johnson and Xie have approached the question from a different but similarly indirect angle. They examined trends in tropical sea surface temperatures (SSTs) and precipitation, which have direct implications for the behavior of the vertical tropical tropospheric temperature profile:
As the SST threshold for convection is tied to convective instability, this threshold must be strongly related to the tropical upper-tropospheric temperature. Observations show that tropospheric temperatures in the tropics approximately follow a moist-adiabatic temperature profile, which suggests an adjustment of upper-tropospheric temperatures in response to surface temperatures in the tropics. This hypothesis of moist-adiabatic lapse rate (MALR) adjustment predicts a close covariability between the SST threshold and tropical mean SST. If true, the variability and long-term trend of the SST threshold may reveal important information about the variability and trends in the tropical troposphere.
Tropical convection and thus precipitation is heavily dependent on sea surface temperatures (SSTs). Thus the absence of increased precipitation is indicative of stability upwards through the troposphere, which suggests that the upper tropical troposphere is indeed warming faster than surface temperatures.
[T]he similarity between the trends of SST and the SST threshold for convection in [the following figure] is consistent with approximate MALR adjustment in observations and inconsistent with reduced upper tropospheric warming relative to the surface, as indicated in some observational data sets. Although the statistical uncertainty of 30-year trends is rather high, the clean relationship between the SST threshold and tropical mean SST at all timescales in both observations and models increases confidence that the tropical atmosphere is warming in a manner that is broadly consistent with theoretical MALR expectations.
Is this the “final word” on amplified tropospheric warming? Of course not. Ideally, instrumental biases and gaps in the satellite and radiosonde records can be sorted out, longer records from newer networks will provide more confident results, and we can get an even clearer picture of what’s going on in the tropical troposphere. In the meantime, however, this is further evidence that things are behaving more or less as we’d expect.
But moreover, these papers illustrate some key aspects of science (and particularly climate science), that could use some emphasizing. Science is iterative, not dictative or supernaturally revelatory. There’s no single, infallible decree. Science is the process by which we strive to best approximate reality. The first results are not necessarily the “best” results, and they certainly are not written in stone. Our monitoring systems, particularly (ironically?) the ones with multidecadal records, were not designed for the kind of questions we may be trying to investigate with them. Or, to paraphrase a certain former Secretary of Defense, you study the world with the instruments you have, not the instruments you might want or wish to have at a later time. Would life be a lot easier if we had designed and implemented global climatic monitoring systems in the 60s and 70s with an eye for explicitly addressing the questions we have now? Of course! But we have to make do with what we’ve got, and that means working with problematic data and finding creative ways to work around them. To that end, it’s worth pointing out, proxies aren’t just used to study the past.
Through comments here and at other blogs, I get the impression that people think using proxies is restricted to paleo questions. The presumption seems to be that in our digital, high-speed, satellite-monitored age, direct observations are the only game in town. As this case shows, however, this is decidedly not true. Indirect methods of assessing an issue are sometimes the only (or only alternate in the case of suspect data) methods available. And that’s not necessarily a bad thing! Sometimes looking at a question from a different angle can avoid some potential complications altogether. And finally, there is a pernicious lie that can be heard in climate denialist circles, typified by remarks like these from Dick Lindzen:
t has become standard in climate science that data in contradiction to alarmism is inevitably ‘corrected’ to bring it closer to alarming models. None of us would argue that this data is perfect, and the corrections are often plausible. What is implausible is that the ‘corrections’ should always bring the data closer to models.
Lindzen’s implication is clear- observational data that don’t support “models” are fraudulently adjusted until they do, ergo climate change is at least partially an artifact of data manipulation. This is, in a word, absurd. First, due to the ludicrous nature of the claim and its inherent absolutism, it’s easily debunked by a single contrary example. Take, for instance, the notorious problem of climate models producing double ITCZs (e.g. Zhang 2006). This is a case in which models produced a result at odds with both theoretical expectations and observations. No one attempted to claim that the models were right about this and theory and observations were wrong.
This does illustrate a germ of truth buried in Lindzen’s conspiratorial falsehood, however. Climate models and theoretical climate dynamics/meteorology are constrained by physics, and for the most part, models tend to agree with physics-based, theoretical underpinnings of meteorology/climate dynamics. When there is an apparent discrepancy between “models” and observations, that often (but not always) means there is a discrepancy between general, theoretical meteorological expectations and the observational data. It’s not a case of trying to reconcile the observations with climate models, but rather trying to reconcile observational data (which often have well known biases) with our physics-based understanding of the climate system.
When people are quick to point out some alleged contradiction between climate models and a data set, they don’t realize that often as not they are pointing out a contradiction between the observations and our fundamental explanations of the climate system irrespective of the question of anthropogenic influence. And far from justifying a position of “nothing to worry about”, significant flaws in our understanding of the climate system would greatly strengthen the case for mitigation from a risk management perspective, as uncertainty and ignorance of consequences increase the relative value of insurance. But that’s a topic for a different day…
References:
- Allen, R.J. and S.C. Sherwood (2008): Warming maximum in the tropical upper troposphere deduced from thermal winds. Nature Geoscience, 1, 399-403, doi:10.1038/ngeo208.
- Bengtsson, L. and K.I. Hodges (2009): On the evaluation of temperature trends in the tropical troposphere. Climate Dynamics, “Online First”, doi:10.1007/s00382-009-0680-y.
- Johnson, N.C. and S.-P. Xie (2010): Changes in the sea surface temperature threshold for tropical convection. Nature Geoscience, 3, 842–845, doi:10.1038/ngeo1008.
- Randel, W.J. and F. Wu (2006): Biases in Stratospheric and Tropospheric Temperature Trends Derived from Historical Radiosonde Data. Journal of Climate, 19, 10, 2094-2104, doi:10.1175/JCLI3717.1.
- Sherwood, S.C., et al. (2008): Robust Tropospheric Warming Revealed by Iteratively Homogenized Radiosonde Data. Journal of Climate, 21, 20, 5336-5352, doi:10.1175/2008JCLI2320.1 .
- Sobel, A. (2010): Raised bar for rain. Nature Geoscience, 3, 821–822, doi:10.1038/ngeo1025.
- Thorne, P.W., et al. (2007): Tropical vertical temperature trends: A real discrepancy? Geophysical Research Letters, 34, L16702, doi:10.1029/2007GL029875.
- Thorne, P.W. (2008): The answer is blowing in the wind. Nature Geoscience, 1, 347-348, doi:10.1038/ngeo209.
- Thorne, P.W., et al. (2010) Tropospheric temperature trends: history of an ongoing controversy. WIRES: Climate Change, in press, doi:10.1002/wcc.80.
- Zhang, G.J., and H. Wang (2006): Toward mitigating the double ITCZ problem in NCAR CCSM3. Geophysical Research Letters, 33, L06709, doi:10.1029/2005GL025229.
YouTube - Monckton Bunkum Part 4 -- Quotes and misquotes
I'll post the following as well just in case any deniers get a Goredon.
I'll post the following as well just in case any deniers get a Goredon.