There is little doubt the air's CO2 concentration has risen significantly since the inception of the Industrial Revolution; and there are few who do not attribute the CO2 increase to the increase in humanity's use of fossil fuels. There is also little doubt the earth has warmed slightly over the same period; but there is no compelling reason to believe that the rise in temperature was caused by the rise in CO2. Furthermore, it is highly unlikely that future increases in the air's CO2 content will produce any global warming; for there are numerous problems with the popular hypothesis that links the two phenomena.
A weak short-term correlation between CO2 and temperature proves nothing about causation. Proponents of the notion that increases in the air's CO2 content lead to global warming point to the past century's weak correlation between atmospheric CO2 concentration and global air temperature as proof of their contention. However, they typically gloss over the fact that correlation does not imply causation, and that a hundred years is not enough time to establish the validity of such a relationship when it comes to earth's temperature history.
The observation that two things have risen together for a period of time says nothing about one trend being the cause of the other. To establish a causal relationship it must be demonstrated that the presumed cause precedes the presumed effect. Furthermore, this relationship should be demonstrable over several cycles of increases and decreases in both parameters. And even when these criteria are met, as in the case of solar/climate relationships, many people are unwilling to acknowledge that variations in the presumed cause truly produced the observed analogous variations in the presumed effect.
In thus considering the seven greatest temperature transitions of the past half-million years - three glacial terminations and four glacial inceptions - we note that increases and decreases in atmospheric CO2 concentration not only did not precede the changes in air temperature, they followed them, and by hundreds to thousands of years! There were also long periods of time when atmospheric CO2 remained unchanged, while air temperature dropped, as well as times when the air's CO2 content dropped, while air temperature remained unchanged or actually rose. Hence, the climate history of the past half-million years provides absolutely no evidence to suggest that the ongoing rise in the air's CO2 concentration will lead to significant global warming.
Strong negative climatic feedbacks prohibit catastrophic warming. Strong negative feedbacks play major roles in earth's climate system. If they did not, no life would exist on the planet, for some perturbation would long ago have sent the world careening into a state of cosmic cold or horrendous heat; and we know from the fossil record that neither of these extremes has ever occurred, even over billions of years, and in spite of a large increase in the luminosity of the sun throughout geologic time.
Consider, in this regard, the water vapor that would be added to the atmosphere by enhanced evaporation in a warmer world. The extra moisture would likely lead to the production of more and higher-water-content clouds, both of which consequences would tend to cool the planet by reflecting more solar radiation back to space.
A warmer world would also mean a warmer ocean, which would likely lead to an increase in the productivity of marine algae or phytoplankton. This phenomenon, in turn, would enhance the biotic production of certain sulfur-based substances that diffuse into the air, where they are oxidized and converted into particles that function as cloud condensation nuclei. The resulting increase in the number of cloud-forming particles would thus produce more and smaller cloud droplets, which are more reflective of incoming solar radiation; and this phenomenon would also tend to cool the planet.
All of these warming-induced cloud-related cooling effects are very powerful. It has been shown, for example, that the warming predicted to result from a doubling of the air's CO2 content may be totally countered by: (1) a mere 1% increase in the reflectivity of the planet, or (2) a 10% increase in the amount of the world's low-level clouds, or (3) a 15 to 20% reduction in the mean droplet radius of earth's boundary-layer clouds, or (4) a 20 to 25% increase in cloud liquid water content. In addition, it has been demonstrated that the warming-induced production of high-level clouds over the equatorial oceans almost totally nullifies that region's powerful water vapor greenhouse effect, which supplies much of the temperature increase in the CO2-induced global warming scenario.
Most of these important negative feedbacks are not adequately represented in state-of-the-art climate models. What is more, many related (and totally ignored!) phenomena are set in motion when the land surfaces of the globe warm. In response to the increase in temperature between 25°N latitude and the equator, for example, the soil-to-air flux of various sulfur gases rises by a factor of 25, as a consequence of warmth-induced increases in soil microbial activity; and this phenomenon can lead to the production of more cloud condensation nuclei just as biological processes over the sea do. Clearly, therefore, any number of combinations of these several negative feedbacks could easily thwart the impetus for warming provided by future increases in the air's CO2 content.
Growth-enhancing effects of CO2 create an impetus for cooling. Carbon dioxide is a powerful aerial fertilizer, directly enhancing the growth of almost all terrestrial plants and many aquatic plants as its atmospheric concentration rises. And just as increased algal productivity at sea increases the emission of sulfur gases to the atmosphere, ultimately leading to more and brighter clouds over the world's oceans, so too do CO2-induced increases in terrestrial plant productivity lead to enhanced emissions of various sulfur gases over land, where they likewise ultimately cool the planet. In addition, many non-sulfur-based biogenic materials of the terrestrial environment play major roles as water- and ice-nucleating aerosols; and the airborne presence of these materials should also be enhanced by rising levels of atmospheric CO2. Hence, it is possible that incorporation of this multifaceted CO2-induced cooling effect into the suite of equations that comprise the current generation of global climate models might actually tip the climatic scales in favor of global cooling in the face of continued growth of anthropogenic CO2 emissions.
There is no evidence for warming-induced increases in extreme weather. Proponents of the CO2-induced global warming hypothesis often predict that extreme weather events such as droughts, floods, and hurricanes will become more numerous and/or extreme in a warmer world; however, there is no evidence to support this claim. In fact, many studies have revealed that the numbers and intensities of extreme weather events have remained relatively constant over the last century of modest global warming or have actually declined. Costs of damages from these phenomena, however, have risen dramatically; but this phenomenon has been demonstrated to be the result of evolving societal, demographic and economic factors.
Elevated levels of atmospheric CO2 are a boon to the biosphere. In lieu of global warming, a little of which would in all probability be good for the planet, where do the above considerations leave us? Simply with the biospheric benefits that come from the aerial fertilization effect of atmospheric CO2 enrichment: enhanced plant growth, increased plant water use efficiency, greater food production for both people and animals, plus a host of other biological benefits too numerous to describe in this short statement.
And these benefits are not mere predictions. They are real. Already, in fact, they are evident in long-term tree-ring records, which reveal a history of increasing forest growth rates that have closely paralleled the progression of the Industrial Revolution. They can also be seen in the slow but inexorable spreading of woody plants into areas where only grasses grew before. In fact, the atmosphere itself bears witness to the increasing prowess of the entire biosphere in the yearly expanding amplitude of the its seasonal CO2 cycle. This oscillatory "breath of the biosphere" - its inhalation of CO2, produced by spring and summer terrestrial plant growth, and its exhalation of CO2, produced by fall and winter biomass decomposition - has been documented to be growing greater and greater each year in response to the ever-increasing growth stimulation provided by the ongoing rise in the air's CO2 content.
Atmospheric CO2 enrichment brings growth and prosperity to man and nature alike. This, then, is what we truly believe will be the result of the ongoing rise in the air's CO2 content: a reinvigorated biosphere characteristic of those prior periods of earth's history when the air's CO2 concentration was much higher than it is today, coupled with a climate not much different from that of the present. Are we right? Only time will tell. But one thing is certain now: there is much more real-world evidence for the encouraging scenario we paint here than for the doom-and-gloom predictions of apocalypse that are preached by those who blindly follow the manifestly less-than-adequate prognostications of imperfect climate models.
Our policy prescription relative to anthropogenic CO2 emissions is thus to leave well enough alone and let nature and humanity take their inextricably intertwined course. All indications are that both will be well served by the ongoing rise in atmospheric CO2.
Supporting references. This brief was written in 1998. References to the voluminous scientific literature that supports the many factual statements of this position paper may be found on our website - www.co2science.org - which we update weekly.