Why Bury Carbon?

Global Warming and the Role of CO2

In a nutshell, the facts are as follows:

• Global mean temperatures (figure 1) have risen anomalously over the last century, and global mean temperature is a key determinant of climate change.
• Since the year 1750, the atmospheric concentration of carbon dioxide (figure 2) and other greenhouse gases has risen by an amount unprecedented in the last several million years.
• Anthropogenic CO², generated by burning fossil fuels (figure 3) and deforestation, fully accounts for the increase in atmospheric and oceanic carbon dioxide in recent times.
• There is a well established physical mechanism, the greenhouse effect, linking rising concentrations of atmospheric CO² with rising temperatures. The observed elevated levels of CO² account quantitatively for recent global temperature rise.

All other mechanisms which might account for the rise in global mean temperatures are either speculative or, where they are well established, insufficient to account for the measured temperature increase. Moreover, even if these alternative mechanisms do contribute to rising temperatures, their contribution would not eliminate the greenhouse warming of CO² which will continue to contribute in any case. The only tenable argument arises if there are other mechanisms which completely swamp the effect of CO² and render it pointless to limit the CO² levels. This does not appear to be the case.

Figure 1: Hadley Centre data showing annual global land-surface temperatures (grey bars), spatially averaged and expressed as deviations from the mean of the temperatures between 1961 and 1990. The smooth curves show decadal moving averages estimated by a variety of independent teams.

Figure 2 : Atmospheric carbon dioxide concentrations, measured in parts per million (ppm), for the last millennium, using ice core data up to 1977 and the direct CO2 measurements (in blue) made in Hawaii by Keeling from 1958 onwards

Figure 3: Anthropogenic CO2 emissions, measured in millions of metric tons of carbon per year.

The Earth's Greenhouse Effect
At the core of the greenhouse mechanism is the behaviour of certain molecules which can absorb, and subsequently re-radiate, infrared radiation. Water molecules have this property and are accompanied in the earth's atmosphere by carbon dioxide (CO₂), methane (CH₄), and small traces of other gases. Most of the sun's radiation comprises photons with frequencies above the infrared which are not strongly susceptible to absorption by the greenhouse gasses (GHGs). These higher energy photons pass through the atmosphere, largely unimpeded by molecular absorption, and warm the earth below. However, by virtue of its being much cooler than the sun, the earth radiates much of its heat energy in the infrared. (The different spectra of incoming and outgoing radiation are shown in figure 4.)

Fig 4: The spectrum of incoming solar radiation is shown in red, and the outgoing thermal radiation of the earth is shown in blue. The absorption bands of the atmosphere are shown below the wavelengths.

Since most of the energy flux leaving the earth's surface consists of infrared photons which are readily absorbed and re-radiated by the GHGs in the atmosphere, individual infrared photons do not propagate far in the lower atmosphere. They have a mean free path which is much shorter than the effective (infrared) optical thickness of the atmosphere. The overall effect of this trapping of infrared is that the lower atmosphere is warmed, raising the total flux of heat energy arriving at the earth's surface (referred to as "radiative forcing") by about 1.7 Wm-2. Indeed, even before industrial emissions of CO₂,the naturally occurring GHGs in the atmosphere (largely water and CO₂) had already caused the earth to warm above a frigid -18oC, which would be its equilibrium temperature in the absence of all greenhouse gases.

Global Mean Temperature Projections
Furthermore, the global warming observed in recent decades is consistent with the projections of detailed numerical climate models. Figure 5 shows estimates of recent global mean temperature with and without the influence of anthropogenic forcing, compared to the data themselves. Clearly this ensemble of models broadly reflects the observed global warming when both natural and anthropogenic forcing is included, but fails to capture this trend when anthropogenic forcing is excluded.

Figure 5: Numerical climate model predictions for mean global temperature, both with the influence of anthropogenic forcing (shown in orange), and without (shown in blue). Actual temperatures are shown in black and the major volcanic eruptions are marked.

Could it be something else?
The question that remains is: could the observed changes, particularly in global mean temperatures, be caused by something other than carbon dioxide? Various mechanisms have been proposed:

Changes in Solar Output
There has been a flurry of speculation in recent years that, while not denying the increase in global mean temperature, the source of global warming is in fact an increase in the output of the sun, which varies with the sunspot cycle and over which we have no control. The tentative basis for this suggestion is that the sunspot activity of the sun has increased significantly since the Maunder Minimum of the second half of the 17^th century when, during a period of 30 years, only about 50 sunspots were observed¹. This compares to a value of about 30,000 for the sum of daily sunspot numbers accumulated over a more representative 30-year period. The Maunder Minimum coincided with the coldest part of the little ice age and this association broadly gives rise to the suggestion that it is the sun, and not CO₂, which caused the recent warming.

However the data do not support this contention. The increase in total solar irradiance since the Maunder Minimum (compared to contemporary minima) is about 0.04%, which amounts to a radiative forcing of about 0.125 Wm^-2, while the increased radiative forcing since the beginning of the industrial era in 1750 is about 0.15Wm^-2. Compared to the radiative forcing of 1.7Wm^-2 due to CO₂, we can see that the contribution of the variation in solar output to global warming is an order of magnitude lower than that of anthropogenic CO₂. So, while there is evidence for a small increase in solar forcing since the 17th century, the magnitude of this forcing is small compared to that of anthropogenic CO₂ and the overall increase in solar forcing is not sufficient to account for recent warming (by a large margin).

Galactic Cosmic Rays Modulated by Solar Activity
Various mechanisms have been proposed in which the modulation of cosmic rays by variations in solar activity influence climate² in some way. However, because of the complex chain of interactions linking variation in the flux of cosmic rays to physical phenomena in the atmosphere, quantitative estimates of the influence of cosmic rays on climate have not been made. A variety of associations between solar cycles and cloud cover have nonetheless been claimed³ although these remain highly controversial. An empirical association between variation in cloud cover and solar activity between 1984 and 1990 continues to be disputed because of uncertainties about the decadal signal itself, the ambiguous phase with solar activity and its separate dependence for low, middle and high clouds. In particular, there is no correlation to global cloud cover after 1994 ^{4 5}without the application of de-trending methods which are subject to question⁶. Furthermore, cloud cover time-series from ship synoptic reports for the multi-decade period 1952 to 1997 do not show any correlation with cosmic ray flux⁷. While there is a small but statistically significant positive correlation⁸ between UK cloud cover and galactic cosmic ray flux for the period 1951 to 2000, cloud cover over the USA for the period 1900 to 1987 exhibits a negative correlation with the cosmic ray flux⁹.

Because of the uncertainty in the mechanisms, it has also been suggested that it may not be just modulation of cosmic rays which plays a role, but rather the direct variation in sea surface temperature with solar irradiance, and also internal variability caused by El Nino¹⁰.

The overall conclusion must be that this phenomenon is very poorly understood. Although it might prove that cosmic rays modulated by the sun do have an influence on cloud cover, and therefore on climate, the evidence is currently tenuous.

The Pacific Decadal Oscillation
A suggestion with some currency is that oscillations in ocean temperature account for recent warming and, furthermore, that these will now start to cool and global warming will be over. The suggestion is based on the Pacific Decadal Oscillation (PDO), a phenomenon in which regions of the Pacific Ocean alternately warm and cool with a period of 20 to 30 years. Figure 6 shows the ocean surface temperature distribution during what are termed the 'warm phase' and the 'cool phase' of the PDO.

Figure 6: Ocean surface temperature distribution during what are termed the 'warm phase' and the 'cool phase' of the Pacific Decadal Oscillation.

I suspect that, to some degree, it is this warm phase/cool phase terminology that has led some to assert that, since we are now going into a cool phase, the Pacific will cool down. This is not the case at all. The term "warm phase" just means that the equatorial Pacific is in the warm part of its cycle. It is interesting to consider the mean sea surface temperatures (SST) in the North Pacific throughout the warm/cool phases of the PDO. They reveal (figure 7 ) that global mean sea surface temperatures exhibit a rising trend regardless of the phase of the PDO.

Figure7: Global mean sea surface annual temperature data for the entire globe (a), and the two hemispheres (c) and (d), shown by the red bars superimposed with the decadal variation, estimated by a variety of teams. (The variation between estimates depends on details of how the raw data are treated).

Nature's Changing Course
It is appealing, of course, to believe that damaging climate change has nothing to do with our activities. If this were the case, then we could abrogate any responsibility to act. This has lead to claims either that recent climate change just reflects the vagaries of nature and has nothing to do with our emissions, or that climate change is natural and our having caused it is of no real consequence The inherent conclusion being that humanity should just adapt to climate change.

The first claim does not stand up. The evidence that anthropogenic emissions play a key role in climate change is substantial. Indeed, anthropogenic emissions have lead to changes of such magnitude and rapidity that known natural mechanisms cannot readily account for them.

As for the second claim, humanity adapting to climate change will come at a great financial and human cost. We have established much of the infrastructure of civilization in regions which are vulnerable to climate change¹¹. This applies to agriculture and water resources as well as to the concentration of extensive development along coastlines.

Figure 8: The coastline of southern/eastern USA showing the potential for inundation by sea level rises of 1.5m (red) and 3m (blue).

The current rate of sea level rise, as measured by the TOPEX/Poseidon satellite, is 3.1mm/year ±0.7mm/year¹² and current projections suggest the possibility of sea level rise of a meter by 2100, depending on the dynamics of the Greenland Ice Sheet. Figure 8 shows the coastline of the southern/eastern USA and the potential for inundation by sea level rise, with the impact of sea level rises of 1.5m and 3m. The value of the infrastructure lying within 1.5 meters of current sea level already runs into trillions of dollars. If global warming continues to accelerate the melting of the Greenland Ice Sheet, as seems likely if we do not act to limit emissions, sea level rises of several meters could occur over a few centuries.

While the cost of mitigating climate change is estimated at a few percent of world GDP, the cost of adaptating to its consequences is estimated to be closer to 20%¹³ . Furthermore, it has to be acknowledged that the longer we leave any real attempt to mitigate, the more we will be forced to adapt. It seems unlikely that we can entirely mitigate the effects of emissions to date. Some combination of mitigation and adaptation is therefore now inevitable. It still remains the case, however, that the more we can mitigate, the lower the overall cost of dealing with future climate change.

Temperature Rise Causes Carbon Dioxide Emissions (rather than the other way round)
The most recent analysis of the striking correlation between global temperatures and atmospheric concentrations of CO₂ in the paleoclimate record indicates that temperature rise precedes the rise in carbon dioxide by several centuries¹⁴. This has been misinterpreted by some to demonstrate that CO₂ is a mere consequence of temperature rise and not a cause. However, as we have discussed, the presence of CO₂ and other greenhouse gases in the atmosphere will inevitably lead to warming. What we are seeing in the paleoclimate is a feedback mechanism. Initial warming induced by variation of the earth's orbit around the sun, does indeed give rise to an increase in the concentration of CO₂, through mechanisms which probably include the reduced solubility of CO₂ in the ocean surface and variation in the biological carbon cycle. The extra CO₂, in turn, contributes to further warming by the greenhouse effect. Without this positive feedback, the variation in temperature induced by the earth's orbital variation alone would be insufficient to account for the transition from glacial to interglacial conditions.

In the case of recent anthropogenic warming, there is no need of a mechanism to initially raise temperatures to release CO₂. We create the CO₂ directly by burning fossil fuels. Moreover, we generate carbon dioxide in such large quantities that it can induce substantial warming, much more rapidly than in most natural episodes in the earth's past.

Conclusions
The case for reducing CO₂ emissions is rational. The fact of a significant rise in mean global temperature over the last century is incontrovertible, as is the fact of a 36% rise in the concentration of carbon dioxide in the atmosphere since the beginning of the industrial revolution. Moreover, the rise since th₂en, in the levels of CO₂ found in the ocean and in the atmosphere, is commensurate with the volume of CO that has been emitted by human activity and is very largely accounted for by the emissions associated with burning fossil fuels.

Complementing these facts is the known physics of the greenhouse effect. Long before anyone was worrying about modern climate change, it had been established that traces of greenhouse gases in the earth's atmosphere caused substantial warming. Indeed this mechanism accounted for the fact that humans found themselves on a planet warm enough to support their existence, rather than in the frigid environment that would have prevailed without the greenhouse effect.

The facts and the physics sit comfortably together as a consistent description which adequately accounts for the observed global warming. None of the other mechanisms proposed to explain global warming, be they variations in solar irradiance, solar modulation of cosmic rays, or uncontrollable natural events, exhibit anything like the quantitative consistency exhibited by the anthropogenic CO₂mechanism described here. Even where the descriptions of these other mechanisms are quantitative, as is the case with solar irradiance, the effects prove small when compared to greenhouse warming, which already comes in at the correct magnitude to fit the facts.

References

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