How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic


The Specter of Species Extinction
Will Global Warming Decimate Earth's Biosphere?

II. Physiological Reasons for Rejecting the CO2-Induced Global Warming Extinction Hypothesis


A. The Adaptability of Plants to Rising Temperature

All else being equal, the global warming extinction scenario would appear to have merit.  After all, if it gets "too hot" for a species of plant or animal where it currently lives, it is only logical that individuals of the heat-stressed species would have to move to a cooler location in order to survive.  In many cases, however, acclimation can adequately substitute for migration, as has been demonstrated by several studies in which the temperatures at which plants grow best rose substantially (by several degrees Centigrade) in response to increases in the air temperature regimes to which they had long been accustomed (Mooney and West, 1964; Strain et al., 1976; Bjorkman et al., 1978; Seemann et al., 1984; Veres and Williams, 1984; El-Sharkawy et al., 1992, Battaglia et al., 1996).  So how does it happen?

One possible way in which adaptation to warmer temperatures may occur is described by Kelly et al. (2003).  In reference to the climate-alarmist view of the Intergovernmental Panel on Climate Change or IPCC (Watson and Team, 2001), they note that "models of future ecological change assume that in situ populations of plants lack the capacity to adapt quickly to warming and as a consequence will be displaced by species better able to exploit the warmer conditions anticipated from 'global warming'."  In contrast to this assumption, they report finding individual trees within a naturally occurring stand of Betula pendula (birch) that are genetically adapted to a range of different temperatures.  As they describe it, they discovered "the existence of 'pre-adapted' individuals in standing tree populations" that "would reduce temperature-based advantages for invading species," which finding, they say, "bring[s] into question assumptions currently used in models of global climate change."

Another perspective on the adaptation vs. migration theme is provided by the work of Loehle (1998), who notes (using forests as an example) that the CO2-induced global warming extinction hypothesis rests on the assumption that the growth rates of trees rise from zero at the cold limits of their natural ranges (their northern boundaries in the Northern Hemisphere) to a broad maximum, after which they decline to zero at the warm limits of their natural ranges (their southern boundaries in the Northern Hemisphere).  Loehle demonstrates that this assumption is only half correct.  It properly describes tree growth dynamics near a Northern Hemispheric forest's northern boundary, but it is an inaccurate representation of tree growth dynamics near a Northern Hemispheric forest's southern boundary.

Loehle notes, for example, that in the Northern Hemisphere (to which we will restrict our discussion for purposes of simplicity), trees planted north of their natural ranges' northern boundaries are only able to grow to maturity within 50-100 miles of those boundaries.  Trees planted south of their natural ranges' southern boundaries, however, often grow to maturity as much as 1000 miles further south (Dressler, 1954; Woodward, 1987, 1988).  In fact, Loehle reports that "many alpine and arctic plants are extremely tolerant of high temperatures, and in general one cannot distinguish between arctic, temperate, and tropical-moist-habitat types on the basis of heat tolerances, with all three types showing damage at 44-52°C (Gauslaa, 1984; Lange and Lange, 1963; Levitt, 1980; Kappen, 1981)."

What Loehle finds from his review of the literature and his experience with various trees in the Unites States, is that as temperatures and growing degree days rise from very low values, the growth rates of Boreal trees at some point begin to rise from zero and continue increasing until they either plateau out at some maximum value or drop only very slowly thereafter, as temperatures rise still higher and growing degree days continue to accumulate.  Trees from the Midwest, by comparison, do not begin to grow until a higher temperature or greater accumulation of growing degree days is reached, after which their growth rates rise considerably higher than those of the colder-adapted Boreal species, until they too either level out or begin to decline ever so slowly.  Last of all, southern species do not begin to grow until even higher temperatures or growing degree day sums are reached, after which their growth rates rise the highest of all before leveling out and exhibiting essentially no decline thereafter, as temperatures and growing degree days continue to climb.

In light of these observations, it is clear that although the northern range limit of a woody species in the Northern Hemisphere is indeed determined by growth-retarding cool growing seasons and frost damage, the southern boundary of a tree's natural range is not determined by temperature, but by competition between the northern species and more southerly-adapted species that have inherently greater growth rates.

Whenever significant long-term warming occurs, therefore, earth's coldest-adapted trees are presented with an opportunity to rapidly extend the cold-limited boundaries of their ranges northward in the Northern Hemisphere, as many studies have demonstrated they have done in the past and are doing now.  Trees at the southern limits of their ranges, however, are little affected by the extra warmth.  As time progresses, they may at some point begin to experience pressure from some of the faster-growing southern species encroaching upon their territory; but this potential challenge is by no means assured of quick success.  As Loehle describes it:

Seedlings of these southern species will not gain much competitive advantage from faster growth in the face of existing stands of northern species, because the existing adult trees have such an advantage due to light interception.  Southern types must wait for gap replacement, disturbances, or stand break up to utilize their faster growth to gain a position in the stand.  Thus the replacement of species will be delayed at least until the existing trees die, which can be hundreds of years...  Furthermore, the faster growing southern species will be initially rare and must spread, perhaps across considerable distances or from initially scattered localities.  Thus, the replacement of forest (southern types replacing northern types) will be an inherently slow process (several to many hundreds of years).

In summing up the significance of this situation, Loehle says that "forests will not suffer catastrophic dieback due to increased temperatures but will rather be replaced gradually by faster growing types."

Another possibility that must be seriously considered is that northern or high-altitude forests will not be replaced at all by southern or low-altitude forests in a warming world.  Rather, the two forest types may merge, creating entirely new forests of greater species diversity, such as those that existed during the warmer Tertiary Period of the Cenozoic Era, when in the western United States many montane taxa regularly grew among mixed conifers and broadleaf schlerophylls (Axelrod 1994a, 1944b, 1956, 1987), creating what could well be called super forest ecosystems, which Axelrod (1988) has described as "much richer than any that exist today."

Possibly helping warmer temperatures to produce this unique biological phenomenon during the Tertiary were the higher atmospheric CO2 concentrations of that period (Volk, 1987), as has been suggested by Idso (1989).  It is a well known fact, for example, that elevated concentrations of atmospheric CO2 significantly stimulate plant growth rates (Kimball, 1983) - especially those of trees (Saxe et al., 1998; Idso and Kimball, 2001) - and that they also greatly enhance their water use efficiencies (Feng, 1999).  Even more important, however, is how atmospheric CO2 enrichment alters plant photosynthetic and growth responses to rising temperatures, as we discuss in the following section.

B. The Extra Help Provided by Rising Atmospheric CO2 Concentrations

It has long been known that photorespiration -- which can "cannibalize" as much as 40-50% of the recently-produced photosynthetic products of C3 plants (Wittwer, 1988) - becomes increasingly more pronounced as air temperature rises (Hanson and Peterson, 1986).  It has also been established that photorespiration is increasingly more inhibited as the air's CO2 content rises (Grodzinski et al., 1987).  Hence, there is a greater potential for rising CO2 concentrations to benefit C3 plants at higher temperatures, as was demonstrated by the early experimental work of Idso et al. (1987) and Mortensen (1987), as well as by the theoretical work of Gifford (1992), Kirschbaum (1994) and Wilks et al. (1995).  In fact, in an analysis of 42 experimental data sets collected by numerous scientists, Idso and Idso (1994) showed that the mean growth enhancement due to a 300-ppm increase in atmospheric CO2 concentration rises from close to zero at an air temperature of 10°C to 100% (doubled growth) at approximately 38°C, while at higher temperatures the growth stimulation rises higher still, as has also been shown by Cannell and Thornley (1998).

Several studies have additionally demonstrated that atmospheric CO2 enrichment tends to alleviate high-temperature stress in plants (Faria, 1996; Nijs and Impens, 1996; Vu et al., 1997); and it has been proven that at temperatures that are high enough to cause plants to die, atmospheric CO2 enrichment can sometimes preserve their lives (Idso et al., 1989, 1995; Baker et al., 1992; Rowland-Bamford et al., 1996; Taub, 2000), just as it can often stave off their demise in the very dry conditions that typically accompany high air temperatures (Tuba et al., 1998; Hamerlynck, et al., 2000; Polley et al., 2002).

A major consequence of these facts is that the optimum temperature (Topt) for plant growth - the temperature at which plants photosynthesize and grow best - generally rises with atmospheric CO2 enrichment (Berry and Bjorkman, 1980; Taiz and Zeiger, 1991).  An example of this phenomenon is presented in Box 1 below, where it can be seen that the increase in atmospheric CO2 concentration utilized in this particular study increases the optimum temperature for photosynthesis in this species from a broad maximum centered at 25°C in ambient air to a well-defined peak at about 36°C in CO2-enriched air.

Box 1: The CO2-Temperature-Growth Interaction

The growth-enhancing effects of elevated CO2 typically increase with rising temperature.  This phenomenon is illustrated by the data of Jurik et al. (1984), who exposed bigtooth aspen leaves to atmospheric CO2 concentrations of 325 and 1935 ppm and measured their photosynthetic rates at a number of different temperatures.  In the figure below, we have reproduced their results and slightly extended the two relationships defined by their data to both warmer and cooler conditions.

At 10°C, elevated CO2 has essentially no effect on net photosynthesis in this particular species, as Idso and Idso (1994) have demonstrated is characteristic of plants in general.  At 25°C, however, where the net photosynthetic rate of the leaves exposed to 325 ppm CO2 is maximal, the extra CO2 of this study boosts the net photosynthetic rate of the foliage by nearly 100%; and at 36°C, where the net photosynthetic rate of the leaves exposed to 1935 ppm CO2 is maximal, the extra CO2 boosts the net photosynthetic rate of the foliage by a whopping 450%.  In addition, it is readily seen that the extra CO2 increases the optimum temperature for net photosynthesis in this species by about 11°C: from 25°C in air of 325 ppm CO2 to 36°C in air of 1935 ppm CO2.

In viewing the warm-temperature projections of the two relationships, it can also be seen that the transition from positive to negative net photosynthesis - which denotes a change from life-sustaining to life-depleting conditions - likely occurs somewhere in the vicinity of 39°C in air of 325 ppm CO2 but somewhere in the vicinity of 50°C in air of 1935 ppm CO2.  Hence, not only was the optimum temperature for the growth of bigtooth aspen greatly increased by the extra CO2 of this experiment, so too was the temperature above which life cannot be sustained increased, and by about the same amount, i.e., 11°C.

How much is plant optimum temperature typically increased by an extra 300 ppm of CO2?  Based largely on theoretical considerations, Long (1991) calculated that such an increase in the air's CO2 concentration should increase Topt, in the mean, by about 5°C, while McMurtrie and Wang (1993) calculated that it should increase it by somewhere between 4 and 8°C.  In Table 1, we report the results of all of the experimental determinations of this number that we could find in the scientific literature.  As can be seen there, the mean increase in Topt for the eleven plants studied is 4.6 ± 1.2°C (3.4 to 5.8°C) for a 300-ppm rise in the air's CO2 concentration.  Hence, both theory and experiment appear to be in reasonably good agreement on this important point.

Table 1.  The increase (Δ) in the optimum temperature for plant growth (Topt, °C) due to various increases in atmospheric CO2 concentration (ppm), along with the increase in Topt due to a 300 ppm increase in atmospheric CO2 concentration (ΔTopt/300) based on the values of ΔTopt and ΔCO2.

SpeciesReferenceΔCO2ΔToptΔTopt/300
Arbutus unedoHarley et al. (1986)2008.012.0
Camissonia brevipesSeeman et al. (1984)6704.01.8
Chenopodium albumSage et al. (1995)4000.00.0
Digitalis lanataStuhlfauth and Fock (1990)65013.06.0
Glycine maxZiska and Bunce (1997)350-2.0-1.7
Lycopersicon esculentumNilsen et al. (1983)65010.04.6
Nerium oleanderBjorkman et al. (1978)4709.05.7
Phaseolus vulgarisCowling and Sage (1998)1875.08.0
Picea abiesRoberntz (2001)3504.13.5
Pinus radiateMcMurtrie et al. (1992)35010.08.6
Populus grandidentataJurik et al. (1984)161011.02.0
Mean   4.6
Standard Error of the Mean   1.2

What is the ultimate implication of the finding that plant optimum temperature rises so dramatically in response to increasing atmospheric CO2 concentration?  It is that if the planet were to warm in response to the ongoing rise in the air's CO2 content -- even to the ungodly degree predicted by the worst-case scenario of the Intergovernmental Panel on Climate Change (5.8°C by 2100) -- the vast majority of earth's plants would likely not feel a need (or only very little need) to migrate towards cooler parts of the globe.  Any warming would obviously provide them an opportunity to move into regions that were previously too cold for them, but it would not force them to move, even at the hottest extremes of their ranges; for as the planet warmed, the rising atmospheric CO2 concentration would work its biological wonders, significantly increasing the temperatures at which most of earth's C3 plants -- which comprise fully 95% of the planet's vegetation (Drake, 1992) -- function best, creating a situation where earth's plant life would actually prefer warmer conditions.

With respect to the C4 and CAM plants that make up the remaining 5% of earth's vegetative cover, most of them are endemic to the planet's hotter environments (De Jong et al., 1982; Drake, 1989; Johnson et al., 1993), which according to the IPCC are expected to warm much less than the cooler regions of the globe.  Hence, the planet's C4 and CAM plants would not face quite as great a thermal challenge as earth's C3 plants in a warming world.  Nevertheless, the work of Chen et al. (1994) suggests that they too may well experience a modest increase in their optimum temperatures as the air's CO2 content rises (a 1.5°C increase in response to a 350-ppm increase in atmospheric CO2 concentration).  Consequently, and in view of the non-CO2-related abilities of earth's vegetation to adapt to rising temperatures discussed in the previous section, plants of all photosynthetic persuasions should be able to successfully adapt to any future warming that could possibly be caused by the enhanced greenhouse effect that may be produced by the CO2 emitted to the air by mankind's burning of fossil fuels.

So what could we logically expect to happen to the biosphere in a world of both rising air temperature and atmospheric CO2 concentration?  We could expect that earth's plants would extend the current cold-limited boundaries of their ranges both poleward in latitude and upward in elevation, but that the heat-limited boundaries of the vast majority of them would remain pretty much as they are now, i.e., unchanged.  Hence, the sizes of the ranges occupied by most of earth's plants would increase.  We additionally hypothesize that many of the animals that depend upon those plants for food and shelter would exhibit analogous behavior.  Hence, with respect to both plants and animals, we would anticipate that nearly everywhere on earth, local biodiversity or species richness would increase in a world of rising air temperature and atmospheric CO2 concentration, as the expanding ranges of the planet's plants and animals overlapped those of their neighbors to an ever-increasing degree.

The implications of these observations are clear: if the planet continues to warm, even at what climate alarmists call "unprecedented rates," we need not worry about earth's plants and animals being unable to migrate to cooler regions of the globe fast enough to avoid extinction, as long as the air's CO2 content continues to rise at its current rate.  So obvious is this conclusion, in fact, that Cowling (1999) has bluntly stated that "maybe we should be less concerned about rising CO2 and rising temperatures and more worried about the possibility that future atmospheric CO2 will suddenly stop increasing, while global temperatures continue rising."