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Forcing Factors (Non-CO2 Greenhouse Gases: Ozone) -- Summary
We hear a lot about the climatic effects (both real and imagined) of the ongoing rise in the air's CO2 content.  But what are the climatic effects of non-CO2 greenhouse gases?  We here summarize the findings of some recent studies that have addressed this question with respect to ozone.

Mickley et al. (2001) used a global three-dimensional model of tropospheric chemistry to calculate preindustrial concentrations of tropospheric ozone, after which they examined the radiative forcing implications of the subsequent upward trend in the concentration of this trace constituent of the atmosphere.  Their model simulations revealed that uncertainties associated with natural emissions of NOx and hydrocarbons led to decreases in previously assumed preindustrial ozone concentrations of 10-20 parts per billion.  Test simulations using the new estimates then yielded, in their words, "a global mean radiative forcing from ozone added to the atmosphere since preindustrial times of 0.72-0.80 watts per square meter, well above the range of forcings (0.3-0.5 watts per square meter) obtained by standard models."  Hence, they concluded that historical increases in tropospheric ozone "may have contributed a much larger fraction of total greenhouse forcing since preindustrial times than is generally assumed."  This conclusion is very important; if correct, it suggests that increases in tropospheric ozone concentration since preindustrial times have provided an increase in radiative forcing that amounts to approximately half of the estimated increase in CO2-induced radiative forcing during the same time period, which suggests that estimates of CO2-induced radiative forcing since preindustrial times are likely considerably overstated.

Shindell et al. (2001) used a version of the Goddard Institute for Space Studies GCM -- which includes a detailed representation of the stratosphere and parameterizations of the response of ozone to solar irradiance, temperature and circulation changes -- to estimate climatic differences between the period of the Maunder Minimum (of solar irradiance, mid-1600s to early 1700s) and a century later, when solar output was relatively high for several decades.  For the globe as a whole, they reported a mean annual near-surface air temperature difference on the order of 0.3 to 0.4C between the model-simulated climates of the two periods, which they say is about the magnitude of change suggested by historical and proxy climate data.  Much larger model-derived temperature differences (on the order of 1 to 2C) were observed for Northern Hemispheric continents in winter; but similar differences were also observed in the corresponding historical and proxy climate records of those regions.  Shindell et al. thus conclude that their ozone-influenced results "provide evidence that relatively small solar forcing may play a significant role in century-scale Northern Hemisphere winter climate change," specifically stating that "colder winter temperatures over the Northern Hemispheric continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) and warmer temperatures during the 12th through 14th centuries (the putative Medieval Warm Period) may have been influenced by long-term solar variations."

Thompson and Solomon (2002) analyzed "30 years (1969-1998) of monthly mean radiosonde data from seven stations located over Antarctica, 32 years (1969-2000) of monthly surface temperature data observations, 30 years (1969-1998) of ground-based total column ozone measurements from Halley station, and 22 years (1979-2000) of tropospheric geopotential height data" in an attempt to describe the nature of climate change in Antarctica since 1969 and determine the reasons for the observed changes.  They found that "at the surface, the Antarctic Peninsula has warmed by several [degrees C] over the past several decades, while the interior of the Antarctic continent has exhibited weak cooling."  They also report that "ice shelves have retreated over the peninsula and sea-ice extent has decreased over the Bellingshausen Sea, while sea-ice concentration has increased and the length of the sea-ice season has increased over much of eastern Antarctica and the Ross Sea," additionally noting there is "a systematic bias toward the high-index polarity of the SAM," or Southern Hemispheric Annular Mode, such that the ring of westerly winds encircling Antarctica has recently been spending more time in its strong-wind phase.

So what does it all mean?  The heightened strength and persistence of the SAM would seem to explain most of the cooling of the bulk of Antarctica over the past several decades, as well as much of the concomitant warming of the Antarctica Peninsula, as the latter location experiences fewer cold-air outbreaks under such conditions while it simultaneously receives increased advective warmth from the Southern Ocean.  As for the strengthening of the SAM, Thompson and Solomon speculate it is related to "recent trends in the lower stratospheric polar vortex, which are due largely to photochemical ozone losses."

Addressing the same subject, Kwok and Comiso (2002) review what is known about Antarctic air temperature trends over the past few decades, as well as (1) trends in the SAM and the extrapolar Southern Oscillation (SO), (2) the roles these phenomena may have played in orchestrating the observed air temperature trends, and (3) what may be the ultimate driver of these phenomana.  Citing the work of King and Harangozo (1998), they report that during the past 20 years, the Antarctic Peninsula has experienced "pronounced warming."  They note, however, that there has been "cooling at a number of weather stations on the coast and plateau of East and West Antarctica (Comiso, 2000)."  In fact, they say that "the analysis of Doran et al. (2002) suggests a net cooling of the Antarctic continent between 1966 and 2000," with the largest cooling centered around the South Pole and the region surrounding Dome C.  Hence, the mean temperature trend of the entire continent over close to the past four decades has clearly been one of cooling.

Over the 17-year period 1982-1998, Kwok and Comiso also report that the SAM index shifted towards more positive values at a rate of 0.22/decade, noting that a positive polarity of the SAM index "is associated with cold anomalies over most of Antarctica with the center of action over the East Antarctic plateau."  Simultaneously, the SO index shifted in a negative direction, indicating "a drift toward a spatial pattern with warmer temperatures around the Antarctic Peninsula, and cooler temperatures over much of the continent."  Together, they say the positive trend in the coupled mode of variability of these two indices (0.3/decade) represents a "significant bias toward positive polarity" that they describe as "remarkable."

Kwok and Comiso additionally report that the tropospheric SAM "has been shown to be related to changes in the lower stratosphere (Thompson and Wallace, 2000)," noting that the high index polarity of the SAM "is associated with the trend toward a cooling and strengthening of the [Southern Hemisphere's] stratospheric polar vortex during the stratosphere's relatively short active season in November, and ozone depletion," which is pretty much the same hypothesis as that put forth by Thompson and Solomon.

Gillett and Thompson (2003) employed a state-of-the-art atmospheric model with high vertical resolution coupled to a mixed-layer ocean model to test this hypothesis.  In doing so, they found that "the seasonality, structure, and amplitude of the observed climate trends are simulated in a model run that is forced solely by prescribed stratospheric ozone depletion [our italics]," providing evidence that "anthropogenic emissions of ozone-depleting gases have had a distinct impact on climate not only at stratospheric levels but at Earth's surface as well."  They also note that "simulations run with increasing greenhouse gases reveal trends in the SAM that are of the same sign as the observed trends," but they report that "the amplitude of the simulated trends is considerably smaller than that observed."

For the portion of the planet covered by Antarctica and much of the Southern Ocean, therefore, we have evidence -- albeit model-produced -- that something other than CO2 plus all other anthropogenic greenhouse gases emitted to date is having a predominant effect on earth's near-surface air temperature.  This observation suggests that other factors of anthropogenic and/or natural origin might also be producing superior and possibly oppositely-directed effects that could readily forestall the type of catastrophic global warming typically predicted by climate alarmists and IPCC functionaries if our usage of fossil fuels is not drastically curtailed.  We also note that the specific mechanism of climate change highlighted in this study was only elucidated within the past couple of years, which suggests there may well be other equally powerful climatic forces at work in the world of which we are still totally ignorant.

In light of these observations, it would appear that our knowledge of things climatic is in no way sufficient to warrant draconian reductions in anthropogenic CO2 emissions, which are known to have many positive biological impacts.  It could be argued, in fact, that it would be better to focus on the twin goals of reducing tropospheric ozone concentrations while restoring stratospheric ozone concentrations, both of which activities have virtues independent of their global warming implications.

Comiso, J.C.  2000.  Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements.  Journal of Climate 13: 1674-1696.

Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N.  2002.  Antarctic climate cooling and terrestrial ecosystem response.  Nature 415: 517-520.

Gillett, N.P. and Thompson, D.W.J.  2003.  Simulation of recent Southern Hemisphere climate change.  Science 302: 273-275.

King, J.C. and Harangozo, S.A.  1998.  Climate change in the western Antarctic Peninsula since 1945: observations and possible causes.  Annals of Glaciology 27: 571-575.

Kwok, R. and Comiso, J.C.  2002.  Spatial patterns of variability in Antarctic surface temperature: Connections to the South Hemisphere Annular Mode and the Southern Oscillation.  Geophysical Research Letters 29: 10.1029/2002GL015415.

Mickley, L.J., Jacob, D.J. and Rind, D.  2001.  Uncertainty in preindustrial abundance of tropospheric ozone: Implications for radiative forcing calculations.  Journal of Geophysical Research 106: 3389-3399.

Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D. and Waple, A.  2001.  Solar forcing of regional climate change during the Maunder Minimum.  Science 294: 2149-2152.

Thompson, D.W.J. and Solomon, S.  2002.  Interpretation of recent Southern Hemisphere climate change.  Science 296: 895-899.

Thompson, D.W.J. and Wallace, J.M.  2000.  Annular modes in extratropical circulation, Part II: Trends.  Journal of Climate 13: 1018-1036.