This is an essay concerning an analysis of UAH satellite data for the lower troposphere. It has been submitted to John Christy of UAH for his comments and these will be added to the page in due course.
John chose not to comment, stating that he is swamped by work and noted that the temperature trend asymmetry has been known for a considerable time. Some background to the topic is added at the end of the essay.
The last thirty-one years climate from satellite measurements of temperature
Much has been said and written about the various trends in global temperature and their relationship to the ever increasing concentration of CO2 in the atmosphere. There are some doubts about the terrestrial records of global temperature, but differences between them are not statistically significant. Since the continuous temperature record of the lower troposphere temperatures is available, it seems appropriate to consider its possible interpretation.
The lower troposphere satellite records from the University of Alabama at Huntsville are used entirely in this exercise and the total global temperature anomaly record, based on the mean of the temperature anomalies over the period from 1979-1997 is shown in Figure 1.
Figure 1 The record of monthly global anomalies over the 1979-2009 period
There is a positive trend and very few of the individual records fall outside the limits shown of ± one standard deviation. The positive trend is statistically highly significant.
A graph of the variations in annual global temperature anomaly and CO2 concentration is shown in Figure 2. Annual values are used in this case because of the considerable seasonal variation in CO2 concentration.
Figure 2 Plots of the annual mean values of global temperature anomalies and CO2 concentrations measured at Mauna Loa
Although it is an approximation to draw straight lines on such graphs, other algebraic possibilities are no better and the two sets of data have a correlation coefficient of 0.64 indicating a possible causal relationship, the warming arising from the extra forcing of the greenhouse effect provided by the extra CO2.
This is not the end of the story by any means and since latitudinal and land/ocean data are available these should be considered.
Figure 3 is a histogram showing the decadal temperature trends in various latitudinal areas of the globe.
Figure 3 Decadal temperature trends of latitudinal areas showing the differences between land and ocean in the various regions
It is very clear that virtually nothing is happening in the Southern Hemisphere and that all the action is in the north. The relatively small global trends cover up much larger ones that are occurring in the north. These are worth extra study.
An updated version of Figure 3 is shown below.
Figure 4 shows a map of the globe with the variations of atmospheric concentration for July 2009.
Figure 4 The global distribution of CO2 concentration, July 2009
The main generation of CO2 is in the northern mid-latitudes and the blue areas indicate the 'sinks'; the oceanic areas where the gas is dissolving in the water. The map is consistent with the BP Statistics of fossil fuel consumption which shows that a remarkable 93% occurs in the northern hemisphere.
The data for the northern extra-tropics, between latitudes 20 N and 70 N show a small difference between the trends for the land and ocean and the overall data for that region may be dealt with without considering that difference.
Figure 5 shows the temperature record for the northern extra-tropics and includes the trend lines for the data and those representing ± one standard deviation.
Figure 5 Temperature data for the northern extra-tropics; 20 N - 70 N
As with the global data very few points fall outside of the ± one standard deviation limits and the trend is statistically extremely significant. The points on the graph beyond the spike of the 1998 El Niño event are of recent intense interest since they show no warming and parts of the global record have been used by some sceptics to claim that there is cooling.
Figure 6 shows the data from 1997 for the northern extra-tropics.
Figure 6 Temperature data for the northern extra-tropics from 1997
Figure 6 does indicate a very small cooling trend, rather than the positive trend that might be expected if the continued increases in atmospheric CO2 concentration are producing their expected warming. This indicates that whatever that warming might be, it is being countered by an equal and opposite cooling trend over the selected period.
That the continual emissions of CO2 into the atmosphere from land change and fossil fuel burning is causing the increase in CO2 concentration is in very little doubt. Figure 7 shows the variations in anthropogenic emissions and the atmospheric CO2 concentration over the 1979-2008 period.
Figure 7 Variations in anthropogenic emissions of carbon and the atmospheric CO2 concentration
The data on which Figure 7 is based show remarkably similar, almost linear trends and statistically the correlation coefficient between the two sets of data is 0.97. This does not prove causality, but offers a very strong suggestion that the emissions of anthropogenic carbon are mainly responsible for the increases in CO2 concentration. There is very compelling evidence for the linkage from the study of the distribution of carbon-12 and carbon-13 isotopes in the atmosphere and in the oceans. Fossil fuels are derived from rotting vegetation that is assisted by bacterial action as well as chemical decomposition. All chemical processes have rates that are dependent to some extent upon the masses of the reacting substances. Both carbon-12 and carbon-13 undergo identical chemical reactions, but the heavier isotope is discriminated against in the products of the reactions. So, in the formation of fossil fuels the carbon-13 isotope has a considerable and measurable lower representation than it does in the vast amount of carbon in the planet. When the fossil fuels are burned, the resulting extra carbon entering the atmosphere has the tell-tale signature of a diluted 13C/12C ratio. The atmospheric carbon is being progressively diluted by the fossil fuel carbon and this is also noticeable in the ocean carbon. This counters the false argument that the extra CO2 entering the atmosphere is natural and is caused by warming of the oceans or the biosphere. If that were the case then the atmospheric concentration of oxygen would also be increasing and that is not occurring. In fact, the atmospheric oxygen content is reducing slightly in accordance with the amount of the gas required to burn the fossil fuels.
Over the last thirty-one years the correlation between surface temperature and CO2 concentration is shown in Figure 8.
Figure 8 A plot of global temperature anomalies against the Mauna Loa CO2 concentrations for the 1979-2008 period
The correlation coefficient is 0.64 and the slope of 0.0077°C increase of temperature for every 1 ppmv increase in CO2 concentration is remarkably similar to that expected from the IPCC formula for forcing:
ΔF = 5.35 ln(c2/c1) for any two CO2 concentrations, c1 and c2, c2 > c1
The forcing originating from an increase of CO2 concentration from 385 to 386 ppmv is 0.0139 W m-2 and this translates to a temperature increase of 0.0139/2 = 0.0069°C.
There are questions about whether such global factors are really relevant because of the observed asymmetry of warming in the 1979-2008 period. Another question to be resolved is that concerning the observed higher concentration of CO2 in the atmosphere of the northern hemisphere and the observed higher rate of warming there. Is this significant and can it be properly explained. It certainly does not appear in the outputs of any of the GCMs. It seems that the excessive CO2 production in the northern hemisphere is not the cause of the hemispherical asymmetry in temperature anomalies and that the latter is due to the greater thermostatic effect of the greater water area in the southern hemisphere.
From this limited data set of thirty-one years of temperature and CO2 data it would seem reasonable to conclude that the observed warming, although asymmetrically distributed, is that expected from the straightforward forcing formula of the IPCC and seems to eliminate any exaggerations that they regard as important. If indeed the forcing formula has any respectability the expected forcing and consequent warming amounts to 2.675 ln (570/385) = 1.05°C if we can find enough fossil fuel to produce a CO2 concentration of 570 ppmv, the latter being twice that of the pre-industrial era. A possible difficulty is that the asymmetric warming, with warming being almost entirely restricted to the northern hemisphere that region could experience a temperature increase of considerably more, maybe twice as much, than 1.05°C. But, as ever, extrapolation of graphs is a very doubtful exercise!
Background to warming asymmetry
A paper in Science 215, 56, (1982) by Bryan, Komro, Manabe and Spelman described a coupled ocean-atmosphere computer model giving results of a 4-fold increase of atmospheric concentration of CO2. The authors admit to reservations about the model and its results, regarding the exercise as work in progress. Nevertheless, their results indicate a variation in latitudinal warming from 3.5°C at the equator to 10.7°C at 80°N.
Using much more sophisticated models, Stouffer, Manabe and Bryan, in a paper in Nature 342, 660, (1989) obtained results for latitudinal variations in warming for an atmosphere in which the CO2 concentration was increased by 1% per year for 100 years. These showed a variation in warming between 2-5°C from the equator to the North Pole and mirrored their previous results. The results for the Southern Hemisphere were significantly different, showing no increase in warming beyond 2°C. They remark that 'the model response exhibits a marked and unexpected inter-hemispheric asymmetry'. Without going into much detail, they ascribe the asymmetry to the slow warming of the southern ocean due to some heat penetrating the surface layer and the more rapid warming of the Northern Hemisphere to the greater predominance of land over ocean coupled with the directions of ocean currents and the reduction in the ice cover of the polar region as the region warms.
The analysis of the temperature measurements by satellite given above outlines the latitudinal asymmetry and, for once, the observations are consistent with the computer results!
A paper by Spencer and Christy in Science 247, 1558, (1990) reported that their satellite measurements showed no obvious trend in global lower tropospheric temperature in the first ten years from 1979 to 1988. The paper was mainly about the precision of their satellite measurements and comparisons with surface data. The precision of their satellite measurements was far better than those for the surface, but there was a good agreement between the two sets, giving confidence in the surface measurements. The authors made no reference to latitudinal differences and their paper marked the start of an international effort to reconcile terrestrial and satellite measurements. A comprehensive review paper on tropospheric temperature trends was published in 2010 by Thorne, Lanzante, Peterson, Siedel and Shine in www.wires.wiley.com/climate change. The temperature trends from various sources for the years since 1979 were collected and were 0.15°C per decade for the observations by the Hadley Centre for the surface measurements and those from NOAA and NASA were in agreement. The temperature trend given by the UAH satellite analysis for the lower troposphere including the surface was 0.14°C per decade, but no attention was given to the latitudinal distribution of the trends.
There is general confidence that the satellite and terrestrial observations of temperature trends are telling the same story. The latest data from the UAH satellite is shown in Figure 9, updated to include the August 2011 value.
Figure 9 The latest UAH record for the lower troposphere
The trend is represented by a curve, although a straight line would have a trend of 0.14°C per decade as mentioned above and it would be imprudent to attempt to extrapolate the fitted curve. Apart from the exceptional El Niño warming of 1998, there seem to be two periods of almost zero trend, one (after 2001) being warmer than the other. The fitted curve seems to be a possible attempt to avoid the conclusion that since 1979 there has been warming. Such a conclusion forms the basis of criticisms by some extreme sceptics.
The main point of this review is to suggest that the global trends, whether from satellites or from terrestrial measurements, hide the distinct asymmetry referred to above showing temperature trends for the Northern Hemisphere that are large enough need to be taken seriously.
A brief literature survey yields the most likely reason for the latitudinal asymmetry as northern warming arising from the North Atlantic Ocean current.